Contributions to the study of “Blindsight”—I. Can stray light account for saccadic localization in patients with postgeniculate field defects?

In the present study the role of stray light was investigated with respect to its contribution to patients' ability to locate a target presented tachistoscopically in their perimetrically blind hemifield. Two patients exhibiting homonymous field defects after unilateral geniculostriate damage were tested under different target-background luminance conditions. The results clearly demonstrate patients' ability to locate the target in their hemianopic field in the absence of any stray light at their field border, as assessed by an electronic photometer. In this condition, the patients never had any visual sensation with regard to the illumination of the target. In conditions where stray light was available at the field border, differing as a function of the target's location, the patients could not locate the target, even though they always reported a light diffusely distributed over the sphere of the perimeter when the target was illuminated. The stray light emitted from the target at different distances from the field was therefore of no help but, rather, hindered the patients in locating the target. Accordingly, the “stray light hypothesis” is rejected as an explanation for patients' ability to locate a light target when presented within their perimetrically blind field region.