Two visual systems in clinical neurology: readaptive role of the primitive system in visual agnosic patients.

In six patients with visual agnosia syndromes there was a striking dissociation between a preserved possibility of recognition of letters or objects in movement versus the recognition by simple visual static presentation. After a review of the literature it is concluded that at least two visual systems may be individualized in humans, i.e. a more recent one (the geniculo-calcarine) and an older one (the tectal-pulvinar-occipito-temporal system). Both from a physiological and clinical point of view, the most primitive system has two parts, i.e. a tectal and a telencephalic part. In visual agnosia, parts of the preserved primitive system remained as single support to compensate the loss of visual recognition for static patterns. The recognition of these two systems has both a theoretical and a practical value.