Temporal dynamics of foveal and peripheral visual discrimination during fixation

The ability to simultaneously monitor stimuli at the center of gaze and across the rest of the visual field during fixation is vital for survival. Here we examine how visual discrimination for concurrently monitored foveal and peripheral stimuli changes during the course of fixation. Two Gabors (8cpd, 2 degrees diam), were presented, one at the center of the display and another 8 degrees away. Participants (N=6) were instructed to saccade to the central stimulus, and after a variable delay (0-450ms), one Gabor briefly (50ms) changed orientation. Subjects reported the direction of the orientation change and its location. To prevent visual fading the peripheral Gabor was jittered throughout the presentation time. Stimulus contrast was adjusted separately for foveal and peripheral stimuli to yield ~65% of correct responses when the orientation change occurred between 150-300 ms after saccade landing. Trials with microsaccades were discarded. Our results show that in the initial 150ms of fixation, performance was higher for foveal vs. peripheral stimuli (0.67±0.07 vs 0.50±0.02, p = 0.002). However, this foveal advantage diminished over time, and eventually performance for foveal stimuli reached chance level after 300 ms. Conversely, performance steadily improved during fixation for peripheral stimuli. 300 ms after saccade landing performance was better for peripheral stimuli (0.50±0.09 vs 0.67±0.10, p = 0.003). Here we show that visual discrimination varies greatly even during short fixation periods. Early on during fixation visual discrimination is better for foveal stimuli whereas later on this pattern flips. As in most circumstances, fixation periods are shorter than 500ms, these results may reflect the natural tendency of the visuomotor system to start preparing for the next saccade after 300 ms of fixation. This study provides critical insights into the interplay between foveal and peripheral processing during fixation and advances our understanding of the temporal dynamics of visual perception.