Saccadic Suppression On Color And Luminance

Objective: Vision is one of the most important sensations of human.Nearly 80%of the information we get from the outside world is from vision.When viewing the world,our eyes make several saccadic eye movements every second to direct the high-resolution fovea of the retina toward objects of interest in the environment.Despite the rapid changes in visual inputs,the brain maintains a stable visual representation through saccadic suppression,which is a temporary decrease in visual sensitivity during saccades.The neural mechanism underlying saccade suppression is still unclear.Particularly,it remains controversial on whether saccadic suppression occurs in the parvocellular pathway.In the present study,we recorded steady-state visual evoked potentials(SSVEPs)to investigate the dynamic changes of visual response during saccades,the effect of saccadic eye movements on visual processing in the magnocellular and parvocellular pathways.Methods: Color-defined(isoluminant)and luminance-defined stimuli were pattern-reverse flickering(square wave)at 7.5 Hz to evoke SSVEPs.The usage of color and luminance stimuli was to bias neural activities towards the parvocellular pathway and magnocellular pathway.In experiment 1,observers(N = 14)were required to make horizontal saccades at a rate of once every 1 to 2 seconds,back and forth between two spots which were separated by 12°.In Experiment 2,we measured the perceptual performance by asking the observer to discriminate a change of contrast in the stimuli.The change was fixed at a level that gave 82% accuracy during stable fixation measured in a pre-test.The observers(N = 11)then made horizontal saccades and,at the same time,discriminated a change in the flicker’s contrast level.Eye movement data and EEGs were simultaneously recorded in both two experiments.Results: In experiment 1,we recorded 53 saccades for each observer in every condition.On average,observers made a saccade every 1.70 seconds,with a mean saccade amplitude of 12.83°.In experiment 2,at the time of saccades,the accuracy dropped to 69.9% and 78.8% for luminance-defined and color-defined stimuli,respectively.Compared with the fixation period before saccade onset,the accuracy of luminance-defined stimuli was significantly decreased during saccades,and the decrease was significantly greater than that of the color-defined stimuli.In the EEGs,there were clear steady-state responses(i.e.,SSVEPs)at the stimulation frequency(7.5 Hz)and harmonic frequencies(e.g.,15,22.5 Hz)in both two experiments.Saccades did strongly modulate SSVEP responses;the reduction started from ～100ms before saccade onset and reached the lowest at around 150 ms.Compared with the fixation period before saccade onset,the reduction in SSVEPs was similar for luminance-defined and color-defined stimuli at the time of saccades.Conclusions: There is no significantly difference in the saccadic suppression between the SSVEPs induced by color and luminance.Since SSVEPs mainly originate from V1,we conclude that saccades suppress both the magnocellular and parvocellular pathways in early stage of visual cortex.However,the behavioral results showed that saccadic suppression on luminance was stronger than that on color.Saccades may,therefore,suppress the magnocellular pathway more than the parvocellular pathway in subsequent processing after V1.