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Tactile Acuity

When an object is too small or too far off in the distance, we cannot identify it visually. We can detect its presence but cannot make out its features. This inability to make out small features is caused by the limited spatial acuity (or spatial resolution) of the visual system. Our visual acuity is tested, using a so-called Snellen chart, by determining what the smallest identifiable letter is. Eye exams are, in effect, tests of how small visual features can be before we can no longer discern them.

The sense of touch is also subject to limited resolution. Indeed, when we run our fingers across surfaces with large features, such as the number pad on a phone, we can discern individual features (e.g., the buttons on the pad). On the other hand, if we scan a surface with small features, like that of an upholstered sofa, we discern texture but not individuated spatial features. This entry covers the tests of tactile spatial acuity, neural basis of tactile spatial acuity, the factors affecting spatial acuity, and why we should measure spatial acuity.

Tests of Tactile Spatial Acuity

Two-Point Threshold

The traditional test of tactile spatial acuity is the two-point threshold (2PT). The test consists of measuring how far apart two points contacting the skin need to be before they are perceived as two points rather than one. The 2PT is administered using a compasslike instrument in which the two rods of the compass have punctate tips. The test consists of pressing the two tips, separated by a predetermined amount, against the skin simultaneously and having subjects judge whether they feel one or two contact points. The test is repeated with a variety of distances between the two tips. The closer the two points are when the subject can still perceive them as two, the higher the acuity. The idea is that, to the extent that you can make out fine spatial features, you will be able to discern two tips as distinct when they are close together. If your tactile spatial resolution is coarse, you will perceive them as a single point.

Recently, however, the 2PT has fallen out of favor because it relies on subjective judgments: the perception of the two contact points falls along a continuum, from a single circular contact point (when the two contact points are adjacent or very close), to an increasingly elongated point (at a range of intermediate separations), to two points (at larger separations). The judgment of whether the two contact points feel distinct is therefore subjective.

One way to get around the subjectivity of the 2PT is to use a two-alternative forced choice task. On each trial, the subject is presented with two stimuli in sequence: One consists of a single tip and the other of two tips. Sometimes the single tip is presented first and the two tips second, sometimes the reverse is true. The subject's task is to judge which of the two stimuli (first or second) comprised two tips. In order to perform this task, the subject has to be able to distinguish one from the other. However, this task is problematic in that nonspatial cues may help subjects perform it. For instance, two nearby tips may feel like a single contact point with a dull tip whereas the single tip feels sharper, a judgment based on an intensive rather than spatial difference.

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