Multiparametric corticofugal modulation of collicular duration-tuned neurons: modulation in the amplitude domain.

The subcortical auditory nuclei contain not only neurons tuned to a specific frequency but also those tuned to multiple parameters characterizing a sound. All these neurons are potentially subject to modulation by descending fibers from the auditory cortex (corticofugal modulation). In the past, we electrically stimulated cortical duration-tuned neurons of the big brown bat, Eptesicus fuscus, and found that its collicular duration-tuned neurons were corticofugally modulated in the frequency and time (duration) domains. In the current paper, we report that they were also corticofugally modulated in the amplitude (intensity) domain. We found the following collicular changes evoked by focal cortical electric stimulation. 1) Corticofugal modulation in the amplitude domain differed depending on whether recorded collicular neurons matched in best frequency (BF) with stimulated cortical neurons. BF-matched neurons decreased their thresholds, whereas BF-unmatched neurons increased their thresholds: the larger the BF difference between the recorded collicular and stimulated cortical neurons, the larger the threshold increase. 2) In general, the dynamic range for amplitude coding was larger in the inferior colliculus than in the auditory cortex. BF-matched neurons increased their dynamic ranges and response magnitude, whereas BF-unmatched neurons decreased them. 3) Single duration-tuned neurons were simultaneously modulated by cortical electric stimulation in the amplitude, frequency and time domains. 4) Corticofugal modulation in these three domains indicates that the contrast of the neural representation of repeatedly delivered sound stimuli is increased.