Text

theses

ETD doctoral

Active vision involves fast eye movements (saccades) with brief inter-saccadic fixations. This presents two interesting problems. (1) During each saccade the moving eye creates motion signals on the retina, and yet we are unaware of this sweeping visual input. (2) Upon each brief fixation the visual system is input with a new scene, and tasked to quickly encode many stimulus features. Regarding the first problem, a decrease in contrast sensitivity during saccades (saccadic suppression) is thought to contribute to our lack of intra-saccadic perception. Chapters 2 & 3 of this thesis seek to further understand the neural mechanisms of saccadic suppression. Regarding the second problem, a post-saccadic change in neural activity is thought to specialize processing of newly fixated stimuli. Chapter 4 investigates the changes in visual response properties that occur after a saccade. Chapter 2 used a signal-detection model that describes the psychophysical phenomenon of saccadic suppression in computational terms. The model is built up of visual detectors in which gain, noise, and spatial uncertainty can be varied. Thus saccadic suppression is recast in terms that provide testable predictions of neural activities. We found that saccadic suppression is the result of reduced detector gain. Chapter 3 studied neural responses from permanently implanted multi-electrode arrays in V1 of macaques. Based on our Chapter 2 results, we looked specifically at how contrast responses change during saccades. We found saccadic gain reduction that begins before saccade onset, suggesting that V1 is a neural site of suppression, and that saccadic gain reduction is the result of a corollary discharge signal. Chapter 4 studied the effect of post-saccadic modulation on contrast response properties. Using model fitting and signal detection measures, we showed that post-saccadic modulations in V1 lead to an increased range of discriminable contrasts. We argue that this increased operating range gives a functional benefit when encountering newly fixated stimuli. Chapter 5 concludes by relating the saccadic gain reduction shown in Chapters 2 & 3 to the post-saccadic response changes shown in Chapter 4 – arguing that saccadic suppression can be viewed as part of a more general process to improve post-saccadic vision.

ETD_6724

doi:10.7282/T3WW7KPS

Ph.D.

Includes bibliographical references

by Jonathan Simon Guez

The author owns the copyright to this work.