An auditory-like region in the motor cortex

Abstract

Left premotor cortex is activated during listening to speech, but whether this reflects a motor mechanism or a response to auditory features is debated. Previously, we showed that left dorsal premotor cortex (dPM) responded to vocal pitch in a degraded speech recognition task, but only when speech was rated as unintelligible. Crucially, vocal pitch was not relevant to the task. Here, we hypothesize that left dPM will respond to vocal pitch for increasingly intelligible speech in a competing speech task that emphasizes pitch for talker segregation. We use fMRI (N = 25) and apply spectrotemporal modulation distortion to modulate pitch in two-talker (male/female) mixtures across two conditions (Competing, Unison), only one of which requires pitch-based segregation (Competing). A Bayesian drift-diffusion model was used to predict speech recognition performance (3-AFC response times) from the pattern of spectrotemporal distortion imposed on each trial. The model’s ‘drift rate’ parameter, a d’-like measure, was strongly associated with vocal pitch for Competing but not Unison. Trial-wise predictions of ‘pitch-restricted’ drift rate—i.e., that component of performance driven only by pitch – were positively associated with activation in left dPM for Competing but not Unison. These findings show that left dPM responds to auditory features during speech recognition.