Whole-body mapping of spatial acuity for pain and touch

The ability to discriminate 2 stimuli close in space, called spatial acuity is a fundamental function of exteroceptive sensory systems. Typically, spatial acuity is not homogeneous across the receptive surface, depending upon receptive field (RF) size and innervation density.1 In the somatosensory system, there is detailed knowledge about the topographical distribution of spatial acuity for touch throughout the whole body.2,3 This information is clinically relevant, as tactile acuity is routinely tested in neurological patients to assess the state of the dorsal column system. Reduced tactile acuity for specific body territories is the hallmark of important clinical conditions (eg, the stocking and glove distribution of impaired acuity in polyneuropathies). In contrast, more than a century after the first description of the spatial acuity for touch across the body,2,3 it is still not known how acuity for pain is distributed throughout the body surface. Technical difficulties in delivering sensory stimuli that are both nociceptive-selective and spatially specific4 underlie this knowledge gap. Radiant heat laser pulses, which excite intraepidermal Aδ- and C-fiber endings without coactivating mechanoreceptors,5 are commonly delivered using beam diameters of 4 to 7mm. However, it is possible to narrow the laser beam to much smaller diameters. Here, we used this approach to provide the first systematic characterization of nociceptive spatial acuity across the body surface. We delivered 2 Nd:YAP laser pulses with a diameter of 1.3mm, and we assessed spatial acuity by measuring the 2-point discrimination (2PD) thresholds for pain across several skin regions. In 2 separate experiments conducted in healthy participants, we evaluated 2PD thresholds using both simultaneous and successive pairs of somatosensory stimuli. In each experiment, we compared 2PD for pain to 2PD for touch in the same volunteers and body sites (Fig 1). Moreover, we studied spatial acuity for pain in a rare participant completely lacking large-myelinated sensory fibers, but with intact Aδ- and C-fiber function,6,7 to test whether the measures of spatial acuity for pain were dependent on the presence of an intact tactile sensory system. FIGURE 1 Method. Spatial acuity was assessed by measuring 2-point discrimination (2PD) thresholds for both pain and touch in 11 body territories of the same healthy volunteers. In the bottom-right panel, an example of staircase13 with simultaneous stimuli is depicted. ...