Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Neuroscience

Collaborative Specialization

Machine Learning in Health and Biomedical Sciences

Supervisor

Pruszynski, J Andrew

2nd Supervisor

Gribble, Paul L

Co-Supervisor

Abstract

Contexts and intentions shape our movement goals, which evolve with new information, often requiring us to replan and modify our actions online. Although it is well-established that stretch reflexes are tuned to movement goals, the timescale at which they update when the goal changes during ongoing movement is largely unknown. Addressing this gap, the current study investigated how quickly and to what extent stretch reflexes are modulated when a new spatial target appeared as participants performed a default reaching action. Mechanical perturbations were applied to the arm randomly after one of four target preview delays (ranging from 0 ms to 200 ms), or not at all, serving as a cue to quickly reach toward the new spatial target. If no perturbation occurred, participants completed the default reach. The results indicate that stretch reflexes in the long-latency epoch (R3; 75-100 ms) are modulated ~120 ms after target onset, nearly matching voluntary response times and faster than previously reported in a postural analog of this study (Yang et al. in Exp Brain Res 211:231–242 2011). Interestingly, small but reliable modulation of reflexes in the short-latency epoch (R1; 20-45 ms) was also observed after both 90 ms and 200 ms of preview delay, suggesting that even the earliest stretch responses can be influenced by new goal information during movement. These findings suggest a close coupling between reflexive and voluntary control during dynamic motor tasks.

Summary for Lay Audience

We often make quick adjustments to our movements based on changing situations, like altering the direction of a tennis swing to meet an unexpected shot or reaching for a glass that has been nudged. Our brain relies on sensory information to seamlessly coordinate these adjustments on the fly while remaining prepared to react accordingly to sudden disturbances — like a bump to the arm. Rapid responses to such bodily impacts, known as stretch reflexes, play an important role in ensuring we achieve our intended movement goal. However, it has been unclear how quickly these reflexes can adjust to a change in our intended goal while we are in motion, for instance, when we plan to quickly redirect our reach to a new target. The present thesis addresses this question and shows that stretch reflexes adjust almost as quickly as our voluntary movements — within approximately 120 milliseconds after a change in the goal. Surprisingly, even the earliest period of the stretch reflex, which is usually considered to be stereotyped and less influenced by the goal, showed signs of adjustment when people were made to wait longer before changing their movement toward the new intended goal. These results suggest that the brain and the body at large are quickly prepared to react in such dynamic situations.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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