Rebound depolarization in single units of the ventral cochlear nucleus: a contribution to grouping by common onset?

Simultaneous grouping by common onset time is believed to be a powerful cue in auditory perception; components that start or stop roughly at the same time are judged as far more likely to have originated from the same source. Here we report a simple experiment designed to simulate a complex psychophysical paradigm first described by Darwin and Sutherland [(1984) Grouping frequency components of vowels. When is a harmonic not a harmonic? Quarterly J of Experimental Psychology: Hum Exp Psychol 36(A):193-208]. It is possible to change the perception of the vowel /I/ to /epsilon/ by manipulating the harmonics around the first formant (F1). Increasing the amplitude of one harmonic around F1 caused the perception of the vowel to change from /I/ to /epsilon/. Extending the increased component before the vowel could, however, greatly reduce this change. The role of neural adaptation in this effect was questioned by repeating the experiment but this time using a 'captor' tone which was switched on with the asynchronous harmonic and off when the vowel started. This time the vowel percept did change in a fashion analogous to the effect of an increase in the amplitude of the fourth harmonic (which is close to F1). This effect was explained by assuming that the captor had grouped with the leading portion of the asynchronous component enabling the remainder of the asynchronous component to be grouped with the remainder of the components. We propose a relatively low-level neuronal explanation for this grouping effect: the captor reduces the neural response to the leading segment of the asynchronous component by activating across-frequency suppression, either from the cochlea, or acting via a wideband inhibitor in the ventral cochlear nucleus. The reduction in neural response results in a release from adaptation with the offset of the captor terminating the inhibition, such that the response to the continuation of that component is now enhanced. Using a simplified paradigm we show that both primary-like and chopper units in the ventral cochlear nucleus of the anesthetized guinea pig may show a rebound in excitation when a captor is positioned so as to stimulate the suppressive sidebands in its receptive field. The strength of the rebound was positively correlated with the strength of the suppression. These and other results are consistent with the view that low-level mechanisms underlie the psychophysical captor effect.