Signal detection analysis of residual vision in a field defect due to a post-geniculate lesion.

A patient's ability to discriminate between 'blank trials' and small visual targets presented tachistoscopically within his blind hemifield was tested at five positions within the scotoma. For each position a Receiver-Operating-Characteristic curve (ROC) was determined. Results show the patient's discrimination between the target and no-target conditions to be better than expected by guessing at three positions along the horizontal meridian, whereas in the natural Blind Spot within the scotoma no significant discrimination is found. Thus, light scatter produced by the visual target cannot explain the observed residual vision. At least three anatomical pathways could be responsible for the discrimination: the retino-collicular projection, the retino-geniculo-striatal projection, and the retino-geniculo-extrastriatal projection.