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Paola Binda, Jonas Knöll, Frank Bremmer, M. Concetta Morrone; Sensitivity to chromatic contrast at the time of saccades. Journal of Vision 2009;9(8):402. doi: 10.1167/9.8.402.
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During saccadic eye movements luminance contrast sensitivity is reduced by up to ten times (for low spatial frequencies), while chromatic contrast sensitivity is enhanced by about a factor of 3, mainly after saccadic offset (Burr et al., Nature 1994, Diamond et al., J.Neurosci. 2000). While the origin of luminance suppression is well explained by an early inhibitory effect on the magnocellular pathways, the enhancement of chromatic sensitivity is still unexplained. Here we demonstrate that the enhancement is not mediated by an active extra-retinal signal, but rather by a facilitation generated by the spurious retinal motion.
Color discrimination (red/green) thresholds were measured for a 1° red-green Gaussian blob equiluminant with the background (Luminance: 18.1 cd/m2; C.I.E. coordinates: x = 0.470; y = 0.452). The stimulus was flashed for 10 ms at various spatial locations along or just above the saccadic path. In the active condition, subjects performed a 15° saccade; in the passive condition they maintained fixation on a mirror (reflecting the monitor) that rotated at saccadic speeds to simulate saccade-induced retinal motion.
Real and simulated saccades produced similar effects on discrimination thresholds. During the actual saccade (real or simulated), sensitivity was enhanced only for stimuli located along the trajectory of the fovea, otherwise it was unaffected. In the period immediately following the saccade sensitivity at all locations was enhanced by about 0.2 log-units (∼factor of 1.6), peaking 100 ms after saccadic onset. The decay of the enhancement lasted longer for the simulated than real saccades (the major difference between the two conditions).
The results demonstrate that saccadic modulation of chromatic sensitivity is not caused by active processes accompanying the execution of eye movements, but is rather a byproduct of the spurious retinal motion that seems to enhance color detection, for reasons yet to be understood.
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