Corticofugal modulation of amplitude domain processing in the midbrain of the big brown bat, Eptesicus fuscus.

Recent studies have shown that the corticofugal system systematically modulates and improves subcortical signal processing in the frequency, time and spatial domains. The present study examined corticofugal modulation of amplitude sensitivity of 113 corticofugally inhibited neurons in the central nucleus of the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus. Cortical electrical stimulation decreased the number of impulses and increased the response latency of these neurons. They had an average of 5.9+/-4.4 kHz best frequency (BF) differences between collicular and electrically stimulated cortical neurons. Cortical electrical stimulation synchronized with sound stimulation for 30 min compressed the rate-amplitude functions of half (56, 49.6%) of these collicular neurons and shifted their minimum thresholds (MT) and dynamic ranges (DR) toward that of electrically stimulated cortical neurons for as long as 40 min. These collicular neurons had an average of 1.6+/-1.4 kHz BF differences. The shift in collicular MT and DR significantly increased with differences in MT and DR between collicular and cortical neurons. Cortical electrical stimulation also shifted the BF and best amplitude (BA) of collicular neurons toward that of cortical neurons. The BF shift increased with BF differences and the BA shift increased with BA differences. These data suggest that the corticofugal system modulates collicular responses on the basis of topographic projections between the IC and auditory cortex. However, corticofugal modulation of collicular amplitude sensitivity is primarily dependent upon the difference but not the absolute amplitude sensitivity between collicular and cortical neurons.