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In a recent study (Venezia et al., 2021), left dorsal premotor cortex (dPM) responded to vocal pitch during a degraded speech recognition task, but only when speech was rated as unintelligible. Crucially, vocal pitch was not relevant to the task. The present fMRI study (N = 25) tests the hypothesis that left dPM will respond to vocal pitch for increasingly intelligible speech in a multi-talker speech recognition task that emphasizes pitch for talker segregation. We applied spectrotemporal modulation distortion to independently modulate vocal pitch and phonetic content in two-talker (male/female) utterances across two conditions (Competing, Unison), only one of which required pitch-based segregation (Competing). A Bayesian hierarchical drift-diffusion model (HDDM) was used to predict speech recognition performance (3-AFC response times, accuracy coded) from the pattern of spectrotemporal distortion imposed on each trial. The model’s drift rate parameter, a d’-like measure of speech recognition performance, was strongly associated with vocal pitch for Competing but not Unison. In a second, Bayesian hierarchical brain-behavior model, we then regressed the HDDM’s posterior predictions of trial-wise drift rate against trial-wise fMRI activation amplitude. A significant positive association with overall drift rate, reflecting contributions from vocal pitch and/or phonetic content, was observed in left dPM in both conditions. A significant positive association with ‘pitch-restricted’ drift rate, reflecting only contributions from vocal pitch, was observed in left dPM but only in the Competing condition. These findings suggest that left dPM: (i) responds to vocal pitch; and (ii) can operate in an auditory-pitch mode and a phonetic-speech mode. |
