Evidence for interactions across frequency channels in the inferior colliculus of awake chinchilla

As a result of cochlear processing, information about acoustic broadband signals is distributed across many parallel frequency channels. Periodic modulations of signal envelopes - conspicuous in particular in harmonic signals - may extend across a wide frequency range and give rise to temporal response patterns in the auditory nerve, particularly useful for recombination of constituents and the separation of the signals from background noise. Herein we report evidence that across frequency processing as necessary for binding of related signal components occurs already in the auditory midbrain of mammals. Extracellular recordings were made from 231 multi and single units in the inferior colliculus of awake chinchillas. Loud pure tones evoked onset type excitation (26%) and suppression of spontaneous rate (60%) not only in the range of the units' characteristic frequency (CF), but also in a frequency range far above CF. About 80% of all units tuned to CFs below 3 kHz gave sustained responses to low level stimuli of high frequencies (>2CF) provided the tones were sinusoidally amplitude modulated (SAM) with a unit specific modulation frequency although none of the spectral components of the amplitude modulation alone was sufficient to evoke such a response, even at high intensities. Low level high carrier SAM responses and wide band onset responses as well as inhibition must have their origin in a non-linear across frequency channel interaction of neuronal information. Many aspects of these responses cannot be explained by peripheral distortion in the cochlea. We therefore propose a mechanism of integration across frequency channels that may originate within the inferior colliculus and/or the nuclei of the lateral lemniscus. This process may lead to the binding of information that shares a common periodicity and may thereby help to distinguish different acoustic objects.