Psychophysical correlates of contralateral efferent suppression. I. The role of the medial olivocochlear system in “central masking” in nonhuman primates

Abstract

An extensive physiological literature, including experimental and clinical studies in humans, demonstrates that activation of the medial olivocochlear (MOC) efferent system, by either contralateral sound or electrical stimulation, can produce significant alterations in cochlear function and suggests a role for the MOC system in influencing the auditory behavior of binaural hearing. The present data are from psychophysical studies in nonhuman primates which seek to determine if the noted physiological changes in response to contralateral acoustic stimulation have a perceptual counterpart. Four juvenile Japanese macaques were trained to respond to the presence of 1-s sinusoids, presented to the test ear, in an operant reinforcement paradigm. Thresholds were compared for frequencies ranging from 1.0 to 4.0 kHz in quiet, with thresholds measured when continuous, two octave-band noise, centered on the test tone frequency, was presented in the contralateral ear. Contralateral noise was presented at levels of 10-60 dB above detection threshold for the test-tone frequency. While some variability was evident across subjects, both in the frequency distribution and magnitude (as a function of contralateral noise level), all subjects exhibited an increase, or suppression of thresholds in the presence of contralateral noise. On average, thresholds increased systematically with contralateral noise level, to a peak of 7 dB. In one subject, the threshold increase seen with contralateral noise was significantly reduced when the MOC was surgically sectioned on the floor of the IVth ventricle. The characteristics of the measured shifts in behavioral thresholds, in the presence of contralateral noise reported here, are qualitatively and quantitatively similar to both efferent physiological suppression effects and psychophysical central masking threshold shifts which have been reported previously. These data suggest that at least some aspects of "central masking" are efferent-mediated peripheral processes, and that the term "central masking" may be incorrect.

Central masking with bilateral cochlear implants

“…Contralateral tones suppress responses of auditory-nerve fiber stimuli near characteristic frequency, but only if the olivocochler bundle is intact (Fex, 1967;Warren and Liberman, 1989). A psychoacoustic study with macaque monkeys indicated that the suppressive effects of central masking could be reduced or eliminated when the MOC bundle was sectioned at the floor of the IVth ventricle (Smith et al, 2000). On the other hand, the bilateral CI subjects in the present report demonstrated central masking despite lacking efferent outer hair cell modulation in their electrically stimulated inner ears.…”

The relationship between precursor level and the temporal effect

“…There is a small amount of behavioral data from animal studies showing efferent-mediated changes in response to sound. Smith et al (2000) found that contralateral noise raised behavioral thresholds for tones in Japanese macaques, and that this effect disappeared with sectioning of the medial olivo-cochlear bundle. May and McQuone (1995) reported that cats showed a decrease in the ability to detect intensity changes in 8-kHz tones presented in ipsilateral noise after sectioning of the medial olivocochlear bundle.…”

Effects of Anesthesia on Efferent-Mediated Adaptation of the DPOAE

“…In four guinea pigs, only one of the agents was used at the same dosages. In two awake guinea pigs, gentamicin was administered at 200 mg/kg IM in order to achieve a pharmacological block of the MOC reex (Smith et al 1994;Avan et al 1996).…”

“…Second, gentamicin, a known blocker of the MOC re¯ex (Smith et al 1994;Avan et al 1996) also greatly reduces these adaptations in guinea pig (Fig. 2).…”

“…Gentamicin has been shown to block the MOC system without ototoxic effects when given as a single injection at a dose of 200 mg/kg (Smith et al 1994;Avan et al 1996), although this block may not be complete (Smith et al 2000). In two awake guinea pigs, DPOAE adaptation was measured before and after gentamicin injection.…”