Is residual vision in monkeys with unilateral lesion in the primary visual cortex like normal, near-threshold vision?

Abstract

Some of the patients with damages in the primary visual cortex (V1) retain their ability to localize visual targets in their affected hemifield (‘blindsight’ or ‘residual vision’). One of the controversies about blindsight is whether it is a form of near-threshold vision or qualitatively different from normal vision. However, the extent of damage of the patients was not necessarily complete and restricted within V1, thus leaving the question unsolved. Here we used monkeys with unilateral lesion in V1 as an animal model of blindsight and examined whether the monkeys' residual vision is a kind of attenuated vision that can be mimicked by presenting near-threshold stimuli to their intact visual hemifield. After the unilateral removal of V1, two monkeys were tested with a visually guided saccade task with a target in one of five possible directions either in the intact or affected hemifield. The monkeys correctly localized the saccadic targets presented in the affected hemifield (70–95 % correct), while the accuracy of the saccade was worse than that of the intact hemifield, as quantified by the variance and the systematic errors in the saccadic end points. Then the monkeys were tested with near-threshold stimuli in the intact hemifield. Near-threshold stimuli were constructed either (1) by reducing luminance contrast or (2) by increasing spatial ambiguity. In both near-threshold conditions (70–90 % correct), the variance and the systematic errors in the saccadic end points were far smaller than that of the affected hemifield. The saccadic reaction time in the near-threshold condition was longer than that of the affected hemifield. These results suggest that the behavioral effects of V1 lesion are not limited to vision, but V1 lesion affected visuomotor processing including the saccade control system and/or decision process. We conclude that the residual vision of monkeys with V1 lesion is not like normal, near-threshold vision.