Doctor of Philosophy (PhD)
Faculty of Science
Jeffrey A. Jones
Whether we are learning how to play a new instrument, song, or even learn a second language, the nervous system relies on various forms of sensory feedback to establish task-specific sensorimotor representations. Over time, the plasticity of the nervous system permits neural reorganization and the formation of an ‘internal model’. It has been suggested that internal models represent neural maps of skilled movement that store the relationship between the motor commands, environment and sensory feedback responsible for their production. These internal representations are often investigated by altering a particular aspect of the sensory feedback associated with a given task. Arguably the most influential contribution to F0 control during speech production is from auditory feedback. For instance, using the frequency-altered feedback (FAF) paradigm (where the pitch of participants’ voices is shifted in frequency) results show that participants will compensate by adjusting their vocal productions in the opposite direction of the perturbation. Moreover, aftereffects are often observed (subsequent responses err in the direction of compensation) when auditory feedback is unexpectedly returned to normal following a series of fixed pitch-shift manipulations. Thus, the evidence from studies on speech production suggests that an internal model represents voice fundamental frequency (F0). However, there is little evidence to suggest that this is also true for vocal control while singing. As a result, the purpose of this dissertation is to examine whether acoustic-motor representations (internal models) control voice F0.
Chapter 1 examined trained singers and untrained singers (nonsingers) sensitivity to subtle manipulations in auditory feedback. Participants produced the consonant-vowel /ta/ while receiving auditory feedback that was shifted up and down in frequency. Results showed that singers and nonsingers compensated to a similar degree when presented with frequency-altered feedback (FAF), however, singers’ F0 values were consistently closer to the intended pitch target. Moreover, singers initiated their compensatory responses when auditory feedback was shifted up or down 6 cents or more, compared to nonsingers who began compensating when feedback was shifted up 26 cents and down 22 cents. Additionally, examination of the first 50 ms of vocalization indicated that participants commenced subsequent vocal utterances, during FAF, near the F0 value on previous shift trials. Interestingly, nonsingers commenced F0 productions below the pitch target and increased their F0 until they matched the note. We conclude that singers and nonsingers rely on an internal model to regulate voice F0, but singers’ models appear to be more sensitive in response to subtle discrepancies in auditory feedback.
Evidence from arm reaching studies suggests that the motor system can acquire multiple internal models, which has been argued to allow an individual to adapt to different perturbations in diverse contexts. In Chapter 2 we show that trained singers can rapidly acquire multiple internal models that accommodate different perturbations to ongoing auditory feedback. Participants heard three musical notes and reproduced them in succession. Adaptation was observed immediately following vocal onset when participants were gradually exposed to altered feedback. Aftereffects were target specific and did not influence vocal productions on subsequent trials. Interestingly, the target notes in Chapter 2 could have served as a contextual cue. Indeed, when the target notes could no longer serve as a contextual cue we observed evidence for trial-by trial adaptation. These findings indicate that the brain is exceptionally sensitive to deviations between auditory feedback and the predicted consequence of a motor command during vocalization. Moreover, these results indicate that when contextually cued the vocal control system may maintain multiple internal models that are capable of independent modification during different tasks or environments.
Auditory feedback has been shown to be influential in the maintenance and control of voice F0. The purpose of Chapter 3 was to address whether task instructions could influence the compensatory responding and sensorimotor adaptation that has been previously found when participants are presented with a series of FAF trials. Trained singers and nonsingers were informed that their auditory feedback would be manipulated in pitch while they sang the target vowel [/ah/] (as in ‘pop’). Participants were instructed to either ‘compensate’ or ‘ignore’ the changes in auditory feedback. Auditory feedback manipulations persisted for the entire vocal production and were either gradually presented (‘ramp’) in -2 cent increments down to -100 cents (1 semitone) or were suddenly (‘constant’) shifted down by 1 semitone. Results indicated that singers and nonsingers could not suppress the compensatory responses to FAF, nor could they reduce the sensorimotor adaptation observed on both ramp and constant FAF trials. Compared to previous research these data suggest that musical training is effective in suppressing compensatory responses only when FAF occurs between 500-2500 ms following vocal onset. Moreover, our data suggest that compensation and adaptation is automatic and is influenced little by conscious control.
Indeed, regardless of whether we manipulate auditory feedback in small or large increments, for a single music note or multiple notes, or whether we modify the task instructions (ignore or compensate), we have shown and that the result is a change in the sensorimotor representations (internal models) underlying voice F0 control while singing. One goal has been to demonstrate the plasticity of the nervous system by examining how changes in singers and non-singers auditory feedback can alter target specific internal representations. Indeed, producing multiple target notes while singing was shown to require participants to employ unique motor commands for each target. Although the human voice has the potential to be initiated at unpredictable frequencies during vocal onset, we found that trained singers consistently produced vocal pitch at frequencies near the desired target, even in the presence of FAF. Overall, it appears that compensation and adaptation to FAF are automatic, influenced little by ‘conscious’ control, and are uniquely associated with the motor commands for specific musical targets.
Keough, Dwayne Nicholas, "The Role of Auditory Feedback on the Control of Voice Fundamental Frequency (F0) while Singing" (2010). Theses and Dissertations (Comprehensive). 1090.