Two-point acuity

Spatial resolution has been studied by the two-point acuity test, in which the skin is poked in two nearby places by a pair of sharp calipers. The subjects are asked whether they perceive a single poke, or two pokes in different places at the same time. The detection thresholds are then arranged by the location on the body to understand how the spatial resolution varies. The sharpest acuity is on the tongue and hands, where points can be distinguished if they are as close as 2 or 3mm. The tips of the tongue and fingers have the highest acuity. For the forehead, the threshold is around 20mm. The back has the lowest acuity, resulting in a threshold of around 60mm. These results have also been shown to correspond directly to the sizes of receptive fields in the somatosensory cortex. For example, neurons that correspond to the back have much larger fields (in terms of skin area) than those of the fingertip.