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Dorion Liston, Leland Stone; Deciphering the relationship between perceptual and motor variability. Journal of Vision 2011;11(11):548. doi: 10.1167/11.11.548.
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The time that elapses between stimulus onset and the initiation of a saccadic eye movement is longer and more variable than can be explained by neural transmission times and synaptic delays (Carpenter, 1981, in: Eye Movements: Cognition & Visual Perception, Earlbaum). In fact, the correlation between response time and stimulus strength is surprisingly weak (r < 0.5), suggesting a relatively large source of internal (neural) noise. In theory, noise underlying response-time variability could arise anywhere in sensorimotor pathways, at any stage from early sensory processing to motor output commands. These loci can be distinguished empirically; if early (shared) visual noise dominates then saccadic response time will correlate with perceived stimulus magnitude whereas if late oculomotor (unshared) noise dominates there should be no such correlation. Methods. Five participants ran a two-stage task in which they first performed a 2AFC saccadic brightness discrimination task (to determine response rate or the inverse of response time) and then a second, method of adjustment, brightness matching task between the remembered chosen stimulus and a variable test stimulus (to determine perceived brightness). Results. We plotted both perceived brightness and response rate (normalized and pooled across participants) as a function of signal strength and used a linear regression model to isolate, on a trial-by-trial basis, the residual effects of internal (neural) noise on the perceptual and motor responses from any external (stimulus brightness) driven variability, including that caused by the stochastic nature of our stimuli (Liston and Stone, 2008, JNS 28:13866–13875). The two internal noise estimates were significantly positively correlated (Pearson's R, p < 0.01). Conclusion. The correlation between the neural noise effects on saccadic response time and perceived brightness is consistent with shared early visual noise jittering brightness perception and saccades in parallel as has been previously observed for motion perception and smooth pursuit (Stone and Krauzlis, JOV 3:725–736, 2003).
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