PERCEPTION AND ACTION IN-DEPTH

Little is known about distance processing in patients with posterior b rain damage. Although many investigators have claimed that distance es timates are normal or abnormal in some of these patients, many of thes e observations were made informally and the examiners often asked for relative, and not absolute, distance estimates. The present investigat ion served two purposes. First, we wanted to contrast the use of dista nce information in peripersonal space for perceptual report as opposed to visuomotor control in our visual form agnosic patient, DF. Second, we wanted to see to what extent her abilities to process distance cue s were dependent on binocular vision, in light of Milner et al.'s (199 1) observations of preserved stereopsis in DF, and Dijkerman et al.'s (1996) and Marotta et al.'s (1997) observations that her visual guidan ce of grasping may be particularly dependent on binocular vision of th e target. We hypothesized that DF's visuomotor responses would show no rmal sensitivity to target distance, while her perceptual estimates wo uld not. In the first experiment, we required DF and two age- and sex- matched control subjects to reach out and grasp black cubes placed at varying distances, or to estimate the distance of the cubes from the h and starting position without making a reaching movement. In the secon d experiment, we required DF and two age-matched control subjects to p oint as rapidly and accurately as possible to small LED targets which differed in spatial location, under binocular and monocular conditions . The results showed that, relative to the control subjects, DF's gras ping movements produced normal peak velocity-distance scaling-when she reached for blocks which varied in depth or pointed to LED targets wh ich were presented at different distances in depth. In contrast, in th e cube experiment, her verbal estimates of object distance were poorly scaled, although they improved slightly under the binocular condition s. The results are discussed in terms of current theories of processin g streams in extrastriate visual cortex and the distinction between ca tegorical and coordinate Spatial processing. (C) 1998 Academic Press.