LEARNING A VISUOMOTOR TRANSFORMATION IN A LOCAL-AREA OF WORK SPACE PRODUCES DIRECTIONAL BIASES IN OTHER AREAS

1. The dependence of directional biases in reaching movements on the i nitial position of the hand was studied in normal human subjects movin g their unseen hand on a horizontal digitizing tablet to visual target s displayed on a vertical computer screen. 2. When initial hand positi ons were to the right of midline, movements were systematically biased clockwise. Biases were counterclockwise for starting points to the le ft. Biases were unaffected by the screen location of the starting and target positions. 3. Vision of the hand in relation to the target befo re movement, as well as practice with vision of the cursor during the movement, temporarily eliminated these biases. The spatial organizatio n of the biases suggests that, without vision of the limb, the nervous system underestimates the distance of the hand from an axis or plane that includes its most common operating location. 4. To test the hypot hesis that such an underestimate might represent an adaptation to a lo cal area of work space or range effect, subjects were trained to reach accurately from right or left positions. After training, movements in itiated from other locations, including ones that were previously erro r free, showed new biases that again represented underestimates of the distance of the initial hand position from the new trained location. 5. We conclude that hand path planning is dependent on learned represe ntations of the location of the hand in the work space.