Electrical stimulation of the inferior colliculus at low rates protects the cochlea from auditory desensitization

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Electrical stimulation of the inferior colliculus at low rates protects the cochlea from auditory desensitization R.

Rajan

Department of Physiology, University of Western Australia, Perth, WA (Australia) (Accepted 6 June 1989) Key words: Inferior colliculus; Electrical stimulation; Cochlea; Olivocochlear; Temporary threshold shift; Protection; Auditory

The effects of inferior collicular (IC) stimulation on cochlear responses were tested with pulsed electrical trains and with 1 min tong continuous bursts. Pulsed trains did not cause any effects at the contralateral cochlea. However, a 1 min burst, containing pulses at low rates, was able to significantly reduce temporary threshold shifts (TI'S) in cochlear sensitivity caused by a loud sound exposure. Intracochlear perfusion of hexamethonium blocked this effect. The time course of the hexamethonium blocking action paralleled its blocking action on the cochlear effects of electrical stimulation at the brainstem of an auditory efferent pathway, the crossed olivocochlear bundle (COCB), The protective IC effects were persistent and TI'S reductions could be obtained even with a 5 min delay between IC stimulus and the loud sound. However, these persistent protective effects did not appear to occur at the cochlea. Finally, electrical stimulation at the IC ipsilateral to a cochlea exposed to loud sound also reduced TTS, but only by smaller amounts and at higher stimulation rates. Thus the IC appears to provide a strong descending influence that modulates the excitability levels of the olivocochlear nuclei in the brainstem. Both crossed and uncrossed OCB appear to be involved and able to reduce TI'S. It is proposed that the protective effects may be due solely to the medial olivocochlear system and possibly only those fibres originating from one of the nuclei of the medial system. INTRODUCTION A n a t o m i c a l studies in a variety of species have shown that the auditory efferent pathways to the cochlea e m a n a t i n g from nuclei o f the superior olivary complex of the b r a i n s t e m receive descending input from the inferior colliculus (IC) of the midbrain 1'4'7'14'17'20'3°'37'38. The presence of such colliculo-olivary pathways suggests that a descending influence can be exerted, firstly, at the nuclei of origin of the olivocochlear pathways, and, ultimately, at the cochlea. Effects at the first site have been claimed by D e s m e d t 6 who p r e s e n t e d an example of a recording from an olivocochlear neuron at the medulla while electrically stimulating at some unidentified higher centre (the so-called centrifugal extra-reticular auditory control system, or C E R A C S ) . Stimulation of the CER A C S even with single electric pulses elicited bursts of spikes at much higher frequency in the olivocochlear n e u r o n e 6. H o w e v e r , the general paucity of information in this r e p o r t means that the possibility of descending control of olivocochlear neurones is still to be clarified. F u r t h e r , it is still to be d e t e r m i n e d w h e t h e r any such control of the olivocochlear neurones is manifested at the level of the cochlea. The p r e s e n t study addresses the question of whether the colliculo-olivary pathways could exert any effects at

the cochlea. The only effects o b t a i n e d with inferior collicular stimulation were protective effects from the neural desensitization at the cochlea caused by a loud sound exposure. The protective effects of IC stimulation, at least in the contralateral cochlea, were similar in magnitude to those o b t a i n e d in recent studies in which an efferent pathway, the crossed olivocochlear bundle (COCB28'29), was stimulated 21'22"25, but were o b t a i n e d at much lower rates of stimulation at the 1C. These results suggest a strong influence from the IC to the cell bodies of the olivocochlear efferent pathways in the brainstem. H o w e v e r , this influence a p p e a r s not to directly activate the efferents but is a facilitatory influence allowing easier activation by a loud sound exposure. These results are the first d e m o n s t r a t i o n of any cochlear effects of electrical stimulation at centres higher than the brainstem and represent a significant m o d u l a t i o n of protective effects 2123.25-27 that may have i m p o r t a n t functional consequences.

MATERIALS AND METHODS Surgical preparation and measurement of cochlear sensitivities After subcutaneous pretreatment with atropine sulphate (0.65 mg/kg), pigmented guinea pigs (150-300 g) were anaesthetized with 30 mg/kg Nembutal (sodium pentobarbitone) i.p. and 0.4 mg Innovar-Vet (fentanyl citrate, Droperidol) i.m. They were tracheostomized, artificially ventilated on Carbogen (95% 0 2, 5% CO2)

Correspondence: R. Rajah. Present address: Department of Psychology, Monash University, Clayton, Vic. 3168, Australia. 0006-8993/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)

193 and administered a muscle relaxant, Alloferin (alcuronium chloride) i.m. at 5 mg/kg. Body temperature was maintained at 38.5 + 1 °C by a thermostatically controlled warming blanket regulated by a rectal probe. Body temperature and heart rate were continuously monitored. Supplementary doses of 0.1 ml Innovar-Vet were delivered hourly and 10 mg/kg Nembutal every 2 h. The animals were mounted between hollow ear bars and the cochleas exposed for placement of electrodes to measure threshold sensitivities to pure tones, as described previously ~3. Pure tone bursts from 2 to 30 kHz (1 ms rise-fall times, 30 ms duration) were delivered at 5/s and the compound action potential (CAP) of the VIIlth nerve monitored through a round window electrode. Using the N 1, the first negative deflection of the CAP, audiograms were constructed for each animal. Only animals with threshold sensitivities from 2 to 30 kHz within 5 dB of normative laboratory data 5"16'24 at each frequency were used in this study. In the majority of experiments in this report, electrical stimulation was carried out at the IC contralateral to the test cochlea. In some experiments stimulation was carried out at the IC ipsilateral to the test cochlea. To expose the appropriate IC, a hole was drilled in the cranium above the occipital cortex of that side using a scalpel blade to reduce any extraneous loud noise. The cortex was gently suctioned away, with tissue wicks saturated with thrombin being used to minimize bleeding. Suctioning was carried out until the dorsal surface of the IC could be seen, and extended rostrally to the border of IC and superior colliculus. Bipolar stimulating electrodes (side-by-side insulated silver-silver chloride wires, with bared 1 mm tips separated by 0.5 mm) mounted on a micromanipulator were lowered to make contact with the dorsal surface of the IC and N~ audiograms remeasured in the test cochlea to ensure that no change had taken place in cochlear sensitivities.

Testing effects on normal cochlear responses To stimulate the IC, the current source was a stimulator and isolation unit built locally. Current level, pulse width and order of leading polarity of pulses could be independently varied at the stimulator. The rate of pulses could be varied on a Neurolog pulse generator (NL300) connected to the stimulator. In preliminary experiments the cochlear effects of bipolar IC stimulation were determined on N~ thresholds and amplitudes to tone bursts in the contralateral cochlea using a stimulus paradigm similar to that traditionally u s e d 6"8~9'18'21'45 at the level of the brainstem to elicit COCB effects at the cochlea. Thus, the electrical stimulus always consisted of bipolar 150/~s wide pulses gated as a short train (100-400 ms) that was presented at 1/s. Pure tone bursts at l/s were delivered after the train, The train length, the rate of pulses in the train, the current level, the delay between the end of the train and the tone burst, and the frequency and intensity level of the tone burst were all independently varied. The effect of the electrical stimulus on the N~ was monitored either visually on an oscilloscope or by averaging amplitudes to 32 presentations of the tone burst at a particular frequency and intensity, with and without the IC stimulus.

In test groups with 5 or 10 rain delay between contralateral IC stimulus and exposure, N~ input-output functions to 14 kHz (and occasionally 10 kHz as well) tone bursts were obtained prior to the IC burst by averaging amplitudes to 32 presentations of the tone burst at various intensities, and then remeasured at the same intensities during the delay between IC stimulus and exposure. The measurements in the delay period commenced 10-15 s after the end of the IC stimulus and were completed within 2-3 min.

Pharmacological testing To confirm that the effects seen in this study were specifically exerted by the olivocochlear pathways, hexamethoniurn, which blocks the traditional cochlear effects of the COCB 3'~1~, was used. The effects and time course of the action of hexamethonium were first tested on COCB effects at the cochlea in group G. The COCB was located at the floor of the fourth ventricle as described previously 2~. In all animals of this group COCB stimulation with l/s, 300 ms long electrical trains with 150/~s pulses at rates of 140-400/s elicited strong effects on N 1 amplitudes (15-28 dB maximal equivalent reductions in stimulus intensity). Then 1 itl of 3.44 mM hexamethonium in saline was perfused through the round window into scala tympani over a period of about 1 rain. Thereafter, regular checks were made of N l thresholds from 6 to 30 kHz as well as the classical effects of the pulsed COCB trains on the N t amplitude 6' 8,9,18,21,45. Normal N~ measures themselves were never affected while the classical COCB effects on the N~ were totally blocked, within 15 rain. Nj thresholds from 6 to 30 kHz were rechecked and always found to be unaltered from initial values. Then the standard monaural loud sound exposure was presented while simultaneously stimulating the COCB continuously at 400 ,uA with bipolar 150/zs pulses delivered at 140/s for the duration of the exposure. These parameters of COCB stimulation have been shown to be very effective in reducing qq'S 2L. At the end of 1 rain the exposure and COCB stimulus were turned off and threshold losses monitored as for all other groups. In group H, hexamethonium was tested on IC effects on TTS. After determining the N~ audiogram and locating bipolar electrodes on the contralateral IC as described above, input-output functions for the N~ were measured at 14 kHz, and occasionally at 10 kHz as well. Then 1 ,ul of 687 mM hexamethonium was perfused into scala tympani. The N~ audiogram from 6 to 30 kHz and the input-output functions were monitored for the next 15 rain. Then the standard exposure was presented to the cochlea while simultaneously stimulating the IC at 20/s. At the end of 1 rain the IC stimulus and the exposure were turned off and threshold losses monitored as before. In the third group in this section (group I) the effects of hexamethonium were tested only on qq'S. After establishing the N~ audiogram and determining N~ input-output functions at 14 kHz, and occasionally 10 kHz as well, 1 Itl of 687 mM hexamethonium was perfused into scala tympani. N l thresholds from 6 to 30 kHz and the input-output functions were then monitored for the next 15 rain before presenting the standard exposure alone.

Monitoring TTS Testing effects on TTS In later experiments the effects of IC stimulation on N 1 TTS were determined using a standard monaural loud sound exposure at 10 kHz, 103 dB sound pressure level (SPL) for 1 min. In the test groups the IC stimulus was always presented as a continuous burst of bipolar 150/~s wide pulses at 400/~A for 1 rain. In groups with stimulation of the IC contralateral to the test cochlea, the stimulus parameters altered in different groups were the rate of stimulation (groups B - F ) and time of presentation (groups J and K) of the IC stimulus with respect to the exposure (stimulus presented 5 or 10 rain prior to exposure). With ipsilateral IC testing only the effects of varying the rate of stimulation (groups L - P ) were determined, with the IC stimulus always presented simultaneous with the standard exposure. In the control group (A) the exposure was presented by itself without any IC stimulus. The various experimental groups are listed in Table I.

In all groups the effects of the loud sound exposure were monitored as detailed previously 23. Briefly, 10 s after the end of the exposure tracking of N l threshold losses commenced at 14 kHz, the frequency of maximum loss 23'24. The second recording was made 15 s postexposure. Thereafter 14 kHz threshold losses were recorded at 15 s intervals until 5 rain postexposure when N~ thresholds losses from 8 to 24 kHz were measured, before returning to track 14 kHz N 1 thresholds at 1 rain intervals. Ten minutes after the exposure, thresholds from 8 to 24 kHz were again measured, following which 14 kHz N 1 thresholds were tracked at 2 rain intervals until the end of the recording session 30 rain postexposure. Threshold losses for the animals of any experimental group are presented as the mean threshold losses for that group, at the designated frequency and postexposure recording time. Student's t-tests were used to determine levels of statistical significance between the various groups.

194 TABLE 1 Experimental groups and 14 kHz N 1 threshold losses (mean + 1 S.D.) at 4 postexposure recording times In all groups the pure tone used to cause TTS was at 10 kHz, 103 dB SPL and was presented for 1 rain. In the control group only this exposure was presented. In all test groups in parts 2 and 4 of the table the exposure was combined with electrical stimulation at the inferior colliculus (IC): the contralateral IC in part 2 and the ipsilateral IC in part 4. The standard bipolar stimulus was at 400pA, with 150 ps pulses presented as a continuous burst at 20/s, simultaneous with the exposure and for the duration of the exposure. Only variations from this standard stimulus are specified below. In the test group in part 3 of the table the exposure was combined with electrical stimulation of the crossed olivocoehlear bundle (COCB) at the floor of the fourth ventricle. In this case the bipolar stimulus was at 400 pA, with 150 ps pulses presented as a continuous burst at 140/s, simultaneous with and for the duration of the exposure. In all cases of hexamethonium pretreatment, 1/A of 3.44 mM hexamethonium in saline was perfused into scala tympani over a period of 1 min. n represents the number of animals in each group. Group

n

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22.04 + 1.96 21.80 + 1.10

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34.00 + 1.00 27.30 + 0.58 24.30 -+ 1.53 21.00 _+2.00 19.67 + 1.16 38.30 _+ 1.16

23.70 -+ 0.58 18.00 ___0.00 15.30 + 1.53 12.00 _+ 1.00 11.33 + 1.53 27.70 _+0.58

18.00 -+ 1.00 13.70 ___0.58 10.30 + 0.58 7.00 _+ 1.00 7.00 + 1.00 21.70 _+0.58

13.30 + 0.58 9.00 -+ 1.00 6.70 + 0.58 2.00 _+ 1.00 3.00 + 1.00 17.30 _+ 1.16

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20.70 + 0.58

15.30 + 0.58

10.70 _+0.58

None

38.30 + 0.58

27.70 + 0.58

22.30 + 0.58

17.30 + 0.58

(3) COCB test group G 4 No IC stimulus. Only COCB stimulus delivered

15 min prior to (exposure + COCB stim.)

39.00 _+ 1.41

28.00 + 1.16

21.75 + 1.26

17.25 + 1.26

(4) lpsilateral IC test groups L 3 No change M 3 Rate increased to 30/s N 3 Rate increased to 50/s O 3 Rate increased to 140/s P 3 Rate increased to 260/s

None None None None None

37.70 -+ 0.58 34.70 -+ 0.58 31.70 + 1.53 30.00_+ 1.00 29.30 _-4_-0.58

28.00 + 1.00 25.00 + 1.00 22.70 + 1.53 20.70+ 1.15 20.00 -+ 1.00

23.00 + 1.00 19.70 + 0.58 18.00 + 1.00 15.00+ 1.00 14.30 -+ 0.58

18.00 + 15.00 + 13.00 + 11.30_+ 10.30 +

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Precautions exercised In all cases of IC stimulation, the electrical stimulus was continuously monitored by displaying on an oscilloscope the voltage changes across resistors placed in series with each of the two bipolar electrodes. In those experiments in which preliminary testing on the effects of gated electrical trains to the IC had been carried out prior to any testing on TI'S, the N~ audiograms was remeasured just prior to the loud sound exposure to ensure that the initial testing had not affected cochlear sensitivities. In all test experiments on TTS, to ensure that the continuous IC burst for 1 min would not cause any blocking of the pre-anaplitier recording Nlresponses , the pre-amplifier was turned off at a switch installed at the input stage during the period of electrical stimulation.

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presented 5 ms after each train. In 5 animals a variety of stimulus and tone burst parameters were used. Current levels r a n g e d f r o m 150 t o 6 0 0 / ~ A , t r a i n l e n g t h s f r o m 100 to 400 m s a n d t h e r a t e o f p u l s e s i n t h e t r a i n s f r o m 20 t o 400 pulses/s. T o n e b u r s t f r e q u e n c i e s r a n g e d f r o m 6 t o 20 k H z , w i t h m o s t t e s t i n g b e i n g d o n e a t 10, 14 a n d 20 k H z , a n d t o n e b u r s t i n t e n s i t i e s f r o m t h r e s h o l d - 50 d B > threshold were always tested with the electrical stimulus. No effects were ever seen on N 1 thresholds or amplitudes (e.g. s e e Fig. 7) e v e n w h e n t h e e l e c t r i c a l p a r a m e t e r s w e r e

RESULTS

s i m i l a r t o t h o s e t h a t a r e o p t i m a l ( c u r r e n t s t r e n g t h 400 Effects of pulsed IC stimulation on r e s p o n s e s in the c o n t r a l a t e r a l c o c h l e a

normal

cochlear

The effects of IC stimulation on normal N 1 thresholds

g A , t r a i n l e n g t h 300 m s , r a t e 2 0 0 - 4 0 0 / s ) t o p r o d u c e N 1 amplitude reductions when applied to the COCB at the f l o o r o f t h e f o u r t h v e n t r i c l e 6,s'9,ls'21"45.

195

Effects of continuous IC stimulation on TTS in the contralateral cochlea Simultaneous presentation experiments. In contrast to

postexposure, were also significantly (P < 0.05) lower in

the lack of cochlear effects of pulsed electrical trains, c o n t i n u o u s electrical stimulation of the IC for 1 min, simultaneous with a standard loud sound exposure to the

1A) resulted in greater reductions in TTS. N~ threshold losses at 14 kHz at all postexposure times in group C were significantly (P < 0.05) lower than the losses in either

contralateral cochlea, reduced the temporary threshold

control group A or the previous test group B. The

test group B. A higher stimulation rate of 5 pulses/s (group C, Fig.

shifts (TTS) in N 1 sensitivity caused by the exposure. This

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threshold losses are listed in Table I for all groups), was d e p e n d e n t on the rate of IC stimulation.

postexposure in group C were significantly (P < 0.05) lower than losses at the same frequencies in either the

E v e n IC stimulation at the very low rate of 2 pulses/s in test group B reduced the TTS caused by the exposure

control group A or group B. W h e n the stimulation rate was increased to 20/s (group

(Fig. 1A). N1 threshold losses at 14 kHz in test group B were always significantly (P < 0.05) lower than losses at

D, Fig. 1A), greater reductions in TI'S were obtained. Threshold losses at 14 kHz in this group were always

corresponding recording times in control group A. Threshold losses from 12 to 24 kHz, recorded 5 min

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Fig. 1. Effect of electrical stimulation of the contralateral inferior colliculus (IC) on temporary threshold shifts (TTS) in cochlear threshold sensitivities caused by a standard loud sound exposure. A: IC stimulation rates of 2-20 pulses/s B: IC stimulation rates of 20-140/s. Capital letters in brackets identify the experimental groups as detailed in Table I. In both parts of the figure the right hand panel presents the recovery of 14 kHz threshold sensitivity at various times after the exposure while the left hand panel presents results for threshold losses from 8 to 24 kHz measured 5 min postexposure. In control group A,only the standard monaural exposure was presented. In the test groups the same exposure was combined with bipolar electrical stimulation of the IC simultaneous with and for the duration of the exposure.

196 previous test groups. Losses from 10 to 24 kHz 5 min postexposure in group D were significantly (P < 0.05) lower than losses at the corresponding frequencies in either the control group or group B with the lowest rate of stimulation. Group D losses from 12 to 16 kHz were significantly (P < 0.05) lower than losses at the corresponding frequencies in group C (stimulation at 5/s). Stimulation at 50 pulses/s (group E, Fig. 1B) produced greater reductions in TTS at the frequencies suffering the greatest TTS. N1 threshold losses at 14 kHz in this group were always significantly (P < 0.05) lower than the losses at the corresponding times in the control group and in all previous test groups. Five minutes after the exposure, losses from 10 to 24 kHz in group E were significantly (P < 0.05) lower than the losses at these frequencies in either the control group or group B with the lowest rate of stimulation. Group E losses from 12 to 16 kHz were significantly (P < 0.05) lower than losses at these frequencies in group C (5 pulses/s) and in group D (20/s). The reductions in TTS appeared to have reached a plateau with stimulation at 50 pulses/s and stimulation at 140/s (group F, Fig. 1B) produced no greater reductions. Although 14 kHz threshold losses in group F were significantly (P < 0.05) lower than losses at corresponding times in the control group and in all previous test groups with stimulation rates less than 50/s, they were similar (P > 0.10) to the losses in group E (50/s). Threshold losses over the affected frequency range 5 min postexposure in group F were also similar (P > 0.10) to losses at the corresponding frequencies in group E. Group F losses from 10 to 24 kHz were significantly (P < 0.05) lower than losses at the corresponding frequencies in the control group and in test group B (2/s). Group F losses from 12 to 18 kHz were significantly (P < 0.05) lower than the losses at corresponding frequencies in group C (5/s) while group F losses from 12 to 16 kHz were significantly (P < 0.05) lower than losses in group D (20/s). Effect of hexamethonium. The time course of hexamethonium action at the cochlea was first established by testing its action on COCB effects obtained with fourth ventricle electrical stimulation in group G. The COCB effects examined were the traditional effects of pulsed gated electrical trains on N 1 amplitudes 6"~'9"1~'z1"45 and the recently demonstrated protective effects of stimulation as a continuous burst simultaneous with, and for the duration of, a loud sound exposure to the cochlea 2I'22. In group G the N 1 amplitude reductions caused by gated electrical trains of pulses applied at the floor of the fourth ventricle were totally blocked 15 min after intracochlear perfusion of hexamethonium (Fig. 2). Once these traditional COCB effects on the N l were blocked, protective COCB effects on TTS were tested by presenting the standard exposure with simultaneous stimulation

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Fig. 2. Action of h e x a m e t h o n i u m on crossed olivocochlear bundle (COCB) effects on the N 1 amplitude obtained with pulsed C O C B trains. In the animals of group G 1/s 300 ms long C O C B trains were presented at the floor of the fourth ventricle at 400/~A, with bipolar 150/~s pulses in each train delivered at the o p t i m u m rate of 400/s. The delay between the end of each train and a test 14 kHz tone burst was 5 ms. The left h a n d panel presents the maximal equivalent dB elevation in N 1 thresholds caused by the C O C B trains while the right hand panel presents the equivalent dB reduction in N~ amplitude to a tone burst set at 57 dB SPL. The time of 0 min indicates the intracochlear perfusion of h e x a m e t h o n i u m . Each symbol represents a different animal.

of the COCB with electrical stimulus parameters (see Materials and Methods) optimized to reduce TTS 21. No reductions in TTS were obtained (Fig. 3) and 14 kHz threshold losses in group G were always similar (P > 0.10) to the losses at corresponding times in control group A presented only the exposure without hexamethonium pretreatment. Threshold losses over the affected frequency range from 8 to 24 kHz 5 min after the exposure were also similar (P > 0.10) between the two groups. The results in hexamethonium-treated group G are compared in Fig. 3 to the results obtained when the same electrical stimulus was applied at the fourth ventricle without hexamethonium pretreatment. As shown previously zl, COCB stimulation with these electrical parameters reduces the TTS caused by the same exposure. Threshold losses at 14 kHz were always significantly (P < 0.05) lower in this group than in control group A presented only the exposure, as were threshold losses from 10 to 24 kHz 5 min after the exposure. More importantly, threshold losses in the group with COCB stimulation without hexamethonium pretreatment were significantly (P < 0.05) lower than the losses in group G pretreated with hexamethonium prior to the same exposure and COCB stimulus, both for threshold losses at 14 kHz at all recording times and for losses over the range from 10 to 24 kHz 5 min after the exposure. Hexamethonium was next tested on the protective effects of contralateral IC stimulation in group H and, as a control, on the exposure alone in group I. In all animals in which hexamethonium was perfused into the cochlea, N 1 thresholds from 6 to 30 kHz, monitored at regular intervals in the period between the perfusion and the

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Fig. 5. Action of hexamethonium on IC effects on "ITS. The figure format is the same as for Fig. 1. Control group A, standard exposure alone; group D, standard exposure combined with IC stimulation at 20 pulses/s. In groups H and I, hexamethonium was perfused into the cochlea 15 min before presenting the same test conditions as in group D and A, respectively. while simultaneously stimulating the 1C at 20 pulses/s for the duration of exposure. The results from this group are illustrated in Fig. 5 with results for control group A (exposure alone) and test group D (the same stimulation and exposure conditions as group H, but without hexam e t h o n i u m pretreatment). Threshold losses at 14 kHz in group H were always similar (P > 0.10) to losses at corresponding times in control group A but were significantly (P < 0.05) higher than losses at corresponding times in group D. Five minutes after the exposure threshold losses across the affected frequency range in group H and control g r o u p A were similar (P > 0.10), while group H losses from 10 to 24 kHz were significantly (P < 0.05) higher than losses at corresponding frequencies in group D. In the control group I, the effect of h e x a m e t h o n i u m on the exposure alone was tested by presenting the standard exposure by itself, 15 min after intracochlear perfusion of hexamethonium. Threshold losses in this group (Fig. 5) were similar (P > 0.10) to those in control group A presented the exposure alone without h e x a m e t h o n i u m pretreatment, both for 14 kHz losses at corresponding recording times in the two groups, and for losses from 8 to 24 kHz 5 min postexposure. The losses in group I were also comparable to those in group H ( h e x a m e t h o n i u m perfusion 15 min prior to presenting the exposure simultaneous with IC stimulation), and in group G ( h e x a m e t h o n i u m perfusion 15 min prior to presenting the same exposure simultaneous with C O C B stimulation). Thus, 14 kHz threshold losses in group l were similar (P > 0.10) to losses at corresponding times in group H and, except for 16 and 18 kHz, so were the losses across the affected frequency range 5 min postexposure. Further, 14 kHz threshold losses at corresponding times in group I and group G and losses across the affected frequency range 5 min postexposure were also similar (P > 0.05). Persistence of the IC effects in delay experiments. The persistence of the protective effect of IC stimulation was

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Fig. 6. Effect of increasing delay between contralateral IC stimulation for 1 rain and the standard monaural exposure. Control group A, exposure alone; group D, exposure was combined with simultaneous electrical stimulation of the IC at 20 pulses/s; group J, IC stimulus presented by itself 5 min before the exposure alone; group K, IC stimulus presented by itself 10 min before the exposure alone. Figure format as for Fig. 1. tested in two groups by presenting the continuous IC stimulus by itself for 1 min at 20 pulses/s and allowing delays of 5 min (group J) or 10 min (group K) to elapse before presenting the standard exposure alone and recording the threshold losses. In the delay period the Na audiogram from 6 to 30 kHz was rechecked at regular intervals. In addition, in most animals of the two groups input-output functions for the N 1 amplitude were measured at 14 kHz, and occasionally at 10 kHz as well, in the period from 15 s to 2 min after the IC stimulus and compared to values recorded prior to the 1C stimulus. The effects of the IC stimulus on TTS were found to be quite persistent. Even with a delay of 5 rain between the stimulus and the exposure, reductions in TTS were obtained (Fig. 6), though not as large reductions as when the stimulus was presented simultaneously with the exposure. N 1 threshold losses at 14 kHz in group J were always significantly (P < 0.05) lower than losses at corresponding times in control group A with exposure alone, though they were significantly (P < 0.05) higher than losses in group D in which the same IC stimulus was presented simultaneously with the exposure. These effects were reflected in the threshold losses measured 5 min after the exposure. Losses from 10 to 24 kHz in group J were significantly (P < 0.05) lower than losses at the same frequencies in control group A. Group J losses from 12-18 kHz were significantly (P < 0.05) higher than losses at these frequencies in group D. With a 10 min delay between IC stimulus and exposure in group K there were no reductions in TTS (Fig. 6). Threshold losses at 14 kHz in this group were always similar (P > 0.10) to losses at corresponding times in control group A but were significantly (P < 0.05) higher than losses at corresponding times in either group D (no delay between stimulus and exposure) or group J (5 min delay). Losses from 10 to 24 kHz, 5 min postexposure in group K were also similar (P > 0.10) to losses at the same

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Effect of continuous IC stimulation on TTS in the ipsilateral cochlea IC stimulation was also able to reduce the T£S to a loud sound exposure in the ipsilateral cochlea. These effects are illustrated in Fig. 8 along with results for control group A presented only the monaural standard exposure. Electrical stimulation of the ipsilateral IC at 20 pulses/s in group L did not produce any reductions in TFS (Fig. 8A): 14 kHz threshold losses at all recording times in this group were similar (P > 0.10) to corresponding losses in control group A as were threshold losses across the affected frequency range from 10 to 24 kHz 5 min postexposure in the two groups. When the rate of stimulation was increased to 30/s (group M) there were some reductions in TTS at the frequencies most affected by the exposure. Threshold losses at 14 kHz at all recording times in group M were significantly (P < 0.05) lower than corresponding losses in either the control group or group L (IC stimulus at 20/s). Losses from 10 to 18 kHz 5 min postexposure in group M were significantly

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( P < 0.05) lower than losses at the same frequencies in either control group A or test group L. Increasing the rate to 50/s in group N p r o d u c e d slightly g r e a t e r reductions in T F S at the frequencies most affected by the e x p o s u r e (Fig. 8A). Threshold losses at 14 k H z were always significantly ( P < 0.05) lower in this group than the c o r r e s p o n d i n g losses in control group A , test group L (20/s) or test group M (30/s). Five minutes after the exposure, losses from 10 to 24 k H z were significantly ( P < 0.05) lower in group N than corresponding losses in control group A or group L (20/s) while group N and group M (30/s) differed significantly (P < 0.05) for losses over a slightly smaller range of frequencies from 12 to 20 kHz. W h e n the rate of stimulation was increased to 140/s in group O (Fig. 8B), greater reductions were o b t a i n e d only at 14 kHz, and only at some recording times. Thus, threshold losses at 14 kHz in group O were significantly (P < 0.05) lower than corresponding losses in group N

(50/s) only at recording times from 2 to 5 min and 15-30 rain postexposure, although they were significantly (P < 0.05) lower at all recording times in group O than corresponding losses in control group A and test groups L (20/s) and M (30/s). These effects were reflected across the affected frequency range 5 min postexposure. A t that time, losses at all frequencies o t h e r than 14 k H z in groups O and N were similar ( P > 0.10) while losses from 10 to 24 kHz in group O were significantly (P < 0.05) lower than corresponding losses in control group A and in test group L (20/s), and losses from 14 to 20 k H z in group O were significantly (P < 0.05) lower than corresponding losses in group M (30/s). A final increase in the stimulation rate to 260/s in group P (Fig. 8B) did not reduce TTS any m o r e than the rate of 140/s (group O). Both the 14 kHz losses at all recording times and losses across the affected frequency range 5 min p o s t e x p o s u r e were similar between the two groups (generally P > 0.10 and always P > 0.05). N 1

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Fig. 9. Comparison of the protective effects of contralateral and ipsilateral IC stimulation on TTS. A: effects across the affected frequency range 5 rain postexposure. In the control group (crosses) only the standard exposure was presented. In group O (squares) and group F (circles) the exposure was combined with ipsilateral or contralateral IC stimulation, respectively, at 140 pulses/s. Apart from the different stimulation sites, the IC stimuli were identical in all other respects. B: relative levels of protection with contralateral compared to ipsilateral IC stimulation. The amount of protection was the difference between control group losses at each frequency and losses at the corresponding frequency in group F (for contralateral IC stimulation) or group O (for ipsilateral IC stimulation). The ratio of these differences is plotted on the abscissa. losses at 14 kHz in group P were always significantly (P < 0.05) lower than corresponding losses in the control group A and in all test groups with rates lower than 140/s. Five minutes after the exposure, losses from 10 to 24 kHz in group P were significantly (P < 0.05) lower than losses in control group A or test group L (20/s). Group P losses from 12 to 24 kHz were significantly (P < 0.05) lower than corresponding losses in group M (30/s) while only the 14 kHz loss 5 min postexposure in group P was significantly (P < 0.05) lower than the corresponding loss in group N (50/s).

Comparison of the effects of IC and COCB stimulation lpsilateral versus contralateral IC stimulation. The results presented above show that reductions in TTS with ipsilateral IC stimulation were smaller (and obtained at higher stimulation rates) than with contralateral IC stimulation. This comparison is made directly in Fig. 9A, for stimulation at a similar relatively high rate (140 pulses/s) producing the maximal reductions obtainable from either site of stimulation. As shown, ipsilateral IC stimulation at 140/s in group O produced less reductions in TTS across the affected frequency range than did contralateral IC stimulation at the same rate in group F. Threshold losses from 10 to 20 kHz 5 min postexposure were significantly (P < 0.05) higher in group O than in group F. Threshold losses at 14 kHz (not illustrated) at all postexposure times in group O were also significantly (P < 0.05) higher than corresponding losses in group F. The ratio of the reductions obtained with contralateral IC stimulation at 140/s to the reductions obtained with ipsilateral IC stimulation at the same rate is illustrated in

Fig. 9B. This ratio was calculated for the frequency range from 10 to 24 kHz from the data presented m Fig. 9A (losses at 8 and 9 kHz in the control and both test groups were zero and were therefore not included). As illustrated, in the range of frequencies most affected by the exposure (12-18 kHz, with threshold losses > 5dB), contralateral IC stimulation produced 1.5-2 times as much reduction in TTS as did ipsilateral IC stimulation. The greatest difference was found at the frequency most affected by the exposure, 14 kHz, at which contralateral IC stimulation produced reductions 2.14 times greater than did ipsilateral IC stimulation at the same optimal rate.

Contralateral 1C stimulation versus COCB stimulation. Contralateral IC stimulation could reduce TTS as much as C O C B stimulation at the floor of the fourth ventricle 21 but at much lower rates of stimulation at the IC. This comparison is illustrated in Fig. 10 for rates of 1C stimulation (from this study) and C O C B stimulation 2l producing comparable reductions in TTS across the affected frequency range 5 min postexposure. Contralateral IC stimulation at 2 pulses/s reduced TTS as much as C O C B stimulation at 50 pulses/s, with no significant (P > 0.10) differences between threshold losses across the affected frequency range 5 min after the exposure in the two groups. Losses at all postexposure times at the frequency most affected, 14 kHz (not illustrated), were also similar (P > 0.05) between the two groups, except for the value recorded 10 s postexposure. Similarly, IC stimulation at 20/s reduced TI'S as much as did C O C B stimulation at 140/s: threshold losses across the affected frequency range 5 min postexposure were similar (P > 0.10) between the two groups as were 14 kHz threshold losses at all postexposure recording times (generally P > 0.10 and always P < U.05) in the two groups. Finally, contralateral IC stimulation at 50/s reduced TTS as much as did C O C B stimulation at 400/s: threshold losses across the affected frequency range 5 rain postexposure were similar (generally P > 0.10 and always P > 0.05) between the two groups as were threshold losses at 14 kHz at all postexposure times (generally P > 0.10 and always P > 0.05) in the two groups. Although equal reductions in TTS could be obtained with lower rates of stimulation of the contralateral IC than with C O C B stimulation at the floor of the fourth ventricle, the effects of the two stimulation sites were similar in other respects, It has already been demonstrated above that intracochlear hexamethonium blocked the protective effects of both sites of stimulation and with a very similar time course. In addition, the persistence of the protective effects of stimulation at either site was also similar. This effect is illustrated in Fig. 11 for IC stimulation at 20/s and C O C B stimulation at 140/s, rates

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at very low stimulation rates. The results obtained with intracochlear perfusion of h e x a m e t h o n i u m demonstrate that the effects are not due to any current spread to the superior colliculus activating the facial nerve 39 and reducing the acoustic input to the cochlea through contractions of the stapedius muscle of the middle ear 9JSJg, but are specifically due to the olivocochlear terminations within the inner ear 3,t°.

Pathways involved in the protective effects In the guinea pig the IC appears to project only ipsilaterally to the brainstem nuclei of origin of the olivocochlear pathways 37-38. Since IC stimulation could reduce TTS in the contralateral and the ipsilateral cochlea, the IC must project ipsilaterally to both the crossed and uncrossed olivocochlear bundles (COCB and U O C B ) , With stimulation of the IC contralateral to the test cochlea, the outflow would be from the IC ipsilaterally to the nuclei of origin of the C O C B . The latter pathway then decussates at the floor of the fourth ventricle to carry the efferent outflow to the contralateral cochlea. With stimulation of the IC ipsilateral to the test cochlea, the IC outflow would go ipsilaterally to the U O C B and this pathway would then carry the outflow to the cochlea on the same side. Thus, both crossed and uncrossed brainstem efferent pathways appear to receive a descending influence from the IC. C u r r e n t views 12'13'40-42 suggest that the olivocochlear

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Fig. 11. Comparison of the time course of the protective effects of contralateral IC stimulation and COCB stimulation. Electrical stimulation at either site was presented for 1 min, either simultaneous with (0 rain delay) the standard exposure or by itself 5 or 10 min prior to the standard exposure, The abcissa represents the difference in the maximum threshold loss (at 14 kHz, 10 s postexposure) in the control group presented only the exposure and the maximum threshold loss in each test group (also at 14 kHz, 10 s postexposure). In all test cases the electrical stimulus was presented at 400 ~A with bipolar 150/~s pulses delivered continuously at either 20 pulses/s (with IC stimulation) or 140 pulses/s (with COCB stimulation) for 1 min. The exposure in all cases was at 10 kHz, 103 dB SPL and was presented for 1 min.

pathways are more usefully classified into a medial and a lateral olivocochlear system (MOCS and LOCS). A p a r t from different nuclei of origin, the M O C S consists of large myelinated fibres that terminate on outer hair cells (OHCs) in the cochlea 12'13'4° 42 while the LOCS consists of small u n m y e l i n a t e d fibres terminating on the afferent dendrites below inner hair cells (IHCs). In the guinea pig almost all crossed olivocochlear fibres are those of the MOCS2.31 36. The protective effects of COCB stimulation2~'z2'25. and of contralateral IC stimulation (the present study), must therefore be exerted by the terminations of MOCS fibres on the OHCs in the cochlea. The effects of ipsilateral IC stimulation may be due to either the LOCS or the MOCS since the uncrossed olivocochlear pathway in the guinea pig consists of fibres

202 of both systems 2'31-3~'. However, since the end result of stimulation of either IC was always to reduce q-TS, it is more parsimonious to assume that the effects of ipsilateral IC stimulation were also exerted by the MOCS, albeit the uncrossed MOCS fibres. Support for this hypothesis is provided by reports that the IC in the guinea pig projects only to the MOCS nuclei in the superior olivary complex and not to the L O C S 37"38, as well as in the relative amounts of protection obtained with the two midbrain stimulation sites and the relative sizes of the crossed and the uncrossed MOCS projections to any cochlea. As shown in Fig. 9B, the maximal reduction in TTS (at 14 kHz) with contralateral IC stimulation (at the most effective rate, 140/s) was twice that obtained with ipsilateral IC stimulation at the same rate. Reductions at other frequencies with threshold losses greater than 5 dB in the control group were at least 1.5 times greater with contralateral IC stimulation than with ipsilateral IC stimulation. Significantly, there are about twice as many crossed MOCS neurones as uncrossed MOCS neurones in the guinea pig 2'31-33. The relative amounts of maximal protection obtained with the two midbrain stimulation sites suggests that, at least in the basal and first cochlear turns where the frequencies examined in this study are represented, the distribution of the terminations of the crossed MOCS (mediating contralateral 1C effects) and the uncrossed MOCS (probably mediating ipsilateral IC effects) are similar and in proportion to the relative numbers of neurones involved. This has previously been suggested for these systems in the cat 11'13'41"42. In the guinea pig this hypothesis receives some support from studies showing that, regardless of the exact medial system nucleus of origin, there are generally about twice as many crossed MOCS fibres as uncrossed MOCS fibres projecting to the basal and first turns of the cochlea 32'3~. Thus, it appears that both the crossed and the uncrossed MOCS can function to reduce threshold losses due to loud sound exposures. In the guinea pig, the IC projections to the MOCS appear to terminate primarily in the ventral nucleus of the trapezoid body (VNTB) with minor projections to the lateral lemniscus 37'3s. It is possible, therefore, that the protective effects of IC stimulation are exercised at the cochlea solely by the MOCS efferents arising from the VNTB 2'31-36. In the discussion that follows only the effects of contralateral 1C stimulation are considered. Since these effects are exerted through the COCB (i.e. the crossed MOCS) the effects of contralateral IC stimulation can be compared to the effects of other manipulations that also act through the COCB to reduce TI"S 2l 23,25-27 To simplify the discussion it will be assumed that the general mechanisms of action of the contralateral IC stimulus

may also be applied to ipsilateral IC stimulation. It will also be assumed that the differences between the end effects of the two sites of midbrain stimulation are related solely to the relative numbers of fibres comprising the crossed and uncrossed MOCS which were suggested to mediate the cochlear effects of contralateral and ipsilateral IC stimulation, respectively.

Mode of action of the IC The results show that although the IC contralaterat to a test cochlea exerts a very strong descending influence on the COCB to that cochlea, this influence does not appear to directly excite the COCB. Pulsed electrical trains to the IC, similar to those that when applied at the floor of the fourth ventricle elicit strong COCB effects at the cochlea 6'8'9"18'2~'45, had no effects at the cochlea. Continuous IC stimulation by itself for 1 min also had no effects on cochlear potentials until a loud sound exposure was subsequently presented (within about 5 min). The latter result has also been demonstrated with the protective effects of direct COCB stimulation at the floor of the fourth ventricle 21'22. It has been shown 2~'22 that the delayed protection from TTS was probably due to a facilitatory action of the electrical stimulus at the cell bodies of origin of the COCB. This facilitatory action was suggested to allow easier activation of the COCB by the subsequent exposure. The descending contralateral IC pathways may provide a similar facilitatory influence at the cell bodies of origin of the COCB, allowing the loud sound exposure to activate the COCB more readily and thereby exert protective effects at the cochlea. Such a mode of action can be applied to both the delayed effects on TTS of IC stimulation as well as the effects when the IC stimulus was simultaneous with the exposure. Evidence for the former is found in the absence of cochlear effects after 1 min of IC stimulation until the loud sound exposure was presented. In the case of IC stimulation simultaneous with the exposure, no direct evidence is available since no cochlear potentials were measured while stimulating the IC continuously for 1 min. However, the absence of any cochlear effects when pulsed electrical trains are applied to the IC with stimulus parameters which are very effective in producing COCB effects at the cochlea when applied directly to the COCB fibres 6"8'9'18'21"45, suggests that IC stimulation does not directly activate the COCB. Recent studies in the cat by Warren and Liberman 43'44 show that MOCS neurones projecting to one ear appear to exhibit integrative facilitatory effects when long lasting tonal or noise stimuli are presented to the contralateral ear. In these studies, the contralateral acoustic stimuli appeared to lower the threshold for activation of the MOCS neurones (possibly the uncrossed MOCS neu-

203 rones, as suggested by these authors43'44). Such integrative facilitatory effects are compatible with the present study and the suggestions advanced in previous reports on C O C B effects o n " I ~ S 21"22'25 27. In the present study, the facilitatory effect of IC stimulation persisted for quite a significant period of time and reductions in TTS (albeit small reductions) could be obtained even with a 5 min delay between IC stimulus and exposure. This represents quite a long term facilitation of the C O C B for a relatively short duration IC stimulus and is similar to the long term effects obtained with other short duration protective manipulations that act through the C O C B 21'23"25"26.

Comparison o f different modes o f activating C O C B effects on TTS The mode of action proposed above for contralateral IC stimulation is more comparable to the mode of action of protective manipulations applied at the contralateral cochlea 23'26'27 than to the mode of action of direct electrical stimulation of the C O C B simultaneous with an exposure 21~22'25. Contralateral cochlear manipulations, like the IC stimulus, do not appear to directly activate the C O C B 23"26'27 but, rather, act in a facilitatory mode at the C O C B cell bodies to allow easier activation by the loud sound exposure 23'26'27. In contrast, it has been argued 22'23 that, with electrical stimulation of the C O C B , this indirect facilitatory mechanism would not apply when the C O C B stimulus was presented simultaneously with the exposure but only in the cases when the same C O C B stimulus preceded the exposure and still resulted in TFS reductions 22"23. In the cases where the C O C B stimulus was simultaneous with the exposure, the protection from TTS could only be due to direct C O C B effects at the cochlea elicited by the C O C B electrical stimulus 22. The fact that reductions in TTS with contralateral IC stimulation simultaneous with the exposure were comparable to reductions obtained with direct C O C B stimulation simultaneous with the exposure (Fig. 10), suggests that, despite the more indirect mode of action of the IC stimulus, the C O C B outflow to the cochlea was similar in both cases. Thus, for example, it is possible that combining the facilitatory effect of IC stimulation at 2 pulses/s with afferent input from the loud sound exposure resulted in C O C B outflow at 50 pulses/s, since direct C O C B stimulation at the floor of the fourth ventricle at REFERENCES 1 Andersen, R.A., Roth, G.L., Aitkin, L.M. and Merzenich, M,M., The efferent projections of the central nucleus of the inferior colliculus in the cat, J. Comp. Neurol., 194 (1980) 649-662. 2 Aschoff, A. and Ostwald, J., Different origins of cochlear

that rate 2~ produced TTS reductions identical to IC stimulation at 2/s. Similarly, the combination of the facilitatory action of IC stimulation at 20/s and afferent input from the exposure may have resulted in C O C B outflow at 140 pulses/s, the rate at which direct C O C B stimulation produced comparable reductions in TTS 21. Finally, it is noteworthy that IC stimulation at 50/s produced as much reduction in TTS as does C O C B stimulation at 400/S 21. It is therefore possible that C O C B neurones can respond at this rate under the combined effects of a facilitatory IC influence at 50/s and afferent input from a loud sound exposure.

Conclusions It was suggested above that the protective effects of contralateral and ipsilaterai IC stimulation, exercised through IC projections to the cells of origin of the crossed MOCS (for contralateral IC effects) and the uncrossed MOCS (for ipsilateral IC effects), were exerted solely by the M O C S fibres originating from the VNTB. In this context it is worth noting that the IC projections in the rabbit 4, the rat 7, the cat 1'3° and the macaque 2° also appear to terminate either solely or mainly in the region of the V N T B which appears to be a source of at least some MOCS fibres in all species studied so far 42. This raises the intriguing possibility that in all species the efferent fibres originating from this nucleus may have a c o m m o n protective function. A corollary to this hypothesis, then, is that the functions of the fibres of the MOCS arising from other nuclei may be different, although still exercised at the level of O H C s in the cochlea. Finally, it is interesting to note that greater reductions in neural threshold losses due to loud sound could be obtained if the effects on TTS of contralaterai and ipsilateral IC stimulation (and, therefore, the crossed and uncrossed components of the MOCS) are assumed to be additive. This possibility requires further testing as it could clarify whether the effects of ipsilateral IC stimulation are exercised through the uncrossed MOCS and whether this efferent system can also be viewed as a protective pathway. Acknowledgements'. This work was supported by grants from the National Health and Medical Research Council of Australia and the Australian Research Grants Scheme and by the laboratory facilities of Dr. B.M. Johnstone. I thank the two reviewers of a previous version of this report for their helpful suggestions and comments.

efferents in some bat species, rats and guinea pigs, J. Cornp. Neurol.. 264 (1987) 56-72. 3 Bobbin, R.P. and Konishi, T., Action of cholinergic and anti-cholinergic drugs at the crossed olivocochlear bundle-hair cell junction, Acta Otolaryngol., 77 (1974) 56-65. 4 Borg, E., A neuroanatomical study of the brainstem auditory system of the rabbit. Part 1I. Descending connections, Acta

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Morphol. Neerl.-Scand., 11 (1973)49-62. 5 Cody, A.R., Robertson, D., Bredberg, G. and Johnstone, B.M., Electrophysiological and morphological changes in the guinea pig cochlea following mechanical trauma to the organ of Corti, Acta Otolaryngol., 89 (1980) 440-452. 6 Desmedt, J.E., Physiological studies of the efferent recurrent system. In W.D. Keidel and W.D. Neff (Eds.), Handbook of Sensor), Physiology, Springer, Berlin, 1975, pp. 219-246. 7 Faye-Lund, H., Projection from the inferior collicuhis to the superior olivary complex in the albino rat. Anat. Embryol., 175 (1986) 35-52. 8 Fex, J., Auditory activity in centrifugal and centripetal cochlear fibres in cat. A study of a feedback system, Acta Physiol. Scand., Suppl. 189, 55 (1962) 1-68. 9 Galambos, R., Suppression of auditory nerve activity by stimulation of efferent fibres to cochlea, J. Neurophysiol., 19 (1956) 424-437. 10 Galley, N., Klinke, R., Oertel, W., Pause, M. and Storch, W.H., The effect of intracochlearly administered acetylcholineblocking agents on the efferent synapses of the cochlea, Brain Research, 64 (1973) 55-63, 1l Gifford, M.L. and Guinan, Jr., J.J., Effects of electrical stimulation of medial olivocochlear neurons on ipsilateral and contralateral cochlear responses, Hearing Res., 29 (1987) 179194. 12 Guinan, Jr., J.J., Warr, W.B. and Norris, B.E., Differential olivocoehlear projections from lateral versus medial zones of the superior olivary complex, J. Comp. Neurol., 221 (1983) 358-370. 13 Guinan, Jr., J.J., Warr, W.B. and Norris, B.E., Topographic organization of the olivocochlear projections from the lateral and medial zones of the superior olivary complex, J. Comp. Neurol., 226 (1984) 21-27. 14 Hashikawa, T., The inferior colliculopontine neurons of the cat in relation to other collicular descending neurons, J. Comp. Neurol., 219 (1983) 241-249. 15 Irvine, D.R.E and Wester, K.G., Middle ear muscle effects on cochlear responses to bone-conducted sound, Acta Physiol. Scand., 91 (1974) 482-496. 16 Johnstone, J.R., Alder, V.A., Johnstone, B.M., Robertson, D. and Yates, G.K., Cochlear action potential and single unit thresholds, J. Acoust. Soc. Am., 65 (1979) 254-257. 17 Kiss, A. and Majorossy, K., Neuron morphology and synaptic architecture in the medial superior olivary nucleus, Exp. Brain Res., 52 (1983) 315-327. 18 Klinke, R. and Galley, N., Efferent innervation of the vestibular and auditory receptors, Physiol. Rev., 54 (1974) 316-357. 19 M011er, A.R., The middle ear. In J.V. Tobias (Ed.), Foundations of Modern Auditory Theory, Vol. 2, Academic, New York, 1972, pp. 133-194. 20 Moore, R.Y. and Goldberg, J.M., Projections of the inferior colliculus in the monkey, Exp. Neurol., 14 (1966) 429-438. 21 Rajan, R., Effect of electrical stimulation of the crossed olivocochlear bundle on temporary threshold shifts in auditory sensitivity. I. Dependence on electrical stimulation parameters, J. Neurophysiol., 60 (1988) 549-568. 22 Rajan, R., Effect of electrical stimulation of the crossed olivocochlear bundle on temporary threshold shifts in auditory sensitivity. II. Dependence on the level of temporary threshold shifts, J. Neurophysiol., 60 (1988) 569-579. 23 Rajan, R. and Johnstone, B.M., Crossed cochlear influences on monaural temporary threshold shifts, Hearing Res., 9 (1983) 279-294. 24 Rajan, R. and Johnstone, B.M., Residual effects in monaural temporary threshold shifts to pure tones, Hearing Res., 12 (1983) 185-197. 25 Rajan, R. and Johnstone, B.M., Electrical stimulation of cochlear efferents at the round window decreases auditory desensitization in guinea pigs. I. Dependence on electrical stimulation parameters, Hearing Res., 36 (1988) 53-73.

26 Rajan, R. and Johnstone, B.M., Binaural acoustic stimulation exercizes protective effects at the cochlea that mimic the effects of electrical stimulation of an auditory efferent pathway. Brain Research, 459 (1988) 241-255. 27 Rajah, R. and Johnstone. B.M., Contralateral cochlear destruction mediates protection from monaural loud sound exposures through the crossed olivocochlear bundle, Hearing Res., 39 (1989) 263-278. 28 Rasmussen, G.L., The olivary peduncle and other fibre projections of the superior olivary complex, J. Comp. Neurol., 84 (1946) 141-219. 29 Rasmussen, G.L., Efferent fibres of the cochlear nerve and cochlear nucleus. In G.L. Rasmussen and W.E Windle (Eds.),

Neural Mechanisms, of the Auditory and Vestibular Systems, 30

31 32

33

34 35

36

37

38

39

40 41

42

43 44 45

Thomas, Springfield, IL, 1960, pp. 105-115. Rasmussen, G.L., Anatomic relationships of the ascending and descending auditory systems. In W.S. Fields and B.R. Alford (Eds.), Neurological Aspects of Auditory and Vestibular Disorders, Thomas, Springfield, IL, 1964, pp. 5-23. Robertson, D., Brainstem location of efferent neurones projecting to the guinea pig cochlea, Hearing Res., 20 (1985) 63-77. Robertson, D., Anderson, C.-J. and Cole, K.S., Segregation of efferent projections to different turns of the guinea pig cochlea, Hearing Res., 25 (1987) 69-76. Robertson, D., Cole, K.S. and Harvey, A.R., Brainstem organization of efferent projections to the guinea pig cochlea studied using the fluorescent tracers fast blue and diamidino yellow, Exp. Brain Res., 66 (1987) 449-457. Strutz, J., Efferent innervation of the cochlea, Ann. Otol. Rhinol. Laryngol., 90 (1981) 158-160. Strutz, J. and Bielenberg, K., Efferent acoustic neurons within the lateral superior olivary nucleus of the guinea pig, Brain Research, 299 (1984) 174-177. Strutz, J. and Spatz, W.B., Superior olivary and extra olivary origin of centrifugal innervation of the cochlea in guinea pig. A horseradish peroxidase study, Neurosci. Lett., 17 (1980) 227230. Syka, J., Druga, R., Popelar, J. and Vlkova, A., Descending central auditory pathway - - structure and function. In Auditory Pathway -- Structure and Function, Satellite Sympsosium Second World Congress of Neuroscienee, Prague, 1987, p. 75. Syka, J., Robertson, D. and Johnstone, B.M., Efferent descending projections from the inferior colliculus in the guinea pig. In Auditory Pathway -- Structure and Function, Satellite Symposium Second World Congress of Neuroscience, Prague, 1987, p. 76. Vidal, P.-P., May, P.J. and Baker, R., Synaptic organization of the tectal-facial pathways in the cat. I. Synaptic potentials following collicular stimulation, J. Neurophysiol., 60 (1988) 769-797. Wart, W.B., Efferent components of the auditory system, Ann. Otorhinolaryngol., Suppl. 74, 89 (1980) 114-120. Warr, W.B. and Guinan, Jr., J.J., Efferent innervation of the organ of corti: two separate systems, Brain Research, 173 (1979) 152-155. Warr, W.B., Guinan, Jr., J.J. and White, J.S., Organization of the efferent fibres. In R.A. Altsehuler, D.W. Hoffman and R.P. Bobbin (Eds.), Neurobiology of Hearing: The Cochlea, Raven, New York, 1986, pp. 333-348. Warren, II1, E.H. and Liberman, M.C., Effects of contralateral sound on auditory-nerve responses. I. Contributions of cochlear efferents, Hearing Res., 37 (1989) 89-104. Warren, III, E.H. and Liberman, M.C., Effects of contralateral sound on auditory-nerve responses. II. Dependence on stimulus variables, Hearing Res., 37 (1989) 105-122. Weiderhold, M.L., Physiology of the olivocochlear system. In R.A. Altschuler, D.W. Hoffman and R:P. Bobbin (Eds.), Neurobiology of Hearing: The Cochlea, Raven, New York, 1986. pp. 349-370.