The value of simultaneous EOG and EEG recording for measurement of saccade-related brain activity
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Introduction. Both to elucidate the cortical mechanism involved in the control of eye movements, and because eye movements cause a large amount of artifacts during electroencephalographic (EEG) recording, it is important to investigate the effects of eye movements on the study of brain activities. The aim of this project is to examine both EEG and electrooculography (EOG) signals during image exploration of eye movements.
Methods. There are two experiments, one to calibrate the EOG signals relative to the amplitude and direction of the EOG, and the other to analyze the properties of the EEG signals as the eyes fixated on a sequence of image features. The eye movement target of Experiment 1 consisted of stimulus squares appearing in one of 25 positions (a 5-by-5 array subtending 34° in each side). Each stimulus was presented for 1.5 s, in random order. The fixation targets of Experiment 2 consisted of 18 white circles appearing in a window (30° by 23°) filled with a) a blank field, b) a white noise pattern, c) a natural image of a face, or d) only the facial features within the target circles with a noise pattern outside them. The targets were each presented for 2s in random order. Participants were required to saccade onto the presented stimulus and maintain fixation until the next stimulus appeared. EEG and EOG were measured simultaneously with a high-density EGI electrode net at a 500 Hz sampling rate. The resultant EOG signals were analyzed for linearity in tracking the fixation targets. The EEG signals were pre-processed and analyzed by Principal Component Analysis (PCA) to reveal visual saccade-related potentials.
Results. The analysis showed a primary EOG component with a saccade-like waveform is measured with anti-symmetry across the facial electrodes for leftward and rightward movements and symmetry for upward and downward movements. These primary components scaled in amplitude and direction with the saccade extent from 5-24°. In the EEG signals, a slow anticipatory component was seen before the saccades. A transient Frontal Eye Field component accompanied the saccades. A posterior component with a single negative peak was found around the time of the saccade, interpretable as representing the neural substrate for saccadic suppression. Occipital components peaking 100-200 ms after the saccade were interpretable as the cortical responses to the targets.
Conclusion. The results show that EOG signals allow accurate analysis of the amplitude and directions of the saccades with a temporal resolution of 500 Hz, which make the EOG from the electrode net a convenient means of measuring eye movements. The findings suggest that the technique allows the measurement of brain responses responsible for, and elicited by, saccadic eye movements. These findings also show that local feature viewing of facial images elicits a range of saccadic control and visual cortical response components.