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Clinical Trial
, 23 (8), 3439-45

Tactile Acuity Is Enhanced in Blindness

Affiliations
Clinical Trial

Tactile Acuity Is Enhanced in Blindness

Daniel Goldreich et al. J Neurosci.

Abstract

Functional imaging studies in blind subjects have shown tactile activation of cortical areas that normally subserve vision, but whether blind people have enhanced tactile acuity has long been controversial. We compared the passive tactile acuity of blind and sighted subjects on a fully automated grating orientation task and used multivariate Bayesian data analysis to determine predictors of acuity. Acuity was significantly superior in blind subjects, independently of the degree of childhood vision, light perception level, or Braille reading. Acuity was strongly dependent on the force of contact between the stimulus surface and the skin, declined with subject age, and was better in women than in men. Despite large intragroup variability, the difference between blind and sighted subjects was highly significant: the average blind subject had the acuity of an average sighted subject of the same gender but 23 years younger. The results suggest that crossmodal plasticity may underlie tactile acuity enhancement in blindness.

Figures

Fig. 1.
Fig. 1.
Testing system schematic. A, Top view. The hand rested palm-down on an upper table, with the index finger positioned over a tunnel (not shown) through which the stimulus surfaces rose. Plastic barriers surrounding the fingertip prevented lateral and forward finger movements. Upward, downward, and backward movements were detected by a force sensor on the fingernail (not shown) and were announced by an error tone. A rotatable disk (dashed circle) just below the table housed the stimulus surfaces (partial expanded view on left). B, Side view of the lower table, concealed during testing. Rotation of the stepper motor positioned the appropriate stimulus surface under the finger, after which the linear actuator moved with velocity V1, freeing the rod supporting mass M to pivot around pointP. The stimulus surface contacted the finger with forceF= Mg and velocityV2 = (V1)(d2/d1). After a 1 sec contact period, the actuator retracted to return the rod and stimulus surface to their resting positions. M= 10 or 50 gm; V2 = 4 cm/sec. For clarity, only 19 of the 40 stimulus surfaces are shown.
Fig. 2.
Fig. 2.
Experimental procedure. A, A single trial. Two grooved surfaces with identical groove widths but orthogonal orientations (grooves either parallel or transverse to the axis of the finger) sequentially contacted the distal pad of the index finger (1 sec contact duration, 2 sec intercontact interval) in random order. The subject indicated the perceived order by pressing one of two response keys with the other hand. B, A single block. Groove width was decreased after two correct (×) responses and increased after a single incorrect (○) response. The block was terminated on the 14th reversal of direction in groove width adjustment (top numbers). The average of groove widths at reversal points 4 through 14 (bracketed region) was the subject's threshold (dotted line; 1.10 mm in this case) (see Materials and Methods). The block shown was performed by a 47-year-old blind woman at 50 gm contact force. Each subject completed 10 such blocks, five at 50 gm contact force alternating with five at 10 gm.
Fig. 3.
Fig. 3.
Effects of repeated testing and contact force.A, Group means ± SEs of sighted and blind subjects on the five 50 gm (filled symbols) and five 10 gm (open symbols) testing blocks. B, Scatter plot showing effect of contact force. Each point represents the average of a subject's five 10 gm thresholds plotted against the average of the subject's five 50 gm thresholds. Dashed lines, x = y; solid lines, best linear fits.
Fig. 4.
Fig. 4.
Effects of blindness, age, and gender.A, Each subject's average threshold (average of all 10 testing blocks) plotted against age. Blind, Red; sighted, blue; women, ○; men, ×. Best-fit linear regression lines are shown for sighted men (SM), sighted women (SW), blind men (BM), and blind women (BW).B, Results of ANCOVA showing posterior PDFs for the effects of vision, age, and gender. Horizontal bars indicate 95% confidence intervals. C, Best estimates for the effects of vision, age, and gender, representing the modes of the PDFs fromB. The 0.33 mm difference between blind and sighted subjects of the same gender (vertical arrows) is equivalent to an age difference of 23 years (horizontal arrows).
Fig. 5.
Fig. 5.
Effects of childhood visual experience, light perception level, and Braille reading. For display purposes only, each subject's average threshold was age adjusted (at 0.014 mm/year) to 45 years old, and thresholds of men were gender adjusted (by subtracting 0.018 mm) to equivalent female values. A, Adjusted average thresholds of sighted (left; dashed line) and different subgroups of blind subjects. B, Adjusted average thresholds of blind nonreaders (left; dashed line) and different subgroups of Braille readers. Histogram bars indicate group means, error bars indicate SEs, and numbers indicate group sizes.

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