3.1.4.2 AUDITORY EFFECTS

Humans near some types of pulsed radar systems haveperceived individual pulses of RFR as audible clicks(without use of electronic receptors). This phenomenon,first investigated by Frey (1961), attracted much interestbecause it has been cited often as evidence that nonthermaleffects can occur and because an initial hypothesis was thata possible mechanism for perception is direct stimulation ofthe central nervous system by RFR.

Many of the results support the hypothesis that a pulseof RFR having the requisite pulse power density and durationcan produce a transient thermal gradient large enough togenerate an elastic shock wave at some boundary betweenregions of dissimilar dielectric properties within the head,and that this shock wave is transmitted to the middle ear,where it is perceived as a click. Persons with impairedhearing are unable to hear such clicks, and experimentalanimals in which the cochlea (inner ear) has been destroyeddo not exhibit brainstem-evoked responses.

Although this topic is presented here under "Studiesof Humans," the results of experiments with animalsubjects and nonbiological targets are also discussed, toavoid fragmenting the descriptions of several studies thatinvolved both human and animal subjects.

Frey (1961) exposed human subjects to either 6-microsecond pulses of 1.3-GHz RFR at 244 pps (0.0015 dutycycle) or 1-microsecond pulses of 3.0-GHz RFR at 400 pps(0.0004 duty cycle). The ambient noise levels wererespectively about 70 and 80 dB, but earplugs decreased thenoise by about 25-30 dB. The mean threshold of average powerdensity for RFR perception was about 0.4 mW/sq cm at 1.3 GHzfor eight subjects and 2 mW/sq cm at 3.0 GHz for sevensubjects. The corresponding peak power densities were about270 and 5000 mW/sq cm. (No variances or other statisticaldata were given.)

The subjects were unable to match the RFR sounds to audiosine waves. With white noise controlled by a variable band-pass-filter, best match was obtained by removing allfrequencies below about 5 kHz.

Four subjects with various degrees of hearing loss (forair-conducted and bone-conducted sound) were tested forperception of the 1.3-GHz RFR. Subject 1 with significanthearing loss of both kinds above about 2 kHz was unable toperceive the RFR sound at intensities 30 times above thethreshold. Subject 2, who had bilateral severe air-conduction loss (about 50 dB) but moderate bone-conductionloss (about 20 dB), was able to perceive the RFR sound atabout threshold level. Subject 3, with tinnitus andbilateral hearing loss ranging from about 10 dB at 250 Hz toabout 70 dB at 8 kHz for air conduction, more severe lossfor bone conduction, and who had been diagnosed as havingneomycin-induced nerve deafness, was unable to perceive theRFR. Subject 4, who had normal bilateral air-conductionhearing to about 4 kHz but severe bilateral bone-conductionloss was also unable to perceive the RFR.

The author had concluded that a necessary condition forperception of RFR as sound was the ability to hear soundabove about 5 kHz, but not necessarily by air conduction.Frey (1962), however, stated that some subjects with anaudiogram notch around 5 kHz (i.e., with adequate hearingfor frequencies above 5 kHz) did not perceive RFR sound. Inthis later study, the RFRs used were 425-MHz pulses of 125,250, 500, 1000, and 2000 microseconds at 27 pps (respectiveduty cycles of 0.0034, 0.0068, 0.0135, 0.027, and 0.054) and2.5-microsecond pulses of 8.9-GHz RFR at 400 pps (duty cycle0.001). The ambient noise levels were 70-90 dB and thesubjects wore Flent antinoise ear stopples, which diminishedthe ambient levels by about 20 dB from 100 Hz to 2 kHz andby about 35 dB at 10 kHz.

The average-power-density thresholds for perception of125-, 250-, 500-, and 1000-microsecond pulses of 425-MHz RFRwere 1.0, 1.9, 3.2, and 7.1 mW/sq cm, respectively; thecorresponding peak power densities were 300, 280, 240, and260 mW/sq cm (again given without statistical data). (Thethreshold for 2000-microsecond pulses of 425-MHz RFR was notdetermined because of inadequate instrumentation.) Thus, allfour 425-MHz peak-power values were comparable to the 1.3-GHz threshold previously found (Frey, 1961), implyinginsensitivity to frequency in this range. The 3-GHz peak-power threshold was much higher, however, about 5 W/sq cm,and the 8.9-GHz RFR was not perceived for peak-powerdensities as high as 25 W/sq cm. The author suggested thatthe perception-threshold rise from 1.3 GHz upward wasrelated to the dependence of penetration depth on frequency.The author, noting the high ambient noise levels, alsosuggested that the thresholds would be lower in quieterenvironments.

Frey (1962) also speculated about the possible sites andmechanisms of detection of pulsed-RFR, including RFR-inducedchanges of the electrical capacitance between the tympanicmembrane and the oval window, detection in the cochlea, andinteraction of RFR with neuron fields in the brain. Thefirst possibility was discounted because of insensitivity ofthe RFR-hearing effect to head orientation relative to theRFR source. He indicated that the then-current experimentalresults were inconclusive relative to the other twopossibilities.

Frey (1967) endeavored to resolve this point by studyingthe potentials evoked in the cat brain by exposure to 10-microsecond pulses within the range 1.2-1.5 GHz. Oneelectrode was implanted in the brain stem of each cat, withtip in the nucleus subthalamicus, formatio reticularis,nucleus olivaris, or nucleus reticularis paramedianus. Theelectrode was of coaxial design to avoid RFR energy pickup.(The author stated that during extensive testing, theelectrode showed no indication of energy pickup.)

The procedure after recovery from surgery (4-6 weeks) wasto anesthetize the cat with Fluothane in oxygen, place it inan exposure chamber lined with RFR-absorbent materials,adjust the anesthesia to a depth that just preventedvoluntary movement, and evaluate the status of the electrodeby monitoring the pattern of brain electrical activity for ashort time. The head of the cat was then exposed to the RFRfrom a horn 50 cm above, and the first 30-ms of brain-stemoutput after each pulse was recorded and averaged oversuccessive pulses with a computer. Synchronization pulses atappropriate repetition frequencies were used to trigger theRFR generator and (with a 1-ms time delay) the oscilloscope,recorder, and computer used for assessing the evokedresponses.

During experimental sessions, exposures of each cat tothe RFR were alternated with 5-min rest periods.Interspersed with the RFR-exposures were runs with CW orpulsed acoustic stimuli at the repetition frequency of theRFR, but no details were given regarding how such stimuliwere presented. The electrical activity in each brain regionwas recorded just before, and a few minutes after,euthanizing the cat.

The author indicated, without giving data, that thethreshold average and peak power densities necessary toevoke brain-stem potentials were about 0.03 and 60 mW/sq cm.These values correspond to a duty cycle of 0.0005 and (for10-microsecond pulses) to a pulse repetition frequency of50. The results presented were representative averagedwaveforms evoked in the four brain-stem regions by the RFRand pulsed acoustic stimuli. These showed that responseswere evoked by both stimuli before but not aftereuthanization, indicating that the responses were notartifactual. The author also stated that no cochlearmicrophonic was apparent in response to the RFR, but thispoint could not be discerned from the waveforms shown.

Aset of four waveforms from the nucleus subthalamicusevoked by RFR at frequencies of 1.2, 1.3, 1.425, and 1.525GHz showed the amplitudes for the two lower frequencies tobe about the same, but that the amplitude was lower at 1.425GHz and almost negligible at 1.525 GHz, results taken by theauthor to indicate that there is a broad range of optimalcarrier frequencies consonant with calculations of RFRpenetration in the head.

Frey and Messenger (1973) exposed humans to pulsed 1.245-GHz RFR at 50 pps in an RFR anechoic chamber. In one set ofexperiments, the average power density was held constant at0.32 mW/sq cm and the pulse width was varied from 10 to 70microseconds in 10-microsecond increments, yielding peakpower densities from 640 to 91 mW/sq cm. In another set, thepeak power density was held constant at 370 mW/sq cm and thepulse width was varied over the same range, yielding averagepower densities from 0.19 to 1.3 mW/sq cm. Four subjectswith clinically normal hearing were each given 3 trials. Thestart of each trial was signaled optically. After a variableinterval of up to 5 seconds, each subject was first given apulsed RFR signal for 2 seconds, the perceived loudness ofwhich was to be taken as reference level 100. About 5seconds later, the test RFR signal was presented for 2seconds and the subject was to signal its numerical loudnessrelative to the reference.

The results were presented as plots of perceived loudnessvs peak power density and vs average power density (all onlogarithmic scales). The point plotted for each testcondition was the median value, without deviations, for allsubjects and repetitions; no individual data were given. Inthe peak-power-density plot, the loudness rose sharply fromabout 3 at 91 mW/sq cm (70-microsecond pulses) to 60 at 125mW/sq cm (50-microsecond pulses); at higher power densities,the sound level rose more slowly to a slightly risingplateau, i.e., to about 100 at 210 mW/sq cm (30-microsecondpulses), 120 at 315 mW/sq cm (20-microsecond pulses), and aslightly lower value at 630 mW/sq cm (10-microsecondpulses). The plateau indicated that there is an optimal bandof pulse widths, within which the perceived loudness dependson the peak power density. The author ascribed the loudnessdiminution at 630 mW/sq cm to 10-microsecond pulses beingshorter than optimal duration. The plot of median loudnessvs average power density showed more scatter, with thepoints ranging from about 60 at 0.19 mW/sq cm (10-microsecond pulses) to a relatively flat maximum of 100 at0.55 mW/sq cm (30-microsecond pulses) and diminishing toabout 40 at 1.29 mW/sq cm (70-microsecond pulses). Theauthor ascribed the dip for 70-microsecond pulses to thisduration being longer than optimum.

From their data, Frey and Messenger (1973) calculatedthat the peak-power-density threshold for perception of RFRpulses is about 80 mW/sq cm, a value lower than thosereported subsequently by Cain and Rissman (1978), discussedlater. In the absence of information on scatter of theresponses by each subject and because subjective judgmentsof relative loudness may be imprecise, the accuracy of theresults of Frey and Messenger (1973) could not beevaluated.

White (1963) reported that when the surface of a body istransiently heated by RFR-absorption (or electronbombardment), elastic waves are produced by surface motiondue to thermal expansion. This process was analyzedtheoretically, with emphasis on the case of an input heatflux varying harmonically with time, to relate the amplitudeof the elastic waves to the characteristics of the inputflux and thermal and elastic properties of the body.Experiments with both electron impact and RFR-absorptionverified the proportionality of the stress wave amplitudeand the absorbed power density, and correlated well with thethermal and elastic properties of the heated medium.

Elastic waves were detected in all metals tested, inseveral carbon-loaded plastics, in water, and in a silver-coated barium titanate piezoelectric crystal. Mixing(production of beat frequencies) was observed when twopulses of different RFR frequencies were absorbedsimultaneously. Comparison of the elastic-wave stressamplitude with radiation pressure showed that the former maybe much greater than the latter, as demonstratedexperimentally. When a barium titanate crystal was used todetect the elastic waves produced, heating by a single 2-microsecond pulse of electrons or RFR produced easilydetectible signals at pulse power densities down to 2 W/sqcm, which corresponded to a computed peak surface-temperature rise of about 0.001 deg C and which producedpiezoelectric-crystal voltages ranging from about 1 to morethan 60 mV per kW/sq cm of absorbed power density.

Foster and Finch (1974) confirmed White's findings thatRFR pulses can produce acoustic transients in water, andshowed by calculation that for short pulses, the peak soundpressure is proportional to the energy per pulse, whereasfor long pulses, it is proportional to the incident powerdensity. Using 2.45-GHz RFR in several pulse-power-densityand pulse-duration combinations and a hydrophone in saline(0.15-N KCl), they found that the transition between the tworegimes occurs for pulse durations between 20 and 25microseconds. The authors noted that the dependence of soundpressure on pulse duration and incident peak power densityis consistent with the results of Frey and Messenger (1973)at 1.245 GHz. They also found that acoustic signals were notobtained in water at 4 deg C (where its thermal expansioncoefficient is essentially zero) and that the polarity ofthe transient acoustic signal between 0 and 4 deg C wasreversed from that for temperatures above 4 deg C. Theseresults support the thermoelastic expansion hypothesis.

Sharp et al. (1974), in an experiment involving shieldingregions of a subject's head from 1.5-GHz RFR pulses with RFRabsorber, noticed that the apparent locus of the perceivedsound moved from the head to the absorber. They thenconfirmed transduction of the RFR by the absorber intoacoustic signals by using a sound-level meter to measure thedelay times for acoustic propagation for distances of 0.3 to0.6 m between the absorber and microphone. The pulses were14-microseconds long and were randomly triggered at about 3pps. By calculation, the power per pulse was 4.5 kW and thatthe pulse power densities were 7.5-15 kW/sq m (750-1500mW/sq cm) for the range of separations above.

Subsequent tests by Sharp et al. (1974) showed thatvarying the carrier frequency from 1.2 to 1.6 GHz or using2.45 GHz made little difference in the level or quality ofthe sound. In addition, detectable sounds could be producedwith various sizes and shapes of absorber, including piecesas small as 4 mm square by 2 mm thick, and in various typesof absorber and in crumpled aluminum foil. The thresholdpulse power for audibility was about 275 W, yieldingestimated pulse power densities in the range 0.46-0.92 kW/sqm (46-92 mW/sq cm). Tests were also done with constant pulserepetition rates up to 500 pps, with the finding: "Thesound produced from the absorber seemed to track therepetition rate of the microwave pulses."

Taylor and Ashleman (1974) surgically prepared threegroups of three cats, for recording potentials evoked byacoustic and RFR stimuli in three brain regions anddetermining the effect of cochlear disablement. Each cat wasfitted on the dorsal surface of the frontal bone with apiezoelectric transducer for presentation of acousticstimuli; it was removed for RFR-exposure. In the cats ofgroup 1, the eighth cranial (vestibulocochlear) nerve wasexposed, a glass pipette microelectrode filled with Ringer'ssolution was inserted into the nerve, and the round windowon the same side was exposed. In those of group 2, a similarelectrode was advanced into the medial geniculate to alocation in the nucleus that yielded acoustically evokedpotentials of appropriate characteristics, and both roundwindows were exposed. In the cats of group 3, a Teflon-covered carbon electrode was placed on the anteriorectosylvian gyrus of the primary auditory cortex and bothround windows were exposed. Connections to the electrodeswere made with carbon leads of high resistance.

The acoustic stimuli were produced by feeding 10-microsecond electric pulses to the piezoelectric transducerat 1 pps. The RFR stimuli were 32-microsecond, 2.45-GHzpulses at 1 pps fed via a directional coupler to a hornplaced 10 cm posterolaterally from the cat's head at 30 degrelative to the sagittal plane. Incident power densitieswere measured with a bolometer.

Following post-surgical stabilization, the lowestpiezoelectric voltage that yielded a response was determinedand the voltage was increased to a level that was maximal inevoking activity. The transducer was then removed, the headof the cat was exposed to the RFR, and the minimum andmaximal levels for evoking responses were determined. Whenresponses to both acoustic and RFR pulses were clearcut, thecochlea was disabled by perforating the round window andaspirating the perilymph, and again the response to eachstimulus was determined. The responses of the medialgeniculate nucleus and auditory cortex were assessed afterdisabling each cochlea. When no evoked response occurredafter disabling the cochlea, the stimulus was raised to themaximum available in each case.

The results indicated that cochlea destruction led tototal loss of evoked potentials to both acoustic and RFRstimuli. Specifically, the eighth-nerve potentials were lostafter unilateral cochlea destruction, the evoked-potentialamplitudes from the medial geniculate and auditory cortexwere markedly attenuated by aspiration of the contralateralcochlea, and disablement of the remaining cochlea resultedin total loss of the potentials.

From their results, the authors concluded: "Webelieve that the data strongly support the contention thatthe microwave auditory effect is exerted on the animal in amanner similar to that of conventional acoustic stimuli.Clearly, the elimination of the first stage of transductionaffects the central nervous system response to both of theseforms of stimulus energy in the same way."

Guy et al. (1975b) determined the power density thresholdand modulation characteristics for the RFR-hearing effect inhuman volunteers, compared the potentials evoked in foursuccessive levels of the cat auditory nervous system by RFRand acoustic pulses, used optical interferometry toquantitate the transduction of RFR pulses to acoustic energyin RFR-absorbing materials, and provided evidence that theRFR-hearing effect is due to direct conversion of RFR energyto acoustic energy in the tissues.

The back of the head of each of two human subjects wasexposed to RFR at 15-30 cm from the aperture of a horn (inthe near field) in an anechoic chamber at an ambient noiselevel of 45 dB, with RFR-absorbent material around thevicinity of the subject to eliminate reflections. The RFRconsisted of 2.45-GHz pulses of duration that was variedfrom 1 to 32 microseconds. For each pulse duration, the RFRwas presented in trains of three pulses per second, with thepulses in each train spaced 100 ms apart. The subject used aswitch to signal perception of an auditory sensation.Standard audiograms taken prior to exposure showed that thehearing threshold of subject 1 was normal and that subject 2had a deep notch at 3.5 kHz for both ears, with similarresults for air and bone conduction.

The results for subject 1 showed that irrespective ofpulse duration, the threshold for perception of the RFR wasa constant peak energy density per pulse (product of peakpower density and pulse duration) of 40 microjoules/sq cm.The corresponding incident average power density (for 3 pps)was 0.12 mW/sq cm. When subject 1 wore ear plugs, thethreshold peak was only 28 microjoules/sq cm per pulse.Based on a spherical model of the head (Johnson and Guy,1972), the threshold peak specific absorbed energy (SAE)corresponding to 40 microjoules/sq cm per pulse was 16mJ/kg. The threshold for a pair of pulses within severalhundred microseconds apart was the same as for one pulsewith the same total energy as the pair. Similar results wereobtained for subject 2 except that the threshold peak energydensity was 135 microjoules/sq cm per pulse or aboutthreefold (5 db) higher than for subject 1.

The authors noted that each pulse was perceivedindividually as a click and that short pulse trains wereheard as chirps of tones corresponding to the pulserecurrence rate. Also, when the pulse generator was keyedmanually, digital (Morse) code transmitted thereby could beinterpreted accurately by the subject.

For the study of cats, each was fitted with a removablepiezoelectric transducer to provide bone-conducted acousticstimuli. A nonperturbing electrode consisting of a glasspipette filled with Ringer's solution was insertedsurgically into the medial geniculate nucleus to a locationthat yielded acoustically evoked responses of the properlatency period. To minimize RFR pickup by theinstrumentation, the electrode and ground connection werecoupled with high-resistance carbon-loaded plastic leadsthrough a low-pass filter to a high-impedance amplifier,oscilloscope, computer of average transients, and x-yplotter.

The acoustical stimuli consisted of pulses 1-30microseconds in duration that were air-conducted from aspeaker 17 cm to the right of the cat's head or weretransducer-induced. The RFR stimuli were pulses, 0.5-32microseconds in duration, of 918-MHz or 2.45-GHz RFR from ahorn or aperture 8 cm from the occipital pole of the cat.Pulses of 8.5-10 GHz RFR were also used, as noted below.Each stimulus was presented at 1 pps. A noise generatorprovided background noise of up to 90 dB in the range 50-15000 Hz.

Representative response curves evoked by 20-microsecondair-conducted and bone-conducted acoustic pulses and by 20-microsecond pulses of 2.45-GHz RFR were displayed and weresimilar. The threshold peak energy-density per pulse forperception of 2.45-GHz RFR varied only from 17.8microjoules/sq cm for 0.5-microsecond pulses to 21.6microjoules/sq cm for 10-microsecond pulses, values abouthalf the human threshold, but it increased more steeply withpulse duration, to 47.0 microjoules/sq cm for 32-microsecondpulses (except for 25-microsecond pulses, for which thethreshold was only 15.2). The peak SAEs were determined byscanning infrared thermography. The values corresponding to21.6 and 47.0 microjoules/sq cm per pulse respectively were12.3 and 26.7 mJ/kg.

The threshold energy-density values for 918-MHz pulseswere 22.6 and 28.3 microjoules/sq cm per pulse respectivelyfor 10- and 32-microsecond pulses (with no dip for 25-microsecond pulses), and the corresponding peak SAEs perpulse were 16.0 and 20.0 mJ/kg.

The RFR thresholds above were obtained with 64 dB ofbackground noise. Increases of the noise level to 80 dB (forpulses up to 10 microseconds) yielded insignificant changesin thresholds. However, the energy-density threshold forbone-conducted acoustic stimulation was about tenfold higherat 80 than 64 dB; for air-conducted acoustic stimulation,the threshold was a hundredfold higher for 70 dB of noiseand was still higher for 80 dB, but by a factor of less thanten.

With 8.5-10 GHz, responses could be evoked only byexposing the brain through a large hole in the skull, withthe RFR horn within a few cm from the hole. The thresholdvalues were a peak incident power density of 14.8-38.8 W/sqcm per pulse, which corresponded to an average power densityof 0.472-1.240 mW/sq cm (for 32-microsecond pulses, 1 pps)and an energy density of 472-1240 microjoules/sq cm perpulse.

In another series of experiments, Guy et al. (1975b)fitted cats with a piezoelectric transducer and inserted anonperturbing electrode in the medial geniculate nucleus asbefore, and also surgically exposed the round window of thecochlea and attached thereto an electrode and leads, both ofhigh-resistance material, for recording the cochlearresponses evoked by acoustic and RFR pulses. The responsesof one cat to an acoustic pulse from a loudspeaker and a2.45-GHz pulse were displayed. The curve obtained forstimulation with the loudspeaker pulse showed the N1 and N2auditory-nerve response and a cochlear microphonic similarto the pulse-induced decaying vibratory movement of theloudspeaker cone, which was determined with an opticalinterferometer. By contrast, the curve evoked by the RFRpulse showed the N1 and N2 response only, and no evidence ofa cochlear microphonic. However, the cochlear microphonicfor another cat stimulated by speaker pulse was of muchlower amplitude (relative to the N1 and N2 response) and wasabsent in the response of another cat stimulatedacoustically with the piezoelectric transducer. Thus, theabsence of an RFR-induced cochlear microphonic does not ruleout theories of the RFR-hearing effect based on transductionof RFR to acoustic energy.

In experiments similar to those of Taylor and Ashleman(1974), Guy et al. (1975b) also studied the effect ofcochlea disablement. Cats were prepared surgically forrecording evoked potentials in the eighth cranial nerve,medial geniculate nucleus, and primary auditory cortex.After establishing that appropriate responses were obtainedwith RFR and acoustic pulses, the cochlea was disabled,which resulted in total loss of all evoked potentials, evenwith the highest available peak acoustic and RFR powers andwith computer averaging of larger numbers of signals.

Guy et al. (1975b) used an interferometer and a lasersource to detect surface movements of several lossymaterials induced by absorption of RFR pulses. (This devicewas also used to observe the speaker-cone movements notedabove.) The results showed that surface displacementamplitude is dependent on the dielectric constant and lossfactor of the material and on its density and elasticproperties. An interesting result noted by the authors wasthe high amplitude obtained in a widely used RFR-absorbentmaterial, a finding ascribed to its relatively low densityand high compressibility.

Chou et al. (1975) studied the induction of cochlearmicrophonics (CM) in the guinea pig by pulses of 918-MHzRFR. For this purpose, they placed a fine RFR-transparentcarbon lead against the round window and cemented it ontothe bulla. An indifferent electrode was fastened to nearbytissue. Only preparations that yielded CM amplitudesexceeding 0.5 mV in response to 70-dB speaker clicks wereused. The head of the guinea pig was placed within a sectionof cylindrical waveguide through a hole. The section wasterminated with a sliding short, adjusted to yield maximumRFR absorption in the head. With this arrangement, theaverage SAE per pulse at 10 kW peak input power was 1.33J/kg, or about an order of magnitude larger than the levelsused in previous studies. The sound level near the animal'shead was about 65 dB, mostly due to the noise from the pulsegenerator.

Each guinea pig was exposed intermittently for 1.5-mindurations to 1-10 microsecond pulses of 918-MHz RFR, 100pps, at various levels of peak power. The responses wereamplified and recorded on a magnetic tape system that had afrequency response up to 80 kHz. After 3-5 hr, the animalwas euthanized and recording of its response was continueduntil the physiological potentials disappeared completely.Recorded responses were processed off-line with a Computerof Averaged Transients.

The electrical responses at the round window of a guineapig stimulated with single acoustic clicks showed that theCM preceded in time the N1 and N2 action potentials, andthat when the polarity of the electrical pulses delivered tothe speaker was inverted, only the polarity of the CM wasreversed. Stimulation of the same animal with single RFRpulses yielded N1 and N2 potentials of about the sameamplitude. In addition, barely discernible was an"electrical event" during the 200-microsecondinterval immediately following the recording artifact causedby the RFR pulse. Time-expansion of this interval showedthis event to be a 50-kHz oscillation of amplitude about 50microvolts, which decayed within the 200-microsecondinterval. This event, which was observed in five guineapigs, was denoted as the RFR-induced CM response.

Comparison of the CM responses to 10-, 5-, and 1-microsecond, 10-kW RFR pulses, which corresponded to SAEs of1.33, 0.67, and 0.133 J/kg per pulse, showed that the CMfrequency remained the same but the amplitude dropped withdecreasing pulse duration and SAE. Also evident was that thestimulus artifact masked the onset of the CM in each case,but that the latency for successive oscillations was aboutthe same for the three pulse widths. These results supportthe conclusions that the CMs are physiological responsestime-locked to the onset of the RFR pulses and are generatedwithin the cochlea, specifically by hair-cellactivation.

With death of an animal, the N1 and N2 responses toacoustic and RFR pulses disappeared, but the RFR-induced CMpersisted for many minutes after the N1 and N2 responses hadgone. The stimulus artifact remained after the CM haddisappeared, indicating that the 50-kHz signal is a genuinephysiological response.

The threshold SAE for producing the RFR-hearing effect inthe guinea-pig head was 20 mJ/kg, about the same order ofmagnitude as for the cat head (10-16 mJ/kg) and the humanhead (16 mJ/kg). The authors surmised that previous failuresto observe RFR-induced CMs in animals may have been due touse of SAEs below the threshold, masking by stimulusartifact, or use of amplifiers with pass bands that did notinclude 50 kHz.

The authors noted that guinea pigs can respond to tonesup to 100 kHz, and suggested that the 50-kHz CM may berelated to the size of the animal's skull. Based on thispremise, the CM frequency would be within the range 15-50kHz for cats and 5-18 kHz for humans.

Guy et al. (1975b) and Lin (1976a,b; 1977a,b,c) analyzedthe postulated mechanisms for the conversion of RFR energyto acoustic energy in lossy dielectric materials. Theyconcluded that pulsed-RFR-induced thermal expansion forces,which are proportional to the square of the peak electricfield, are much larger than the radiation pressure or theelectrostriction produced by the same RFR pulses and cangenerate in the head acoustic waves of the requisitemagnitude for the hearing effect.

In Lin (1977c), equations developed for a spherical modelof the head consisting of brain-equivalent material wereused to obtain the acoustic resonant frequencies generatedin the heads of guinea pigs, cats, and human adults andinfants by exposure to RFR pulses. The results showed thatthe (fundamental and higher-harmonic) frequencies producedby RFR pulses are independent of the carrier frequency, butare dependent on head size, and specifically that thefundamental frequency is inversely proportional to theradius of the head.

Predicted from the equations was a fundamental frequencyof 45 kHz for a 2-cm (guinea-pig) head, which was close tothe 50-kHz experimental value found by Chou et al. (1975).For a 3-cm head, the predicted fundamental was 30 kHz, ascompared with 38 kHz found experimentally for a typical cat.For humans, the predicted fundamental frequencies were 13kHz for an adult and 18 kHz for an infant.

Chou et al. (1977), using the method described in Chou etal. (1975), recorded CMs from the round windows of guineapigs and cats of various sizes induced by exposure to 10-microsecond pulses of 918-MHz and 2.45-GHz RFR. Exposureswere done with horn applicators and the cylindricalwaveguide system described above. In the guinea pigs, the CMfrequency varied inversely with body mass; in the cats,however, there was no consistent variation of CM frequencywith body mass. The authors noted that although head mass,skull mass, skull dimensions, skull thickness, andcerebellar-cavity dimensions all increase with body mass,the brain cavity and bulla dimensions increase onlyslightly. They found that CM frequency correlates well withthe length of the brain cavity but not with the otherdimensions of the head or skull. The average thresholdenergies per pulse for CM responses were 10 mJ/kg for adultcats, 2.5 for kittens, and 7.5 for adult guinea pigs.

Cain and Rissman (1978) used 3.0-GHz RFR pulses to studythe auditory effect in two cats, two chinchillas, onebeagle, and eight human volunteers. For the animals, surfaceor brainstem-implanted electrodes were used to measure theresponses evoked by audio clicks from a speaker and theresponses to 5-, 10-, and 15-microsecond pulses.

The threshold peak power densities were 2.2 W/sq cm for5-microsecond pulses, 1.3 W/sq cm for 10-microsecond pulses,and 0.58 W/sq cm for 15-microsecond pulses for one cat, andrespectively 2.8, 1.3, and 0.58 W/kg for the other cat. Thecorresponding threshold peak power densities for the beaglewere 1.8, 0.30, and 0.20 W/sq cm. The values were 2.8, 2.0,and 0.58 W/sq cm for one chinchilla and 2.2, 1.0, and 0.50W/sq cm for the other. Thus, for corresponding pulsedurations, the beagle had the lowest thresholds and thelowest absolute threshold (irrespective of pulseduration).

The authors found that depending on pulse width, therange of threshold energy density for RFR perception was8.7-14 microjoules/sq cm per pulse for the cats and 7.5-20microjoules/sq cm for the chinchillas, and that thethreshold averaged 5.0 microjoules/sq cm for the beagle. For10-microsecond pulses, the threshold pulse power densitieswere 1.3 W/sq cm for both cats, 1 and 2 W/sq cm for the twochinchillas, and 300 mW/sq cm for the beagle.

The eight humans were given standard audiograms for bothair-conducted and bone-conducted sound. In addition, becauseaudiograms do not test hearing above 8 kHz, binaural hearingthresholds were determined for seven of the subjects fortone frequencies in the range 1-20 kHz. The RFR pulses werepresented at 0.5 pps. Each subject wore foam ear muffsduring exposure, to reduce the ambient noise level, whichwas 45 dB.

Subjects 1-5 could hear 15-microsecond pulses as clicks;their peak power density thresholds were respectively 300,300, 300, 600, and 1000 mW/sq cm, and their energy densitythresholds were 4.5, 4.5, 4.5, 9.0, and 15.0 microjoules/sqcm. Subjects 1-5 could also hear 10-microsecond pulses, withpeak power density thresholds of 1800, 225, 600, 2000, and2000 mW/sq cm, respectively, and energy density thresholdsof 18.0, 2.3, 6.0, 20.0, and 20.0 microjoules/sq cm. Subject1 was the only one able to perceive 5-microsecond pulses,with a threshold peak power density and energy density of2500 mW/sq cm and 12.5 microjoules/sq cm. The other threesubjects, 6-8, could not hear these pulses at the highestavailable peak power density but could perceive 20-microsecond pulses.

The authors found no correlation between the results andthe standard audiograms. However, they did note that astrong correlation existed between RFR perception andhearing ability above 8 kHz as determined from the binauralthresholds. They also stated that their results areconsistent with the hypothesis that an induced pressure wavein the human head in response to short RFR pulses (less than20 microseconds) contains a significant portion of itsenergy at frequencies above 8 kHz.

In summary of these results with humans, only subjects 1-3 were able to perceive 15-microsecond pulses at a pulse-power-density threshold as low as 300 mW/sq cm (energy-density-threshold of 4.5 microjoules/sq cm); of this group,only subject 2 could hear 10-microsecond pulses, with 225mW/sq cm (2.3 microjoules/sq cm) as the threshold; thethresholds for the other subjects were much higher than 300mW/sq cm. The latter value of pulse power density can betaken as the nominal human threshold for the RFR hearingeffect (e.g., for environmental assessments). It should benoted that the thresholds cited were for an ambient noiselevel of 45 dB and could be higher in noisier environments.It is also noteworthy that these investigators exposed thehuman volunteers to pulse power densities as high as 2,000mW/sq cm without apparent ill effects.

Lebovitz and Seaman (1977) studied the responses insingle auditory units of cats to acoustic clicks and pulsesof 915-MHz RFR. For this purpose, the posterolateral aspectof the cerebellum was removed and a recording micropipettewas inserted into the proximal portion of the eighth nerve.Acoustic clicks ranging from 25 to 200 microseconds induration, but typically 70 microseconds, were presented toone ear at 10 clicks per second, with the contralateral earstoppled. The durations of the RFR pulses were in the samerange, the repetition rates were 10 pps or less, and thepulses were delivered at forward peak powers of up to 70 Wwith an applicator, yielding average power densities thatnever exceeded 1 mW/sq cm.

Medullary SARs were determined by euthanizing the catsafter completing several experiments, letting the medullarytemperature fall to about 30 deg C, exposing the heads ofthe cats to an appropriate level of CW RFR for periods of upto 80 seconds, inserting a thermistor into the medullaimmediately before and after exposure, deriving the coolingcurves, and using them to determine the linear relationshipof temperature rise to exposure duration. From the slope ofthis line, 0.011 deg per second, and the effective forwardpower, 48.6 W, the normalized SAR was 0.94 W/kg per mW/sqcm. The energy absorbed per pulse was then calculated fromthe SAR and the peak power and duration.

Of 133 auditory units studied, 63 were responsive to bothstimuli, and additional dose responses were obtained for thelatter units as long as each showed stable responses. Theapparent absence of response of 70 units to RFR was ascribedto the limited range of intensities available, about 10 dBas compared with 50-80 dB for the acoustic clicks. For atypical single auditory unit that did respond to RFR, theresponse was very similar to its response to acousticclicks, differing primarily only in amplitude. The lowestRFR-energy-absorption threshold for a single unit was 4mJ/kg. The latency interval between stimulation and responseincreased with decreasing acoustic or RFR stimulusintensity. The smallest latencies observed were 1.5-2 ms,with values up to 5 ms for near-threshold intensities.

The authors noted that the characteristic frequency (CF)of a unit is the frequency at which its response thresholdis lowest and that the CF is determined by the mechanicalproperties of that part of the basilar membrane to which thecell is most directly related. Therefore, for responses to aclick that show multiple peaks, the interpeak interval(i.e., the period between the first and second peaks) isabout the same as the oscillation period of the basilarmembrane and the former is a measure of the latter. Theresults showed high correlation between unit CFs foracoustic and RFR stimuli, an indication that the samemechanical factors within the cochlea are involved.

Chou and Galambos (1979) investigated the effects in 10guinea pigs of external-ear blocking, middle-ear damping,and middle-ear destruction on brainstem-evoked responses(BERs) to both acoustic and RFR stimuli. The basicmeasurement technique was to record the amplitudes andlatencies of the BERs to acoustic stimuli and RFR with apair of carbon-loaded Teflon electrodes (Chou and Guy,1979a), one of which was attached to the left mastoidprocess and the other to the skin.

The head of each guinea pig was exposed to 10-microsecondpulses of 918-MHz RFR at 30 pps in a cylindrical waveguidesystem (Chou et al., 1975) at energies ranging from 0.027 to11.05 J/kg per pulse. All 10 animals were exposed to 0.1-msacoustical pulses at 30 pps from a piezoelectric tweeterplaced 15 cm from the nose (air conduction). For threeanimals, comparisons were also made between BERs to airborneand bone-conducted acoustic stimuli, using a piezoelectrictransducer in contact with the frontal bone for thelatter.

BERs were recorded after each of the following successivetreatments: (1) blocking of the left external meatus withmineral-oil-soaked cotton balls, (2) alteration of themechanical damping of the ossicular chain by filling thebulla with mineral oil, (3) destruction of the middle ear bycutting the ossicular chain and piercing the tympanicmembrane, and (4) destruction of the cochlea by piercing theround window.

Treatments 1 and 2 reduced the airborne acoustically-stimulated BERs but not the RFR-induced BERs. Treatment 3further reduced the airborne acoustic BERs, and also reducedthe bone-conducted acoustic BERs and the RFR BERS to alesser extent than the airborne acoustic BERs. Aftertreatment 4 (cochlea destruction), no BERs were obtainedfrom either acoustic or RFR stimuli.

These results constitute strong evidence that activationof the cochlea is necessary for auditory perception ofpulsed RFR. The similar BERs obtained from bone-conducted-acoustic and RFR stimuli after destruction of the middle earand the much lower BERs obtained for airborne-acousticstimuli support the hypothesis that perception is due totransduction of RFR into acoustic waves that travel via boneconduction to the cochlea or are generated directly in thecochlea itself.

Chou and Guy (1979b) performed similar experiments withBERs induced in guinea pigs, to determine the RFR thresholdsfor BERs. The input-power thresholds for BERs induced by918-MHz RFR were determined for pulse widths of 10 to 500microseconds, and the values were divided by the cross-sectional area of the cylindrical waveguide (about 320 sqcm) to obtain the corresponding threshold peak incidentpower densities. Also derived was the incident energydensity per pulse for each pulse width, and the pulserepetition frequency (30 pps) was used to calculate theincident average power density. Lastly, the threshold SAEper pulse was obtained from the incident energy density perpulse by dividing the latter by the mass of the animal'shead.

The authors found that the threshold energy density forevoking BERs was essentially constant (1.56-1.87microjoules/sq cm per pulse) for pulse durations of 10, 20,and 30 microseconds. The threshold SAE was 5 mJ/kg per pulseand the corresponding incident peak power densities were156, 78, and 62.4 mW/sq cm, respectively. For pulsedurations longer than 30 microseconds, however, thethreshold SAE increased with pulse duration, and for pulseslonger than 70 microseconds, the threshold peak powerdensity for evoking BERs was essentially constant (90 mW/sqcm), with corresponding pulse-width related increases ofenergy density per pulse.

The waveforms of the RFR and acoustic BERs were found tobe similar except for the longer latency of the latter (dueto the longer sound-propagation path). Despite the largedifferences in pulse width, the latencies of the RFR-inducedBERs were about the same, indicating that the evokedresponse is time-locked to the onset of the RFR pulse. Chouand Guy indicated that their experimental results agreedwell with the predictions of the thermal expansiontheory.

In a subsequent study, Chou et al. (1985a) exposedanesthetized rats to 2.45-GHz RFR pulses within a circularlypolarized waveguide (Guy et al., 1979) in threeorientations: (1) body along the waveguide axis and headtoward the source, (2) body across the waveguide, and (3)body along the waveguide axis and head away from the source.The BERs were recorded with carbon-loaded Teflon electrodes,one at the vertex of the rat's head and another at eitherthe left or right mastoid process (behind the pinna).

In one experiment, exposures were to pulses 10, 5, 2, and1 microseconds in duration at 10 pps and a fixed peak powerof 4 kW (spatially averaged peak power density of 12.3 W/sqcm) in the first orientation. The corresponding energydensities were 123.4, 61.7, 24.7, and 12.3 microjoules/sqcm. Representative BERs from one rat showed amplitudes thatdecreased with decreasing pulse duration or energy density.The responses were similar to those obtained from guineapigs by Chou and Galambos (1979), but the latency of thepeak BER was shorter in rats.

In another experiment, rats were exposed to RFR in eachorientation, and the largest responses were obtained in thefirst orientation. In this experiment, exposures were topulses of 1, 2, 5, and 10 microseconds at 10 pps anddifferent peak powers, to yield various energy densities.The BER amplitudes at the four pulse durations and constantenergy density were about the same, indicating that theresponse is dependent on energy per pulse and not on pulseduration. The threshold energy density (in the firstorientation) was 1.5 to 3 microjoules/sq cm per pulse. Basedon calorimetric data, the whole-body SAE was in the range0.9-1.8 mJ/kg. The corresponding peak power densities forthe four pulse durations were in the ranges 1.5-3, 0.75-1.5,0.3-0.6, and 0.15-0.30 W/sq cm, respectively. The authorsnoted that these peak power densities were for exposure inthe circularly polarized waveguide and that free-spaceexposure would require about threefold higher values.

Lin et al. (1979b) studied BERs induced in cats byacoustic and RFR pulses and the alterations of the BERs bythe successive coagulative formation of lesions in severalbrainstem regions. Under anesthesia, the dorsal aspect ofthe skull of each cat was surgically exposed and severalstainless-steel electrodes (100-250 microns in diameter)were advanced stereotaxically into the selected brainstemnuclei to locations that yielded maximal evoked potentials.In addition, a stainless-steel screw electrode was fastenedto the skull at the vertex and a reference gold-pinelectrode was placed near the lowest part of the rightpinna.

Acoustic pulses about 70 dB above threshold sound level,generated by feeding currents 0.1 ms in duration into a pairof commercial earphones, were presented binaurally at 10 to100 pps. Pulses of 2.45-GHz RFR, 0.5 to 25 microseconds wideand up to 10 kW peak, were delivered at 10-100 pps to thedorsal or frontal surface of the head with a small-diameter(15-mm), dielectrically loaded, direct-contact, diathermyapplicator. The bioelectric activities at the vertex and ateach brainstem location were fed through amplifiers having apassband of 80 Hz to 10 kHz and were summed with a signal-averaging computer. The first 10 ms of averaged responseswere displayed on a video monitor and photographed.

Brainstem lesions were produced in succession at the tipsof electrodes inserted in the inferior colliculus nucleus,lateral lemniscus, and superior olivary nucleus. The BERswere recorded after each lesion and compared with theprelesion BERs, as were microwave-evoked potentials (MEPs)and acoustically-evoked potentials (AEPs) recorded by thevertex electrode. At the end of each experiment, each catwas euthanized and its brain was fixed, removed, embedded inparaffin, and sectioned to ascertain the exact locations ofthe lesions and electrode tracks.

For each brainstem region, the pre-lesion RFR-induced BERwas similar to the acoustic BER, but with no significantpropagation delay for the RFR-induced BER relative to thestimulus pulses. The vertex MEPs showed four successivecycles, designated sequentially as Waves I, II, III, and IV.Of these, Wave III was often the largest and Wave IV thesmallest. The sources of the waves (as well as for the AEPs)were thought by the authors to be the volume-conductedaction potentials generated in the cochlea and auditorybrainstem nuclei.

To determine the effects on the MEPs of changing thepulse repetition rate (PRR), 10-microsecond RFR pulses at 5kW were applied sequentially at PRRs of 10, 25, 50, and 100pps and then in reverse order. Minimal or no changes inlatency were evident, but the amplitudes of the MEP waveswere found to decrease with increasing PRR, a reversibleeffect.

Next, 10-microsecond pulses with peak powers of 10 to 2kW were applied at 10 pps. Again, any changes in latency ofthe MEPs were minimal. The amplitudes of the MEPs decreasedwith decreasing peak power, but not in the same proportionfor each wave. In the example presented (for one cat), theamplitude of Wave I at 10 kW was larger than of Wave II, butdecreased faster with decreasing power than for Wave II, sothe Wave-II amplitude at 4 kW was larger than that of WaveI, also a reversible effect.

The effects of pulse-duration changes on the MEPs weredetermined with 5-kW pulses of widths 2.5 to 25 microsecondsat 10 pps. Once more, the latency changes were minimal.However, wave amplitudes increased with increasing pulsedurations to a plateau for about 10-microsecond and longerpulses, and the temporal relationships among all of thewaves were not altered.

Using 25-microsecond, 10-kW pulses at 10 pps, the effectsof the lesions formed successively in the inferiorcolliculus, lateral lemniscus, and superior olive on the BERrecorded by the electrode in each region were compared withthe pre-lesion BER from that electrode. Also compared werethe pre- and post-lesion MEPs recorded by the vertexelectrode. The results for one cat were presented.

Each successive lesion yielded decreases in the BERamplitudes from all regions. The most pronounced effect wason the BER from the inferior colliculus nucleus followinglesion formation in that region; the BERs from the other twobrainstem regions and the MEP remained practically unchangedafter producing a lesion in the inferior colliculus nucleus.Similarly, the lesion in the lateral lemniscus yielded thelargest effect on the BER recorded by the electrode in thatregion, with minor changes in the BERs from the otherauditory structures except for the inferior colliculus. TheBER amplitudes from the superior olive were less affected bythe lesions in the inferior colliculus and the laterallemniscus, reflecting the distal location of the superiorolive in the auditory pathway, but its BER amplitude wasdrastically reduced by the lesion in its nucleus. Theamplitudes of the vertex MEPs were altered after eachsuccessive lesion, indicating that they were a function ofthe integrity of brainstem nuclei along the auditorypathway.

Tyazhelov et al. (1979) studied the qualities of theapparent sounds perceived by humans from exposure to 800-MHzpulsed RFR. The parietal area of the head was exposed to theopen end of a waveguide fed from a 500-W source. The ambientnoise level did not exceed 40 dB and was reduced by pluggingthe subjects' ears with stoppels or sound-conducting tubes.The pulse durations used ranged from 5 to 150 microseconds.The pulses were presented either continuously at 50 to 2000pps (the latter for short pulse durations, to limit theaverage power density) or in trains of duration 0.1 to 0.5seconds at rates of 0.2 to 2.0 trains per second. Eachsubject could be presented with sinusoidal audiofrequency(AF) sound waves independently of, or concurrently with, thepulsed RFR and could adjust the amplitude, frequency, andphase of the AF signal to match the timbre and loudness ofthe perceived RFR. Acoustic signals were presented to thesubject by means of a pair of small hollow tubes extendingfrom a speaker to the ears.

The high-frequency auditory limit (HFAL) of each subjectfor sinusoidal tones from 1 kHz upward was tested first.Three of the subjects had HFALs below 10 kHz and could notperceive 10-30 microsecond RFR pulses, results consonantwith those of Cain and Rissman (1978). Of 15 subjects withHFALs above 10 kHz, only one could not perceive the RFRpulses.

All of the perceptive subjects reported that 10-30microsecond pulses delivered at 1000 to 12,000 pps at peakpower densities exceeding 500 mW/sq cm produced sound ofpolytonal character that seemed to originate in the head,and that the quality of the sound changed with increasingpulse repetition rate (PRR) in a complex manner. Loudnessdiminished sharply and became more monotonal as the PRR wasincreased from 6000 to 8000 pps. However, no more than threedistinguishable tonal transitions occurred. Subjects withHFALs below 15 kHz were unable to distinguish between thesounds perceived from a 5000-pps and a 10,000-pps signal,and subjects with more extended HFALs reported that thepitch for a 5000-pps signal was higher than for a 10,000-ppssignal.

The subjects were able to detect small (5%) shifts of PRRonly in the 8000-pps region; at lower PRRs, the subjectserred on 100% of tests to detect the direction of PRRchange, indicating that increases of PRR were oftenperceived as decreases in frequency. For pulses of constantpeak amplitude, loudness was perceived to increase withduration from 5 to 50 microseconds, decrease from 70 to 100microseconds, and increase again for 100 microseconds andupward. Such perceptual patterns were exemplified by plotsof threshold pulse power (normalized to the 10-kHz PRRthreshold) vs PRR for a subject with a 14-kHz HFAL and foranother subject with a 17-kHz HFAL. These curves wereroughly W-shaped, with central relative maxima at about 8kHz. A plot of mean threshold pulse power (normalized to thethreshold at 50-microsecond pulse duration) was alsopresented for subjects unable to perceive sounds for pulseslonger than 50 microseconds. This curve was also W-shaped,with a central relative maximum within the pulse range 100-120 microseconds.

After subjects matched the pitch and timbre of a 2-kHzacoustic tone to the perceived sound of a train of RFRpulses at 2000 pps, they were asked to match the loudness ofthe acoustic tone with the loudness of the perceived pulsesas the pulse duration was varied between 5 and 150microseconds while the peak power was held constant. Noactual data were given. Instead, the relationship betweenthe ratio of acoustic signal amplitude to the pulse powerfor the subjects (both quantities normalized to theirrespective thresholds) were merged into a shaded areabounded by two straight lines through the origin on a graphthat also displayed a straight line through the originstated by the authors to represent the theoreticalrelationship between these quantities as predicted from thethermoelastic model. The entire shaded area was above thetheoretical line, i.e., the ratios of acoustic amplitude topulse power for all of these subjects were reported to belarger than predicted.

When acoustic tones above 8 kHz were presentedconcurrently with 10- to 30-microsecond pulses at PRRsslightly above or below 8 kHz, the subjects reported hearingbeat-frequency notes. Also, for a PRR of 800 pps, similarbeat frequencies were perceived when the acoustic frequencywas set slightly above or below harmonics of the PRR.Moreover, when the tone and PRR frequencies were matched andthe subjects were allowed to vary the phase of the acoustictone, cancellation of perception of the two stimuli could beachieved. By proper phasing, subjects with HFALs below 15kHz could also obtain cancellation between a 10-kHz acousticsignal and a 5-kHz train of pulses.

The authors also reported that the sensorycharacteristics (pitch and timbre) evoked by RFR pulses lessthan 50 microseconds in duration persisted when subjects'heads were lowered into seawater, with loudness diminishingroughly in proportion to immersion depth and vanishingentirely with total immersion. For pulses longer than 50microseconds, even partial immersion resulted in loss ofperception.

In their discussion, the authors suggested that many oftheir results are consistent with the thermoelastichypothesis, but that others, such as the suppression of theperception of a 5000-pps train of RFR pulses by a 10-kHzacoustic tone, are at variance with that model.

Frey and Coren (1979) endeavored to detect surfacemovements purportedly induced in heads of animals by RFRpulses, using dynamic time-averaged interferometricholography. In this technique, a hologram of an object invibratory motion is recorded for a long interval relative toone period of the vibration. Such a hologram will containdata on the spatial distribution of the time-averagedamplitude of motion of the object. Thus, nodal regions willappear brightest and antinodal regions appear darkest, withregions of intermediate intensity inversely related to theirsurface displacement.

Each animal studied was euthanized, placed with itsabdomen on the surface of a vibrationally-isolated block ofcommercial RFR-absorbent material (or concrete in sometests), and exposed to RFR from above with a horn as soon asthere was no detectable heartbeat or respiration. A set of30 holograms was made for each animal, 15 each duringalternating RFR- and sham-exposures. First, holograms duringthree RFR- and three sham-exposures were made after removingthe hair from the dorsal surface of the head and thevicinity of the left pinna. Next, the skin over those areaswas removed and six holograms of the musculature revealedthereby were made. Then, the muscle tissue was removed fromthe dorsal surface and mastoid area and six holograms weremade of the skull. Six more holograms were made of the brainafter removing the dorsal surface of the skull. The last sixholograms were made of the base of the skull cavity (dorsalsurface of a bulla) after removing the brain.

In the first of two experiments, 10 Sprague-Dawley ratswere exposed to 25-microsecond pulses of 1.275-GHz RFR at apeak power density of 1.7 W/sq cm and a PRR of 50 pps. Anadditional set of holograms was made at 100 pps for five ofthe rats. In the second experiment, 16 adult guinea pigswere used. Of these, eight were exposed to 1.1-GHz pulses ina 2x2x2 factorial design with peak power densities 1.25 and8.5 W/sq cm, pulse durations 10 and 20 microseconds, andPRRs 25 and 50 pps. The other eight were exposed to 1.2-GHzpulses in the same design. The holograms for three of theguinea pigs exposed at 8.5 W/sq cm showed that movement wasengendered in the RFR absorber supporting the animals by theRFR pulses, so the support was replaced with a cementblock.

The authors indicated that they could not detect anydifferences between the holograms obtained from each animalduring RFR exposure and the holograms from the same surfacesof the same animal taken during sham exposure. (Thecomparisons were made on a coded and blind basis.) Theyconcluded therefrom that the hypothesis of RFR transductioninto elastic waves in the head and propagation of the soundto the cochlea by bone conduction, predicted by otherinvestigators, is untenable. Instead, they suggested thatthe transduction site is more likely to be in the cochleaitself rather than elsewhere in the head.

The authors did not provide specific information aboutthe appearances of the holograms or the differences soughtbetween holograms of RFR- and sham-exposed surfaces,rendering it difficult to assess the validity of thesenegative findings. Also, the adequacy of the sensitivity ofthis holographic technique for detecting such movements wasdisputed by Chou et al. (1980a), with a response by Frey andCoren (1980). Based on the description of the holographictechnique, one would expect that the brightness of a surfacehaving non-uniform optical reflectance would appear non-uniform even if the surface were stationary. Also, a surfacehaving uniform reflectance and moving as a unit, i.e.,without motion of any area relative to another, would appearuniformly illuminated (but of lower brightness than if thesame surface were stationary).

Amore fundamental question is whether or not thesuccessive removal of skin, musculature, etc., would alterthe characteristics of possible RFR-to-elastic-wavetransduction wherever it occurred in the head. For example,suppose transduction takes place at the inner or outersurface of the skull with the skin and musculature intact(which may render the motion undetectable with thisholographic technique). Would baring the skull by theremoval of skin and musculature alter the characteristics ofthe transduction process significantly?

Olsen and Hammer (1980) used a hydrophone transducerimplanted in a rectangular muscle-equivalent model to detectacoustical responses to exposure of the model to pulsed RFR.The model consisted of about 15 kg of polyethylene powder,water, sodium chloride, and gelling agent in proportionsgiven by Guy (1971) and contained within an open rectangularpolystyrene box. It was exposed to 0.5-microsecond 5.7-GHzpulses at 7-ms intervals (143 pps) at 5.5 cm from astandard-gain horn (about 0.07 of the conventional far-fielddistance) at an average power density of 120 mW/sq cm. Thecorresponding pulse power density exceeded 1.5 kW/sq cm. Theauthors noted that in the near field, the on-axis powerdensity is a strongly oscillating function of the distancefrom the horn, and they found that the amplitude of thethermoelastic waves exhibited such behavior when thedistance between the horn and hydrophone was slightlyincreased or decreased.

The SAR was determined calorimetrically at several depthswithin the model. The results were about 95 W/kg at 1 mm, 50W/kg at 1 cm, and 5 W/kg at 2 cm, showing that most of theRFR energy was absorbed within the first 2 cm. Thehydrophone used was directional and sensitive in thefrequency range 50-700 kHz, and was inserted into the modelon the axis of the horn at 15.24 cm from the exposedsurface. The hydrophone output was fed to a broadbandfilter, amplified, and monitored with an oscilloscope. Forcomparison with prior studies, measurements with thehydrophone were also made in 1% saline (12 kg) in thepolystyrene box.

The response of the muscle-equivalent model to the RFRpulses was a rapidly decaying thermoelastic wave lastingabout 10 microseconds and a narrower RFR artifact, thelatter serving as a convenient marker for measuring the timedelay corresponding to the acoustic propagation speed of thethermoelastic wave. The delay between the thermoelasticresponse and the artifact was 85 microseconds. Using thedepth of the hydrophone to calculate the propagation (group)speed yielded about 1800 m/s.

Asecond wave delayed from the first by about 380microseconds was also evident. The authors ascribed thiswave to two successive reflections, from the back and frontsurfaces of the model, for a total distance traveled (oneround trip from the hydrophone) of twice the dimension ofbox parallel to the propagation direction, or 60.96 cm, adistance that yielded a propagation speed of 1600 m/s. Theynoted that transduction of the pulses into thermoelasticwaves at the surface was tacitly assumed in the calculationof the higher speed, but that transduction actually occurs1-2 cm from the surface. Taking 1.5 cm as the locus oftransduction yields a speed of 1616 m/s for the 85-microsecond delay (with no change for the twice-reflectedwave).

The amplitude of the twice-reflected wave was about 20%of the initial hydrophone response amplitude, whichpermitted the authors to estimate the acoustic attenuationwithin the model. Excluding reflection losses, estimated tobe less than 10%, yielded an upper-bound loss of 14 dB for a61-cm slab of muscle-equivalent material.

For studying transduced waves in saline, the hydrophonewas placed 7.62 cm from the exposed surface. Unlike therapidly decaying wave in the muscle-equivalent material, theRFR pulses yielded a ringing response by the hydrophone. The61-cm-round-trip delay time between the initial and twice-reflected waves was 400 microseconds, which yielded apropagation speed of 1525 m/s, consonant with values foundby others (Lin, 1978). Also prominent was a wave delayed byonly 90 microseconds from the initial wave, apparently dueto reflections from the hydrophone itself and from the frontsurface of the saline back to the hydrophone. (An analogouswave of intermediate delay time was barely discernible inthe oscilloscope trace for the muscle-equivalentmaterial.)

The authors suggested that the presence of ringing in thesaline model indicated the induction of higher-frequencyacoustic components and that the absence of ringing in themuscle-equivalent model may be because of a thinner RFR-absorption profile and/or greater high-frequency damping forthe simulated muscle tissue.

Olsen and Hammer (1981) performed similar measurements ina rectangular muscle-equivalent model. However, the 0.5-microsecond, 5.7-GHz pulses were triggered at 760-microsecond intervals or twice the round trip time observedpreviously, to reinforce the thermoelectric waves.(Interpulse intervals of 380 microseconds could not be usedbecause of equipment limitations.) An amplitude enhancementfactor of about 3 was obtained at the end of a burst of fourpulses.

Also studied by Olsen and Hammer (1981) was a sphericalbrain-equivalent model 10 cm in diameter. The model wasexposed to 1.10-GHz RFR from an open section of waveguide,either as single pulses of 4-kW peak power and duration thatwas varied or as bursts of three such pulses with anadjustable interpulse interval. For a nominal 10-microsecondpulse, the SAR was 824 W/kg at the center of the sphere and653 W/kg at the surface facing the source. A hydrophone wasplaced at the center of the model to detect thermoelasticwaves.

Exposure of the model to single 14-microsecond pulsesyielded ringing for each pulse, with a fundamental frequencyof about 16 kHz and a time constant of about 500microseconds, the latter said to be consistent with theattenuation obtained in the rectangular model. A plot ofhydrophone response vs pulse duration over the range 10-60microseconds showed maximum response for 20-microsecondpulses. Three-pulse bursts at burst frequencies ranging from10 to 30 kHz yielded higher amplitudes than single pulses,with maximum enhancement at 16 kHz, as expected.

Olsen and Lin (1981) performed similar studies ofspherical brain-equivalent models 6, 10, and 14 cm indiameter exposed to 10-kW, 1.10-GHz single pulses and burstsof three pulses from an open section of waveguide. Toincrease the amplitude of the thermoelastic waves in the 6-cm sphere, it was placed directly against the waveguideopening.

Aplot of peak hydrophone responses of the 6-cm sphere tobursts of 10-microsecond pulses vs burst frequency showedmaximum response at 25.5 kHz. The corresponding data for the10-cm sphere were the same as in Olsen and Hammer (1981).The response of the 14-cm sphere to single pulses wasringing at a fundamental frequency slightly above 10 kHz,and was maximum for 35-microsecond pulses. The responses ofthat sphere to bursts of 35-microsecond pulses had a peak at11.5 kHz. Plots of the experimentally determined fundamentalresonant frequencies for the three models were on the curveof resonant frequency vs head radius derived from thethermoelastic theory for a homogeneous brain sphere withstress-free boundaries, thereby supporting that theory.

In a subsequent study, Olsen and Lin (1983) surgicallyimplanted disk hydrophone transducers 3.2 mm in diameter and0.5 mm thick in the brains of rats, guinea pigs, and cats tomeasure the stress waves induced by RFR pulses. Connectionsto the transducers were made with coaxial cable 2.5 mm indiameter.

In the experiments with cats and guinea pigs, thehydrophone transducer was implanted about 1.5 cm deep in thebrain of the anesthetized animal through a hole in the skullon the left side of the head near the top of the parietalbone. Next, the animal was taken to an anechoic chamber andexposed to 0.5-microsecond, 2 kW-peak pulses of 5.7-GHz RFRat 2 pps from a standard-gain horn, and to acoustic clicks.The output cable of the hydrophone was not connected duringthese stimuli; instead, metallic and nonmetallic electrodes(at unspecified head locations) were used to detect andcompare brainstem potentials in response to eachstimulus.

After the brainstem potentials were measured, thehydrophone output cable was connected to an oscilloscope andthe animal was exposed to several series of 5.7-GHz RFRpulses at 14 pps. The animal was then removed from thechamber and placed next to a 3-kW-peak 2.45-GHz source,where its head was exposed to 2.5-microsecond pulses with asurface applicator. The output of the hydrophone wasrecorded during each exposure.

In the experiments with rats, brainstem potentials inresponse to 5.7-GHz pulses and acoustic clicks were measuredbefore the hydrophone was implanted. After implantation,hydrophone signals were recorded during exposure to 0.5-microsecond, 5.7-GHz pulses in the anechoic chamber and to5-6-microsecond, 2.45-GHz pulses with the applicator.

Representative hydrophone output waveforms for one catand one guinea pig for the two RFR frequencies werepresented, which showed that the shorter 5.7-GHz pulsesstimulated vibrations having more of the higher-frequencycomponents than the 2.45-GHz pulses. In addition, varyingthe 2.45-GHz burst frequency for the cat yielded maximumresponse near 40 kHz. Hydrophone output waveforms for sixrats were also presented and were characterized similarly.In addition, a distinct vibration near 60 kHz, the computedfundamental mode of the rat brain, was discernible.

From their results, the authors concluded that RFR pulsesdo induce acoustic pressure waves in the brain, confirmingprevious predictions, particularly regarding the fundamentalradial oscillation of the rat brain near 60 kHz. They alsonoted that the theoretically predicted frequencies areindependent of heating patterns, but are functions only ofthe propagation speed of pressure waves and the size of thehead. Open to question, however, is whether the use ofcoaxial cable and other metal leads introduced significantartifact.

Wilson et al. (1980) used C-14-labeled 2-deoxy-D-glucose(C-14-DG) to prepare autoradiographic maps of brain activityin 11 rats, to study the effects of exposure to RFR,acoustic clicks, and infrared radiation (IR) on the auditorysystem. In nine of the rats, the left bulla was opened, theossicles were removed, and the bulla was packed withgelfoam. In the other two rats, one cochlea was destroyed byinserting a blunt probe through the round window. Theseoperations, done prior to exposure, were to abolish orattenuate the response of one side of the auditory system toairborne sound.

Each rat was exposed to only one stimulus in a sound-isolation chamber for 45 min, while the rat was restrainedwithin a cylindrical cage of RFR-transparent mesh. Justbefore exposure, each rat was injected with C-14-DG. Oncompletion of exposure, the rat was euthanized and its brainwas quickly removed, frozen, and sectioned in the frontalplane (30-micron slices). Autoradiographs of C-14-DG uptakethroughout the brain were prepared and examined fordifferences in optical densities resulting from exposure tothe various stimuli. Also, autoradiographs of representativesections through the auditory and vestibular nuclei wereidentified and enlarged, and the identified sections werestained with cresyl violet.

Two of four rats were stimulated with acoustic clicks at87 dB SPL from a loudspeaker driven by 100-microsecondpulses at 10 pps. One of the remaining rats was exposed toIR from two heat lamps at a level stated to mimic the totalthermal load induced by RFR exposure. Specifically, thevoltage applied to the lamps was adjusted to yield atemperature-increase rate of a saline-filled beaker thatmatched the rate obtained from exposure to 918-GHz RFR at 10mW/sq cm. (The authors did note that the spatial heatingprofiles for the IR and RFR were dissimilar.) The fourth ratwas held in the sound-isolation chamber withoutstimulation.

Autoradiographs of these four rats (denoted as controls,i.e., not exposed to RFR) were qualitatively similar; allshowed bilateral asymmetry in C-14-DG uptake by the inferiorcolliculus and medial geniculate body, with higher uptake onthe side contralateral to the intact middle ear. As noted bythe authors, this form of asymmetry was expected becausemost ascending pathways from one cochlea lead to the centralnucleus of the contralateral inferior colliculus. Not clearwas why the autoradiographs for two such different stimuli(acoustic clicks and IR) and those taken in their absencewere so similar to one another.

In an initial experiment, one of the rats with leftossicles removed was exposed to 20-microsecond pulses, 10pps, of 2.45-GHz RFR at an average power density of 2.5mW/sq cm, for a peak power density of 12.5 W/sq cm. Theautoradiographs of this rat from the inferior colliculus,unlike those for the control rats, showed bilateral symmetryin C-14-DG uptake, taken as indicating the utility of the C-14-DG method for demonstrating a known effect of RFRexposure on brain activity. The four remaining rats withleft ossicles removed were then exposed to 918-MHz CW RFR,two each at 2.5 and 10 mW/sq cm, to identify possibleeffects of the CW RFR. The corresponding SARs in themidbrain were 1.1 and 4.4 W/kg, determined thermometricallywith rat carcasses. For both CW RFR levels, C-14-DG uptakein the inferior colliculus was also bilaterally symmetricand the autoradiographs were "surprisinglysimilar" to those for the pulsed RFR.

The authors stated: "To exclude the possibility thatCW microwave radiation produced this result by direct actionon brain tissue, additional data were obtained from twoanimals in which one cochlea was destroyed. In both animals,the uptake of C-14-DG was greatest at the inferiorcolliculus contralateral to the intact cochlea. The degreeof asymmetry at the inferior colliculus was, in fact, atleast as great as that found in any of the control animals.This finding, coupled with the finding of a bilateralsymmetry of C-14-DG uptake in the auditory pathways ofanimals with one middle ear ablated, demonstrated that CWmicrowave radiation acts at some site within the cochlea ineliciting auditory responses."

C-14-DG uptake in other structures of the auditorysystem, such as the lateral superior olive, medial superiorolive, or cochlear nucleus, showed no bilateral differencesexcept in the two rats with one cochlea destroyed.Autoradiographs from regions outside of the auditory systemwere bilaterally symmetric and showed no stimulus-relatedqualitative differences.

In their discussion, the authors suggested that theactivity of the rat's auditory system in response to CW RFR,determined by integrated C-14-DG uptake during a 45-minperiod, is an effect distinct from the thermoelasticresponses to RFR pulses, and that the CW interaction appearsto occur somewhere within the cochlea. They estimated that asteady-state increase in intracochlear temperature ofbetween 0.1 and 0.5 deg C would be induced in live ratsexposed to 918-MHz CW RFR at 2.5 mW/sq cm, suggesting thatsuch increases in temperature may be effective in alteringauditory activity. In this context, it is reiterated thatthe average power density of the 2.45-GHz pulsed RFR used inthis study was also 2.5 mW/sq cm and that the midbrain SARwas 1.1 W/kg.

In conclusion, the preponderance of experimental resultsindicates that auditory perception of RFR pulses is due toinduction of thermoelastic waves in the head, rather than todirect brain stimulation by the RFR. Also, becauseindividual pulses can be perceived, it is not meaningful tocalculate average power densities for two or more widelyspaced pulses and cite such values as evidence that theeffect is nonthermal in nature. The response to CW RFRreported by Wilson et al. (1980) is an effect distinct fromthe thermoelastic responses to pulses and possibly isrelated to an intracochlear temperature rise of 0.1 to 0.5deg C.

REFERENCES:

Cain, C.A. and W.J. Rissman
MAMMALIAN AUDITORYRESPONSES TO 3.0 GHz MICROWAVE PULSES
IEEE Trans.Biomed. Eng., Vol. 25, No. 3, pp. 288-293 (1978)

Chou, C.-K., R. Galambos, A.W. Guy, and R.H. Lovely
COCHLEAR MICROPHONICS GENERATED BY MICROWAVE PULSES
J.Microwave Power, Vol. 10, No. 4, pp. 361-367 (1975)

Chou, C.-K., A.W. Guy, and R. Galambos
CHARACTERISTICS OF MICROWAVE-INDUCED COCHLEARMICROPHONICS
Radio Sci., Vol. 12, No. 6S, pp. 221-227(1977)

Chou, C.-K. and R. Galambos
MIDDLE-EAR STRUCTURESCONTRIBUTE LITTLE TO AUDITORY PERCEPTION OF MICROWAVES
J. Microwave Power, Vol. 14, No. 4, pp. 321-326 (1979)

Chou, C.-K. and A.W. Guy
CARBON-LOADED TEFLONELECTRODES FOR CHRONIC EEG RECORDINGS IN MICROWAVERESEARCH
J. Microwave Power, Vol. 14, No. 4, pp. 399-404(1979a)

Chou, C.-K., and A.W. Guy
MICROWAVE-INDUCED AUDITORYRESPONSES IN GUINEA PIGS: RELATIONSHIP OF THRESHOLD ANDMICROWAVE-PULSE DURATION
Radio Sci., Vol. 14, No. 6S,pp. 193-197 (1979b)

Chou, C.-K., A.W. Guy, K.R. Foster, R. Galambos, and D.R.Justesen
HOLOGRAPHIC ASSESSMENT OF MICROWAVE HEARING
Science, Vol. 209, pp. 1143-1144 (5 Sept 1980a)

Chou, C.-K., K.-C. Yee, and A.W. Guy
AUDITORYRESPONSE IN RATS EXPOSED TO 2,450 MHZ ELECTROMAGNETIC WAVESIN A CIRCULARLY POLARIZED WAVEGUIDE
Bioelectromagnetics,Vol. 6, No. 3, pp. 323-326 (1985a)

Foster, K.R. and E.D. Finch
MICROWAVE HEARING:EVIDENCE FOR THERMOACOUSTIC AUDITORY STIMULATION BY PULSEDMICROWAVES
Science, Vol. 185, pp. 256-258 (19 July1974)

Frey, A.H.
AUDITORY SYSTEM RESPONSE TO RADIO-FREQUENCY ENERGY
Aerospace Med., Vol. 32, pp. 1140-1142(1961)

Frey, A.H.
HUMAN AUDITORY SYSTEM RESPONSE TOMODULATED ELECTROMAGNETIC ENERGY
J. Appl. Physiol., Vol.17, No. 4, pp. 689-692 (1962)

Frey, A.H.
MAIN STEM EVOKED RESPONSES ASSOCIATED WITHLOW-INTENSITY PULSED UHF ENERGY
J. Appl. Physiol., Vol.23, No. 6, pp. 984-988 (1967)

Frey, A.H. and R. Messenger, Jr.
HUMAN PERCEPTION OFILLUMINATION WITH PULSED ULTRAHIGH-FREQUENCY ELECTROMAGNETICENERGY
Science, Vol. 181, pp. 356-358 (27 July 1973)

Frey, A.H. and E. Coren
HOLOGRAPHIC ASSESSMENT OF AHYPOTHESIZED MICROWAVE HEARING MECHANISM
Science, Vol.206, pp. 232-234 (12 Oct 1979)

Frey, A.H. and E. Coren
HOLOGRAPHIC ASSESSMENT OFMICROWAVE HEARING [A response]
Science, Vol. 209, pp.1144-1145 (5 Sept 1980)

Guy, A.W.
ANALYSIS OF ELECTROMAGNETIC FIELDS INDUCEDIN BIOLOGICAL TISSUES BY THERMOGRAPHIC STUDIES ON EQUIVALENTPHANTOM MODELS
IEEE Trans. Microwave Theory Tech., Vol.19, No. 2, pp. 205-214 (1971)

Guy, A.W., C.-K. Chou, J.C. Lin, and D. Christensen
MICROWAVE-INDUCED ACOUSTIC EFFECTS IN MAMMALIAN AUDITORYSYSTEMS AND PHYSICAL MATERIALS
Ann. N.Y. Acad. Sci., Vol247, pp. 194-218 (1975b)

Guy, A.W., J. Wallace, and J. McDougall
CIRCULARLYPOLARIZED 2450 MHZ WAVEGUIDE SYSTEM FOR CHRONIC EXPOSURE OFSMALL ANIMALS TO MICROWAVES
Radio Sci., Vol. 14, No. 6S,pp. 63-74 (1979)

Johnson, C.C. and A.W. Guy
NONIONIZINGELECTROMAGNETIC WAVE EFFECTS IN BIOLOGICAL MATERIALS ANDSYSTEMS
Proc. IEEE, Vol. 60, No. 6, pp. 692-718(1972)

Lebovitz, R.M. and R.L. Seaman
MICROWAVE HEARING: THERESPONSE OF SINGLE AUDITORY NEURONS IN THE CAT TO PULSEDMICROWAVE RADIATION
Radio Sci., Vol. 12, No. 6S, pp.229-236 (1977)

Lin, J.C.
MICROWAVE AUDITORY EFFECT--A COMPARISON OFSOME POSSIBLE TRANSDUCTION MECHANISMS
J. MicrowavePower, Vol. 11, No. 1, pp. 77-81 (1976a)

Lin, J.C.
MICROWAVE-INDUCED HEARING: SOME PRELIMINARYTHEORETICAL OBSERVATIONS
J. Microwave Power, Vol. 11,No. 3, pp. 295-298 (1976b)

Lin, J.C.
ON MICROWAVE-INDUCED HEARING SENSATION
IEEE Trans. Microwave Theory Tech., Vol. 25, No. 7, pp. 605-613 (1977a)

Lin, J.C.
FURTHER STUDIES ON THE MICROWAVE AUDITORYEFFECT
IEEE Trans. Microwave Theory Tech., Vol. 25, No.7, pp. 938-943 (1977b)

Lin, J.C.
THEORETICAL CALCULATION OF FREQUENCIES ANDTHRESHOLDS OF MICROWAVE-
INDUCED AUDITORY SIGNALS
Radio Sci., Vol. 12, No. 6S, pp. 237-242 (1977c)

Lin, J.C.
MICROWAVE AUDITORY EFFECTS ANDAPPLICATIONS
Charles C. Thomas, Springfield, IL, p. 108(1978)

Lin, J.C., R.J. Meltzer, and F.K. Redding
MICROWAVE-EVOKED BRAINSTEM POTENTIALS IN CATS
J. Microwave Power,Vol. 14, No. 3, pp. 291-296 (1979b)

Olsen, R.G. and W.C. Hammer
MICROWAVE-INDUCEDPRESSURE WAVES IN A MODEL OF MUSCLE TISSUE
Bioelectromagnetics, Vol. 1, No. 1, pp. 45-54 (1980)

Olsen, R.G. and W.C. Hammer
EVIDENCE FOR MICROWAVE-INDUCED ACOUSTICAL RESONANCES IN BIOLOGICAL MATERIAL
J.Microwave Power, Vol. 16, Nos. 3 & 4, pp. 263-269(1981)

Olsen, R.G. and J.C. Lin
MICROWAVE PULSE-INDUCEDACOUSTIC RESONANCES IN SPHERICAL HEAD MODELS
IEEE Trans.Microwave Theory Tech., Vol. 29, No. 10, pp. 1114-1117(1981)

Olsen, R.G. and J.C. Lin
MICROWAVE-INDUCED PRESSUREWAVES IN MAMMALIAN BRAINS
IEEE Trans. Biomed. Eng., Vol.30, No. 5, pp. 289-294 (1983)

Sharp, J.C., H.M. Grove, and O.P. Gandhi
GENERATIONOF ACOUSTIC SIGNALS BY PULSED MICROWAVE ENERGY
IEEETrans. Microwave Theory Tech., Vol. 22, No. 5, pp. 583-584(1974)

Taylor, E.M. and B.T. Ashleman
ANALYSIS OF CENTRALNERVOUS SYSTEM INVOLVEMENT IN THE MICROWAVE AUDITORYEFFECT
Brain Res., Vol. 74, pp. 201-208 (1974)

Tyazhelov, V.V., R.E. Tigranian, E.O. Khizhniak, and I.G.Akoev
SOME PECULIARITIES OF AUDITORY SENSATIONS EVOKEDBY PULSED MICROWAVE FIELDS
Radio Sci., Vol. 14, No. 6S,pp. 259-263 (1979)

White, R.M.
GENERATION OF ELASTIC WAVES BY TRANSIENTSURFACE HEATING
J. Appl. Phys., Vol. 34, No. 12, pp.3559-3567 (1963)

Wilson, B.S., J.M. Zook, W.T. Joines, and J.H.Casseday

ALTERATIONS IN ACTIVITY AT AUDITORY NUCLEI OF THE RATINDUCED BY EXPOSURE TO MICROWAVE RADIATION:
AUTORADIOGRAPHIC EVIDENCE USING [C-14] 2-DEOXY-D-GLUCOSE
Brain Res., Vol. 187, pp. 291-306 (1980)