Residual vision in a scotoma. A follow-up study of 'form' discrimination.

This study is a follow-up of a patient, D.B., who was reported (Weiskrantz et al., 1974) to be able to discriminate between simple visual forms within the scotoma caused by a lesion in calcarine cortex. Among other capacities, he was able to discriminate between lines of different orientation in the frontal plane. Given the reported deficits for form discrimination but a high sensitivity for orientation discrimination in primates without striate cortex, the question arises whether D.B.'s apparent 'form' discrimination arises from an ability to discriminate the orientations of components of the figures. It is shown in the first experiment, by using the optic disc as a control, that his ability to detect stimuli in his scotoma cannot be due to stray light falling upon the intact field. Next, his ability to discriminate orientations is confirmed, and an orientation threshold determined. A range of form discriminations is presented varying in degree of orientation cues of their components. It is confirmed that he can discriminate those forms originally studied, in which such differences are large, but not when orientation cues are small or minimal. Finally, his ability to compare two forms both projected within his scotoma is examined. Even when the components of the two forms have large orientation differences and are highly discriminable when presented successfully, D.B. appears to be unable to make a 'same-different' comparison when both are presented simultaneously. The evidence is interpreted against D.B.'s having a residual capacity for form discrimination.