Whole-body mapping of spatial acuity for pain and touch

Abstract

Objective: Tactile spatial acuity is routinely tested in neurology to assess the state of the dorsal column system. In contrast, spatial acuity for pain is not assessed, having never been systematically characterized. More than a century after the initial description of tactile acuity across the body, we provide the first systematic whole-body mapping of spatial acuity for pain.

Methods: We evaluated the 2-point discrimination thresholds for both nociceptive-selective and tactile stimuli across several skin regions. Thresholds were estimated using pairs of simultaneous stimuli, and also using successive stimuli.

Results and interpretation: These two approaches produced convergent results. The fingertip was the area of highest spatial acuity, for both pain and touch. On the glabrous skin of the hand, the gradient of spatial acuity for pain followed that observed for touch. On the hairy skin of the upper limb, spatial acuity for pain and touch followed opposite proximal-distal gradients, consistent with the known innervation density of this body territory. Finally, by testing spatial acuity for pain in a rare participant completely lacking Aβ fibers, we demonstrate that spatial acuity for pain does not rely on a functioning system of tactile primary afferents. This study represents the first systematic characterization of spatial acuity for pain across multiple regions of the body surface.

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 staircase with simultaneous stimuli is depicted. For each modality, we delivered either single stimuli (25% of trials) or 2 simultaneous stimuli (75% of trials). Participants were required to discriminate whether they felt 1 or 2 stimuli, and the 2PD threshold was defined 3× for each territory, using the method of limits.

FIGURE 2

Mean 2-point discrimination (2PD) thresholds for pain and touch across the body surface. Thresholds were measured in 2 separate groups of participants, using either simultaneous stimuli (top panel, n = 16) or successive stimuli (bottom panel, n = 10). Error bars express variability (standard error) across participants. [Color figure can be viewed in the online issue, which is available at www.annalsofneurology.org.]

FIGURE 3

Individual 2-point discrimination (2PD) thresholds for touch (top panels) and pain (bottom panels) across the body surface, using either simultaneous (left panels, n = 16) or successive stimuli (right panels, n = 10). Thin lines depict single participants. The thick black line represents the group average. [Color figure can be viewed in the online issue, which is available at www.annalsofneurology.org.]

FIGURE 4

Gradients of 2-point discrimination (2PD) thresholds on the hairy and glabrous skin of the upper (left panels) and lower limbs (right panels), for pain and touch. 2PD was assessed using either simultaneous stimuli (top panels, n = 16) or successive stimuli (bottom panels, n = 10), in separate experiments. Error bars express variability (standard error) across participants. [Color figure can be viewed in the online issue, which is available at www.annalsofneurology.org.]

FIGURE 5

Comparison of 2-point discrimination (2PD) for touch, as measured by Weinstein, by Weber, and in the present study. Both Weinstein and Weber used simultaneous stimuli. In the present study, we used both simultaneous and successive stimuli. Error bars express variability (standard error) across participants. [Color figure can be viewed in the online issue, which is available at www.annalsofneurology.org.]