Two-point orientation discrimination versus the traditional two-point test for tactile spatial acuity assessment

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.

Keywords: neurological examination; psychophysics; reliability and validity; sensory testing; somatosensory discrimination; spatial acuity; tactile perception.

Figure 1

Calipers. (A) The calipers used in the study, opened to a tip separation of 4.5 mm. (B) Magnified images of the caliper tips above a scale with marks at 0.5 mm intervals. Top: closed caliper tips (0 mm separation); bottom: tips opened to 2.0 mm separation.

Figure 2

Two-interval forced-choice perceptual tasks. (A) The four test locations are indicated (dashed outlines): forearm, palm, finger base, and fingertip. (B) 2PD task (shown on palm as an example). Participants reported whether the two-point stimulus preceded (upper) or followed (lower) the one-point stimulus. (C) 2POD task (shown on palm as an example). Participants reported whether the horizontally oriented two-point stimulus preceded (upper) or followed (lower) the vertically oriented two-point stimulus.

Figure 3

Bayesian adaptive testing procedure. Plots illustrate the trial-by-trial performance (upper) and gamma parameter posterior density (lower) for experiments done on the finger base of one participant. (A) 2PD task. (B) 2POD task. Crosses represent incorrect responses; circles, correct responses. Note that, at zero mm tip separation, this participant answered correctly on 21/25 = 84% of 2PD trials, compared to 6/13 = 46% of 2POD trials. The Bayesian adaptive procedure does not follow a preset stimulus sequence or simple staircase algorithm, but rather selects the separation on each trial that is expected to provide the most information regarding the shape of the participant’s psychometric function.

Figure 4

Mean performance by task and body site. Proportion correct versus caliper tip separation on (A) 2PD and (B) 2POD. Data points are means across all participants. For illustration purposes, all curves have been extended to 40 mm on the x-axis.

Figure 5

Mean γ-parameters by task and body site. Gray bars, mean 2PD γ; white bars, mean 2POD γ. Error bars: ±1 SE. Dotted line: γ = 0.5.

Figure 6

Mean 95%-correct thresholds by task and body site. Gray bars, mean 2PD 95%-correct thresholds; white bars, mean 2POD 95%-correct thresholds. Error bars: ±1 SE.

Figure 7

95%-correct thresholds versus receptor spacing. Participants’ 95%-correct thresholds for 2PD (A) and 2POD (B) plotted against estimated SA-1 receptive field spacing (Johansson and Vallbo, ; Olausson et al., 2000): fingertip (1.20 mm), finger base (1.77 mm), palm (3.53 mm), forearm (5.00 mm). Data points show individual participant performance; dashed lines connect group means.

Figure 8

Neural response magnitude cues in the 2PD task. The three panels show hypothetical activity profiles of a population of central somatosensory neurons in response to three stimulus configurations: (A) a single point, (B) two closely spaced points, and (C) two points separated by a greater distance. We assume that the activity of central neurons reflects approximately that of the SA-1 afferents, described in Vega-Bermudez and Johnson (1999). In the textbook view of the 2PD task, the stimulus configurations illustrated in (A) and (B) would be indistinguishable from one another, because both configurations result in a single peak of neural activity. However, the neurophysiological data (Vega-Bermudez and Johnson, 1999) suggest that the population response in (B) is of lower magnitude than in (A), a cue that allows the participant to distinguish (A) from (B) by non-spatial means. In (C), the two activity peaks are indeed distinguishable spatially; in addition, because each activity peak in (C) has equal height to the single peak in (A), the total population response in (C) is greater that in (A), giving rise to another magnitude cue.

Figure 9

Advantage of 2POD over 2PD for tactile spatial acuity assessment. Each panel depicts idealized circular receptive fields of nine SA-1 afferents; for clarity, only non-overlapping fields are shown. Asterisks represent point stimuli. In 2PD, the participant attempts to distinguish between a single point (A) and two points separated by some distance, e.g., (B) or (C). For illustration, we assume that a single point evokes 100 action potentials per second in the central SA-1. When two points fall within the same receptive field (B), they evoke fewer action potentials than the single point. For instance, two points at 1 mm separation evoke on average 88% the firing rate of a single point (Vega-Bermudez and Johnson, 1999). Thus, the participant can distinguish one from two points based on the number of action potentials (magnitude cue), even when the two points cannot be individually perceived. When separated by a greater distance (C), the two points can be perceived, because they fall within separate receptive fields (spatial cue). In addition, the magnitude cue has reversed direction, as the total number of action potentials in the two-point condition (200) is twice that in the one-point condition. Thus, the two-point task conveys spatial information at larger separations but is contaminated by a magnitude cue at all separations. In 2POD, the participant attempts to distinguish between two points separated horizontally and two points separated vertically by the same distance: (B) vs. (D), or (C) vs. (E). These stimuli evoke an equal number of action potentials, eliminating the magnitude cue and forcing the participant to rely on purely spatial information. When the points fall within a single receptive field, as in (B) and (D), their orientation is indistinguishable. When the points fall within distinct receptive fields, as in (C) and (E), their orientation is distinguishable.