The spatial resolution of the nociceptive system.

Background and aims: A systematic study of the spatial resolution of the nociceptive system across different body districts is surprisingly lacking, especially considering the recent description of a fovea for pain at the fingertips (Mancini et al Curr Biol 2013). In this psychophysical study we assessed the spatial acuity of ten body regions, using the two-point discrimination (2PD) of Nd:YAP laser pulses that selectively activate Aδ cutaneous afferents. Methods: Using two Nd:YAP lasers we delivered either one or two simultaneous pulses (diameter: 1.3 mm) of identical energy on each of the following body sites: forehead, volar forearm, hand dorsum, hand palm, fingertip, shoulder, lower back, upper thigh, calf, and foot dorsum. The 2PD task consisted in judging whether one or two stimuli were delivered, using ascending and descending staircases of varying spatial distance between the two pulses. Single stimuli were included as catch trials. All stimuli elicited a clear pinprick sensation related to the activation of Aδ afferents. Results: We found a proximal-distal gradient of spatial acuity for nociceptive stimuli, with higher acuity for proximal stimuli, an observation in agreement with innervation density of skin nociceptors and perceptual threshold of Aδ stimuli. Two distal regions (fingertips and palm) represented an exception, and showed maximal spatial acuity (see also Mancini et al., 2013). Conclusions: This study provides the first systematic description of spatial acuity of the nociceptive system across the entire body in healthy participants.