Academic journal article Perception and Psychophysics

Tactile Spatial Acuity Differs between Fingers: A Study Comparing Two Testing Paradigms

Academic journal article Perception and Psychophysics

Tactile Spatial Acuity Differs between Fingers: A Study Comparing Two Testing Paradigms

Article excerpt

Tactile spatial acuity (TSA) is a reliable and reproducible measure of somatosensory system function that has been used to study a broad range of subject populations. Although TSA is most often assessed at the fingertip, published studies employing identical stimuli disagree on whether TSA differs between the fingers of neurologically normal subjects. Using a validated grating orientation discrimination task, we determined TSA bilaterally at the index and ring fingers of 16 healthy young adults. Motivated by earlier work, we utilized two stimulus presentation paradigms, the method of constant stimuli (MCS) and a staircase (SC) method. We found that TSA was significantly higher (the discrimination threshold was lower) at the index than at the ring finger, which was consistent with a prior study. Although mean thresholds at both fingers were higher when measured with the SC than with the MCS paradigm, this difference did not reach statistical significance (p = .14). These findings should facilitate both design and interpretation of future studies investigating TSA.

Tactile spatial acuity (TSA) is a reliable indicator of somatosensory system integrity, and it is highly correlated with subjective tactile perceptual dysfunction (Johnson & Hsiao, 1992; Johnson & Phillips, 1981; Van Boven & Johnson, 1994a, 1994b). A well-established and reproducible measure of TSA is the psychophysical threshold for discrimination of grating orientation (Johnson & Phillips, 1981 ; Sathian & Zangaladze, 1996; Van Boven & Johnson, 1994a, 1994b). To obtain this measure, gratings varying in spatial frequency are applied to a body part and the subject states die stimulus orientation with respect to a bodycentered axis, in a forced choice paradigm, resulting in a grating orientation discrimination threshold (GOT). This measure of TSA has been applied to the study of peripheral nerve injury (Van Boven & Johnson, 1994b), Braille readers (Goldreich & Kanics, 2003; Grant, Thiagarajah, & Sathian, 2000; Van Boven, Hamilton, Kaufrman, Keenan, & Pascual-Leone, 2000), Parkinson's disease (Sathian, Zangaladze, Green, Vitek, & DeLong, 1997), dyslexia (Grant, Zangaladze, Thiagarajah, & Sathian, 1999), and age-related changes in spatial acuity (Sathian et al., 1997; Tremblay, Backman, Cuenco, Vant, & Wassef, 2000; Vega-Bermudez & Johnson, 2004), as well as in the study of other subject populations. Surprisingly, despite the frequent application of this measure at the human fingertip, there is no consensus on whether TSA differs between the fingers of neurologically normal subjects. Furthermore, different stimulus presentation paradigms may produce different GOT values.

Two studies have reported normative GOT values at the different fingers. One study (Sathian & Zangaladze, 1996; N = 7 subjects; mean age, 27.6 years) found equivalent thresholds at the first through the fourth digit, whereas another study (Vega-Bermudez & Johnson, 2001 ; 8 subjects; mean age, 34.8 years) found that the GOT increased significantly from the second to the third digit, and from the third to the fourth digit. Studies comparing the GOT at the index fingertip between a variety of subject populations and nonelderly normal controls (<55 years) have reported mean normal values ranging from 0.82 mm (Sathian & Zangaladze, 1997) up to 1.57 mm (Van Boven et al., 2000; see their Figure 2). In the latter study, a GOT of 0.97 mm in blind Braille readers was interpreted to represent supranormal performance (Van Boven et al., 2000).

Vega-Bermudez and Johnson (2004) have recently shown that in young subjects, natural variation in fingertip skin conformance accounts for about 50% of the statistical variation in the tactile GOT. Another factor that may account for some between-studies variation in normal values is the testing method. The two stimulus presentation paradigms utilized in these studies, the method of constant stimuli (MCS) and the staircase (SC) method, both result in a quantitative psychophysical threshold. …

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