An electrophysiological study of active vision
Spera, Karen M.
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In active vision, information is obtained at brief fixations separated by saccadic eye movements. Nonetheless, our visual experience is not of discrete snapshots, but of a complete and integrated percept. In this dissertation, scalp recorded saccade-related potentials (SRPs) were recorded in adults in order to examine the dynamic course of activation associated with active vision. The spatiotemporal course of transcranial current flow at the scalp was examined with scalp current density analysis. Two experiments were conducted. In Experiment I, SRPs associated with self-paced saccades of 5° to 50° were recorded to determine the saccade size for the optimal temporal dispersion of saccade and fixation related activity. This activity was found to be relatively independent from one another in SRPs associated with saccades of 30° and greater. A series of four saccade related SRP components were identified between 70 ms and 130 ms following saccade onset, while a series of three fixation related components were identified between 100 ms and 225 ms following saccade offset.;In Experiment II, the content of the pre- and post-saccade fields was varied in order to examine saccade and fixation related activity in relative isolation. Saccade related activation consisted of a mid-occipital current maximum approximately 110 ms after saccade onset, which was followed (75 ms later) by an occipital-parietal focus of maximum current. It is suggested that this activation reflects both efferent and afferent activity, which functions in visual suppression and contributes to visual stability during eye movements. Activation associated with fixation, focused over lateral occipital regions 100 ms after saccade completion, was suggested to reflect the initial representation of visual information at fixation. A focus of current maximum over the parietal region followed 50 ms later and was hypothesized to reflect an internal spatial reference, which contributes to perceptual stability, as well as the anticipation of the subsequent saccade. A second lateral occipital current maximum was observed 210 ms following saccade completion, It was suggested that this activity might underlie the active maintenance of fixation. The findings of this dissertation demonstrate that SRPs can provide much needed evidence concerning the neural mechanisms that underlie our visual experience.
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