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Laura Pérez Zapata, Antonio Aznar-Casanova, Hans Supèr; Dissociation of eye movement signals and perception during fixation. Journal of Vision 2010;10(7):549. doi: 10.1167/10.7.549.
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We perceive our visual surrounding by continuously making saccadic eye movements. Saccades are guided by visual information that is previously selected by the visual system. Both bottom-up and top-down information guides saccades. The role of top-down control in gaze is demonstrated by studies showing that object perception, task demands, behavioral relevance, memory, and attention are important factors in guiding saccadic eye movements. It is therefore not surprising that many sensory, associational and motor areas of the visual system are involved in the control of oculomotor behavior. In some perceptual illusions the perceived location of a target differs from the physical location. Here we asked the question whether in such a case saccade programming is governed by bottom-up (physical target location) and/or by top-down (the perceived target location) signals. For this we used induced depth stimuli where a target is perceived further away than it in reality is. In this way we can test whether saccades are directed towards the perceptual or to the physical location. At the onset of fixation, observers saccade and converge their eyes towards the perceived target location. Only after fixation (>∼200 ms), the visual system is able to correct this ‘depth illusion’ by a fixational saccade and vergence eye movements. When observers maintain fixation but the target swaps between near and far location no differences in eye gaze and vergence were observed. Thus the saccadic adjustment and vergence are saccade dependent. This adjustment does not represent a correction due to under or overshoot of the saccade. Neither does it reflect an adjustment to vergence related to disturbance to the saccade. Thus eye movements (saccades and vergence) are guided by the perceived stimulus and during fixation eye movement signals are guided by the physical stimulus.
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