What can errors tell us about body representations?

In this review, we examine how tactile misperceptions provide evidence regarding body representations. First, we propose that tactile detection and localization are serial processes, in contrast to parallel processing hypotheses based on patients with numbsense. Second, we discuss how information in primary somatosensory maps projects to body size and shape representations to localize touch on the skin surface, and how responses after use-dependent plasticity reflect changes in this mapping. Third, we review situations in which our body representations are inconsistent with our actual body shape, specifically discussing phantom limb phenomena and anesthetization. We discuss problems with the traditional remapping hypothesis in amputees, factors that modulate perceived body size and shape, and how changes in perceived body form influence tactile localization. Finally, we review studies in which brain-damaged individuals perceive touch on the opposite side of the body, and demonstrate how interhemispheric mechanisms can give rise to these anomalous percepts.

Phantom touch: How to unmask sensory unawareness after stroke.

Comprehending the nature of tactile disorders following brain damage is crucial to understand how the brain constructs sensory awareness. Stroke patients may be unaware of being touched on the affected hand if, simultaneously, they are touched on the unaffected hand (i.e., tactile extinction). More rarely, they feel touches on the two hands, when they are solely touched on the unaffected hand (i.e., synchiria). Using a novel assessment tool, we investigated whether in stroke patients with apparent intact tactile awareness on standard evaluation, tactile extinction might be possibly masked by phantom (synchiric) sensations (i.e., elicited by ipsilesional stimulation) arising exclusively during Double Simultaneous Stimulation (DSS). Patients with right (n = 17) and left (n = 8) hemisphere lesions and age-matched healthy controls (n = 13) were tested with the Tactile Quadrant Stimulation test, consisting in delivering unilateral or bilateral touches to one of four quadrants, identified on the participants' hands. In DSS trials, stimuli were applied to asymmetric quadrants. Participants reported the side(s) and then pointed to the site(s) of stimulation. We found that, with the exception of one patient who showed tactile extinction, about 50% of patients with overall intact tactile perception on classical evaluation, although reporting two stimuli in DSS, failed in pointing to the correct contralesional stimulated site. They reported the felt sensation in positions that corresponded to the ipsilesional stimulated sites. Thus apparent detections of contralesional touches in DSS were accounted for by 'phantom' sensations of ipsilesional stimulation that masked unawareness of contralesional touches when classic assessment was applied. Preliminary lesion analyses indicate that the symptom was associated with damage to structures often affected in tactile extinction. These findings, while unveiling important underestimation of the patients' neurological condition, provide a framework for investigating bihemispheric contributions to altered tactile perception following stroke.

Phantoms on the hands: Influence of the body on brief synchiric visual percepts.

Recent studies have found preferential responses for brief, transient visual stimuli near the hands, suggesting a link between magnocellular visual processing and peripersonal representations. We report an individual with a right hemisphere lesion whose illusory phantom percepts may be attributable to an impairment in the peripersonal system specific to transient visual stimuli. When presented with a single, brief (250 ms) visual stimulus to her ipsilesional side, she reported visual percepts on both sides - synchiria. These contralesional phantoms were significantly more frequent when visual stimuli were presented on the hands versus off the hands. We next manipulated stimulus duration to examine the relationship between these phantom percepts and transient visual processing. We found a significant position by duration interaction, with substantially more phantom synchiric percepts on the hands for brief compared to sustained stimuli. This deficit provides novel evidence both for preferential processing of transient visual stimuli near the hands, and for mechanisms that, when damaged, result in phantom percepts.

Increased attentional load moves the left to the right.

INTRODUCTION: Unilateral brain damage can heterogeneously alter spatial processing. Very often brain-lesioned patients fail to report (neglect) items appearing within the contralesional space. Much less often patients mislocalize items' spatial position. We investigated whether a top-down attentional load manipulation (dual-tasking), known to result in contralesional omissions even in apparently unimpaired cases, might also induce spatial mislocalizations. METHOD: Nine right-hemisphere-damaged patients performed three computer-based tasks encompassing different levels of attentional load. The side of appearance of visual targets had to be reported either in isolation or while processing additional information (visual or auditory dual task). Spatial mislocalizations (from the contralesional hemispace towards the ipsilesional unaffected one) were then contrasted with omissions both within and across tasks, at individual as well as at group level. RESULTS: The representation of ipsilesional targets was accurate and not affected by dual-tasking requirements. Contralesional targets were instead often omitted and, under dual-task conditions, also mislocalized by four patients. Three cases reported a significant number of left targets as appearing on the right (alloesthesia). Two of these patients perceived more targets (albeit to a wrong spatial location) under dual- than under single-task load. In a fourth patient, increased visual load resulted in synchiria, the (mis)perception of single, contralesional targets as being two (one on each side). CONCLUSIONS: When the neural circuitry subtending spatial processing is damaged, an increase in task load can lead to either a disregard or a bias in the processing of contralesional hemispace. The spatial bias subtending mislocalizations seems to index a more severe deficit than neglect, as if contralesional space would be completely erased rather than merely ignored.