Fast and slow feedback loops for the visual correction of spatial errors in a pointing task: a reappraisal.

Experimental evidence supporting the idea that central and peripheral vision play a distinctive role in the on-line visual guidance of reaching movements is reappraised. The central retina, highly sensitive for the discrimination of relative position cues, subserves the error-detecting mechanisms that encode the discrete rate of change of location of the moving hand relative to the stationary target, feeding the slow corrective feedback loops that allow the accurate homing in of the hand on the target. The peripheral retina, mainly sensitive for the detection of continuous motion cues, is swept by the image of the hand moving towards the fovea, which is itself strongly anchored to the target during the whole course of the movement, thus providing a directional error signal used for fast correction of the movement trajectory. This interpretation fits a two-system model of motion perception derived from psychophysical data. It also fits anatomical and physiological data concerning the central distribution of static and kinetic cues through two separate visual channels. However, the way in which reafferent visual information involved in both feedback loops is further conveyed to the control system of arm-hand movement remains largely unknown.