Spontaneous variability predicts compensative motor response in vocal pitch control.

Our motor system uses sensory feedback to keep desired performance. From this view, motor fluctuation is not simply 'noise' inevitably caused in the nervous system but would play a role in generating variations to explore better outcomes via sensory feedback. Vocalization system offers a good model for studying such sensory-motor interactions since we regulate vocalization by hearing our own voice. This behavior is typically observed as compensatory responses in vocalized pitch, or fundamental frequency (fo), when artificial fo shifts were induced in the auditory feedback. However, the relationship between adaptive regulation and motor exploration in vocalization has remained unclear. Here we investigated behavioral variability in spontaneous vocal fo and compensatory responses against fo shifts in the feedback, and demonstrated that larger spontaneous fluctuation correlates with greater compensation in vocal fo. This correlation was found in slow components (≤ 5 Hz) of the spontaneous fluctuation but not in fast components (between 6 and 30 Hz), and the slow one was amplified during the compensatory responses. Furthermore, the compensatory ratio was reduced when large fo shifts were applied to the auditory feedback, as if reflecting the range of motor exploration. All these findings consistently suggest the functional role of motor variability in the exploration of better vocal outcomes.