Awareness of space mediates crossed-hands analgesia

Crossing the hands over the midline reduces the perceived intensity of nociceptive stimuli applied onto the hands. It is unclear how much of this effect relies on the representation of the space surrounding our body. Here we used the crossed hands paradigm in patients with unilateral spatial neglect, a neuropsychological condition characterized by the inability to detect, attend and respond to contralesional (most often left) stimuli, as consequence of a brain damage affecting spatial attention and representation. Importantly, neglect patients have a spared ability to process stimuli in the non-affected space, as such providing the model to investigate the effect of space representation in nociceptive processing. Thirty patients with brain damage (16 with spared representation of space, who acted as controls, and 14 with spatial deficits) rated mechanical punctate pinprick stimuli applied onto the crossed or uncrossed hands. We hypothesized that if the so-called ‘crossed-hands analgesia’ is based on representation on space patients with unilateral spatial neglect would not show any reduction of the perceived intensity of the stimuli when applied onto the crossing hands. In line with this hypothesis, we observed the ‘crossed-hands analgesia’ effect in patients with brain damage but spared representation of space, but not in neglect patients, that instead perceived as more intense stimuli applied onto the left contralesional hand when the hand was positioned in the right healthy space. These results indicate that space representation plays a fundamental role in the processing of nociceptive stimuli