The Two-Point Threshold

The two-point threshold, or compass test, has long been used as a measure of tactile spatial resolution; however, since it was first developed, there have been problems associated with its use. Some of these problems include setting an appropriate criterion for responding “two,” extreme variability both within and between subjects, and the ability of subjects to discriminate two points from one at separations well below the two-point threshold. Recent neurophysiological results have clarified some of the neural mechanisms responsible for spatial resolution and demonstrated the inadequacy of the two-point threshold as a measure of spatial mechanisms. Several new methods may overcome these problems and provide a valid measure of spatial resolution and a reflection of neural mechanisms.