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Asymmetry in corticofugal modulation of frequency-tuning in mustached bat auditory system.

Focal electric stimulation of the auditory cortex is well suited for exploration of the function of the corticofugal (descending) system and the neural mechanism of plasticity in the central auditory system, because it evokes changes in frequency-tuning, called best frequency (BF) shifts, as does auditory fear conditioning. The Doppler-shifted constant frequency (DSCF) area of the primary auditory cortex of the mustached bat is highly specialized for fine frequency analysis. Focal electric stimulation of the DSCF area evokes the BF shifts of ipsilateral cortical and collicular neurons away from the BF of stimulated neurons, whereas the stimulation evokes the BF shifts of contralateral cortical and collicular neurons either toward or away from the stimulated BF. The direction of contralateral BF shifts shows a flip-flop, depending on the spatial relationship between the stimulated and recorded neurons. This asymmetry in corticofugal modulation is mostly, if not totally, created by two subdivisions of the stimulated DSCF area that transmit signals to the contralateral DSCF area, presumably through the corpus callosum. This intriguing asymmetry in corticofugal modulation presumably functions for equalization of the reorganization of the frequency maps of the DSCF areas and subcortical auditory nuclei on both sides.