A hand in blindsight: hand placement near target improves size perception in the blind visual field

Bimodal visual-tactile neurons respond to visual and/or tactile stimuli presented near the hands, arms, and face. The strength of bimodal-cell response to a visual stimulus depends on its proximity to the hand. We tested the hypothesis that hand proximity to a visual stimulus would influence unconscious residual vision in the blind field. MB is a 26-year-old man with a dense upper-left quadrantanopia following surgical treatment that severed Meyer's loop, and SB is a 35-year-old man with a dense left hemianopia following childhood meningitis. Testing for "hand-assisted" blindsight in MB allowed us to evaluate whether phenomena related to hand proximity are dependent on information relayed through the optic radiations. We presented target objects of different sizes to the blind fields of these gentlemen, and asked them to estimate target size with their right hand by adjusting thumb-finger distance or reaching and grasping the object. Concurrently, we varied the location of the resting left hand. In the hand-far condition, the left hand rested on their lap, and in the hand-near condition, it was placed just to the left of the target object. In the hand-far condition, both patients showed no scaling of either size-estimation aperture (SEA) or peak grip aperture (PGA) to object size. In the hand-near condition, however, both PGA and SEA were scaled to object size. Additional studies of MB suggest that hand proximity effects cannot be attributed solely to changes in spatial attention. These results imply that if bimodal cells are recruited when the hand is placed nearby the visual target, they must receive information from cortical areas that do not depend on the integrity of the optic radiations.