TEMPORAL PROCESSING ACROSS FREQUENCY CHANNELS BY FM SELECTIVE AUDITORY NEURONS CAN ACCOUNT FOR FM RATE SELECTIVITY

Auditory neurons tuned to the direction and rate of frequency modulati ons (FM) might underlie the encoding of frequency sweeps in animal voc alizations and formant transitions in human speech. We examined the re lationship between FM direction and rate selectivity and the precise t emporal interactions of excitatory and inhibitory sideband inputs. Ext racellular single-unit recordings were made in the auditory midbrains of eight mustached bats. Up- and down-sweeping linear FM stimuli were presented at different modulation rates in order to determine FM selec tivity. Brief tone pairs with varying interstimulus delays were presen ted in a forward masking paradigm to examine the relative timing of ex citatory and inhibitory inputs. In the 33 units for which tone pair da ta were collected, a correspondence existed between FM rate selectivit y and the time delays between paired tones. Moreover, FM directional s electivity was strongly linked to rate selectivity, because directiona l preferences were expressed only at certain rates and not others. We discuss how abnormalities in the relative timing of inputs could alter or abolish the selectivity of such neurons, and how such a mechanism could account for the perceptual deficits for formant transitions seen in certain children with phonological deficits. (C) 1998 Published by Elsevier Science B.V. All rights reserved.