Sources of "overadditivity" in prism adaptation.

Abstract

In two experiments, tests of visual shift (VS), proprioceptive shift (PS), and negative aftereffect (NA)were made following 15-min exposure to 20-D (base-right) displacement. In Experiment 1, subjects engaged in saggital pointing (hand exposure) at single or multiple (three) exposure targets, while in Experiment 2, subjects experienced hand exposure or hallway exploration (hall exposure) factorially combined with single or multiple (three) NA test targets. In both experiments, additivity (VS+PS = NA) appeared for multiple target conditions while single target conditions produced “overadditivity” (VS+ PS >NA). This additivity effect for target conditions did not interact with exposure conditions in Experiment 2, although VS was greater in hall exposure and PS was greater in hand exposure. The presence of additivity with multiple targets was supported by a higher correlation between the NA test and each component test with multiple targets in both experiments. These results are interpreted as indicating two causes of overadditivity: specific motor response learning with single-exposure targets and association of a single NA test target with a cognitive shift in egocentric straight-ahead.