Doctor of Philosophy
Dr. Melvyn A. Goodale
The degree to which something stands out against the background of its environment communicates important information. The phenomenon of camouflage is a testament of the degree to which visual salience and probability of survival tend to overlap. Salient stimuli often elicit fast, reflexive movements in order to catch prey or avoid a predator. The overarching goal of the work presented in this thesis is to investigate how the physical salience of visual stimuli influence the programming and execution of reaching movements. I approached this question by recording kinematics and muscle responses during reaching movements. Broadly, this thesis investigates the effect of the physical salience of targets on the magnitude and latency of involuntary, spatially tuned muscle responses toward those targets.
In Chapters 2 and 3, subjects reached toward an array of potential targets on a touchscreen. The final target was cued only after the reaching movement was initiated. From trial to trial, targets differed in their numerosity (i.e., how many on the left versus the right) and in their salience (i.e., their relative contrast with the background). Different amounts of delay were introduced between the appearance of the targets and the cue to move. The results from these two studies demonstrate that the physical salience of (i.e., the luminance contrast differences between) targets influences the timing and the magnitude of involuntary deviations toward the most salient target(s) during reaching movements. At the level of individual subjects, the degree to which someone involuntarily reached toward the salient stimulus was predicted by the relationship between processing speeds for the different target contrasts.
In Chapter 4, subjects reached toward individual targets that varied in luminance contrast. Muscle activity in the right pectoralis major was recorded with intramuscular electrodes. Consistent with past studies, there was a consistent muscle response that was time-locked to the appearance of the target, regardless of the reaction time for the ensuing reaching movement. The same processing speed differences and magnitude modulations observed in Chapters 2 and 3 (due to different luminance contrast values of the targets) were observed in these stimulus-locked muscle responses. Further testing revealed that stimulus-locked responses were elicited by a delayed, spatially uninformative go-cue.
Wood, Daniel K., "Reaching for the light: The prioritization of conspicuous visual stimuli for reflexive target-directed reaching" (2013). Electronic Thesis and Dissertation Repository. 1759.