I know where you are secretly attending! The topography of human visual attention revealed with fMRI

Previous studies have described the topography of attention-related activation in retinotopic visual cortex for an attended target at one or a few locations within the subject's field of view. However a complete description for all locations in the visual field is lacking. In this human fMRI study, we describe the complete topography of attention-related cortical activation throughout the central 28 degrees of the visual field and compare it with previous models. We cataloged separate fMRI-based maps of attentional topography in medial occipital visual cortex when subjects covertly attended to each target location in an array of three concentric rings of six targets each. We combined the attentional maps for each of the 18 target locations for each subject into a unique composite display to identify common principles of attentional organization for different target locations. Attentional activation was universally highest at the attended target but spread to other segments in a manner depending on eccentricity and/or target size. For targets scaled in size with eccentricity, the attentional effects spread circumferentially as a gradient whose full width at half maximum was consistently 1.5 times the target width at each eccentricity. For targets in the inner (1.8-8.5 degrees) and middle (8.5-15.6 degrees) rings, attention also tended to spread outward, radially to other segments. For targets in the outer ring (15.6-28 degrees), the radial spread of attention was primarily inward to targets of the middle ring. We propose an "attentional landscape" model that is more complex than a "spotlight" or simple "gradient" model but includes aspects of both. Finally, we asked subjects to secretly attend to one of the 18 targets without informing the investigator. We then show that it is possible to determine the target of attentional scrutiny from the pattern of brain activation alone with 100% accuracy. Together, these results provide a comprehensive, quantitative and behaviorally relevant account of the macroscopic cortical topography of visuospatial attention. We also show the pattern of attentional enhancement as it would appear distributed within the observer's field of view thereby permitting direct observation of a neurophysiological correlate of a purely mental phenomenon, the "window of attention".