Visual capacity in the hemianopic field following a restricted occipital ablation.

DAMAGE to striate cortex and neighbouring regions in man is well known to produce severe loss of visual capacity in correlated legions of the visual field (Holmes, 1918; Teuber, Battersby and Bender, 1960). Depending on the duration, nature and extent of the damage, and the method of measurement, the resultant blindness may be more or less absolute. In the most extreme examples, a patient may acknowledge no visual information or only the onset or offset of a light in the scotoma. In less extreme cases vigorously moving or flickering stimuli may be seen (Riddoch, 1917). It is generally held, however, that man is more severely impaired by damage to the visual cortex than the monkey, even though the anatomical organizations of the visual pathways and cortex are closely similar. Indeed, the more thoroughly the monkey has been studied, the more remarkable has been the extent of residual visual capacity found: for example, the ability to discriminate twodimensional patterns even when the whole of the striate cortex has been removed, with complete retrograde degeneration of the dorsal lateral geniculate nuclei (Pasik and Pasik, 1971). It has even been suggested that the monkey without striate cortex may be capable of qualitatively normal pattern vision but with reduced visual acuity (Weiskrantz, 1972; cf. Ward and Masterton, 1970). Recent animal results, however, also suggest strongly that a severe penalty is inflicted when damage to neighbouring posterior cortical areas is added to that of the striate cortex (Pasik and Pasik, 1971). For this reason, human cases in which damage is relatively restricted to area 17 (the only known cortical projection from the dorsal lateral geniculate nucleus), with only minimal damage to surrounding tissue, have a renewed interest. There also have been suggestions, based on animal research, that the type of visual information processed by the direct pathway from the retina to the mid-brain is qualitatively different from that by the geniculo-cortical route (the "two visual system" hypothesis: Schneider, 1969; Trevarthen, 1968; Ingle, 1967). The former, it has been claimed, is primarily concerned with the detection of "salient" visual events in space and the control of ocular fixation, and the latter with the identification