Abstract
Functional magnetic resonance imaging (fMRI) provides a noninvasive method to study inferred neural responses to visual stimuli in humans. However, many experiments require the ability to localize this response to specific spatial locations in a visual scene rather than averaging the response over a large cortical area. In order to localize blood oxygen-level dependent (BOLD) responses on a fine spatial scale in primary visual cortex (V1), we used high resolution fMRI (1.5mm isotropic voxels). Four sets of target and flanking Gabor patches centered on the four corners of a square were presented at 3 degrees eccentricity (each patch subtended 0.75 degree of visual angle; target and flanker separated by 1 degree). Two separate block localizer scans with target alone and target with flanker conditions were used to define regions of interest (ROIs), and event-related scans were used to estimate the hemodynamic response to targets-alone, targets with flankers, and flankers-alone stimulus sets. The BOLD response to these stimuli was measured with a gradient echo (GE) sequence at 3 Tesla (3T) and a spin echo (SE) sequence at 7 Tesla (7T). At 3T, in an ROI defined by the target-alone localizer, the BOLD response to the flankers alone was just as strong as the BOLD response to the target stimuli. This indicates a failure of the 3T GE BOLD response to isolate stimuli whose cortical representation is separated by approximately 5mm on the cortex. With 7T SE BOLD, however, the stimuli were more clearly separable: the flanker-alone stimulus evoked a much weaker response in the target ROI than the target stimulus. SE techniques at 7T reduce large vein effects, thus allowing for improved spatial accuracy in BOLD signal experiments investigating neural response modulation at specific spatial locations in V1.
NIH-NPCS Graduate Student Training Fellowship, MIND Foundation, BTRR P41 RR008079, P30 NS057091.