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, 31 (19), 7028-37

Tactile Spatial Acuity Enhancement in Blindness: Evidence for Experience-Dependent Mechanisms

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Tactile Spatial Acuity Enhancement in Blindness: Evidence for Experience-Dependent Mechanisms

Michael Wong et al. J Neurosci.

Abstract

Tactile spatial acuity is enhanced in blindness, according to several studies, but the cause of this enhancement has been controversial. Two competing hypotheses are the tactile experience hypothesis (reliance on the sense of touch drives tactile-acuity enhancement) and the visual deprivation hypothesis (the absence of vision itself drives tactile-acuity enhancement). Here, we performed experiments to distinguish between these two hypotheses. We used force-controlled grating orientation tasks to compare the passive (finger stationary) tactile spatial acuity of 28 profoundly blind and 55 normally sighted humans on the index, middle, and ring fingers of each hand, and on the lips. The tactile experience hypothesis predicted that blind participants would outperform the sighted on the fingers, and that Braille reading would correlate with tactile acuity. The visual deprivation hypothesis predicted that blind participants would outperform the sighted on fingers and lips. Consistent with the tactile experience hypothesis, the blind significantly outperformed the sighted on all fingers, but not on the lips. Additionally, among blind participants, proficient Braille readers on their preferred reading index finger outperformed nonreaders. Finally, proficient Braille readers performed better with their preferred reading index finger than with the opposite index finger, and their acuity on the preferred reading finger correlated with their weekly reading time. These results clearly implicate reliance on the sense of touch as the trigger for tactile spatial acuity enhancement in the blind, and suggest the action of underlying experience-dependent neural mechanisms such as somatosensory and/or cross-modal cortical plasticity.

Figures

Figure 1.
Figure 1.
Grating orientation task. A, Finger testing. A computer-controlled rod rotated to press the selected stimulus surface against the fingertip with 50 g of force. The gratings are shown aligned horizontally. B, Lip testing. JVP Domes were held within a plastic tube (dotted rectangle). Two silicone rubber o-rings (thick black lines) surrounding the dome shaft and contacting the inner wall of the tube provided stability with minimal friction, allowing the shaft to slide backwards slightly upon lip contact. A sensor (gray rectangle) at the rear of the tube monitored the force with which the investigator pressed the JVP Dome orthogonally against the lip surface (right). Participants kept their mouths slightly open during stimulus application. The target force was 50 g. The gratings are shown aligned vertically. Images in A and B are not drawn to scale.
Figure 2.
Figure 2.
Adaptive psychophysical procedure. A, Correct (+) and incorrect (o) answers of a 30-year-old sighted male tested on the right middle finger. B, Best estimate of the participant's psychometric function. C, Bayesian posterior probability distribution for the participant's 76%-correct GOT threshold (groove width for which d′ = 1; see dotted lines in B).
Figure 3.
Figure 3.
GOT thresholds of blind and sighted participants on fingers and lip. A, Dominant hand and side of lip corresponding to dominant hand. B, Nondominant hand and side of lip corresponding to nondominant hand. Threshold values for all participants were adjusted to those of a sex-neutral 39-year-old (the mean age of the participant sample). Means ± 1 SE.
Figure 4.
Figure 4.
Effects among blind participants of blindness characteristics. A, Braille expertise [proficient (Prof.), novice, nonreader (Non.)]. B, blindness onset [congenital (Con.), early, late]. C, Light perception (residual, none). Left, Preferred reading or dominant hand; right, opposite hand. Threshold values for all participants were adjusted to those of a sex-neutral 39-year-old. Numbers of participants in each subgroup are indicated in parentheses. Bars show mean threshold ± 1 SE, on index (I), middle (M), and ring (R) fingers, and lip (L).
Figure 5.
Figure 5.
GOT thresholds of proficient Braille readers on all six fingers. A, Two-index-finger readers (n = 10). B, One-index-finger readers (n = 9). Gray bars, Mean threshold of each finger on the preferred reading hand; white bars, mean threshold of each finger on the opposite hand. Means ± 1 SE.
Figure 6.
Figure 6.
GOT thresholds of proficient Braille readers versus Braille reading hours per week (h/w). A, All proficient readers (n = 19). B, Those who read with both index fingers (n = 10). C, Those who read with a single index finger (n = 9). Circles, Fingers on preferred reading hand; squares, fingers on opposite hand; left, index fingers; right, middle and ring fingers. Regression lines are shown for each finger (solid, preferred reading hand; dotted, opposite hand). Threshold values were adjusted to those of a sex-neutral 39-year-old.
Figure 7.
Figure 7.
Index finger GOT thresholds of participants from the current study and from Goldreich and Kanics (2003). A, Blind Braille readers, blind nonreaders, and sighted participants' thresholds combined across the two studies. Thresholds for Braille readers are from the preferred reading index finger; for blind nonreaders and sighted participants, from the index finger of the dominant hand. B, Braille readers' GOT thresholds versus Braille reading hours per week (h/w). Filled circles and dashed regression line, Current study participants; open circles and dotted regression line, participants from Goldreich and Kanics (2003); solid line, regression on data from all participants, combined across studies. Threshold values are 70.71%-correct thresholds, adjusted to those of a sex-neutral 39-year-old.

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