Delay and temporal integration in medial olivocochlear bundle activation in humans

Objective: Contralateral suppression of the transient-evoked otoacoustic em issions (TEOAEs) provides a means of studying auditory efferent function, b ut the temporal dynamics of the reflex are not fully understood. The most f undamental parameter is the time-course of activation of contralateral supp ression. The stimulus parameters are likely to be important; this may inclu de temporal dynamics of the suppressor itself. This investigation thus was devoted to the further study of 1) delay of contralateral suppression of TE OAEs-effect of delay of the ipsilateral probe-and 2) temporal variation of the suppressor-effect of amplitude modulation of the contralateral noise st imulus. Design: Measurements were made in three samples of normal-hearing subjects (N-total = 71), employing well-established methods of TEOAE assessment. Results: 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 les s than 60 msec, nevertheless, were systematic and readily fitted by a slopi ng straight line (dB suppression versus time) reminiscent of the concept of threshold power integration. The onset of suppression may thus be characte rized by a time constant. The delay of suppression also was found to be red uced by contralateral amplitude-modulated noise. Conclusions: These findings reinforce a growing consensus in the literature that, despite initiation perhaps some milliseconds after onset of the cont ralateral stimulus, there is a substantial delay, i.e., in the tens of mill iseconds, before maximal suppression is achieved. The exact time constant o f this effect appears to depend upon the combination of probe and suppresso r levels, including the temporality of the suppressor. These factors are li kely to delimit the role/influence of this reflex in real-world function, f avoring perhaps more-or-less sustained suppression that is activated in a t ime-varying sound environment.