Short-term memory for reaching to visual targets: Psychophysical evidence for body-centered reference frames

Pointing to a remembered visual target involves the transformation of visual information into an appropriate motor output, with a passage through short-term memory storage. In an attempt to identify the reference frames used to represent the target position during the memory period, we measured errors in pointing to remembered three-dimensional (3D) targets. Subjects pointed after a fixed delay to remembered targets distributed within a 22 mm radius volume, Conditions varied in terms of lighting (dim light or total darkness), delay duration (0.5, 5.0, and 8.0 sec), effector hand (left or right), and workspace location. Pointing errors were quantified by 3D constant and variable errors and by a novel measure of local distortion in the mapping from target to endpoint positions. The orientation of variable errors differed significantly between light and dark conditions. Increasing the memory delay in darkness evoked a reorientation of variable errors, whereas in the light, the viewer-centered variability changed only in magnitude. Local distortion measurements revealed an anisotropic contraction of endpoint positions toward an "average" response along an axis that points between the eyes and the effector arm. This local contraction was present in both lighting conditions. The magnitude of the contraction remained constant for the two memory delays in the light but increased significantly for the longer delays in darkness. These data argue for the separate storage of distance and direction information within short-term memory, in a reference frame tied to the eyes and the effector arm.