Two-point discriminability: relation to properties of the somatosensory system

The ability to resolve two closely spaced cutaneous stimuli presumably depends upon the degree of overlap between the two populations of responding neurons. The degree of overlap is determined by receptive field (RF) geometry and location, and by interactive factors such as lateral inhibition. In this paper, we first consider some aspects of RFs that would be expected to influence two-point acuity. In some somatosensory brain regions, relatively few RFs overlap the body midline. As would be expected, discrimination is enhanced for two points straddling the backbone. This does not simply reflect a mediolateral gradient of acuity, as we found higher acuity laterally. On the limbs, where RFs are elongated along the length of the limb, transverse two-point acuity was greater than longitudinal acuity. However, on the back, where RFs are fairly round, there was an even larger orientation effect, with two-point acuity greater for stimuli longitudinal than for stimuli transverse to the spine. Thus, the substantial variation of two-point acuity with stimulus orientation on the back cannot be explained by RF geometry alone. We discuss the possibility that differences in lateral inhibition and degree of similarity in dermatomal composition contribute to the observed stimulus orientation effects.