Right brain-damaged (RBD) patients may present a deficit known as ‘extinction’: they fail to report a stimulation in the contralesional hemispace when cuncurrently delivered with an ipsilesional stimulus. Cross-modal extinction has been documented between different sensory modalities, such as touch and vision. We recently reported an extinction-like phenomenon in healthy people, in the tactile modality. Here we report cross-modal extinction phenomenon in normal subjects, between vision and touch.
Subjects were presented with visual stimulations (30 ms) on a computer monitor, and touches (electro-cutaneous, 100 µs) on the index finger of the left or right hand placed just below the monitor. Visual stimuli could be either near (left/right bottom corner) or far from (left/right upper corner) each hand. Visual and tactile stimulations could be delivered singlhy or simultaneously combined on both sides (a visual stimulaiton on the right and a touch on the left, or viceversa). Subjects were asked to discriminate stimulations (left, right or both), irrespective of the sensory modality, by making a saccadic eye-movement (to a leftward, rightward, or upperward square from the fixation point), recorded by an eyetracking system. Results showed that subjects perceived tactile stimulation better when delivered alone than simultaneously with a visual stimulation on the contralateral hemispace, particularly when it was presented close to the hand. Subjects’ performance on unilateral touches were less affected by visual stimuli when presented far from the hands. These results suggest that cross-modal visuo-tacile extinction-like phenomena are physiologically present in the healthy brain. Pathological extinction could thus be an exaggerated form of normal, spatially modulated competition between inputs from different sensory modalities.