Functional dissociation between perception and action is evident early in life

The functional distinction between vision for perception and vision for action is well documented in the mature visual system. Ganel and colleagues recently provided direct evidence for this dissociation, showing that while visual processing for perception follows Weber's fundamental law of psychophysics, action violates this law. We tracked the developmental trajectory of this functional dissociation, asking whether the qualitatively different pattern observed in adults of adherence of perception but not of action to Weber's law would also be evident early in life. Children aged 5-8 and adults were asked to either estimate the size of discs (perception) or grasp discs (action) varying in diameter. Interestingly, variability of perceptual estimates increased as a function of object size in accord with Weber's law, while variability of grasping did not scale with object size, at all ages tested. This provides the first clear evidence for an early emergence of the dissociation between perception and action.