Feature search in persons with severe visual impairment

Feature search performance was measured in visually impaired (VI) and age-matched controls with normal vision (NV). All VI subjects were legally blind. The task was to search for a 2 degrees x 2 degrees square target among smaller 1 degrees x1 degrees distracters. Targets and distracters were white and presented on a dark background that subtended 69 degrees by 58 degrees . Three field-sizes (10 degrees , 20 degrees , and 40 degrees ) and three set sizes (8-, 16-, and 32-items) were tested. The VI subjects searched more slowly than the NV subjects, but the reaction time of both groups of subjects did not rise with increasing number of items. The latter is consistent with a parallel search. Both groups searched more slowly when field-size increased, but the VI group was affected more by the increase than the NV group.