Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Neuroscience

Supervisor

Goodale, Mel A.

2nd Supervisor

Corneil, Brian D.

Co-Supervisor

Abstract

To respond swiftly to urgent visual stimuli, the brain must balance speed and accuracy. A key part of this process is the express visuomotor response (EVR), a rapid muscle reflex occurring 80–120 ms after stimulus onset. This thesis explores the EVR’s role in visually-guided reaching in humans and offers converging evidence that it is initiated by the superior colliculus (SC).

In my first study (Chapter 2), I examined whether the EVR shares the SC’s preference for evolutionarily relevant stimuli, particularly faces. I found that the EVR is uniquely modulated within tasks involving face stimuli despite occurring earlier than face detection in the occipitotemporal cortex. In Chapter 3, I found that the EVR contributes to the initiation of short-latency arm movements, regardless of whether they are made from rest or to correct an ongoing movement. This result challenges long-held assumptions that ongoing reaching movements are corrected by a distinct neural circuit, suggesting instead a single nested neural system. In Chapter 4, I used transcranial magnetic stimulation to investigate claims that the posterior parietal cortex (PPC) is the main driver of online movement correction. I found that PPC stimulation had no impact on the muscle recruitment driving online correction, including the EVR. Instead, the results suggest that prior disruptions from PPC stimulation may reflect inadvertent activation of the corticospinal tract, causing disruptive motor evoked potentials.

Together, these three studies support the hypothesis that the SC mediates the EVR to initiate short-latency visually-guided movements.

Summary for Lay Audience

When something suddenly appears in our field of vision, like a ball flying toward us, our brain has to react quickly. This thesis explores one key part of this process: a fast muscle reflex called the express visuomotor response (EVR).

In the first study (Chapter 2), I looked at whether this fast muscle response is especially sensitive to faces. I found that the EVR reacted strongly to face-like images, and did so even before the part of the brain normally responsible for recognizing faces had time to respond. This result is consistent with the hypothesis that the superior colliculus in the brainstem is responsible for producing the EVR. Next, I examined whether the EVR explains why we are quicker at correcting movements that are already in progress. The results showed that the EVR is part of both new and corrected movements, meaning our brain uses the same system in both situations. The difference in speed comes not from a different pathway, but from the fact that the arm is already moving. Finally, I used brain stimulation to test whether another brain area, known as the posterior parietal cortex (PPC), is the main driver of these rapid adjustments. I found that stimulating the PPC did not affect the EVR. Instead, the effects seen in earlier studies may have resulted from accidentally stimulating a nearby brain region that directly controls movement.

Overall, my research shows that a deep brain structure called the superior colliculus may play a key role in helping us move quickly in response to what we see. It also highlights the importance of using careful methods when studying how the brain controls movement.

Creative Commons License

Creative Commons Attribution-No Derivative Works 4.0 License
This work is licensed under a Creative Commons Attribution-No Derivative Works 4.0 License.

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