Sensory feedback modulates Weber's law of both perception and action.

Weber's law states that estimation noise is proportional to stimulus intensity. Although this holds in perception, it appears absent in visually guided actions where response variability does not scale with object size. This discrepancy is often attributed to dissociated visual processing for perception and action. Here, we explore an alternative explanation: It is the influence of sensory feedback on motor output that causes this apparent violation. Our research investigated response variability across repeated grasps relative to object size and found that the variability pattern is contingent on sensory feedback. Pantomime grasps with neither online visual feedback nor final haptic feedback showed variability that scaled with object size, as expected by Weber's law. However, this scaling diminished when sensory feedback was available, either directly present in the movement (Experiment 1) or in adjacent movements in the same block (Experiment 2). Moreover, a simple visual cue indicating performance error similarly reduced the scaling of variability with object size in manual size estimates, the perceptual counterpart of grasping responses (Experiment 3). These results support the hypothesis that sensory feedback modulates motor responses and their associated variability across both action and perception tasks. Post hoc analyses indicated that the reduced scaling of response variability with object size could be due to changes in motor mapping, the process mapping visual size estimates to motor outputs. Consequently, the absence of Weber's law in action responses might not indicate distinct visual processing but rather adaptive changes in motor strategies based on sensory feedback.