The temporal order judgement of tactile and nociceptive stimuli is impaired by crossing the hands over the body midline.

Crossing the hands over the midline impairs the ability to correctly judge the order of a pair of tactile stimuli, delivered in rapid succession, one to each hand. This impairment, termed crossed-hands deficit, has been attributed to a mismatch between the somatotopic and body-centred frames of reference, onto which somatosensory stimuli are automatically mapped. Whether or not such crossed-hands deficit occurs also when delivering nociceptive stimuli has not been previously investigated. In this study, participants performed a temporal order judgement (TOJ) task in which pairs of either nociceptive or tactile stimuli were delivered, one to each hand, while their arms were either crossed over the body midline or uncrossed. We observed that crossing the hands over the midline significantly decreases the ability to determine the stimulus order when a pair of nociceptive stimuli is delivered to the hands, and that this crossed-hands deficit has a temporal profile similar to that observed for tactile stimuli. These findings suggest that similar mechanisms for integrating somatotopic and body-centred frames of reference underlie the ability to localise both nociceptive and tactile stimuli in space.

The analgesic effect of crossing the arms.

The ability to determine precisely the location of sensory stimuli is fundamental to how we interact with the world; indeed, to our survival. Crossing the hands over the body midline impairs this ability to localize tactile stimuli. We hypothesized that crossing the arms would modulate the intensity of pain evoked by noxious stimulation of the hand. In two separate experiments, we show (1) that the intensity of both laser-evoked painful sensations and electrically-evoked nonpainful sensations were decreased when the arms were crossed over the midline, and (2) that these effects were associated with changes in the multimodal cortical processing of somatosensory information. Critically, there was no change in the somatosensory-specific cortical processing of somatosensory information. Besides studies showing relief of phantom limb pain using mirrors, this is the first evidence that impeding the processes by which the brain localises a noxious stimulus can reduce pain, and that this effect reflects modulation of multimodal neural activities. By showing that the neural mechanisms by which pain emerges from nociception represent a possible target for analgesia, we raise the possibility of novel approaches to the treatment of painful clinical conditions. Crossing the arms over the midline impairs multimodal processing of somatosensory stimuli and induces significant analgesia to noxious hand stimulation.

When the whole is more than the sum of the parts: evidence from visuospatial neglect.

One possible pathological mechanism underlying the rightward bisection error of right-brain-damaged patients with left spatial neglect is a leftward relaxation of the spatial representational medium. This view was originally based on the finding that patients with left neglect, required to extend horizontal segments, in order to double their original length, may exhibit a relative left overextension of the drawn lines (Bisiach et al., 1994). We investigated this putative distortion of representational space using a 16 cm 'line segmentation' task (Experiment I). Were the representation of space relaxed contralesionally, a progressive increase from right to left of the size of the drawn segments would be expected. Right-brain-damaged patients with left unilateral neglect (N=12) performed the segmentation task with no left versus right differences, as right-brain-damaged patients without neglect (N=8), and neurologically unimpaired control subjects (N=10), did. Experiments 2 and 3 explored the effects of sample length (1, 2, 4, and 8cm), by which the 16cm lines had to be segmented. Neglect patients produced longer left-sided segments only for the 8 cm sample (i.e. half of the length of the segment). This set of experiments suggests an impairment in the segmentation task only with the larger (8 cm) sample, when a more global level of processing may be involved. Experiment 4 assessed this hypothesis by a 'part/whole' bisection task, using 8 cm lines, presented either embedded in a longer 16 cm line or in isolation. Neglect patients made a larger rightward bisection error when the segment was not embedded. The suggestion is made that the lateral distortion of the representation of space in neglect patients (i.e., a leftward relaxation of the spatial medium) concerns tasks where a more 'global' representation of the visual stimulus has to be set up. The different demands of the segmentation and bisection tasks are discussed.