The size of the receptive fields for light touch can be measured by the two-point threshold test. In this procedure, the two points on a pair of calipers are simultaneously positioned on the skin and one determines the minimum distance between the two caliper points that can be perceived as separate points of stimulation. This is called the two-point discrimination threshold.If the distance is very small, each caliper point touches the receptive field of only one sensory neuron. If the distance between stimulation points is less than this threshold, only one point of stimulation can be felt. Thus, the two-point discrimination threshold is a measure of tactile acuity. The magnitude of two-point discrimination thresholds varies from place to place on the body and is smallest where touch receptors are most abundant.Stimulus points on the back, for instance, must be separated by at least 65 mm before they can be distinguished as separate, whereas on the fingeips two stimuli are recognized if they are separated by as little as 2 mm. Blind individuals benefit from the tactile acuity of fingeips to facilitate the ability to read Braille; the dots forming Braille symbols are separated by 2.5 mm. Two-point discrimination is used to test the integrity of the dorsal column (medial lemniscus) system, the central pathway for touch and proprioception.