Academic journal article Perception and Psychophysics

Tactile Spatial Sensitivity and Anisotropy

Academic journal article Perception and Psychophysics

Tactile Spatial Sensitivity and Anisotropy

Article excerpt

A gap detection task was examined for its usefulness as a measure of tactile spatial sensitivity and as a measure of anisotropy. In Experiment 1, sensitivity was measured with a gap detection task both with and without a latex glove at three locations on the hand: the fingerpad, fmgerbase, and palm. Results showed that sensitivity varied as a function of location and was correlated with changes in the density of innervation of the primary afferent fibers. In accord with other measures of spatial sensitivity, the glove had a moderate effect on sensitivity in the gap detection task. The results both with and without the glove were more similar to those obtained using another measure of spatial sensitivity, the grating orientation task, than to those obtained using the smooth-grooved task, which is considered an intensive measure. In Experiments 2-4, anisotropy was examined using the gap detection and grating orientation tasks, as well as the smooth-grooved task. Locations on the index finger, palm, and arm were tested. Results indicated that anisotropy was revealed only by tasks that relied on spatial cues. The differences between spatial sensitivity measured in the proximal-distal orientation as compared with the lateral-medial orientation varied by location and were as much as 2.35/1. The results are discussed in terms of what they may reveal about the underlying mechanisms responsible for tactile anisotropy.

The attempt to develop measures of tactile spatial acuity has a long history. For over 150 years, the standard measure of tactile spatial acuity has been the two-point threshold (Weber, 1834/1996). From the time of its early use, however, researchers have recognized the inherent problems with the two-point threshold as a measure of tactile spatial acuity (Boring, 1942; Friedline, 1918; Tawney, 1895; Weber, 1834/1996; for reviews, see Craig & Johnson, 2000; Johnson, Van Boven, & Hsiao, 1994; see also Van Boven & Johnson, 1994b). More recently, there have been efforts by several researchers to develop new psychophysical measures of tactile spatial acuity (Craig, 1999; Craig & Kisner, 1998; Craig & Lyle, 2001; Gibson & Craig, 2002; Johnson & Phillips, 1981; Stevens & Choo, 1996; Stevens & Patterson, 1995).

One of these new measures that has received considerable attention is the grating orientation (GR/OR) task (Johnson & Phillips, 1981). In the GR/OR task, subjects discriminate the orientation of a square-wave grating placed on the skin. Performance is measured as a function of the groove width of the grating. It has been shown that performance on the GR/OR task is correlated with the density of innervation of the primary afferent fibers (Gibson & Craig, 2002; Van Boven & Johnson, 1994a, 1994b). In one study that used the GR/OR task, subjects were tested before and after oral surgery that had produced nerve damage. Return to normal performance on the GR/OR task closely paralleled the rate of reinnervation of the primary afferent fibers, as well as the subject's self-report of normal sensory function (Van Boven & Johnson, 1994b). The GR/OR task has also been used as a measure of spatial acuity at a number of different locations on the skin that are known to vary in their density of innervation (Craig, 1999; Craig & Kisner, 1998; Craig & Lyle, 2001; Essock, Krebs, & Prather, 1997; Gibson & Craig, 2002; Johnson & Phillips, 1981; Patel, Essick, & Kelly, 1997; Sathian & Zangaladze, 1996; Van Boven & Johnson, 1994a). In each case, performance on the GR/OR task roughly correlated with changes in density of innervation of the primary afferent fibers. The two types of fibers that convey spatial information are rapidly adapting (RA) and slowly adapting Type I (SAI). Of the two, the SAI fibers are believed to be responsible for the finest spatial information (Johnson & Hsiao, 1992). The density of the SAI fibers is of more interest in determining spatial acuity. …

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