THE NATURE AND LIMITS OF ORIENTATION AND PATTERN PROCESSING SUPPORTING VISUOMOTOR CONTROL IN A VISUAL FORM AGNOSIC

We have previously reported that a patient (DF) with visual form agnos ia shows accurate guidance of hand and finger movements with respect t o the size, orientation, and shape of the objects to which her movemen ts are directed. Despite this, she is unable to indicate any knowledge about these object properties. In the present study, we investigated the extent to which DF is able to use visual shape or pattern to guide her hand movements. In the first experiment, are found that when pres ented with a stimulus aperture cut in the shape of the letter T, DF wa s able to guide a T-shaped form into it on about half of the trials, a cross a range of different stimulus orientations. On the remaining tri als, her responses were almost always perpendicular to the correct ori entation. Thus, the visual information guiding the rotation of DF's ha nd appears to be limited to a single orientation. In other words, the visuomotor transformations mediating her hand rotation appear to be un able to combine the orientations of the stem and the top of the T, alt hough they are sensitive to the orientation of the element(s) that com prise the T. In a second experiment, we examined her ability to use di fferent sources of visual information to guide her hand rotation. In t his experiment, DF was required to guide the leading edge of a hand-he ld card onto a rectangular target positioned at different orientations on a flat surface. Here the orientation of her hand was determined pr imarily by the predominant orientation of the luminance edge elements present in the stimulus, rather than by information about orientation that was conveyed by nonluminance boundaries. Little evidence was foun d for an ability to use contour boundaries defined by Gestalt principl es of grouping (good continuation or similarity) or ''nonaccidental'' image properties (colinearity) to guide her movements. We have argued elsewhere that the dorsal visual pathway from occipital to parietal co rtex may underlie these preserved visuomotor skills in DF. If so, the limitations in her ability to use different kinds of ''pattern'' infor mation to guide her hand rotation suggest that such information may ne ed to be transmitted from the ventral visual stream to these parietal areas to enable the full range of prehensive acts in the intact indivi dual.