The speed of visual discrimination differs between foveola and perifovea: a combined EEG and behavioral investigation

: Despite the vivid experience of homogeneous vision, our visual system is inherently endowed with highly inhomogeneous structures. Although the temporal characteristics of visual responses vary with eccentricity, the connection between this variation, the speed of visual processing, and its underlying neurophysiological mechanisms remains a topic of debate. Here, we performed simultaneous recordings of high-precision gaze positions and EEG activity to investigate how foveal and perifoveal stimulation impact reaction times (RTs) and visual evoked potentials (VEPs). Volunteers discriminated the position and orientation of a U-shaped figure with the aperture facing either upward or downward. Stimuli were presented briefly (50 ms) either in the foveola (0.33◦) or perifovea (6.5◦), to the right or left of the fixation point. Stimulus size in the perifovea condition was adjusted according to the cortical magnification factor (stimulus size: 0.2◦ and 0.75◦ for the foveola and perifovea condition, respectively). When stimuli were equated for sensitivity and cortical area of stimulation, we observed faster RTs in the perifovea condition (16.8±4 ms) compared to the foveola. The analysis of the VEP revealed a similar effect for the N1 response (11.0±4 ms), a parieto-occipital component associated with discriminative processing and influenced by spatial attention. Overall, our findings suggest that visual discrimination speeds vary across eccentricities, with faster processing and shorter latency of early visual responses in the perifovea compared to the foveola.Significance Statement This study investigates how visual processing speeds vary with eccentricity. By recording high-precision gaze positions and EEG activity, we asked volunteers to discriminate the orientation of a U-shaped figure briefly presented foveally or perifoveally. We found that visual stimuli presented perifoveally led to faster reaction times and shorter visual evoked potentials compared to a foveal presentation. In particular, reaction times for perifoveal stimuli were faster by 16.8±4 ms, with similar effects observed in the N1 VEP component (11.0±4 ms). These findings suggest that early visual processing is quicker in the perifovea, highlighting the presence of effective temporal inhomogeneities across the retina.