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Adam P. Morris, Charles C. Liu, Simon J. Cropper, Jason D. Forte, Jason B. Mattingley; Psychophysical summation of visual motion across eye movements reflects decision processes, not sensory integration. Journal of Vision 2008;8(17):70. doi: 10.1167/8.17.70.
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Human vision remains perceptually stable despite frequent displacements of the retinal image associated with saccadic eye movements. In the current study, we evaluated recent claims that this visual stability relies on the integration of sensory inputs over time in environmental rather than retinal coordinates. These claims were inspired by a psychophysical phenomenon in which perceptual sensitivity for a motion stimulus presented after a saccade is enhanced when preceded by a priming motion stimulus prior to the saccade at the same spatial position. We characterized the mechanism underlying this psychophysical summation by examining its spatial and directional specificity, as well as its modulation by temporal uncertainty. Summation was similar irrespective of whether the two motion signals occupied the same or different locations in space, and whether they contained the same or opposite directions of motion. Further, the introduction of a tone that announced the onset of motion was sufficient to abolish summation. These data are inconsistent with a sensory-level explanation for psychophysical summation across eye movements and argue against a role for the underlying mechanism in realizing visual stability. Instead, our findings are consistent with a model in which abstracted representations of independent sensory inputs are combined statistically at the level of decision-making.
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