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Gaelle Desbordes, Michele Rucci; Discrimination of briefly presented stimuli in the presence and absence of fixational eye movements. Journal of Vision 2004;4(8):739. doi: 10.1167/4.8.739.
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Our eyes are never completely at rest. During the periods of visual fixation, small eye movements keep the projection of the visual scene on the retina in constant motion. Yet, while it is known that images tend to fade over a period of several seconds when retinal image motion is eliminated, surprisingly little is known about the possible function of retinal jitter during the brief periods of visual fixation that occur in natural viewing conditions. In this study, we analyzed the effect of fixational eye movements in a visual discrimination task. Human subjects were asked to discriminate the orientation of a small bar (30′) that was displayed for short durations comparable to the periods of natural visual fixation. The bar was randomly tilted by +45 or −45 and embedded in a noisy, low contrast background. Subject eye movements were recorded by a Dual Purkinje Image eye-tracker. Three experimental conditions were investigated: (A) Normal eye movements; (B) Retinal stabilization: Retinal image motion was eliminated by a stimulus deflector coupled with the eyetracker that shifted the image according to subject eye movements; (C) Simulated eye movements: Retinal image motion was eliminated as in (B) while fixational instability was simulated by moving the image on the screen to follow prerecorded traces of eye movements. For all subjects, percentages of correct discrimination were significantly lower under conditions of visual stabilization than in the presence of the normally or artificially moving retinal image. Experimental results are compared to the results of computer simulations in which models of LGN and V1 cells simulated neuronal activity in the experimental trials given the stimulus and subject eye movements. These results suggest that fixational instability may be an important component how visual information is acquired and encoded in the brain.
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