The present study describes a technique for analysis of vocal responses to auditory feedback pitch perturbations in which individual trials are first sorted according to response direction and then separately averaged in groups of upward or downward responses. In experiment 1, the stimulus direction was predictable (all upward) but magnitude was randomized between þ100, þ200, or þ500 cents (unpredictable). Results showed that pitch-shift stimuli (PSS) of þ100 and þ200 cents elicited significantly larger opposing (compensatory) responses than þ500 cent stimuli, but no such effect was observed for "following" responses. In experiment 2, subjects were tested in three blocks of trials where for the first two, PSS magnitude and direction were predictable (block 1þ100 and block 2-100 cents), and in block 3, the magnitude was predictable (6100 cents) but direction was randomized (upward or downward). Results showed there were slightly more opposing than following responses for predictable PSS direction, but randomized directions led to significantly more opposing than following responses. Results suggest that predictability of stimulus direction and magnitude can modulate vocal responses to feedback pitch perturbations. The function and causes of the opposing and following responses are unknown, but there may be two different neural mechanisms involved in their production.
Accurate vocal production relies on several factors including sensory feedback and the ability to predict future challenges to the control processes. Repetitive patterns of perturbations in sensory feedback by themselves elicit implicit expectations in the vocal control system regarding the timing, quality and direction of perturbations. In the present study, the predictability of voice pitch-shifted auditory feedback was experimentally manipulated. A block of trials where all pitch-shift stimuli were upward, and therefore predictable was contrasted against an unpredictable block of trials in which the stimulus direction was randomized between upward and downward pitch-shifts. It was found that predictable perturbations in voice auditory feedback led to a reduction in the proportion of compensatory vocal responses, which might be indicative of a reduction in vocal control. The predictable perturbations also led to a reduction in the magnitude of the N1 component of cortical Event Related Potentials (ERP) that was associated with the reflexive compensations to the perturbations. We hypothesize that formation of expectancy in our study is accompanied by involuntary allocation of attentional resources occurring as a result of habituation or learning, that in turn trigger limited and controlled exploration-related motor variability in the vocal control system.
Models of neural mechanisms of voice control propose that voice auditory feedback is compared with an internal representation of the predicted voice output that is based on efference copies of motor commands. In addition, sensory memory from previous productions can also help the brain form predictions about incoming auditory feedback during vocal production. Previous studies have shown that the auditory evoked neural responses are maximally dampened in conditions where the incoming feedback closely matches the internal predictions. The present study aimed to determine if the predictability of stimulus direction influences the ERP responses to pitch shifts in voice auditory feedback. Subjects were tested with all upward stimuli, all downward stimuli, or randomized stimulus directions in separate blocks of trials. The N100 ERP response had a greater amplitude in conditions where the stimulus direction was randomized (unpredictable) compared to constant direction stimulus conditions (predictable), regardless of whether the stimuli were upwards or downwards. These findings suggest that auditory neural responses to predictable stimulus direction are dampened compared with unpredictable stimuli possibly because the predictable stimuli are suppressed by the internal predictions formed by the efference copy and/or sensory memory mechanisms.
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