Context-Dependent Modulation of Timbre Encoding in the Human Auditory Brainstem

Music is often polyphonic and requires perceptual segregation of concurrently occurring instruments. Little is known about how concurrent instrument tones are processed subcortically, so that they can be perceptually segregated. Accordingly, the goal of this study was to understand how the timbre of one instrument impacts subsequent processing of that same instrument when combined with a second instrument, using the frequency following response (FFRs). Participants were presented with sequences that consisted of a priming tone that was either a guitar or a piano tone, immediately followed by a composite tone consisting of both instruments. EEG data was averaged into two separate FFRs. Both FFRs were evoked by the combined piano and guitar tone; the only difference between the two FFRs was which tone came first: guitar or piano. The FFR was analyzed using the strength of stimulus-to-response correlation (SRC) to both instrument tones in isolation and spectral power analyses at harmonic frequencies for each instrument in isolation. Results suggest instrument-specific neural priming. Guitar primed FFRs correlated more strongly with the guitar tone compared to the piano tone, (p<.001, d=1.54). Guitar priming enhanced SRC between combined-tone responses and guitar stimuli compared to piano-primed conditions (p<.001, d=0.76). Piano priming showed no significant SRC enhancement (p=0.29). Spectral analysis revealed instrument-specific enhancements: guitar priming increased spectral power at the harmonics in the guitar tone, while piano priming enhanced power at the harmonics in the piano, both p<.001. These findings demonstrate timbre-based priming influences on the FFR and highlighting context sensitivity in early auditory processing.