A neurological dissociation between perceiving objects and grasping them

Abstract

Studies of the visual capacity of neurological patients have provided evidence for a dissociation between the perceptual report of a visual stimulus and the ability to direct spatially accurate movements toward that stimulus. Some patients with damage to the parietal lobe, for example, are unable to reach accurately towards visual targets that they unequivocally report seeing. Conversely, some patients with extensive damage to primary visual cortex can make accurate pointing movements or saccades toward a stimulus presented in their 'blind' scotoma. But in investigations of visuomotor control in patients with visual disorders, little consideration has been given to complex acts such as manual prehension. Grasping a three-dimensional object requires knowledge not only of the object's spatial location, but also of its form, orientation and size. We have examined a patient with a profound disorder in the perception of such object qualities. Our quantitative analyses demonstrate strikingly accurate guidance of hand and finger movements directed at the very objects whose qualities she fails to perceive. These data suggest that the neural substrates for the visual perception of object qualities such as shape, orientation and size are distinct from those underlying the use of those qualities in the control of manual skills.

The removal of binocular cues disrupts the calibration of grasping in patients with visual form agnosia

“…What cues could they be using? Despite their profound form perception deficit, DF and JW can still detect the movement of objects and contours in the visual array (Milner et al 1991;unpublished observations). It is possible, therefore, that they are able to use retinal motion (from head movements) to compute distance for transporting the limb to the correct location -even though this cue was not used (and perhaps could not be used) to calibrate their retinal image size.…”

“…Detailed descriptions of her residual perceptual abilities are available elsewhere (Goodale et al 1991;Milner et al 1993). Magnetic resonance imaging revealed evidence of diffuse brain damage consistent with discrimination, at judging the symmetry of shapes (Vecera and Behrmann, in press) and at performing simple visual image segmentation of overlapping shapes.…”

“…Even though DF's 'low-level' visual abilities are reasonably intact, she can no longer recognize even the simplest of geometric shapes. Nevertheless, despite her profound inability to perceive the size, shape and orientation of visual objects, DF can direct accurate and well-formed grasping movements, indistinguishable from those shown by normal subjects, towards the very same objects she cannot identify or discriminate (Goodale et al 1991;Milner and Goodale 1993). What visual information is she using to calibrate these apparently normal grasping movements?…”

Frames of Reference for Perception and Action in the Human Visual System

“…Remarkably, however, DF shows strikingly accurate guidance of her hand and finger movements when she attempts to pick up the very objects she cannot identify. Thus, when she reaches out to grasp objects of different sizes, her hand opens wider mid-flight for larger objects than it does for smaller ones, just as it does in people with normal vision (26). Similarly, she rotates her hand and wrist quite normally when she reaches out to grasp objects in different orientations, and she places her fingers correctly on the boundaries of objects of different shapes (24,26).…”

“…Thus, when she reaches out to grasp objects of different sizes, her hand opens wider mid-flight for larger objects than it does for smaller ones, just as it does in people with normal vision (26). Similarly, she rotates her hand and wrist quite normally when she reaches out to grasp objects in different orientations, and she places her fingers correctly on the boundaries of objects of different shapes (24,26). At the same time, she is quite unable to describe or distinguish between any of these objects when they are presented to her in discrimination tests.…”