VISUAL CAPACITY IN THE HEMIANOPIC FIELD FOLLOWING A RESTRICTED OCCIPITAL ABLATION

Abstract

DAMAGE to striate cortex and neighbouring regions in man is well known to producesevere loss of visual capacity in correlated legions of the visual field (Holmes, 1918;Teuber, Battersby and Bender, 1960). Depending on the duration, nature andextent of the damage, and the method of measurement, the resultant blindnessmay be more or less absolute. In the most extreme examples, a patient mayacknowledge no visual information or only the onset or offset of a light in thescotoma. In less extreme cases vigorously moving or flickering stimuli may be seen(Riddoch, 1917). It is generally held, however, that man is more severely impairedby damage to the visual cortex than the monkey, even though the anatomicalorganizations of the visual pathways and cortex are closely similar. Indeed, the morethoroughly the monkey has been studied, the more remarkable has been the extentof residual visual capacity found: for example, the ability to discriminate twodimensionalpatterns even when the whole of the striate cortex has been removed,with complete retrograde degeneration of the dorsal lateral geniculate nuclei (Pasikand Pasik, 1971). It has even been suggested that the monkey without striate cortexmay be capable of qualitatively normal pattern vision but with reduced visualacuity (Weiskrantz, 1972; cf. Ward and Masterton, 1970). Recent animal results,however, also suggest strongly that a severe penalty is inflicted when damage toneighbouring posterior cortical areas is added to that of the striate cortex (Pasik andPasik, 1971). For this reason, human cases in which damage is relatively restrictedto area 17 (the only known cortical projection from the dorsal lateral geniculatenucleus), with only minimal damage to surrounding tissue, have a renewed interest.