Crossed Olivocochlear Bundle Research Articles

Electrophysiological Studies on the Spatial Distribution of the Crossed Olivocochlear Bundle along the Guniea Pig Cochlea

Changes in the electrical potentials in the cochlea were recorded at various locations along the cochlea, when the crossed olivocochlear bundle (COCB) was stimulated electrically on the floor of the fourth ventricle. Cochlear microphonic (CM) was recorded from Turns I, II, and III, and endocochlear potential (EP) from Turns I and III. Responses of the individual auditory nerve fibers were recorded in the modiolus. The site of the termination of these fibers could be ascertained by their best frequencies. Tetanic stimulation of COCB terminated 10 msec prior to the tone bursts. The effect of COCB stimulation on slow potentials in the scala media and augmentation of CM decreased as a function of distance between recording site and the round window. Decreases in discharge rate showed a wide variation among the fibers with best frequencies above 10 kHz. The greatest decrease in discharge rate was obtained in fibers with best frequencies between 7 and 9 kHz. Effectiveness of efferent stimulation on discharge rate of auditory fibers decreased gradually as their best frequencies decreased below 7 kHz. [This work was supported by the PHS.]

Further evidence for peripheral activation of olivocochlear bundle endings.

Function of the olivocochlear bundle (OCB) has been studied in anesthetized guinea pigs, using differential cochlear recording. Low rates of efferent bundle stimulation (EBS) are highly effective in equalizing the two microphonic modulation envelope maxima (peaks) seen in the double modulation response to two-tone stimulation. EBS pulses, time-locked to a 20-Hz modulating tone, are effective in increasing the amplitude of the smaller peak of a 400-Hz double modulation response, provided they occur during the time interval of the larger peak. Using sequential two-tone stimuli, it has been found that there is a transient period of microphonic enhancement immediately following a short burst of moderately intense low-frequency tonal stimulation. The increase of the microphonic response to a weak test tone presented immediately after a low-frequency tone pip is similar in both amplitude and rate of decay to that produced by crossed olivocochlear bundle (COCB) stimulation. The microphonic enhancement effects produced by low-frequency tonal stimulation and low-rate COCB stimulation are additive; higher frequency tones—e.g., 1000 Hz—reduce the microphonic enhancement effectiveness of COCB stimulation when presented concurrently. These findings suggest peripheral reflex mechanisms for firing OCB endings.

Behavioral investigation of some possible effects of sectioning the crossed olivocochlear bundle.

Due to the paucity of behavioral data bearing on the possible role of the efferent auditory system, an investigation of some possible functions of the crossed olivocochlear bundle (OCB) has been undertaken, using cats. Behavioral correlates of transection of the crossed OCB were sought in such measures as absolute thresholds in quiet, effective masking over differing noise levels and frequencies, and temporary threshold shifts. The following results were obtained after transection of crossed OCB: (1) Absolute thresholds were unchanged; (2) amount of masking at 1000 Hz and 2000 Hz increased (though the shift was not statistically significant with the small number of subjects tested); and (3) TTS did not differ greatly for the transected animals.

Electrical Stimulation of the Crossed Olivocochlear Bundle: Its Effect on Spontaneous Firings of Auditory-Nerve Fibers

In anesthetized cats, the response rate of single auditory-nerve fibers to low-level tones at the characteristic frequency remains markedly reduced during trains of shocks to the crossed olivocochlear bundle (COCB). For most fibers an identical COCB shock train does not result in maintained reduction of “spontaneous” activity. In cats with low thresholds for N1 responses to clicks, fibers with characteristic frequencies within a restricted range exhibit partial suppression of “spontaneous” activity during COCB stimulation. These frequencies are in the most sensitive portion of the cat's audiogram. Our present view is that part of the “spontaneous” activity in these very sensitive fibers represents responses to low-level acoustic noise and can be suppressed by COCB stimulation. Presumably, in less sensitive cats the “spontaneous” discharges of auditory-nerve fibers are not generated by any acoustic input and are not suppressed by COCB stimulation. [This study was supported by grants from the National Institutes of Health and the Joint Services Electronics Program.]

Electrical Stimulation of the Crossed Olivocochlear Bundle—Its Effect on Responses of Auditory-Nerve Fibers to Tone Bursts

In anesthetized cats, spike responses of single auditory-nerve fibers during tone-burst stimulation were recorded for tones at different sound-pressure levels (SPL). Every other tone burst was preceded by a train of shocks to the crossed olivocochlear bundle (COCB). For each fiber two curves of number-of-spikes vs SPL were plotted, one with and one without shocks to the COCB. The two curves are nearly parallel in the region where they are rapidly changing, the curve obtained with shocks to the COCB being effectively shifted to higher SPL. The size of this shift varies considerably among fibers even within a single preparation and has been plotted against characteristic frequency, threshold, and rate of spontaneous discharge. [Supported in part by the Joint Services Electronics Program, the National Science Foundation, the National Institutes of Health, Department of Health, Education, and Welfare, and the National Aeronautics and Space Administration; and in part by a research grant and a predoctoral fellowship (to MLW) from the National Institutes of Health, Public Health Service.]