Abstract
Abstract: :
Purpose: To investigate whether the presence of a briefly flashed intrasaccadic Gabor patch influences the modulation of the perisaccadic gamma range EEG. Backgorund: In previous studies we provided evidence that gamma power of the human EEG is increasing during the voluntary saccade peaking just before the new fixation is achieved. Gamma modulation is evident in the light and in the blindfolded subjects, suggesting that intrasaccadic gamma is not the direct correlate of the actual visual input. To test this hypothesis we evaluated the possible effect of a intrasaccadic visual stimulus on gamma power modulation. Methods: EEG was recorded using 26 electrodes. Eye movements were recorded with EOG and infrared eye position signals (ISCAN). Viewing a high resolution monitor (320), subjects (n=6) were instructed to execute voluntary saccades when the central fixation point was replaced with a vertically oriented Gabor patch (1cpd, C=0.15, size: 10o,SD=1.1). The saccadic target was an identical patch 13o from midline presented for 2000 ms. Once the saccade was initiated, with a delay of 15+4 ms, an identical patch with variable contrast was presented for 20 ms in the saccadic path. Detection threshold was established using MCS. Stimulus contrast ranged from 0 to 90%. Trials with steps, blinks or when the stimulus failed to appear in the middle of the saccade were excluded (about 1 in 10). For each accepted saccade, single trial perisaccadic EEG was analyzed in 6 time windows (preceding, during, and following) each saccade. Gamma was analyzed for the contrast level when the subject detected it at the level of 66.6 %.To quantify gamma power Continuous Wavelet Transform was followed by Hilbert transform. General linear model was used for statistical evaluation. Results: The contrast threshold for detecting the intrasaccadic Gabor patch ranged from 30% to 75% for different subjects. Gamma power analysis was performed separately for correct and false positives and negatives and for each of the individual time segments (pre–, intra– and postsaccadic window). There was no difference in perisaccadic gamma power irrespective to the presence of the intrasaccadic visual stimulus or its subjective detection. Conclusions: There is no change in the perisaccadic (pre–, intra– or postsaccadic) gamma power by the brief presentation of a near threshold visual stimulus, whether it is detected or not. The lack of a relationship between the presence and "subjective" detectability of a brief change and the shape of perisaccadic modulation suggests that intrasaccadic gamma may represent an internal monitoring signal, possibly as a result of corollary discharge.
Keywords: eye movements: saccades and pursuits • electrophysiology: non-clinical • vision and action