Coding of sound envelopes by inhibitory rebound in neurons of the superiorolivary complex in the unanesthetized rabbit

Most natural sounds (e.g., speech) are complex and have amplitude envelopes that fluctuate rapidly. A number of studies have examined the neural codin g of envelopes, but little attention has been paid to the superior olivary complex (SOC), a constellation of nuclei that receive information from the cochlear nucleus. We studied two classes of predominantly monaural neurons: those that displayed a sustained response to tone bursts and those that ga ve only a response to the tone offset. Our results demonstrate that the off neurons in the SOC can encode the pattern of amplitude-modulated sounds wi th high synchrony that is superior to sustained neurons. The upper cutoff f requency and highest modulation frequency at which significant synchrony wa s present were, on average, slightly higher for off neurons compared with s ustained neurons. Finally, most sustained and off neurons encoded the level of pure tones over a wider range cf intensities than those reported for au ditory nerve fibers and cochlear nucleus neurons. A traditional view of inh ibition is, that it attenuates or terminates neural activity. Although this holds true for off neurons, the robust discharge when inhibition is releas ed adds a new dimension. Far simple sounds (i.e., pulse tones), the off res ponse can code a wide range of sound levels. For complex sounds, the off re sponse becomes entrained to each modulation, resulting in a precise tempora l coding of the envelope.