Spatio-temporal representations during eye movements and their neuronal correlates
During fast ballistic eye movements, so-called saccades, our visual perception undergoes a range of distinct changes. Sensitivity to luminance contrasts is reduced (saccadic suppression) and the localization of stimuli can be shifted in the direction of a saccade or is compressed around the saccade...
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|Summary:||During fast ballistic eye movements, so-called saccades, our visual perception undergoes a range of distinct changes. Sensitivity to luminance contrasts is reduced (saccadic suppression) and the localization of stimuli can be shifted in the direction of a saccade or is compressed around the saccade target. The temporal order of two stimuli can be perceived as inverted and the duration in between can be underestimated. The duration of a target change close to the saccade target can be overestimated, when the change occurs during the saccade (chronostasis). In my thesis I investigated the spatial and temporal profiles of peri-saccadic changes in human visual perception and explored how these might result from changes in neural activity of the macaque middle temporal area (MT). I found that peri-saccadic contrast sensitivity was only reduced by a constant factor across space when the data was analyzed in retinal coordinates (as opposed to screen coordinates), indicating that saccadic suppression occurs in an eye-centered frame of reference. I demonstrated that the found variations of saccadic suppression with the location of the stimulus appear to cause variations in the spatio-temporal pattern of another peri-saccadic misperception: chronostasis. I was able to show that, unlike previously assumed, the saccadic overestimation of time is not a spatially localized disturbance of time perception but instead spans across the whole visual field. I further determined that chronostasis is not dependent on the eye movement itself, but is rather a consequence of the visual stimulation induced by it. This result clearly segregates chronostasis from other peri-saccadic perceptual changes like saccadic suppression and the compression of space. To relate these findings to a potential neuronal basis of saccadic suppression and time perception, I measured neuronal responses of single cells in MT of an awake behaving macaque. The results provide relevant insight into the processing of stationary stimuli and pairs of stimuli during fixation and saccades in MT. Responses to the second of a pair of stimuli were strongly suppressed and response latencies increased even at onset asynchronies of about 100ms. The increase in latency is an important difference to the temporal dynamics previously reported in other brain areas as the frontal eye field in the frontal cortex and the superior colliculus in the midbrain. During saccades, response latencies to single high luminance stimuli remained unchanged. For stimuli shown during the second half of the saccade, the average responses were reduced. By comparison with responses to single stimuli at different luminance levels during fixation, I was able to show that the peri-saccadic response reduction found in MT quantitatively fit to what could be expected from known psychophysical measurements of peri-saccadic contrast sensitivity. Responses that were already reduced due to a preceding stimulus were however not subject to further reductions, indicating a possible interaction of these two response modulations. Saccadic suppression occurs in an eye-centered frame of reference with changes in perception compatible to changes in single cell activity in the macaque monkey MT. The peri-saccadic overestimation of time is influenced by saccadic suppression and the saccade-induced visual changes, but is not dependent on eye-movement related signals.|