Modification of acoustic and tactile startle by single microwave pulses

Physiology& Behavior,Vol. 55, No. 3, pp. 587-595, 1994 Copyright© 1994 ElsevierScienceLtd Printedin the LISA.All fights reserved 0031-9384/94$6.00 + .00

Pergamon

Modification of Acoustic and Tactile Startle by Single Microwave P u l s e s I RONALD

L. S E A M A N , .1 D O L O R E S

A. BEBLOJ" A N D T H O M A S

G. R A S L E A R : ~

*Louisiana Tech University, Ruston, LA 71272-0001, tERC BioServices Corporation, Gaithersburg, MD 20879, and $Walter Reed Army Institute of Research, Washington, DC 20307-5100

R e c e i v e d 14 D e c e m b e r 1992 SEAMAN, R. L., D. A. BEBLO AND T. G. RASLEAR. Modification of acoustic and tactile startle by single microwavepulses. PHYSIOL BEHAV 55(3) 587-595, 1994.--Single microwave pulses at 1.25 GHz were delivered to the head and neck of male Long-Evans rats as a prestimulns to acoustic and tactile startle. For acoustic startle, pulses averaging 0.96/zs in duration were tested with two specific absorption rate (specific absorption) ranges, 15.0-30.0 kW/kg (16.0-44.2 mJ/kg) and 35.5-86.0 kW/kg (66.6-141.8 mJ/kg), delivered 201, 101, 51, 3, and 1 ms before and 1 ms after onset of a startling noise. The low-intensity pulse did not affect peak amplitude, integral, or latency of the whole-body startle response. The high-intensity pulse at 101 and 51 ms inhibited the startle response by decreasing peak amplitude and integral; at 201 and 51 ms latency was increased. The high-intensity pulse at 1 ms enhanced the startle response by increasing peak amplitude and at 3 ms by increasing integral. For tactile startle, either microwave pulses averaging 7.82/as in duration and 55.9-113.3 kW/kg (525.0-1055.7 m.l/kg) or 94 dB SPL clicks were delivered 157, 107, 57, and 7 ms before and 43 ms after onset of a startling air burst. The microwave pulse at 57 ms inhibited the startle response by decreasing peak amplitude; at 157, 107, 57, and 7 ms it increased latency. The microwave pulse at 43 ms after onset enhanced the startle response by increasing peak amplitude. The acoustic click at 157 and 57 ms inhibited the startle response by decreasing peak amplitude; at 157, 107, and 57 ms it increased latency. The microwave pulse inhibition and enhancement of startle were similar to previously reported effects of sensory stimuli delivered at similar lead times, indicating the possibility that action was mediated by sensory stimulation. Acoustic startle Tactile startle Microwave pulse Response amplitude Response latency

Prestimulus

THE acoustic startle response consists of a sequence of reflexive muscular contractions elicited by sudden, intense acoustic stimuli in many mammals, including humans (10,11,31). The response is complete within 8 0 - 5 0 0 ms, depending on species and particular type of response recorded. Its robustness in the rat makes startle reliable and easy to measure. Similar motor responses occur in tactile startle, elicited in the laboratory with bursts of air directed at the dorsal surface of the rat (6,41,51). Startle can be modified by preceding brief sensory stimuli, prestimuli or prepulses (21,25). In the rat, acoustic, tactile, and photic prestimuli have been shown to modify acoustic startle (19,38,42) and acoustic prestimuli have been shown to modify tactile startle (34,52). Modification by sensory prestimuli is sensitive enough to be used to determine sensory thresholds (2,12,19,21,54). Because many details of the neural circuitry and the pharmacology of startle and its modification are known (10,11,30,32,37,50), a change in response can ultimately be interpreted in terms of neural circuits. Microwave pulses delivered repetitively have been used in experiments with animal behavior. Rats in shuttlebox experiments prefer the side shielded from microwave pulses or from

Lead time

Inhibition

Enhancement

sound (14,18). In rat operant behavior, microwave pulse and acoustic cue stimuli substitute for one another (29). Rat and monkey operant task behaviors are not affected unless thermalizing intensities are used (1,9). Indeed, a number of reported effects of pulsed microwaves, including those on animal behavior, seem to depend on heating capability rather than on pulse characteristics (8,39). However, single nonthermalizing microwave pulses impinging on the head evoke a click perception, called microwave hearing (7,16,33). Also, modification of acoustic startle by single microwave pulses has been observed in studies of a preliminary nature. In the mouse, pulses of 1 - 1 0 / ~ s delivered 5 - 5 0 0 ms before a startling stimulus reduce startle amplitude, as indicated by movement of the base of the tail (53). In the rat, pulses of 0 . 8 - 1 / z s delivered 100 ms before the startling stimulus reduce amplitude and increase latency of the wholebody response (44). The present experiments use modification of rat acoustic and tactile startle to test the effectiveness of single microwave pulses as prestimuli. Modification of whole-body response peak amplitude by a microwave pulse has been presented in preliminary

1 Requests for reprints should be addressed to Dr. Ronald L. Seaman, Department of Biomedical Engineering, Louisiana Tech University, P.O. Box 3185, Ruston, LA 71272-0001. 587

588

SEAMAN, BEBLO AND RASLEAR

form for both types of startle (45,46). More detailed and extended analyses of peak amplitude are reported here along with analyses of response integral and latency. GENERAL METHOD

tie experiments and 15.1-19.4 W/kg/W, for animals in tactile startle experiments. The SAR and the specific absorption (SA) were computed for each microwave pulse using the normalized SAR, body mass, and detected power levels. The SA, which indicates deposited energy, is commonly used as a pulse dose measure.

Subjects Subjects were male Long-Evans rats housed in facilities with lighting on a 12/12 light/dark cycle with lights on at 0600 h. Animals were in group cages during a 10-day quarantine upon arrival and in individual cages subsequently. Animals were provided food and water ad lib except when food was restricted to 15 g daily as necessary to maintain body mass at a relatively constant value. This restriction began after quarantine when body mass reached 260-280 g. The procedure maintained animals in a size range to fit into the animal holder, prevent head-to-tail turning in the holder, and provide consistent microwave dose for a period of 7 - 1 0 days of conditioning and testing. Food restriction did not result in any unusual behavior or adverse health condition. An animal was handled a number of times and conditioned to being in an animal holder during two 20-min periods in a simulated exposure device before its first test session.

Experimental Apparatus The experimental apparatus consisted of a number of subsystems. Detailed descriptions of the subsystems are contained in a technical report that can be obtained from the first author (43). Stimulus delivery, data acquisition, and data storage were largely under control of a computer using custom software and equipped with an analog and digital I/O board and an IEEE 488 interface board. To aid data analysis, a continuous recording was made throughout each test session on a Gould TA2000 chart recorder. Recorded information included the digital signals used for stimulus control, the motion sensor signal, and descriptive text. Sound levels were measured with a Briiel and Kjaer 2230 sound level meter and 4155 half-inch microphone.

Microwave Exposure Device and Stimuli Microwave pulses at 1.25 GHz were generated by an Epsco PH40K microwave pulse signal source with a 1705HB plug-in. Pulse intensity and duration were manually set on the signal source. The coaxial output of the plug-in was connected through a circulator, a low-pass filter, and coaxial cable to WR-650 waveguide. A waveguide tuner was used to maximize absorbed power in the animal. Forward and reflected powers were sampled with crystal detectors using a waveguide bidirectional coupler and monitored on a Tektronix 2430A digital oscilloscope. Pulse waveforms stored in an ASCII data file after each trial were later used to determine actual pulse intensities and durations. The waveguide terminated with a shorting screen inside a sound-attenuating chamber. The terminal section of waveguide accepted the animal holder through a round hole in one broad wall to position the head of the animal inside the waveguide. Acoustic stimuli were delivered by a loudspeaker mounted on the opposite broad wall over a screened hole. The terminal section of waveguide and its attachments are referred to as the exposure device. The specific absorption rate (SAR), a widely accepted measure of microwave dose, was determined in separate dosimetry experiments. This was done by using the rate of rise of head temperatures measured with a special probe at the onset of microwave power to calculate SAR in an established formula (28). The maximum head SAR normalized to net power to the exposure device was 15.4-20.1 W/kg/W, for animals in acoustic star-

Startle Apparatus An animal was tested in one of a number of identical cylindrical holders that positioned the head within the exposure device and restricted extraneous motion. Made from Plexiglas, the animal holder was 18.9 cm long, and had nominal i.d and o.d. of 7 and 7.6 cm with a front wall of 3.2-mm Plexiglas. The cylinder and front wall had several holes to allow air exchange and sound transmission. The back wall was a door of 3.2-ram Plexiglas that was inserted through slots in the top of the cylinder after animal entry. In the holder, an animal rested on a rectangular platform made from 3.2-mm Plexiglas and supported by short sections of rubber tubing. A small rigid tube was attached along the inside top of the cylinder. A hole in the tube 2.6 cm from the front of the holder delivered downward air bursts for tactile startle. A 4.2-cm vertical space was between the platform and the tube. The hole was estimated to be 0.2-1.5 cm from the back of the neck of the animal. A strip of piezoelectric film attached to the bottom of the platform provided a sensitive sensor of whole-body motion. The voltage signal from the film was monitored on a Tektronix 2430A digital oscilloscope and the chart recorder. The response waveform from the oscilloscope was stored in an ASCII data file after each trial. The stored waveform was 409.6 ms long, 0.4 ms per point, and included 25.2 ms of pretrigger data.

Procedure For a test session, an animal was placed in a clean animal holder and the holder was placed in the exposure device. Electrical connection to the piezoelectric film and compressed air line connection to the animal holder tubing were made. The door to the sound-attenuating chamber was then closed, placing the animal in darkness and a low-level acoustic environment. An animal was removed and returned to its home cage immediately after a test session. Specific procedures for each experiment are described below.

Data Analysis Characteristics of startle responses were extracted from waveforms stored in the ASCII data files using spreadsheet software. Four points on each response waveform were determined. One point was the initial negative inflection used to indicate response latency. The other three points were peaks of the first three negative waves, as identifiable, with the largest taken to indicate response peak amplitude. Absolute values of waveform data points were summed to compute an integral measure of response amplitude. This was done over the entire posttrigger period of 384 ms for acoustic startle and over the last 334 ms for tactile startle. Latency values were corrected to account for delay in arrival of startling stimuli. Measured latency for acoustic startle was reduced 1 ms for acoustic propagation delay and rise time of the startling noise burst. Measured latency for tactile startle was reduced 56.8 ms for the average arrival delay of the startling air burst. The delay between trigger signal and air burst can be seen in the lower trace of Fig. I(A).

MICROWAVE PULSES AND STARTLE

Median values of response parameters for each condition were analyzed. Variations in the microwave pulse at the signal source made it necessary to delete trials from analysis, as described below for each experiment. Trials were deleted on the basis of pulse parameters without knowledge of experimental condition or resuit. Medians for peak amplitude, response integral, and latency were derived from remaining trials. Statistical tests were applied directly to medians of corrected response latencies. Peak amplitude medians and response integral medians for each animal were first normalized by summing them and dividing by the respective sum (24). The transformation maintained the proportionate differences among conditions for each animal and weighted animals equally in subsequent statistical tests. EXPERIMENTS 1A AND 1B: ACOUSTICSTARTLEWITH SINGLE MICROWAVE PULSES AS PRESTIMULI Click prestimuli reduce startle amplitude for lead times of 8 64 ms and increase amplitude for lead times of 1 - 4 ms (17).

589

Therefore, microwave pulses causing click perceptions through microwave hearing should modify startle in similar ways. A preliminary study had shown that 1-#s pulses depositing 30 mJ/kg and 100 mJ/kg in the head and neck modified rat acoustic startle (44). Therefore, microwave pulses with similar energies at lead times of 201 to - 1 ms were tested here. Method

Twenty-four animals were tested with body mass of 254-312 g during test sessions. The startling acoustic stimulus was a 50ms burst of noise with 106 dB peak SPL at the animal head position. The noise burst contained frequencies of 100 Hz to 40 kHz and was effective in eliciting startle in all animals tested. The acoustic background noise at the head position was 67 dBA. Duration of the microwave pulses was set at a nominal 1 #s. With delay of arrival and rise time of the startling noise burst taken into account, effective prestimulus lead times were 201, 101, 51, 3, 1, and - 1 ms.

A.

lOOmV

50

mV

B°

] 5 ms

50

mV

FIG. 1. Sound and pressure wavefonns associated with the air burst used for the startling tactile stimulus. In each pair of waveforms, the upper waveform is sound level meter output and the lower waveform is the voltage signal from a piezoelectric film similar to that used in the animal holder. The film voltage oscillates at the mechanical natural frequency of the film with the onset indicative of air burst arrival.(A) A single air burst with trigger (T) at the onset of solenoid valve trigger. Note the delay between the trigger and the waveform onsets. (B) Averages of responses to 16 air bursts. Trigger (T) was at the initialpressure increase. Note the reduced average of the random sounds.

590

SEAMAN, BEBLO AND RASLEAR

0.40

0.35

High

Intensity Pulse

..o..o o.~o

(3.

0.25

:~ 0 . 1 5

**

o.1o z

0.05 0.00

ST 201 101 51

3

1

-1

PRESTIMULUS CONDtTION

FIG. 2. Acoustic startle peak amplitude (mean _ SEM) for the highintensity microwave pulse prestimulus. Prestimulus condition is designated by prestimulus lead time in milliseconds and by ST for the noprestimulus condition. *Indicates difference relative to the ST condition at the 0.05 level; ** at the 0.01 level. One group of 12 animals was tested in Experiment 1A with pulses of low intensity (average SAR of 21.24 kW/kg). Another group of 12 animals was tested in Experiment 1B with pulses of high intensity (average SAR of 64.28 kW/kg). Eight conditions were tested in each experiment. Condition ST was the startling noise burst delivered alone. Six conditions consisted of a startling noise burst preceded by a microwave pulse with the above lead times. The remaining condition was the microwave pulse delivered alone. Testing began after a 5 - 1 0 - m i n (15 min for one session in high-intensity pulse experiment) acclimation period in the exposure device. Following four ST trials not included in analysis, two blocks of trials were presented in a test session. Each block contained one presentation, or trial, of each of the eight conditions in random sequence. An intertrial interval of 2 - 3 min was necessary for transfer of data from the digital oscilloscopes to an ASCII data file. A total of 10 blocks of trials were presented to each animal over five sessions on 5 different days. After deletion of trials on the basis of microwave pulse parameters, medians of peak amplitude, response integral, and latency were tested for influence of lead time condition with one-factor repeated-measures analysis of variance (ANOVA) with a significance level of 0.05. As appropriate, post hoc Tukey's protected t-tests were applied.

Results No startle response was seen for the low-intensity or the highintensity microwave pulse delivered alone. This condition was not included in subsequent analysis of results, leaving seven conditions for each experiment. For the low-intensity microwave pulse in Experiment 1A, 23 trials were deleted over the seven conditions (847 trials) because pulse duration was less than 0.7 #s, pulse net energy was less than 1 m J, or percentage power absorbed was less than 80% and to restrict the SAR range. This resulted in the low-intensity prestimulus having pulse duration of 0.70-1.26 #s (0.96/.,s average), SAR of 15.0-30.0 kW/kg (21.24 kW/kg average), and SA of 16.0-44.2 mJ/kg (26.30 mJ/kg average) in remaining trials. The maximum numbers of deleted trials were 10 for an animal and five for a condition. The low-intensity microwave pulse did not modify responses. Means of peak amplitudes were 0.122-0.157, with that of the 101-ms condition being the smallest, F(6, 66) < 1, p = 0.57. Means of the response integrals, 0.133-0.150, were similar across lead time condition, F(6, 66) < 1, p = 0.78. For equal normalized values for all seven conditions, the means would all

be 0.143 for peak amplitude and response integral. Mean latencies were 12.2-12.9 ms, F(6, 66) = 1.929, p = 0.09. Based on these results, no significant modification of acoustic startle response amplitude or latency occurred with the low-intensity microwave pulse prestimulus for the lead times tested. For the high-intensity microwave pulse in Experiment 1B, 94 trials were deleted over the seven conditions (868 trials) because pulse duration was less than 0.7 #s, pulse net energy was less than 4 m J, o r percentage power absorbed was less than 80% and to restrict the SAR range. This resulted in the high-intensity prestimulus having pulse duration of 0.73-1.20 #s (0.96 #s average), SAR of 35.5-86.0 kW/kg (64.28 kW/kg average), and SA of 66.6-141.8 mJ/kg (88.66 mJ/kg average) in remaining trials. The maximum numbers of deleted trials were 20 for an animal and 18 for a condition. The high-intensity microwave pulse modified response amplitude. Lead time condition influenced peak amplitude, shown in Fig. 2, F(6, 66) = 7.328, p < 0.0001. Means for the 101- and 51-ms conditions were smaller than the mean for the ST condition (t-test, p < 0.01) and the mean for the 1-ms condition was larger than the ST mean (p < 0.05). Also, the mean for the 1ms condition was larger than the means for the other five conditions (p < 0.01) and the mean for the 51-ms condition was smaller than the mean for the 3-ms condition (p < 0.05). Lead time condition also influenced response integral, shown in Fig. 3, F(6, 66) = 13.446, p < 0.0001. Means for the 101- and 51ms conditions were smaller than the mean for the ST condition (p < 0.05) and the mean for the 3-ms condition was larger than the ST mean (p < 0.01). Also, the mean for the 3-ms condition was larger than the means for the other five conditions (p < 0.01) and the means for 201-, 101-, and 51-ms conditions were each smaller than the means for the 1- and - 1 - m s conditions (p < 0.05 o r p < 0.01). The high-intensity microwave pulse also modified response latency, shown in Fig. 4, F(6, 66) = 3.776, p = 0.0027. Means for the 201- and 51-ms conditions were larger than the mean for the ST condition (t-test, p < 0.01). Also, the mean for the 201ms condition was larger than means for the 1- and - 1 - m s conditions (p < 0.01) and than means for the 101- and 3-ms conditions (p < 0.05). In addition, the mean for the 5 l-ms condition was larger than the mean for the 1-ms condition (p < 0.05). Based on these results for the high-intensity microwave pulse, response inhibition occurred for lead times of 201, 101, and 51 ms indicated by reduced amplitude, increased latency, or both. Although both mean amplitude measures at the 201 ms lead time were also smaller than the respective ST condition mean, neither

0.40

High Intensity Pulse

0.35 tY

0.30 0.25

z

0.20 0.15

nO 0.10 z 0.05

0.00

ST 201 101 51

3

1

-1

PRESTIMULUS CONDITION

FIG. 3. Acoustic startle response integral (mean __ SEM) for the highintensity microwave pulse prestimulus. Prestimulus condition is designated by prestimulus lead time in milliseconds and by ST for the noprestimulus condition. *Indicates difference relative to the ST condition at the 0.05 level; ** at the 0.01 level.

MICROWAVE PULSES AND STARTLE

20 18

591

High Intensity Pulse

16 14

**

~ I0 z 8 Ld

4 2 0

ST 201 101 51

3

1

-1

PRESTIMULUS CONDITION FIG. 4. Acoustic startle latency (mean _+ S E M ) for the high-intensity

microwave pulse prestimulus. Prestimulus condition is designated by prestimulus lead time in milliseconds and by ST for the no-prestimulus condition. *Indicates difference relative to the ST condition at the 0.05 level; ** at the 0.01 level. difference was significant at the 0.05 level. Response enhancement occurred for lead times of 1 and 3 ms, indicated by increased amplitude measures without a change in latency. EXPERIMENT 2: TACTILESTARTLEWITH SINGLEMICROWAVE PUI.~ES AND ACOUSTICCLICKS AS PRESTIMULI Modification of tactile startle was studied to generalize the results of Experiments 1A and lB. Certain acoustic prestimuli modify tactile startle elicited by air bursts (34,52). Microwave pulses generating click perceptions might act in a similar manner. An acoustic click prestimulus was added to the experiment because no information on a click as a prestimulus to tactile startle was found. A wide range of lead times was used for the same reason. To increase the likelihood of action, the microwave pulse had intensity similar to the high intensity used with acoustic startle and a longer duration.

Testing began after a 4.7-7-rain acclimation period in the exposure device. Following three ST trials not included in analysis, four to six trials of each condition (all clicks or all microwave pulses) were delivered in random order. The intertrial interval was again 2 - 3 rain to allow transfer of data from the digital oscilloscopes to an ASCII data file. After deletion of trials on the basis of microwave pulse parameters, medians of peak amplitude, response integral, and latency were tested for influence of lead time and prestimulus type with two-factor ANOVA with repeated measures on lead time and a significance level of 0.05. As appropriate, post hoc Tukey's protected t-tests were applied. Results Data from three animals were not included in the analysis because of technical difficulties related to background noise in the exposure device and storage of responses in data files. Over the six conditions (272 trials) for the microwave pulse prestimuIns in the 11 remaining animals, 18 trials were deleted for which pulse duration was less than 6/zs, pulse net energy was less than 32 mJ, or percentage power absorbed was less than 80%. This resulted in the microwave pulse prestimulns having duration of 6.80-8.36 /zs (7.82 /zs average), SAR of 55.9-113.3 kW/kg (76.95 kW/kg average), and SA of 525.0-1055.7 rnJ/kg (663.36 rrd/kg average) in remaining trials. The maximum numbers of deleted trials were four for an animal and nine for a condition. All 273 trials for the click prestimulus in the 11 animals were included in the analysis. Both the acoustic click and the microwave pulse modified response amplitude. Lead time condition influenced peak amplitude, shown in Fig. 5, F(5, 120) = 7.517,p < 0.0001. PrestimuIns type and interaction of condition and type did not influence peak amplitude [F(1, 120) < 1 andF(5, 120) = 1.151,p = 0.34, respectively]. For the acoustic click, means for the 157- and 57ms conditions were smaller than the mean for the ST condition (t-test, p < 0.05 and p < 0.01, respectively). Also, the mean for the 57-ms condition was smaller than means for the 7- and - 4 3 -

Method 0.30

Fourteen animals were tested with body mass of 250-304 g during test sessions. The startling stimulus was a burst of compressed air. The tube inside the animal holder was connected through plastic tubing to a solenoid valve opened by a computergenerated trigger. The pressure at the valve, 4 8 - 5 2 psi (330-350 kPa), created an air burst stimulus at the animal (Fig. 1) with an average delay of 56.8 ms (51.0-58.9 ms range) from the trigger. The air burst was effective in eliciting startle in all animals tested. The acoustic background noise at the head position was 50-55 dBA. The microwave pulses had average SAR of 76.95 kW/kg and duration set at a nominal 8/zs. Acoustic clicks produced by rectangular voltage pulses applied to a speaker were 94 dB peak SPL at the head position. With the average arrival delay of the air burst taken into account, effective prestimulus lead times were 157, 107, 57, 7, and - 4 3 ms. The variation of about 8 ms around the average arrival time was smaller than the 50-ms interval between lead times. All 14 animals were used in the same experiment with microwave and click prestimuli in separate test sessions. Six conditions were presented in each test session. Condition ST was the starfling air burst delivered alone. Five conditions consisted of a startling air burst and a prestimulus with the above lead times. Each animal was tested with acoustic clicks and microwave pulses on different days. Seven randomly selected animals were tested with acoustic clicks first, the other seven with microwave pulses first.

D /--

Acoustic

0.25

CLick

_~ 0.20 (3. <

0.15

:~ 0.10

(E o 0.05 z 0.00 ST

157

107

57

7

-43

PRESTIMULUS CONDITION 0.30 Microwave Pulse 0.25 D h-~ 0.20 (3. •,,< ~ o.15

o.1o fig

o z

0.05 O.O0 ST

157

107

57

7

-43

PRESTIMULUS CONDITION

FIG. 5. Tactile startle peak amplitude (mean _+ SEM) for acoustic click and microwavepulse prestimuli. Prestimuluscondition is designatedby prestimulus lead time in milliseconds and by ST for the no-prestimulus condition. *Indicates difference relative to the ST condition at the 0.05 level; ** at the 0.01 level.

592

SEAMAN, BEBLO AND RASLEAR

ms conditions (p < 0.01). For the microwave pulse, the mean for the 57-ms condition was smaller than the mean for the ST condition and the mean for the -43-ms condition was larger than the ST mean (2 < 0.05). Also, the mean for the 57-ms condition was smaller than means for the 107- and -43-ms conditions (p < 0.05 andp < 0.01, respectively). In addition, the mean for the -43-ms condition was larger than the means for the 157- and 7ms conditions (t9 < 0.01 a n d p < 0.05, respectively). Response integral results are shown in Fig. 6. The integral shows small decreases in mean for the 157-, 107-, and 57-ms conditions for the acoustic click prestimulus and a trend to increase across lead time for the microwave pulse prestimulus. However, lead time condition, prestimulus type, and interaction of condition and type did not influence the response integral [F(5, 120) = 2.199,p = 0.06; F(1, 120) < 1; and F(5, 120) = 1.210, p = 0.31, respectively]. The acoustic click and the microwave pulse modified response latency, shown in Fig. 7, with both prestimuli causing longer latencies. As for peak amplitude, lead time condition influenced latency, F(5, 120) = 4.075, p = 0.0019, but prestimulus type and interaction of condition and type did not [F(1, 120) < 1 and F(5, 120) < 1, respectively]. For the acoustic click, means for the 157- and 107-ms conditions (t-test, p < 0.01) and the mean for the 57-ms condition (p < 0.05) were larger than the mean for the ST condition. Also, means for the 157- and 107-ms conditions were each larger than means for the 7- and -43-ms conditions (p < 0.01) and the mean for the 107-ms condition was larger than the mean for the 57-ms condition (2 < 0.05). In addition, the mean for the 57-ms condition was larger than means for the 7- and -43-ms conditions (t9 < 0.05). For the microwave pulse, means for the 157-, 107-, and 57-ms (p < 0.01) and the mean for the 7-ms condition (p < 0.05) were larger than the mean for the ST condition. Also, means for these four conditions bore the same respective relationships to the mean for the - 4 3 ms condition• The decreased amplitudes and increased latencies of responses occurring with acoustic clicks and microwave pulses pre-

0.30 Acoustic Click

0.25 o

0.20

z

0.15

(3c

A c o u s t i c Click

30 v

25

20 ,.z, 15 5 0 ST

157 107

57

7

-43

PRESTIMULU5 CONDITION

v

4O 35 30 25

Microweve Pulse

20 ,.z, 15 <,1o 5 o ST

157 107

57

7

-43

PRESTIMULUS CONDITION

FIG. 7. Tactile startle latency (mean _ SEM) for acoustic click and microwave pulse prestimuli.Prestimuluscondition is designatedby prestimulus lead time in milliseconds and by ST for the no-prestimulus condition. *Indicatesdifference relative to the ST condition at the 0.05 level; ** at the 0.01 level. ceding the air burst indicated inhibition of tactile startle. For the acoustic click prestimulus, response inhibition occurred at lead times of 157 and 57 ms as indicated by decreased amplitude and increased latency, and at lead time of 107 ms as indicated by increased latency with a nonsignificant decrease in amplitude. For the microwave pulse prestimulus, response inhibition occurred at lead times of 157, 107, and 7 ms as indicated by increased latency, and at lead time of 57 ms as indicated by decreased amplitude and increased latency. Also, for the microwave pulse prestimulus, response enhancement occurred at lead time of - 4 3 ms as reflected in increased amplitude without a change in latency. Although the trend in response integral with lead time followed that of the peak amplitude for the acoustic click prestimulus, the mean response integral was not significantly different from that of the respective no-prestimulus condition for any lead time condition for either type of prestimulus.

0.10 DISCUSSION

0.05 0.00 ST

157 107

57

7

-43

PRESTIMULUS CONDITION 0.30 Microwave Pulse t~

4O 35

0.25

w 0.20 o

0.15 m of

0.10

° z

0.05 0.00 ST

157 107

57

7

-43

PRESTIMULUS CONDITION

FIG. 6. Tactile startle responseintegral (mean _ SEM) for acousticclick and microwavepulse prestimuli.Prestimuluscondition is designatedby prestimulus lead time in millisecondsand by ST for the no-prestimulus condition.

The inhibition of acoustic startle by the high-intensity microwave pulse at lead times of 51-201 ms is consistent with inhibition seen in the rat with sensory stimuli of different modalities delivered at these lead times (19,21,38,42). Inhibition occurs in the form of reduced amplitude and either unchanged or increased latency of response (21,26,35). Although effectiveness depends on the details of a particular experiment, pronounced inhibition occurs for sensory prestimuli with lead times of 50-200 ms, with a global reported range of roughly 8-2000 ms. Lead times of 1 - 3 ms for enhancement of acoustic startle by the high-intensity microwave pulse are consistent with those for enhancement previously seen with sensory stimuli. Enhancement for prestimuli with lead time of less than 10 ms is usually in the form of decreased latency without change in amplitude (21). Latency reduction of the rat whole-body acoustic startle response occurs for acoustic and photic prestimuli with lead time of 4 ms (22,42,49). Forelimb EMG amplitude is increased and its latency is reduced for prestimulus lead times of 0, 5, and 10 ms (26).

MICROWAVE PULSES AND STARTLE

An acoustic click of 92 dB peak SPL at lead times of 1 - 4 ms increases acoustic startle amplitude (17). The same click at lead times of 8 - 6 4 ms reduces the amplitude but has no effect when delivered simultaneously with, or 1 or 16 ms after onset of, the startling stimulus. The click also produces a weak enhancement of response amplitude when delivered 2, 4, or 8 ms after the onset of the startling stimulus but only in the initial test session. Although lead times for microwave pulse prestimuli were not identical to lead times in the acoustic click experiments, the pattern of inhibition and enhancement for comparable lead times are quite similar. This may indicate a similar mode of action by acoustic click and microwave pulse prestimuli, possibly through the auditory system as discussed below. However, caution should be used in making this type of comparison because electric foot shock also produces a similar pattern of response modification with lead time (17). Lead times of 57, 107, and 157 ms for acoustic clicks and microwave pulses effective in producing inhibition of tactile startle are consistent with those previously reported for inhibition of tactile startle. The significant reductions of 30-40% in peak amplitude at lead times of 57 and 157 ms (Fig. 5) are similar to the reduction seen with 20-ms, 80-dBA noise delivered 100 ms before onset of an air burst (34). These lead times are also similar to the 51-201-ms range effective for inhibition of acoustic startle described above. The microwave pulse lead time of 7 ms that increased response latency is shorter than reported inhibitory lead times for tactile and most acoustic startle studies but is similar to the 8-ms lead time of an acoustic click effective in reducing acoustic startle amplitude (17). The lack of significant influence of prestimulus type and its lack of interaction with lead time indicate that acoustic clicks and microwave pulses had similar effects on tactile startle. Enhancement of tactile startle for the microwave pulse delivered 43 ms after the onset of the startling air burst was unexpected. Based on rat acoustic startle responses, the tactile startle response was expected to last about 100 ms and a stimulus delivered 43 ms after onset of the startling stimulus was expected to have no effect. However, recorded tactile startle responses often lasted longer than 200 ms. For the no-prestimulus ST condition, maximum latencies to the first, second, and third peaks in the response were 96.4, 107.6, and 105.6 ms, respectively, with corresponding mean latencies of 34.5, 40.6, and 49.6 ms. Thus, the microwave pulse at - 4 3 ms occurred during many tactile startle responses, indicating the possibility of action on ongoing neuromuscular activity underlying the whole-body response. Air pressure was most likely the primary startling tactile stimulus. The air pressure was larger than pressures previously reported effective in rats (6,30,34). On the other hand, the generated sound lacked the critical property of reaching a level of about 90 dB SPL within 12 ms to be an effective startling stimulus (11,13). Because many tactile startle responses started, and peaked, before the sound peak, they were most likely caused by the air pressure component even if the sound were capable of eliciting startle. However, the possibility that the pressure and sound components of the air burst shown in Fig. 1 acted together in some way to elicit startle cannot be excluded. A primary role for the air burst acoustic component has been suggested (51). The -43-ms lead time is the only lead time for which prestimuli occurred during the air burst sound and thus the lead time for which presence of sound might have affected a result. If the sound prevented response enhancement by the acoustic click due to auditory masking but not by the microwave pulse, then a nonmaskable or nonauditory interaction may be indicated. The inhibition and enhancement of tactile startle resemble in some respects modification of the human blink reflex (2). Inhi-

593

bition in the form of decreased amplitude occurs for photic, shock, and vibration prestimuli delivered more than 100 ms before an intense acoustic stimulus (3,4,15). Enhancement in the form of increased amplitude and/or decreased latency occurs with the same prestimuli delivered with lead times less than 100 ms. A similar trend in response amplitude is seen for blinks elicited by air puffs preceded by photic prestimuli (40). Photic or acoustic stimuli delivered at the same time as an air puff increase blink amplitude (23). Similar enhancement is seen with blinks elicited by electrical stimuli preceded by or followed by acoustic white noise (5). Blinks elicited by glabellar taps are inhibited by acoustic prestimuli with lead time of 200 ms but are augmented by the same stimuli presented simultaneously (20). The decreased amplitude of tactile startle found for both acoustic click and microwave pulse prestimuli (Fig. 5) at longer lead times is similar to that reported for the blink reflex except that the effective lead time of 57 ms is shorter. The increased latencies seen for both types of prestimuli (Fig. 7) is a feature not reported for inhibition of the blink reflex. The enhancement of tactile startle manifested by increased amplitude with microwave pulses at - 4 3 ms differs from blink enhancement by being effective at a later time. The absence of enhancement of tactile startle at lead time of 7 ms and the lack of reduced latency as part of the enhancement are also different. The different effective ranges may be because of a shift in effective lead times, perhaps due to species differences or to the types of stimuli. Causing larger reductions in peak amplitude and larger increases in latency, the click seemed more effective than the microwave pulse at the longer lead times, 57-157 ms, to produce inhibition in tactile startle. On the other hand, the microwave pulse seemed more effective at the short lead time of 7 ms and at 43 ms after startling stimulus onset. These differences may indicate different effects on the neural circuits involved in tactile startle. Besides the end points measured, no basis currently exists for comparing equivalence of the two prestimuli, e.g., in terms of auditory stimulation. Assessing differences that might have led to differences in effectiveness is therefore not possible. A comparison of results for the two amplitude measures of whole-body startle response is interesting. The peak amplitude measure indicates the magnitude of limb extension in the early part of the response, typically during the first 20-30 ms for acoustic startle and during the first 3 0 - 4 0 ms for tactile startle. The response integral contains magnitude information over most of the response waveform. Because both measures indicate response magnitude, we expect them to change in the same way. Indeed, the integral contains peak amplitude information. However, the integral is influenced by response events occurring after initial peaks. In these experiments, peak amplitude and response integral measures generally give similar results with the integral showing less sensitivity. One exception involves lead times of 3 and 1 ms for the high-intensity microwave pulse with acoustic startle (Figs. 2 and 3). The increase in peak amplitude at 1 ms indicates an interaction affecting the early part of the response and the increase in response integral at 3 ms indicates an interaction affecting primarily the later parts of the response. Further, these differences may indicate a difference in action of the microwave pulse on the neural circuits responsible for acoustic startle. Another exception is the absence of change in response integral for the microwave pulse in tactile startle while the peak amplitude is reduced for a lead time of 57 ms and increased for a lead time of - 4 3 ms (Figs. 5 and 6). These differences may indicate an effect primarily on the early events of the tactile startle response. The microwave pulses averaging 0.96 #s in the acoustic startle experiments and 7.82 #s in the tactile startle experiment are

594

SEAMAN, BEBLO A N D R A S L E A R

the briefest prestimuli so far reported to modify startle. Gaps in background noise of 1 - 4 ms inhibit startle (27). Depending on lead time, nonreflexogenic acoustic clicks and electric shocks of millisecond duration can inhibit or enhance startle (17,38). Although the microwave pulses were roughly three orders of magnitude shorter than these previously used prestimuli, such pulses are capable of generating intracranial pressure waves that outlast the pulse (33). However, pressure waves measured in heads of large rats have durations of 4 0 - 1 0 0 #s (36), also shorter than effective noise gaps, dicks, and shocks. Microwave pulses could have acted to alter the startle response in a number of ways. Pulses similar to those used here cause rapid increases in temperature of 10-6-10-4°C (7,33). Although this small temperature increment is inconsequential in terms of heat load, the rapid rate of temperature change creates a pressure pulse by means of thermal expansion. A pressure wave launched by this thermoelastic process is thought to be responsible for auditory perception of microwave pulses. Microwave pulses of sufficient intensity result in auditory system stimulation, or microwave hearing (7,16,33,39,47,48,55). Energies of the low-and high-intensity microwave pulses for acoustic startle (16.0-44.2 mJ/kg and 66.6-141.8 mJ/kg, respectively) and the microwave pulse for tactile startle (525.01055.7 mJ/kg) were larger than microwave hearing thresholds of 0.12-16 mJ/kg in various species. Thus, auditory stimulation is likely to have occurred in these experiments. Energies of the lowintensity microwave pulse in acoustic startle may have been small enough that the 67-dBA background noise masked perception and thus modification did not occur. However, auditory system involvement was not specifically tested here. Common findings in these experiments were that the occurrence of effect and the type of effect of a microwave pulse depended on the temporal relationship between the pulse and the startling stimulus. For the 1-#s pulse in acoustic startle, response amplitude decreased for the 101- and 51-ms lead times but increased for the 3- and 1-ms lead times. Response latency increased for the 201- and 51-ms lead times but did not change for other lead times. For the 8-#s pulse in tactile startle, response amplitude decreased for the 57-ms lead time but increased for the - 4 3 - m s lead time. Latency increased for all cases of a preceding pulse, 7 - 1 5 7 - m s lead times, but did not change at the - 4 3 - m s lead time. As described above, these patterns are similar to those commonly observed for sensory prestimuli and are ev-

idence for action through sensory systems. Because of the thermal mass of head contents, the thermal diffusion time constant can be expected to be much longer than the 201-ms maximum lead time for microwave pulse prestimuli used here. A thermal action of a microwave pulse would thus be expected to be relatively constant over lead times used. The observed temporal dependencies and the difference in effects are thus evidence against gross thermal actions. In summary, the results show that a single microwave pulse of sufficient intensity can modify the amplitude and/or latency of whole-body acoustic or tactile startle of the rat depending on the timing between it and the startling stimulus. The modifications raise the possibility that similar single pulses can modify other behaviors when delivered at a particular time relative to other stimuli. Thermal loading, which seems to be necessary for a number of behavioral effects of untimed microwave pulses (1,8,9,39), would not be a factor. Although the microwave pulse effects reported here could be due to sound generated in the head, this possibility was not directly tested. Given our knowledge of microwave hearing, the similarities in modification by a microwave pulse to modification by sensory prestimuli are evidence for such action. ACKNOWLEDGEMENTS The Walter Reed Army Institute for Research (WRAIR) sponsored this research under contract DAMD17-89-C-9021 to ERC BioServices Corporation. The WRAIR Department of Microwave Research (DMR), COL E.C. Elson, Chief, provided additional support through the U.S. Army Research Office under contract DAAL03-86-D-0001, Delivery Orders 1542 and 2776, a summer faculty program administered by Battelle. We thank ERC personnel Dr. Yahya Akyel for valuable advice on experimental design; Dennis Brown, Dave Varle, Mike Belt, Bill York, and Michelle DeAngelis for technical advice and assistance; and Doug Larrick for computer software. We are also grateful to Shashank Dhadphale of the Louisiana Tech Department of Biomedical Engineering for data extraction and graphics and to two anonymous reviewers for valuable comments. The work adhered to principles of the Guide for the Care and Use of Laboratory Animals, NIH publication 85-23. All procedures were performed under protocols approved by the WRAIR Laboratory Animal Care and Use Committee. Opinions and conclusions obtained in this work are those of the authors and do not necessarily reflect the position of the Department of the Army or the Department of Defense (Para 4-3, AR 360-5).

REFERENCES 1. Akyel, Y.; Hunt, E. L.; Gambrill, C.; Vargas, C., Jr. Immediate postexposure effects of high-peak-power microwave pulses on operant behavior of Wistar rats. Bioelectromagnetics 12:183-195; 1991. 2. Anthony, B.J. In the blink of an eye: Implications of reflex modification for information processing. In: AcHes, P. K.; Jennings, J. R.; Coles, M. G. H., eds. Advances in psyehophysiology, vol. 1. Greenwich, CT: JAI Press Inc.; 1985:167-218. 3. Blumenthal, T. D.; Crescheider,G. A. Modification of the acoustic startle reflex by a tactile prepolse: The effects of stimulus onset asynchrony and prepulse intensity. Psycbophysiology 24:320-327; 1987. 4. Blumenthal, T. D.; Tolomeo, E. A. Bidirectional influences of vibrotactile stimuli on modification of the human acoustic startle reflex. Psychobiology 17:315-322; 1989. 5. Boelhouwer, A. J. W.; Teurlings, R. J. M. A.; Brunia, C. H. M. The effect of an acoustic warning stimulus upon the electrically elicited blink reflex in humans. Psychophysiology 28:133-139; 1991. 6. Casto, R.; Nguyen, T.; Printz, M. P. Characterization of cardiovascular and behavioral responses to alerting stimuli in rats. Am. J. Physiol. 256:Rl121-Rl126; 1989.

7. Chou, C. K.; Guy, A. W.; Galambos, R. Auditory perception of radio-frequency electromagnetic fields. J. Acoust. Soc. Am. 71:1321-1334; 1982. 8. D'Andrea, J. A. Microwave radiation absorption: Behavioral effects. Health Phys. 61:29-40; 1991. 9. D'Andrea, J. A.; Cobb, B. L.; de Lorge, J. O. Lack of behavioral effects in the rhesus monkey: High peak microwave pulses at 1.3 GHz. Bioelectromagnetics 10:65-76; 1989. 10. Davis, M. Neurochemical modulation of sensory-motor reactivity: Acoustic and tactile startle reflexes. Neurosci. Biobehav. Rev. 4:241-263; 1980. 11. Davis, M. The mammalian startle response. In: Eaton, R. C., ed. Neural mechanisms of startle behavior. New York: Plenum Press; 1984:287-351. 12. Fechter, L. D.; Sheppard, L.; Young, J. S.; Zeger, S. Sensory threshold estimation from a continuously graded response produced by reflex modification audiometry. J. Acoust. Soc. Am. 84:179-185; 1988. 13. Fleshier, M. Adequate acoustic stimulus for startle reaction in the rat. J. Comp. Physiol. Psychol. 60:200-207; 1965.

M I C R O W A V E PULSES A N D S T A R T L E

14. Frey, A. H.; Feld, S. R. Avoidance by rats of illumination with low power nonionizing electromagnetic energy. J. Comp. Physiol. Psychol. 89:183-188; 1975. 15. Graham, F. K. Control of reflex blink excitability. In: Thompson, R. F.; Hicks, L. H.; Shvyrkov, V. B., eds. Neural mechanisms of goal-directed behavior and learning. New York: Academic Press; 1980:511-519. 16. Guy, A. W.; Chou, C. K.; Lin, J. C.; Christensen, D. Microwaveinduced acoustic effects in mammalian auditory systems and physical materials. Ann. NY Acad. Sci. 247:194-215; 1975. 17. Hammond, G. R.; Leitner, D. S. Augmentation of the rat's acoustic startle reflex by nonreflexogenic stimuli. Behav. Neurosci. 104:841848; 1990. 18. Hjeresen, D. L.; Doctor, S. R.; Sheldon, R. L. Shuttlebox side preference as mediated by pulsed microwave and conventional auditory cues. In: Proceedings of the 1978 Symposium on Electromagnetic Fields in Biological Systems. Edmonton: 1MPI, Ltd.; 1979:194214. 19. Hoffman, H. S. Methodological factors in the behavioral analysis of startle. In: Eaton, R. C., ed. Neural mechanisms of startle behavior. New York: Plenum Press; 1984:267-285. 20. Hoffman, H. S.; Cohen, M. E.; Stitt, C. L. Acoustic augmentation and inhibition of the human eyeblink. J. Exp. Psychol. [Hum. Percept. Perf.] 7:1357-1362; 1981. 21. Hoffman, H. S.; Ison, J. R. Reflex modification in the domain of startle: I. Some empirical findings and their implications for how the nervous system processes sensory input. Psychol. Rev. 87:175-189; 1980. 22. Hoffman, H. S.; Searle, J. L. Acoustic and temporal factors in the evocation startle. J. Acoust. Soc. Am. 43:269-282; 1968. 23. Hoffman, H. S.; StiR, C. L. Augmentation of the air puff-elicited eye blink by concurrent visual and acoustic input. Bull. Psychonom. Soc. 15:115-117; 1980. 24. Ison, J. R.; Bowen, G. P.; Kellogg, C. Potentiation of acoustic startle behavior in the rat (Ratms norvegicus) at the onset of darkness. J. Comp. Psychol. 105:3-9; 1991. 25. Ison, J. R.; Hoffman, H. S. Reflex modification in the domain of startle: II. The anomalous history of a robust and ubiquitous phenomenon. Psychol. Bull. 94:3-17; 1983. 26. Ison, J. R.; McAdam, D. W.; Hammond, G. R. Latency and amplitude changes in the acoustic startle reflex of the rat produced by variation in auditory prestimulation. Physiol. Behav. 10:1035-1039; 1973. 27. Ison, J. R.; O'Connor, K.; Bowen, G. P.; Bocirnea, A. Temporal resolution of gaps in noise by the rat is lost with functional decortication. Behav. Neurosci. 105:33-40; 1991. 28. Johnson, C. C.; Guy, A. W. Nonionizing electromagnetic wave effects in biological materials and systems. Proc. IEEE 60:692-718; 1972. 29. Johnson, R. B.; Myers, D. E.; Guy, A. W.; Lovely, R. H.; Galambos, R. Discriminative control of appetitive behavior by pulsed microwave radiation in rats. In: Johnson, C. C.; Shore, M. L., eds. Biological effects of electromagnetic waves. Selected Papers of the USNC/URSI Annual Meeting, Boulder, CO, Oct. 20-23, 1975. Rockville, Maryland: HEW (FDA) 77-8010; 1976:238-256. 30. Kehne, J. H.; McCIoskey, T. C.; Taylor, V. L.; Black, C. K.; Fadayel, G. M.; Schrnidt, C. J. Effects of the serotonin releasers 3,4-methylenedioxymethamphetamine (MDMA), 4-choloramphetamine (PCA) and fenfluramine on acoustic and tactile startle reflexes in rats. J. Pharmacol. Exp. Ther. 260:78-89; 1992. 31. Landis, C.; Hunt, W. A. The startle pattern. New York: Farrar and Rinehart; 1939. 32. Leitner, D. S.; Cohen, M. E. Role of the inferior colliculus in the inhibition of acoustic startle in the rat. Physiol. Behav. 34:65-70; 1985. 33. Lin, J. C. Auditory perception of pulsed microwave radiation. In: Gandhi, O. P., ed. Biological effects and medical applications of electromagnetic energy. Englewood Cliffs, NJ: Prentice Hall; 1990:277-318.

595

34. Mansbach, R. S.; Geyer, M. A. Effects of phencyclidine and phencyclidine biologs on sensorimotor gating in the rat. Neuropsychopharmacology 2:299-308; 1989. 35. Mansbach, R. S.; Geyer, M. A. Parametric determinants in prestimulus modification of acoustic startle--interaction with ketaminc. Psychopharmacology (Berlin) 105:162-168; 1991. 36. Olsen, R. G.; Lin, J. C. Microwave-induced pressure waves in mammalian brains. IEEE Trans. Biomed. Eng. BME-30:289-293; 1983. 37. Pellet, J. Neural organization in the brainstem circuit mediating the primary acoustic head startle: An electrophysiological study in the rat. Physiol. Behav. 48:727-739; 1990. 38. Pinckney, L. A. Inhibition of the startle reflex in the rat by prior tactile stimulation. Anita. Learn. Behav. 4:467-472; 1976. 39. Postow, E.; Swicord, M. L. Modulated fields and "window" effects. In: Polk, C.; Postow, E., eds. CRC handbook of biological effects of electromagnetic fields. Boca Raton, FL: CRC Press; 1986:425460. 40. Reiter, L. A.; Ison, J. R. Inhibition of the human eyeblink reflex: An evaluation of the sensitivity of the Wendt-Yerkes method for threshold detection. J. Exp. Psychol. ]Hum. Percept. Perf.] 3:325336; 1977. 41. Rettig, R.; Geyer, M. A.; Printz, M. P. Cardiovascular concomitants of tactile and acoustic startle responses in spontaneously hypertensive and normotensive rats. Physiol. Behav. 36:1123-1128; 1986. 42. Schwartz, G. M.; Hoffman, H. S.; Stitt, C. L.; Marsh, R. R. Modification of the rat's acoustic startle response by antecedent visual stimulation. J. Exp. Psychol. [Anim. Behav. Proc.] 2:28-37; 1976. 43. Seaman, R. L. Modification of acoustic and tactile startle by microwave pulses. Final technical report, ERC BioServices Corporation, Gaithersburg, MD; September 1992. 44. Seaman, R. L.; Beblo, D. A. Modification of acoustic startle by microwave pulses in the rat: A preliminary report. Bioelectromagnetics 13:323-328; 1992. 45. Seaman, R. L.; Beblo, D. A.; Raslear, T. G. Modification of acoustic startle amplitude by microwave pulses. In: Blank, M., ed. Electricity and magnetism in biology and medicine. San Francisco: San Francisco Press; 1993:662-664. 46. Seaman, R. L.; Beblo, D. A.; Raslear, T. G. Modification of tactile startle amplitude by microwave pulses. In: Blank, M., ed. Electricity and magnetism in biology and medicine. San Francisco: San Francisco Press; 1993:665-667. 47. Seaman, R. L.; Lebovitz, R. M. Auditory unit responses to singlepulse and twin-pulse microwave stimuli. Hear. Res. 26:105-116; 1987. 48. Seaman, R. L.; Lebovitz, R. M. Thresholds of cat cochlear nucleus neurons to microwave pulses. Bioelectromagnetics 10:147-160; 1989. 49. Stitt, C. L.; Hoffman, H. S.; Marsh, R. R.; Boskoff, K. J. Modification of the rat's startle reaction by an antecedent change in the acoustic environment. J. Comp. Physiol. Psychol. 86:826-836; 1974. 50. Swerdlow, N. R.; Geyer, M. A. Prepulse inhibition of acoustic startle in rats after lesions of the pedunculopontine tegmental nucleus. Behay. Neurosci. 107:104-117; 1993. 51. Taylor, B. K.; Casto, R.; Printz, M. P. Dissociation of tactile and acoustic components in air puff startle. Physiol. Behav. 49:527-532; 1991. 52. Vorhees, C. V.; Minck, D. R. Long-term effects of prenatal phenytoin exposure on offspring behavior in rats. Neurotoxicol. Teratol. 11:295-305; 1989. 53. Wachtel, H.; Beblo, D.; Vargas, C., Jr,; Bassen, H.; Brown, D. Single microwave pulses can suppress startle reflexes in mice. Proc. Ann. Int. Conf. IEEE Eng. Med. Biol. Soc. 10:911-912; 1988. 54. Wecker, J. R.; Ison, J. R.; Foss, J. A. Reflex modification as a test for sensory function. Neurobehav. Toxicol. Teratol. 7:733-738; 1985. 55. Wilson, B. S.; Joines, W. T. Mechanisms and physiological significance of microwave action on the auditory system. J. Bioelectricity 4:495-525; 1985.