Implicit processing of somatosensory stimuli disclosed by a perceptual after-effect

A right-brain damaged patient (GR) showing tactile extinction, neglect and no hypesthesia, was asked to compare, in 30 trials, the size of two spheres of different size simultaneously put into each hand. Accuracy was poor because GR was unable to feel the sphere in his left hand and could only report the size of the right sphere. GR was then asked to compare two spheres of identical sizes although again unable to perceive the left sphere he was deceived by a perceptual after-effect also observed in normal subjects, and reported incorrectly the size of the right sphere. The left sphere although unperceived, influenced the perception of the right hand, thus giving the first evidence for an implicit processing of extinguished somatosensory information.