HAPTIC LOCALIZATION AND THE INTERNAL REPRESENTATION OF THE HAND IN-SPACE

As the hand actively explores the environment, contact with an object leads to neuronal activity in the topographic maps of somatosensory co rtex. However, the brain must combine this somatotopically encoded tac tile information with an internal representation of the hand's locatio n in space if it is to determine the position of the object in three-d imensional space (3-D haptic localization). To investigate the fidelit y of this internal representation in human subjects, a small tactual s timulator, light enough to be worn on the subject's hand, was used to present a brief mechanical pulse (6-ms duration) to the right index fi nger before, during, or after a fast, visually evoked movement of the right hand. In experiment 1, subjects responded by pointing to the per ceived location of the mechanical stimulus in 3-D space. Stimuli prese nted shortly before or during the visually evoked movement were system atically mislocalized, with the reported location of the stimulus appr oximately equal to the location occupied by the hand 90 ms after stimu lus onset. This pattern of errors indicates a representation of the mo vement that fails to account for the change in the hand's location dur ing somatosensory delays and, in some subjects, inaccurately depicts t he velocity of the actual movement. In experiment 2, subjects were ins tructed to verbally indicate the perceived temporal relationship of th e stimulus and the visually evoked movement (i.e., by reporting whethe r the stimulus was presented ''before,'' ''during,'' or ''after'' the movement). On average, stimuli presented in the 38-ms period before mo vement onset were more likely to be perceived as having occurred durin g rather than before the movement. Similarly, stimuli in the 145-ms pe riod before movement termination were more likely to be perceived as h aving occurred after rather than during the movement. The analogous fi ndings of experiments 1 and 2 indicate that the same inaccurate repres entation of dynamic hand position is used to both localize tactual sti muli in 3-D space and construct the perception of arm movement.