An investigation into perceptual completion in blind areas of the visual field

The perceptual completion phenomenon refers to seeing a figure as complete when part of it falls in a blind area of the visual field. This phenomenon and the conditions for its occurrence were examined in three different situations: over the retinal blind spot, across the midline of the visual field in commissurotomized patients, and in the blind visual field in hemianopic patients. Responses of complete perception were obtained in each of the three conditions, but they were not all the result of a genuine perceptual completion process. Reports of vision over the retinal blind spot qualified as genuine perceptual completion. In the other two situations, reports suggestive of completion could be entirely eliminated in all but one hemianopic patient by manipulating parameters inherently involved in the evaluation of perceptual completion. The nature of perception, the characteristics and properties of the experimental task and stimuli, the mode of response, the patient's mental status, and eccentric eye fixation, all contributed towards reports of complete perception for reasons other than a process of completing the missing information. Evidence of completion was found in one patient who could see more distinctly in his affected field with bilateral stimulation than with stimulation of the affected field alone. The results suggest that perceptual completion in the hemianopic field is not a common phenomenon and that reports of complete perception are determined by many factors, not all related to seeing more than is objectively available. The results also indicate that the hemianopic defect does not function as an extended blind spot. Possible mechanisms underlying perceptual completion over the blind spot and the hemianopic field are suggested.