The corticofugal system for hearing: Recent progress

Peripheral auditory neurons are tuned to single frequencies of sound. In th e central auditory system, excitatory (or facilitatory) and inhibitory neur al interactions take place at multiple levels and produce neurons with shar p level-tolerant frequency-tuning curves, neurons tuned to parameters other than frequency, cochleotopic (frequency) maps, which are different from th e peripheral cochleotopic map, and computational maps. The mechanisms to cr eate the response properties of these neurons have been considered to be so lely caused by divergent and convergent projections of neurons in the ascen ding auditory system. The recent research on the corticofugal (descending) auditory system, however, indicates that the corticofugal system adjusts an d improves auditory signal processing by modulating neural responses and ma ps. The corticofugal function consists of at least the following subfunctio ns. (i) Egocentric selection for short-term modulation of auditory signal p rocessing according to auditory experience. Egocentric selection, based on focused positive feedback associated with widespread lateral inhibition, is mediated by the cortical neural net working together with the corticofugal system. (ii) Reorganization for long-term modulation of the processing of behaviorally relevant auditory signals. Reorganization is based on egocentr ic selection working together with nonauditory systems. (iii) Gain control based on overall excitatory. facilitatory, or inhibitory corticofugal modul ation, Egocentric selection can be viewed as selective gain control. (iv) S haping (or even creation) of response properties of neurons, Filter propert ies of neurons in the frequency, amplitude, time, and spatial domains can b e sharpened by the corticofugal system. Sharpening of tuning is one of the functions of egocentric selection.