GRASPING WITHOUT FORM DISCRIMINATION IN A HEMIANOPIC FIELD

ALTHOUGH patients with blindsight are usually unable to discriminate f orms, recent neuropsychological data have suggested that they could st ill use some form attributes in object-oriented actions. One patient w ith a complete right hemianopia due to a medial occipital lesion has b een tested for his capacities to process orientation and size of visua l objects. He was presented with either a slot of variable orientation or with rectangular objects of the same surface but variable length. His performance was studied in three types of tasks: motor, in which h e had to insert a card in the slot or to grasp the rectangle between t humb and indexfinger; verbal, in which forced-choice verbal guesses we re required; and matching, which required matching orientation or size with wrist or fingers. Although responses were at chance level in the two latter conditions, motor responses were systematically influenced by both orientation and size of the stimulus. These data provide furt her evidence for two dissociable modes of visual information processin g dealing respectively with 'what ''' the object is vs 'how' to grasp it. They also indicate that the neural pathway controlling visuomotor transformation in humans is much less dependent on V1 input than the p athway involved in visual discrimination and identification.