The role of memory in visually guided reaching.

People can be shown to use memorized location information to move their hand to a target location if no visual information is available. However, for several reasons, memorized information may be imprecise and inaccurate. Here, we study whether and to what extent humans use the remembered location of an object to plan reaching movements when the target is visible. Subjects sequentially picked up and moved two different virtual, "magnetic" target objects from a target region into a virtual trash bin with their index fingers. In one third of the trials, we perturbed the position of the second target by 1 cm while the finger was transporting the first target to the trash. Subjects never noticed this. Although the second target was visible in the periphery, subjects' movements were biased to its initial (remembered) position. The first part of subjects' movements was predictable from a weighted sum of the visible and remembered target positions. For high contrast targets, subjects initially weigh visual and remembered information about target position in an average ratio of 0.67 to 0.33. Over the course of the movement, weight given to memory decreased. Diminishing the contrast of the targets substantially increased the weight that subjects gave to the remembered location. Thus, even when peripheral visual information is available, humans use the remembered location of an object to plan goal-directed movements. In contrast to previous suggestions in the literature, our results indicate that absolute location is remembered quite well.