Oxyopia Abstract

June 19, 2009
Friday, 4:00 PM
489 Minor Hall

David Ress, PhD
Associate Professor and Director of Research, Department of Neurobiology, Center for Perceptual Systems, University of Texas at Austin
Host: Michael Silver

Title

Topography of visual stimulation and attention in human superior colliculus

Abstract

Neural microstimulation experiments in the superior colliculus (SC) of primates enhanced performance in a spatially selective manner while they maintained fixation, suggesting a role for covert visual attention in SC (e.g., Muller et al., 2004; Cavanaugh, et al., 2006). We performed experiments to determine if retinotopically organized signals corresponding to covert visual attention were present in human SC. At the same time, we developed methods to examine the laminar distribution of hemodynamic activity in the depth of the SC. We measured the retinotopic organization of SC to visual stimulation using a 90° wedge of moving dots (eccentricity 2-10°, speed 1.5 °/s) that slowly rotated (24-s period) around fixation. Subjects were cued to perform a difficult speed-discrimination task within the wedge. To measure the retinotopy of covert attention, we used a full-field of moving dots that had no systematic variation throughout. However, subjects were now cued to perform the speed-discrimination task on a 90° wedge-shaped region, and the cued region rotated slowly around fixation. High-resolution fMRI data (1.2 mm isometric voxels) were acquired (3 s/volume) on 8 slices covering SC. These data were then aligned to a high-resolution (0.6-mm voxels) T1-weighted reference volume. The SC was segmented in this volume so that data could be averaged through the depth of SC, and to permit laminar analysis. We obtained retinotopic maps of both direct visual stimulation and covert attention in SC. These maps were in good registration with each other, and showed a similar pattern to those observed in the monkey. Laminar analysis indicates that responses for both kinds of stimuli extend in a similar fashion through both superficial and intermediate layers of SC. These observations have shown good session-to-session reliability, with similar results in three subjects so far. We conclude that signals corresponding to covert visual attention are present in both superficial and intermediate layers of SC. These signals are retinotopically organized in register with the signals produced by direct visual stimulation.

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