During normal viewing, the projection of the visual scene on the retina is never stationary. Even during the brief periods of visual fixation, small eye movements combine with movements of the head and the body to keep the retinal image in motion. Fixational instability may be an important component of how information about a visual scene is acquired and represented in the brain. By affecting the temporal structure of neuronal responses, it may contribute to the encoding of visual information in a way that goes beyond simply refreshing neuronal responses to prevent the fading of unchanging stimuli. Modeling studies that simulated neuronal responses during oculomotor activity suggest specific mechanisms by which this may occur. This proposal combines visual psychophysics and neuronal modeling to examine the influence of fixational eye movements on the identification of stimuli presented for periods comparable to the brief durations of visual fixation. Psychophysical experiments with human subjects will compare identification performances in the presence and absence of retinal image motion and correlate the fine structure of eye movements to visual identification. Experiments will rely both on a traditional technique of retinal stabilization as well as on a new system of gaze-contingent display that will allow selective elimination of eye movements during the periods of fixation following saccades. Neuronal models will simulate the responses of neurons in the retina, LGN and V1. For every experimental trial, a simulation will replicate neuronal activity during the sequence of eye movements performed by the subject. Subject performance and the information about the stimulus transmitted by neuronal firing will be compared on a trial-by-trial basis. A number of visual disorders manifest abnormal fixational eye movements. In addition to advance our basic understanding of visual perception, a comprehension of the functional implications of fixational instability may lead to treatment of the visual impairments commonly associated with such conditions. ? ? ?
