INDEPENDENT CODING OF TARGET DISTANCE AND DIRECTION IN VISUOSPATIAL WORKING-MEMORY

The organization of manual reaching movements suggests considerable in dependence in the initial programming with respect to the direction an d the distance of the intended movement. It was hypothesized that shor t-term memory for a visually-presented location within reaching space, ill the absence of other allocentric reference points, might also be represented in a motoric code, showing similar independence in the enc oding of direction and distance. This hypothesis was tested in two exp eriments, using adult human subjects who were required to remember the location of a briefly presented luminous spot, Stimuli were presented in the dark, thus providing purely egocentric spatial information. Af ter the specified delay, subjects: were instructed to point to the rem embered location. In Exp. 1, temporal decay of location memory was stu died, over a range of 4-30 s. The results showed that (a) memory for b oth the direction and the distance of the visual target location decli ned over time, al about the same rate for both parameters; however, (b ) errors of distance were much greater in the left than in the ri,eht, hemispace, whereas direction errors showed no such effect; (c) the di stance and direction errors were essentially uncorrelated, at all dela ys. These findings suggest independent representation of these two par ameters in working memory. In Exp. 2 the subjects were required to rem ember the locations of two visual stimuli presented sequentially, one after the other. Only after both stimuli had been presented did the su bject receive a signal from the experimenter as to which one was to be pointed to. The results showed that the encoding of a second location selectively interfered with memory for the direction but not for the distance of the to-be-remembered target location. As in Exp. II direct ion and distance errors were again uncorrelated. The results of both e xperiments indicate that memory for egocentrically-specified visual lo cations can encode the direction and distance of the target independen tly. Use of motor-related representation in spatial working memory is thus strongly suggested. The findings are discussed in the context of multiple representations of space in visuo-spatial short-term memory.