TOPOGRAPHY OF COVERT ATTENTION IN HUMAN SUPERIOR COLLICULUS

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Introduction: 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., M¿ller et al, 2004; Cavenaugh, et al., 2006). Previous fMRI experiments have shown a rough retinotopic organization in human SC corresponding to direct visual stimulus (Shneider & Kastner, 2005). 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. Methods: We measured the retinotopic organization of SC to direct visual stimulation using a wedge of flickering dots (eccentricity 19¿, azimuthal width 144¿) that slowly rotated (24-s period) around the fixation mark. To measure the retinotopy of covert attention, we used a full-field flickering (4-Hz) grating (1 cpd) stimulus at 60% contrast. Subjects were cued to perform a difficult contrast decrement-discrimination task within a region (eccentricity 19¿, azimuthal width 90¿) that slowly rotated (24-s period) in azimuth around fixation. High-resolution fMRI (1.3 mm cubic voxels) data were acquired (3 s/volume) on 8 slices within SC (orientation perpendicular to the neuraxis) using a 3 shot spiral sequence and a TE of 45 ms. 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 the surface of the SC could be computationally flattened, and to permit computation of a depth map for laminar analysis