Delay and temporal integration in medial olivocochlear bundle activation in humans.

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Contralateral suppression of the transient-evoked otoacoustic emissions (TEOAEs) provides a means of studying auditory efferent function, but the temporal dynamics of the reflex are not fully understood. The most fundamental parameter is the time-course of activation of contralateral suppression. The stimulus parameters are likely to be important; this may include temporal dynamics of the suppressor itself. This investigation thus was devoted to the further study of 1) delay of contralateral suppression of TEOAEs-effect of delay of the ipsilateral probe-and 2) temporal variation of the suppressor-effect of amplitude modulation of the contralateral noise stimulus. Measurements were made in three samples of normal-hearing subjects (N(total) = 71), employing well-established methods of TEOAE assessment. Statistically significant contralateral suppression occurred some 60 msec after onset of the contralateral noise; thereafter, the effect was essentially constant (i.e., to >180 msec). The results for click delays less than 60 msec, nevertheless, were systematic and readily fitted by a sloping straight line (dB suppression versus time) reminiscent of the concept of threshold power integration. The onset of suppression may thus be characterized by a time constant. The delay of suppression also was found to be reduced by contralateral amplitude-modulated noise. These findings reinforce a growing consensus in the literature that, despite initiation perhaps some milliseconds after onset of the contralateral stimulus, there is a substantial delay, i.e., in the tens of milliseconds, before maximal suppression is achieved. The exact time constant of this effect appears to depend upon the combination of probe and suppressor levels, including the temporality of the suppressor. These factors are likely to delimit the role/influence of this reflex in real-world function, favoring perhaps more-or-less sustained suppression that is activated in a time-varying sound environment.