Investigating multisensory spatial cognition through the phenomenon of extinction

The study of pathologic behavioral phenomena following damage to the central nervous system has substantially contributed to our understanding of the normal organization of cognitive brain functions. The investigation of clinical extinction in neurological patients, for example, revealed that events occurring in the immediate proximity to the body (i.e. in near peripersonal space) undergo a high degree of multisensory processing. Indeed, visual–tactile extinction is most severe near a given body part: tactile stimuli on the contralesional hand are more severely extinguished by visual stimuli presented near, as compared to far, from the ipsilesional hand. Here, extinction-based evidence will first be reported to illustrate the peculiar nature of multisensory processing occurring in pericorporeal space. Second, the plastic features of such processing following tool-use will be presented. Our findings in patients with visual–tactile extinction show that the use of a tool as an extension of reachable space may dynamically modulate the multisensory processing of farther, non-reachable sectors of space. Finally, a normal model of the clinical phenomenon of tactile extinction is proposed and demonstrated. We show that, despite accurate and comparable perception of single stimulation to the right or left hand, somatosensory performance is lower when reporting double simultaneous stimuli in neurologically healthy participants, thus replicating the typical extinction patient’s behavior. This intrinsic limitation of the somatosensory perceptual system was exploited to further investigate multisensory interactions between proprioceptive–kinesthetic and tactile inputs in normal subjects. In this respect, we demonstrate that experiencing the functional property of a tool may selectively modulate the tactile extinction-like phenomenon in healthy subjects. These findings, in addition to validating the normal model of tactile extinction, may open new perspectives for investigating the pervasive aspects of multisensory processing in humans and their neurophysiological bases.