No abstract

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.

Abstract& Tactile±visual links in spatial attention were examined by presenting spatially nonpredictive tactile cues to the left or right hand, shortly prior to visual targets in the left or right hemifield. To examine the spatial coordinates of any crossmodal links, different postures were examined. The hands were either uncrossed, or crossed so that the left hand lay in the right visual field and vice versa. Visual judgments were better on the side where the stimulated hand lay, though this effect was somewhat smaller with longer intervals between cue and target, and with crossed hands. Event-related brain potentials (ERPs) showed a similar pattern. Larger amplitude occipital N1 components were obtained for visual events on the same side as the preceding tactile cue, at ipsilateral electrode sites. Negativities in the Nd2 interval at midline and lateral central sites, and in the Nd1 interval at electrode Pz, were also enhanced for the cued side. As in the psychophysical results, ERP cueing effects during the crossed posture were determined by the side of space in which the stimulated hand lay, not by the anatomical side of the initial hemispheric projection for the tactile cue. These results demonstrate that crossmodal links in spatial attention can influence sensory brain responses as early as the N1, and that these links operate in a spatial frame-of-reference that can remap between the modalities across changes in posture. &

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.