Abstract
Background and Goal. Covert spatial attention modulates behavioral and neural sensitivity. fMRI studies have reported that endogenous (voluntary) attention increases BOLD amplitude, shifts population receptive fields (pRFs) and alters pRF sizes. Here, we used a combined fMRI/psychophysics experiment to investigate these effects concurrently, at polar angle locations that typically show discriminability differences. Methods. In every trial, a precue directed participants to either attend to one of four isoeccentric (6°) locations on the cardinal meridians, or to distribute attention across four locations. 300 ms after the precue, a stimulus for mapping pRFs (a contrast pattern masked by a bar aperture) was presented for 1 or 2 s. Shortly after, four small, low-contrast Gabor patches appeared and participants discriminated the orientation of the target Gabor indicated by a response cue. PRF models were solved for voxels in V1-hV4 and V3A/B. Results. Focal attention improved behavioral performance at the cued location and decreased performance at the other locations to the same extent across the four locations. In all visual field maps, BOLD amplitude increased for voxels with pRF centers near the attended location, and decreased at unattended locations. The amplitude changes were independent of mapping stimulus location, reflecting a baseline shift rather than a multiplicative gain. The magnitude and spatial spread of amplitude changes were similar across locations and maps. pRF centers shifted slightly towards the cued location and there was a trend for smaller peripheral pRF sizes in the focal than the distributed attention condition. These two effects increased across the visual hierarchy. Conclusions. We observed a pronounced attention-related baseline shift in BOLD response, accompanied by small but detectable changes in properties of visual field maps. Our results suggest that endogenous spatial attention, prior to target appearance, primarily affects visual cortex by a retinotopic change in mean neural activity.