Abstract
Two-point discrimination is widely used to measure tactile spatial acuity. The validity of the two-point threshold as a spatial acuity measure rests on the assumption that two points can be distinguished from one only when the two points are sufficiently separated to evoke spatially distinguishable foci of neural activity. However, some previous research has challenged this view, suggesting instead that two-point task performance benefits from an unintended non-spatial cue, allowing spuriously good performance at small tip separations. We compared the traditional two-point task to an equally convenient alternative task in which participants attempt to discern the orientation (vertical or horizontal) of two points of contact. We used precision digital readout calipers to administer two-interval forced-choice versions of both tasks to 24 neurologically healthy adults, on the fingertip, finger base, palm, and forearm. We used Bayesian adaptive testing to estimate the participants’ psychometric functions on the two tasks. Traditional two-point performance remained significantly above chance levels even at zero point separation. In contrast, two-point orientation discrimination approached chance as point separation approached zero, as expected for a valid measure of tactile spatial acuity. Traditional two-point performance was so inflated at small point separations that 75%-correct thresholds could be determined on all tested sites for fewer than half of participants. The 95%-correct thresholds on the two tasks were similar, and correlated with receptive field spacing. In keeping with previous critiques, we conclude that the traditional two-point task provides an unintended non-spatial cue, resulting in spuriously good performance at small spatial separations. Unlike two-point discrimination, two-point orientation discrimination rigorously measures tactile spatial acuity. We recommend the use of two-point orientation discrimination for neurological assessment.
Highlights
Two-point discrimination (2PD) has been used to measure tactile spatial acuity ever since E
The 2PD task is widely used clinically (Dellon, 1981; American Society for Surgery of the Hand, 1983; Van Boven and Johnson, 1994; Lundborg and Rosen, 2004; Jerosch-Herold, 2005; Campbell et al, 2013) and has been used in several research laboratories to characterize tactile spatial acuity in healthy populations (Godde et al, 2000; Kennett et al, 2001; Dinse et al, 2006; Boles and Givens, 2011)
Our results confirm that the 2PD threshold is not a pure measure of spatial acuity
Summary
Two-point discrimination (2PD) has been used to measure tactile spatial acuity ever since E. It has been assumed that two points are distinguishable from one only when the two points are sufficiently separated to evoke spatially distinct foci of neural activity (Mountcastle and Bard, 1968; Vallbo and Johansson, 1978). In the “textbook view” of the 2PD task, two points that fall closely together, for instance within a single afferent receptive field, will evoke only one locus of neural activity and will be misperceived as a single point (Brodal, 2010; Purves et al, 2012). The threshold separation at which neurologically healthy individuals can correctly identify two points has been assumed by many to reflect the size and spacing of cutaneous receptive fields, the innervation density of slowly adapting type-I (SA1) afferents, the tactile axons that convey fine spatial information (Johnson, 2001)
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