Abstract
Adaptive behavior often requires monitoring multiple spatial locations at once. Research has shown that attention can be divided concurrently to noncontiguous regions of space, though this ability is strongly constrained by the organization of the visual cortex: selecting two locations across the visual half-fields is easier than dividing attention within a single visual half-field (Alvarez & Cavanagh, 2005). How does spatial attention operate within a single hemifield? We examined the attentional profile when participants were asked to select two noncontiguous locations within a visual quadrant, and compared this to when participants attended to one broad spatial region that encompassed the same locations as the divided attention condition. Using EEG, we measured early attentional enhancement of visual processing at each of these locations using the steady-state visual evoked potential (SSVEP). Participants (N=31) were cued to continuously attend to one location (single focus), two noncontiguous locations (divided focus), or three locations (broad focus) to perform a visual discrimination task. Each of these locations flickered at a distinct frequency to elicit separable SSVEP signals. We computed the signal-to-noise ratio (SNR) for each flicker frequency from their frequency power measured at occipital electrodes. We found clear attentional modulations when participants attended a single location, replicating previous work. Critically, we did not find differences in attentional enhancement between the divided and broad attention conditions, suggesting that when participants were instructed to split attention within a visual quadrant, they were unable to suppress the intermediate, uncued location, but instead enlarged their attentional focus just as in the broad attention condition. These results add to evidence that attention cannot be effectively divided within a single visual half-field, and is instead tuned broadly across relevant and irrelevant locations.