Research on sensory perception now often considers more than one sense at a time. This approach reflects real-world situations, such as when a visible object touches us. Indeed, vision and touch show great interdependence: the sight of a body part can reduce tactile target detection times [1], visual and tactile attentional systems are spatially linked [2], and the texture of surfaces that are actively touched with the fingertips is perceived using both vision and touch [3]. However, these previous findings might be mediated by spatial attention [1, 2] or by improved guidance of movement [3] via visually enhanced body position sense [4--6]. Here, we investigate the direct effects of viewing the body on passive touch. We measured tactile two-point discrimination thresholds [7] on the forearm while manipulating the visibility of the arm but holding gaze direction constant. The spatial resolution of touch was better when the arm was visible than when it was not. Tactile performance was further improved when the view of the arm was magnified. In contrast, performance was not improved by viewing a neutral object at the arm's location, ruling out improved spatial orienting as a possible account. Controls confirmed that no information about the tactile stimulation was provided by visibility of the arm. This visual enhancement of touch may point to online reorganization of tactile receptive fields.

The perceived size of objects touching different regions of skin varies across the body surface by much less than is predicted from variations in tactile receptor density. Here we show that altering the visual experience of the body alters perceived tactile distances. We propose that the brain attempts to preserve tactile size constancy by rescaling the primary, distorted body-surface representation into object-centered space according to visual experience of the body.

Over 150 years ago, E.H. Weber declared that experience showed that tactile acuity was not affected by viewing the stimulated body part. However, more recent investigations suggest that cross-modal links do exist between the senses. Viewing the stimulated body site improves performance on tactile discrimination and detection tasks and enhances tactile acuity. Here, we show that vision modulates somatosensory cortex activity, as measured by somatosensory event-related potentials (ERPs). This modulation is greatest when tactile stimulation is task relevant. Visual modulation is not present in the P50 component reflecting the primary afferent input to the cortex but appears in the subsequent N80 component, which has also been localized to SI, the primary somatosensory cortex. Furthermore, we replicate previous findings that noninformative vision improves spatial acuity. These results are consistent with a hypothesis that vision modulates cortical processing of tactile stimuli via back projections from multimodal cortical areas. Several neurophysiological studies suggest that primary and secondary somatosensory cortex (SI and SII, respectively) activity can be modulated by spatial and tactile attention and by visual cues. To our knowledge, this is the first demonstration of direct modulation of somatosensory cortex activity by a noninformative view of the stimulated body site with concomitant enhancement of tactile acuity in normal subjects.

The brain keeps track of the changing positions of body parts in space using a spatial body schema. When subjects localise a tactile stimulus on the skin, they might either use a somatotopic body map, or use a body schema to identify the location of the stimulation in external space. Healthy subjects were touched on the fingertips, with the hands in one of two postures: either the right hand was vertically above the left, or the fingers of both hands were interwoven. Subjects made speeded verbal responses to identify either the finger or the hand that was touched. Interweaving the fingers significantly impaired hand identification across several experiments, but had no effect on finger identification. Our results suggest that identification of fingers occurs in a somatotopic representation or finger schema. Identification of hands uses a general body schema, and is influenced by external spatial location. This dissociation implies that touches on the finger can only be identified with a particular hand after a process of assigning fingers to hands. This assignment is based on external spatial location. Our results suggest a role of the body schema in the identification of structural body parts from touch.

Perception of our own bodies is based on integration of visual and tactile inputs, notably by neurons in the brain's parietal lobes. Here we report a behavioural consequence of this integration process. Simply viewing the arm can speed up reactions to an invisible tactile stimulus on the arm. We observed this visual enhancement effect only when a tactile task required spatial computation within a topographic map of the body surface and the judgements made were close to the limits of performance. This effect of viewing the body surface was absent or reversed in tasks that either did not require a spatial computation or in which judgements were well above performance limits. We consider possible mechanisms by which vision may influence tactile processing.

An abnormal sense of agency is among the most characteristic yet perplexing positive symptoms of schizophrenia. Schizophrenics may either attribute the consequences of their own actions to the intentions of others (delusions of influence), or may perceive themselves as causing events which they do not in fact control (megalomania). Previous reports have often described inaccurate agency judgements in schizophrenia, but have not identified the disordered neural mechanisms or psychological processes underlying these judgements. We report the perceived time of a voluntary action and its consequence in eight schizophrenic patients and matched controls. The patients showed an unusually strong binding effect between actions and consequences. Specifically, the temporal interval between action and consequence appeared shorter for patients than for controls. Patients may overassociate their actions with subsequent events, experiencing their actions as having unusual causal efficacy. Disorders of agency may reflect an underlying abnormality in the experience of voluntary action.

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An abnormal sense of agency is among the most characteristic yet perplexing positive symptoms of schizophrenia. Schizophrenics may either attribute the consequences of their own actions to the intentions of others (delusions of influence), or may perceive themselves as causing events which they do not in fact control (megalomania). Previous reports have often described inaccurate agency judgements in schizophrenia, but have not identified the disordered neural mechanisms or psychological processes underlying these judgements. We report the perceived time of a voluntary action and its consequence in eight schizophrenic patients and matched controls. The patients showed an unusually strong binding effect between actions and consequences. Specifically, the temporal interval between action and consequence appeared shorter for patients than for controls. Patients may overassociate their actions with subsequent events, experiencing their actions as having unusual causal efficacy. Disorders of agency may reflect an underlying abnormality in the experience of voluntary action.