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 modulations (FM) might underlie the encoding of frequency sweeps in animal vocalizations and formant transitions in human speech. We examined the relationship between FM direction and rate selectivity and the precise temporal interactions of excitatory and inhibitory sideband inputs. Extracellular 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 selectivity. Brief tone pairs with varying interstimulus delays were presented in a forward masking paradigm to examine the relative timing of excitatory and inhibitory inputs. In the 33 units for which tone pair data were collected, a correspondence existed between FM rate selectivity and the time delays between paired tones. Moreover, FM directional selectivity was strongly linked to rate selectivity, because directional 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.