| The eyes are constantly moving, even during fixation of gaze. It is known that fixational eye movements help prevent fading of the stimulus and improve fine spatial discrimination. However, the specific functions of microsaccades in high-acuity visual tasks have remained topics of ongoing debates. In this thesis, the roles of microsaccades and gaze drifts in high-acuity tasks were examined by means of psychophysical experiments and computational modeling.;Two psychophysical experiments were designed to understand possible contributions of fixational eye movements in high-acuity tasks. In the first experiment, observers judged whether the top portion of a vernier-like stimulus was to the left or to the right of the bottom portion. Stimuli were embedded in a noise field and were either viewed normally or during stabilization of retinal images. Results showed that retinal image motions normally caused by gaze drifts facilitated task performance.;In the second experiment, subjects performed a virtual needle-threading task. Eye movements were recorded as subjects watched a computer display and used a joypad to align a "thread" with the position of a gap in a "needle" toward which the thread was being moved. For comparison, eye movements were also recorded in fixation and free-viewing tasks. The characteristics of fixational eye movements were task-dependent. As the thread approached the needle, microsaccades were used to move back and forth between the two objects. Adjustments of the thread position often occurred following microsaccades. Microsaccades were frequent and small during fixation, but rare and larger during free-viewing.;Modeling studies examined the effect of fixational eye movements on the statistics of neural activity in a model of the retina. Neuronal models simulated the evoked responses of parvocellular ganglion cells in the macaque retina. Fixational eye movements synchronized the responses of arrays of ganglion cells in a way that predicted the perceptual reports of subjects in our experiments.;The findings from the psychophysical experiments reveal the importance of both microsaccades and drifts in high-acuity tasks. The results from the modeling study suggest possible neural mechanism by which observed perceptual improvements might occur. |
