Neural representation of pitch salience in the human brainstem revealed by psychophysical and electrophysiological indices.

Acoustically, pitch is related to the temporal regularity or periodicity of a sound. Perceptual and electrophysiologic studies have revealed that pitch salience grows systematically with increasing stimulus periodicity. The aim of this study is to show that information relevant to pitch salience is already encoded in the phase-locked neural activity of brainstem neurons in order to demonstrate that the neural manifestation of pitch salience emerges well before cortical involvement. Brainstem frequency following responses (FFRs) were recorded from participants in response to linguistic tones, which varied only in their degree of pitch salience. Neural pitch strength was computed from FFRs using autocorrelation algorithms. In addition, behavioral frequency difference limens (F0 DLs) were measured from each participant to obtain a perceptual estimate related to pitch salience. Brainstem neural pitch strength increased systematically with increasing temporal regularity in stimulus periodicity, indicating more robust encoding for salient pitch. F0 DLs decreased with increasing stimulus periodicity revealing better pitch change detection for more salient stimuli. FFR neural pitch strength and behavioral F0 DLs were negatively correlated suggesting that subcortical processing can, in part, predict an individual's behavioral judgments of pitch salience. These data imply that changes to the acoustic periodicity of a stimulus directly influence brainstem encoding and the corresponding behavioral responses to pitch. We infer that information related to pitch salience may emerge early along the auditory pathway and is likely rooted in pre-attentive, sensory-level processing.