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PURE TONE ~ ~ S H O L D S

FOR THE RHESUS MONKEY *

ELAMMINO~ BRENDA LONSBURY,MARTIN and

Auditory t ~ s h o l d s were measured for 18 ears from 13 rhesus monkeys using a simple reactiontime procedure. The t h r e ~ o l d contour was a smooth W-shaped function with flse~ at the extreme frequendes and around 4 kHz and was comparable in shape with previously reported threshoMs for this animal. Standard deviations averaged 5.3 dB. Keywords: auditory thresh01ds; rhesus monkey; hearing.

INTRODUCTION

in recent years the rhesus monkey (Macacc~ mulatto) has been the most commonly u~d nonhuman prirn~e for animal research. Normative data on this anhnal, therefore, are of importance to the scientific community. The recent ban by the Indian government on the export ~!" rhesus monkeys to the United States make,~ the future supply of this species uncertain, again emphasizing the need for normative da~a on the rhesus monkey ~ c a ~ l eomp ns can be made between ,t~asimportant researc| model and

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ed behavioral pure tone thresholds for a rhesus monkey a n ~ b e r of experimenters have published threshold ~ [1,2,4-6,8-11]. Most of these studies used ations in : e X ~ ~ n t a l procedures from different laboral~rable. Most of the early studies were limited to the the m o n ~ ' s receptive field; Stebbins et al. [i 1] were er frequency limits of heating for the macaque. lave t~een measuring auditory thresholds in rhesus menused for a variety of experimental puposes, all of the ini~re,c¢:~l~cted using a stmdard set of psychoacousfic pro-

. . . . . . . . ;6 from the National Institutes of Hea|th, the Office of NavM Research, and a grant

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cedures [8]. Here we pre~nt data from 18 ears from 13 monkeys ~owing stable hearing behavior and from whict~ a complete set of thresholds was collected, METHODS

Subjects were I3 rhesus monkeys weighing 2.9-7.1 kg. At the time these data were collected, none of the monkeys had any known history of acoustic, biocbe~cal, su or other trauma to the test ear or to Me central nervous system. Three monkeys had undergone surg~.~ry involving the contrahteral ear, mastoid, or both, but t h ~ h a d no measurable effect on hearing in the tested ear. The external auditory meati were checked periodically in most of the monkeys and were found to be normal. Small amounts of wax were typically seen in the ear canals: the canals were not cleaned except for occasional removal of wax to facilitate further observation of the meatus. The laboratory apparatus, stimulus presentation and calibration procedures, and behavioral procedures were very similar for all subjects. They have been detailed elsewhere [7,8] and will be summarized here with detai3s of minor variations in equipment and procedure. The monkeys were tested in double.walled, sound-attenuated rooms (Industrial Acoustics Corporation). During testing, they sat in a primate restraining chair with their heads further restrained. At all other times they were housed in cages. Acoustic stimuli were presented through Telephonics TDH-49 or Beyer Dynamic DT 48 ear speakers fitted with hard rubber circumaural ear cushions which were positioned tightly against the monkey's head. Stimuli were calibrated using 2 mm t outside diameter) probe tubes positioned just lateral to the external auditory meatus. "Ihe probe tubes in turn were calibrated using a minimum volume coupler. Behavioral programming was controlled by either solid-state logic circuits or computer. In the former case, stimuli were controlled manually using a Hewlett-Packard oscillator and attenuator, and stimulus intensities were changed every five trials. In experiments controlled by the computer, stimulus presentation was also computer controlled via a Rockland programmable waveform generator and a Grason-Stadler programmable attenuator. Under computer control, stimuli were changed on every trial. The protocol for data collection was as follows. Monkeys were trained in the conventional manner to perform the simple reaction-time task and were then run for several weeks using suprathreshold and near-threshold stimuli until stable responding was achieved. Threshold testing was then begun with about 50% of the presented stimuli being near threshold and the remainder being distributed throughout the dynamic range of hearing for thc monkey. Stimuli were selected in 2 dB steps near threshold and 10 dB steps above threshold. Catch (no tone) trials were also presented in each session. Data for this paper were collected only after the monkey showed stable threshold heating over a period of at least 2 wk. The criteria for stable threshold hearing were: (I) smooth pefithreshold functions rising from less than 20% correct to greater than 80% correct over a range of 8 dB or less; and (2) thresholds varying by no more than 8 dB ove~ the 2-wk period. Of 15 monkeys trained in tiffs manner, 2 failed to meet these criteria and were excluded from our sample. Ttlresholds were measured daily over a period ofseveral weeks with 2 - 5 frequencies being tested each day using 10-20 trials per intensity. Final thresh.

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old calculations for a given frequency were made by combinLng data from 4 or more days so that each threshold was calculated from perithreshold functions using 40 or more trails per intensity distributed over four or more sessions. Thresholds were ~eff~ed as the points on the pe::ithreshold functions half-way between the guess rate and the 100% correct point with "correct' responses being def'med as those with reaction times less than I ~ msec. RESULTS

The average threshold contour for the monkeys in our sample is shown in Fig. 1, which gives means and standard deviations for all ears tested. For frequencies spaced at octave intervals from 125 Hz to 32 kHz, the data represent 18 ears. For octave steps from 1.414 kHz to 45.3 kHz, data are for 8 ears. We also tested 2 monkeys at 707 Hz and 4 monkeys at 10 and 40 kHz. The threshold contour is a fairly smooth W-shaped function with predominant rises in threshold at the extreme frequencies and iE~ the middle frequencies peaking at 4 kHz. The low-frequency slope up to 1.4 kHz is about 9.4 dB/ octave. Above 32 kHz the contour rim,,; rapidly; the 60 dB point is at 41 kUz and the 70 dB point is at 43 kHz. The general shape of this pooled threshold contour is representa. tire of the shapes of each of the individual contours. The 4 kHz elevation in threshold was seen in all 18 ears with threshold at 4 kHz being an average of 10.2 dB higher than threshold for two neighboring octaves (range = 4.6-19.5 dB). Other minor bumps in the threshold contours for individual ears were sometimes seen at frequencies above 4 kHz. In

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Fig. 1. Threshold contour for the rhesu~ monkey. Mean thresholds ± I SD are shown for all ears tested.

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3 monkeys we saw s m ~ e~vatiom in t ~ e at IO $ o 11,3t4 kHz), but only 4 subjects were tested at t 0 ~ ....... Standard clematis dB. ance was greate~ at standard deviat range and 2 points ,~,.,,. ,,,o...u . . . . . . . . ~ -~ ............... - . . . . . feted by an average of only 0.05 dB and the moment coefficient of skewnessavemgcd 0.09. DISCUSSION

This threshold contour is in close a~eement with that reported prev~ u~$ from laboratory using almos~ identical procedures [8] and is similar in ~ p e t o those fouM by other investigators, especially Behar et al. [ 1] and Stebbins et al. [ 11 ]. Our threshdds are higher by an average of 8 - 9 dB than those reported by l~har et al. and Ste et al. at frequencies of 1 kHz and below, and are lower ~ n those reported by Stebbins et al. at higher frequencies. Our population size was sufficiently large to give a reasonable estimate of vatiabfli~ of thresholds in a population of 'normal' heating rhesus monkeys. Our standard de~fions averaged about 5.3 dB. This value is slightly lower than that reported for a large population of 'normal' human subjects by Dadson and King [3]. Their standard deviations averaged about 7.5 dB. These differences might best be explained by two factors: first, our subjects received a great deal more training than those of Dason and King; and second, our fllreshold and calibration values were computed on the basis of many more trials and sessions than theirs. Even with ranges of 8 - 2 9 dB in our threshold data, some of the thresholds reported by other laboratories fall completely outside the ravge of our data. This reflects the di!ificulty of comparing results from laboratories that u ~ different stimulus presentation aad calibration procedures. For example, using the same human subjects in the same laboratory, we have shown differences in measured thresholds of up to 20 dB due simply to differences in ear cushions and calibration procedures (appendix A in ref. [8]). If a thre,~h' old standard is to be established for the rhesus monkey, it will have to accompany V~e establishment of a standardized set of threshold measurement procedures. REFERENCES " ,~i ' " 0 f We rhesus monkey. [1] Behar, I., Cronhotm, J.N. and Loeb, M, (!965): Auditory sen~Im~ty J. Comp. Physiol. Psychol. 59.426-428. [2] Clack, T. D. and lierman, P,N, (1963): Sing!e-level psychophysical measuring auditory thresholds in the monkey. J, Aud, Res, 3.175-183, [3] Dadson, R.S. and King, ~.H. (1952): :A detezm~ation of:the norm~ thre~old ofhear~g and its relation to the standardization of audiometer' L [41 Dalton Jr., L.W,, Taylor, H., and Alien, :J:N: (t969): Auditory th~sholds m the rhesus monkey using a ¢lo~d..system helmet. J. Aud, Res, 9, I78-182. ,

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r three species of monkey. J. Comp. Psychol. 3 5 , 2 5 5 time procedure for measure. nent of hear-

dty in noise[

an and monkey. J. Comp. Physiol. Psychol. 57, 89"93.

[11] Stebbins, W,C., Green, S. and Miller,F.L. (1966): Auditory sensitivityof the monkey. Science 153, I646-1647. [ 12] Wendt, G.R. (1934): Auditory acuity vf monkeys. Comp. Psyehol. Monogr. 10, 1-51.

Pure tone thresholds for the rhesus monkey.

43 PURE TONE ~ ~ S H O L D S FOR THE RHESUS MONKEY * ELAMMINO~ BRENDA LONSBURY,MARTIN and Auditory t ~ s h o l d s were measured for 18 ears from...
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