Preserved visual imagery in visual form agnosia.

We investigated the ability of a patient (D.F.) with profound visual form agnosia to perform a variety of tasks requiring visual imagery. Despite her inability to discriminate between objects and patterns of different shapes, sizes, and orientations, D.F. showed quite normal visual imagery involving these same 'visual' properties when the images were drawn from long-term memory. Thus, she was able both to scan mental images in search of particular features and to form new images by combining several known images. While there is growing evidence that perception and imagery share common neural substrates, the fact that D.F. shows intact visual imagery in the face of a massive perceptual deficit in form vision challenges recent suggestions that these two psychological processes share common input pathways in early vision. It is suggested that regions in the occipitotemporal pathway may be important for the generation of visual images while regions in the posterior parietal system might be involved in the manipulation of these images.