Human medial olivocochlear reflex: effects as functions of contralateral, ipsilateral, and bilateral elicitor bandwidths

Abstract

Animal studies have led to the view that the acoustic medial olivocochlear (MOC) efferent reflex provides sharply tuned frequency-specific feedback that inhibits cochlear amplification. To determine if MOC activation is indeed narrow band, we measured the MOC effects in humans elicited by 60-dB sound pressure level (SPL) contralateral, ipsilateral, and bilateral noise bands as a function of noise bandwidth from 1/2 to 6.7 octaves. MOC effects were quantified by the change in stimulus frequency otoacoustic emissions from 40 dB SPL probe tones near 0.5, 1, and 4 kHz. In a second experiment, the noise bands were centered 2 octaves below probe frequencies near 1 and 4 kHz. In all cases, the MOC effects increased as elicitor bandwidth increased, with the effect saturating at about 4 octaves. Generally, the MOC effects increased as the probe frequency decreased, opposite expectations based on MOC innervation density in the cochlea. Bilateral-elicitor effects were always the largest. The ratio of ipsilateral/contralateral effects depended on elicitor bandwidth; the ratio was large for narrow-band probe-centered elicitors and approximately unity for wide-band elicitors. In another experiment, the MOC effects from increasing elicitor bandwidths were calculated from measurements of the MOC effects from adjacent half-octave noise bands. The predicted bandwidth function agreed well with the measured bandwidth function for contralateral elicitors, but overestimated it for ipsilateral and bilateral elicitors. Overall, the results indicate that (1) the MOC reflexes integrate excitation from almost the entire cochlear length, (2) as elicitor bandwidth is increased, the excitation from newly stimulated cochlear regions more than overcomes the reduced excitation at frequencies in the center of the elicitor band, and (3) contralateral, ipsilateral, and bilateral elicitors show MOC reflex spatial summation over most of the cochlea, but ipsilateral spatial summation is less additive than contralateral.

FIG. 1

MOC effect as a function of elicitor bandwidth for noise bands centered at the probe frequency. ΔSFOAEn magnitudes from bilateral (circles), ipsilateral (×’s), and contralateral (triangles) 60-dB SPL elicitors were measured in the postelicitor window. The squares are ΔSFOAEn magnitudes from the same contralateral elicitors except measured in a during-elicitor window. ΔSFOAEn is the change in the SFOAE normalized by the magnitude of the SFOAE. BBN = broadband noise (0.1–10 kHz, or 6.67 octaves). Note that BBN is centered only for the 1-kHz probe. For probe tones at 0.5, 1, and 4 kHz, data from five, 15 and nine ears were included in the averages. Error bars are standard errors of the mean. Horizontal lines: solid = noise-floor mean, dotted = 1 standard deviation above the noise mean. Average values that were statistically different from the noise floor are indicated by the small symbols: one symbol = P ≤ 0.05, two symbols = P ≤ 0.01, three symbols = P ≤ 0.001.

FIG. 2

MOC effect as a function of elicitor bandwidth for noise bands centered 2 octaves below the probe frequency. ΔSFOAEn magnitudes from bilateral (circles), ipsilateral (×’s), and contralateral (triangles) 60-dB SPL elicitors were measured in a during-elicitor window. For probe tones at 1 and 4 kHz, data from six and three ears were included in the averages. Error bars are standard errors of the mean. Horizontal lines: solid = noise-floor mean, dotted = 1 standard deviation above the noise mean. Average values that were statistically different from the noise floor are indicated by the small symbols: one symbol = P ≤ 0.05, two symbols = P ≤ 0.01, three symbols = P ≤ 0.001.

FIG. 3

MOC effects tended to decrease as probe frequency increased. ΔSFOAEn magnitudes from centered noise bands (top) measured in the postelicitor window and from off-centered noise bands (bottom) measured in a during-elicitor window. Error bars are standard errors of the mean.

FIG. 4

MOC effects (ΔSFOAEn magnitudes) calculated by summing the effects of half-octave band elicitors (dashed lines and ×’s) compared to measured effects (solid lines) from 60-dB SPL elicitors of different bandwidths (see “Methods”). The calculated effects were similar to the measured effects for contralateral elicitors but were greater than the measured effects for ipsilateral and bilateral elicitors. Error bars represent standard errors of the mean.