Two-point discrimination

Abstract: 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 characterised. 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. We evaluated the two-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. 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. ANN NEUROL 2014. © 2014 American Neurological Association.

Abstract: Both moving and static two-point discrimination are valid measurements of functional sensibility in the hand. The present study investigated the interobserver variability of these measurements when two observers used the same testing instrument and the same definition of the testing procedure's end point. In testing 30 nerve-injured patients, interobserver variability was found to vary within 1 mm, or less, for moving two-point discrimination in 93.3% of the measurements and for static two-point discrimination in 86.8% of the measurements. In only two of the 30 measurements of moving two-point discrimination and in four of the measurements for static two-point discrimination were interobserver differences equal to 2 mm. Measurements for both moving and static two-point discrimination were analyzed statistically by linear regression and correlation co-efficient techniques. The measurements for two-point discrimination obtained with the protocol employed in this study were found to have an interobserver reliability at the p less than 0.00001 level.

Abstract: Spatial acuity over 13 regions of the body was assessed cross-sectionally in 122 male and female subjects between 8 and 87 years of age. Of two measures, the primary one was a threshold for detecting a gap between two points (a refinement of the conventional two-point threshold). The secondary one was a threshold of point localization in 7 of these 13 body regions. The two measures yielded similar pictures of body acuity and age-related changes in acuity, and they agreed in essentials with an early acuity map dating back to Weber in 1835, as cited and confirmed experimentally by Weinstein (1968). To this acuity map, the present study added the dimension of age. The main finding was that aging is much harder on some body regions than on others. Declining acuity with age was found to characterize all regions to one degree or another, but the hands and feet turned out to be far more vulnerable than the more central regions, including the very acute lip and tongue. Deterioration of acuity in the great toe (averaging 400% between youth and advanced age) and fingertip (averaging 130%) may adversely affect such diverse activities as braille reading, grasping, and maintaining balance. The acuity map determined by gap discrimination was essentially the same for males and females; however, males gave significantly smaller localization thresholds than females. In two body regions tested (fingertip and upper lip), children significantly outperformed young adults at gap discrimination.

Abstract: In assessing the prospects for surgical and other kinds of rehabilitation in tetraplegia and stroke patients the two-point discrimination test, correctly performed by experienced examiners, is of great value. Valid and repeatable results depend on exact technique and proper tools. There is a firm correlation between the thresholds of two-point discrimination on the pulps of the fingers and the accuracy of position sense at the metacarpophalangeal and interphalangeal joints. Microneurography has shown that cutaneous receptors have proprioceptive as well as exteroceptive functions. Thus the two-point discrimination test can be used as a measure of proprioceptive function. The results can be expressed numerically. A two-point threshold less than 10 mm on the pulp is a valid measure of useful finger proprioception. It also shows tactile gnosis, necessary for precision sensory grips.