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Abstract

A central goal of sensation and perception is to direct our interactions with the environment. During most voluntary motor actions that are driven by sensory input we consciously experience an internal representation of the visual world. This leads to the question how faithful this internal representation is, and how precise our actions are compared to this reference. To answer that question, we studied the relationship between the perceived speed, which is the experiential representation of the stimulus, and the speed of smooth-pursuit eye movements, the motor action.
We determined psychophysical thresholds for detecting small perturbations in the speed of Gabor patterns (1 cycle deg-1) moving at a base speed of 4 deg s-1. At the same time we recorded eye-movement traces and used an ideal-observer analysis to compute analogous 'oculometric' thresholds. Our results show a remarkable agreement between perceptual judgments for speed discrimination and the fine gradations in eye-movement speed, with psychophysical and oculometric functions exhibiting the same slope. However, there was no correlation between perceptual errors and eye-movement errors on a trial-by-trial basis. We conclude that the motor system and perception share the same constraints in their analysis of motion signals, but they act independently and have different sources of noise.