Stimulus representations in neural priority maps are equally enhanced by attention independent of the number of attended locations

When spatial attention is distributed across multiple visual field locations, performance in visual tasks is often impaired. This bottleneck is evident in behavioral and neural studies, especially when using complex stimuli, and is echoed in behavioral and neural measurements of visual working memory. Some studies have suggested that this is because distributing attention results in lower attentional enhancement in visual cortex compared to focused attention (e.g., McMains & Somers, 2005), while others support a bottleneck at a post-perceptual decision-making stage (e.g., White et al, 2017; Harrison et al., 2022; Chen & Seidemann, 2012). To characterize how stimulus representations in neural priority maps reflect these constraints and discriminate between these models, we scanned participants with fMRI while they performed a selective attention task in which they were cued on each trial to discriminate a target that appeared at the fixation point, 1 cued location, or 2 cued locations. Using a spatial inverted encoding model, we reconstructed images of priority maps from retinotopic brain regions which contained representations of each stimulus. Comparing map activation between focal attention and fixation conditions replicated the canonical finding that attention to one stimulus caused map activation enhancement at the attended stimulus location. Next, we examined map activation when both stimuli were attended. Strikingly, both stimulus representations were enhanced when attended, with an equivalent increase in map activation as observed with focused attention directed to a single stimulus. This pattern was consistent across retinotopic cortex, with no evidence for graded attentional enhancement in any region. Thus, our results are consistent with a model whereby fMRI signals are enhanced when a stimulus is attended, and the degree of enhancement does not wane as the number of attended stimuli increases. Such a ‘relevance’ marker may be used to identify neural populations for selective readout from relevant locations during decision-making.