CORTICOFUGAL MODULATION OF FREQUENCY PROCESSING IN BAT AUDITORY-SYSTEM

Auditory signals are transmitted from the inner ear through the brains tem to the higher auditory regions of the brain. Neurons throughout th e auditory system are tuned to stimulus frequency, and in many auditor y regions are arranged in topographical maps with respect to their pre ferred frequency, These properties are assumed to arise from the inter actions of convergent and divergent projections ascending from lower t o higher auditory areas(1); such a view, however, ignores the possible role of descending projections from cortical to subcortical regions(2 -10). In the bat auditory system, such corticofugal connections modula te neuronal activity to improve the processing of echo-delay informati on(11,12), a specialized feature. Here we show that corticofugal proje ctions are also involved in the most common type of auditory processin g, frequency tuning. When cortical neurons tuned to a specific frequen cy are inactivated, the auditory responses of subcortical neurons tune d to the same frequency are reduced. Moreover, the responses of other subcortical neurons tuned to different frequencies are increased, and their preferred frequencies are shifted towards that of the inactivate d cortical neurons. Thus the corticofugal system mediates a positive f eedback which, in combination with widespread lateral inhibition, shar pens and adjusts the tuning of neurons at earlier stages in the audito ry processing pathway.