Searching for subcortical indications of interaction between dynamic pitch and timbre cues via the frequency-following response

Pitch and timbre cues, specifically the fundamental frequency (F0) and spectral centroid (SC), respectively, interact in listeners’ perception. For example, changes in SC can influence the perceived pitch. F0 and SC appear to interact in the auditory cortex. We investigated whether these cues interact in the auditory brainstem, via frequency-following responses (FFRs). Four dynamic stimuli were constructed with either congruent (both ascending or descending) or incongruent movements of F0 and SC across their 400-ms duration. The nominal values for F0 and SC were 150 and 750 Hz, and the ranges of F0 and SC movements corresponded to 10 times the just noticeable difference. Dynamically varying F0s were encoded faithfully in the FFRs, independent of cue congruency. This suggests a lack of interaction between F0 and SC in FFR. Furthermore, although the two congruent stimuli were time-reverse of each other, the cross-correlation function between FFRs to the two stimuli exhibited temporal asymmetry for some participants. Similar asymmetry was also observed for FFRs to incongruent stimuli. The asymmetry in cross-correlations of FFRs was not observed for the corresponding cross-correlations of the stimuli. This suggests a temporal asymmetry in the stimulus-encoding processes of the subcortical auditory system, which is heterogenous across listeners.