Changes in the speed of visual processing between foveola and perifovea: a combined behavioral and EEG investigation

Despite the vivid experience of a homogeneous vision, our visual system is inherently endowed with highly inhomogeneous structures. While the temporal profile of visual responses is known to vary with eccentricity, its link with the speed of visual processing and its neurophysiological underpinnings is still debated. By Simultaneously acquiring gaze position – by means of a digital dual-Purkinje eye tracker – and EEG activity, we investigated how foveal and perifoveal stimulation affects reaction times (RTs) and visual evoked potentials (VEPs). Subjects (n=15) discriminated the position and orientation of a U-letter, with the aperture oriented upward or downward. Stimuli were flashed for 50 ms either in the foveola (0.33°) or perifovea (6.5°), either to the right or left of the fixation. Stimuli in the perifovea were scaled according to the cortical magnification factor (stimulus size: 0.33° and 1.25° for foveola and perifovea condition, respectively). When stimuli were matched for sensitivity and the cortical area of stimulation was comparable, RTs in the perifovea condition were faster than in the foveola (14.6±4.8 ms, p<0.01). The VEP analysis revealed an effect of the same order of magnitude for the N1 response – a parieto-occipital visual related component reflecting discriminative processing and influenced by spatial attention – peaking earlier in time (14.0±5.0 ms, p<0.05) when the stimulus was presented in the perifovea, compared to the foveola. Notably, interhemispheric transfer time did not vary with eccentricity, suggesting that the observed latency differences were not due to differences in interareal transmission time, but reflected differences in local visual processing. Overall, our findings support the hypothesis of uneven speeds of visual processing across eccentricities, with faster processing and shorter response latencies in perifovea than in the foveola.