The dynamic effect of saccades in the visual cortex: evidence from fMRI, sTMS and EEG studies

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While much has been learned about the role of frontal and parietal cortices in saccades and in visuospatial attention, the role of the visual cortex remains an enigma. Traditionally, the striate cortex was considered a way station to transmit static information about the features and retinotopical loci of elementary visual stimuli. Recent multielectrode studies in the monkey, however, established visual context dependent dynamic properties of the striate cortex, beyond retinotopy. Our studies evaluate eye movement related modulation of the visual cortex. We have studied the frontal, parietal and occipital cortex using functional MRI (fMRI) in normal observers. Our results show that in addition to the frontal cortex, the striate cortex and the precuneus are active in association with quick, darting eye movements called saccades (S) even in the dark without visual input. Single pulse Transcranial Magnetic Stimulation (sTMS) reveals that the conscious perception of a visual cue is a prerequisite to execute a cued saccade, or antisaccade. Wavelet analysis of the perisaccadic EEG reveals a burst of high frequency components of the occipital cortex, both in light and in dark. Apparently, there is a non-visual stimulus dependent saccadic role of the striate cortex. Even though saccades are executed constantly by awake humans, we are not aware of a shifting visual scene by saccades. We propose that the visual maintenance of stability in association with saccades and blinks relies on dynamic modulation within the striate cortex.