Frequency-dependent modulation of foveal contrast sensitivity by fine-scale exogenously triggered attention.

Exogenous attention is a rapid, involuntary mechanism that automatically reallocates processing resources toward salient stimuli. It enhances visual sensitivity in the vicinity of the salient stimulus, both in extrafoveal regions and within the high-acuity foveola. While the spatial frequencies (SFs) modulated by exogenous attention in extrafoveal vision are well characterized, it remains unknown how this mechanism operates within the foveola, which can resolve SFs up to 30 cycles per degree (CPD). Here, we examined which SFs were enhanced by fine-grained deployments of exogenous attention within this highest-acuity region of the visual field. Using high-precision eye-tracking to precisely localize gaze during attentional allocation, we found that exogenous attention at the foveal scale selectively enhances contrast sensitivity for low- to mid-range SFs (4-8 CPD), with no significant benefits for higher SFs (12-20 CPD). In contrast, attention-related benefits on asymptotic performance at the highest contrast were observed across a wide range of SFs. These results indicate that, despite the high-resolution capacity of the foveola, exogenous attention remains an inflexible mechanism that, even at this scale, selectively enhances contrast gain for lower SFs-mirroring its behavior in extrafoveal vision.