Implicit processing of somaesthetic information

We present the case of a patient with a lesion of the thalamus who was completely anaesthetized on his right side. He was unable to detect and describe a tactile stimulus applied to his affected right arm, but could direct his normal left hand toward the specific right hand site where the stimulus had been applied when so instructed ('blind touch'). Strikingly, this pointing ability disappeared when the patient had to indicate on a picture of an arm where the stimulus was applied, and when he had to name the stimulus location during his pointing. Similar results were also obtained for localizing the unfelt fingertip ('blind proprioception'). Neuropsychological case studies have demonstrated that brain lesions can produce reciprocal dissociations between object identification (what is the object) and object-oriented action (how to direct a movement to the object). Along these lines, it is suggested that our patient exhibited a dissociation between a 'where' system and a 'how' system for tactile and proprioceptive stimuli.