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. 2010 Oct 27;30(43):14288-98.
doi: 10.1523/JNEUROSCI.1447-10.2010.

Vibrotactile Masking Experiments Reveal Accelerated Somatosensory Processing in Congenitally Blind Braille Readers

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Free PMC article

Vibrotactile Masking Experiments Reveal Accelerated Somatosensory Processing in Congenitally Blind Braille Readers

Arindam Bhattacharjee et al. J Neurosci. .
Free PMC article

Abstract

Braille reading is a demanding task that requires the identification of rapidly varying tactile patterns. During proficient reading, neighboring characters impact the fingertip at ∼100 ms intervals, and adjacent raised dots within a character at 50 ms intervals. Because the brain requires time to interpret afferent sensorineural activity, among other reasons, tactile stimuli separated by such short temporal intervals pose a challenge to perception. How, then, do proficient Braille readers successfully interpret inputs arising from their fingertips at such rapid rates? We hypothesized that somatosensory perceptual consolidation occurs more rapidly in proficient Braille readers. If so, Braille readers should outperform sighted participants on masking tasks, which demand rapid perceptual processing, but would not necessarily outperform the sighted on tests of simple vibrotactile sensitivity. To investigate, we conducted two-interval forced-choice vibrotactile detection, amplitude discrimination, and masking tasks on the index fingertips of 89 sighted and 57 profoundly blind humans. Sighted and blind participants had similar unmasked detection (25 ms target tap) and amplitude discrimination (compared with 100 μm reference tap) thresholds, but congenitally blind Braille readers, the fastest readers among the blind participants, exhibited significantly less masking than the sighted (masker, 50 Hz, 50 μm; target-masker delays, ±50 and ±100 ms). Indeed, Braille reading speed correlated significantly and specifically with masking task performance, and in particular with the backward masking decay time constant. We conclude that vibrotactile sensitivity is unchanged but that perceptual processing is accelerated in congenitally blind Braille readers.

Figures

Figure 1.
Figure 1.
Perceptual tasks. A, Simple vibrotactile. B, Amplitude discrimination. C, Backward masking, target–masker delay, 100 ms. D, Backward masking, target–masker delay, 50 ms. E, Forward masking, masker–target delay, 100 ms. F, Forward masking, masker–target delay, 50 ms. In all tasks except amplitude discrimination, the participant's goal was to detect which interval contained the target tap (a 20 Hz sinusoidal pulse, 25 ms in duration, with amplitude ranging from 1 to 100 μm). In amplitude discrimination, the goal was to select the interval with the stronger tap. A and C–F show only the interval containing the tap.
Figure 2.
Figure 2.
Adaptive psychophysical procedure. A, Performance of a participant (∘, correct; x, incorrect) on the simple vibrotactile detection task. The participant reported the perceived target interval by pressing one of two response keys. Target amplitude for each trial was selected using the modified ψ method. B, Most probable psychometric function, given the participant's performance. The dashed vertical line shows the stimulus level corresponding to 76% correct response probability. C, Posterior PDF for the participant's vibrotactile detection threshold. The mean of the PDF was taken as the best estimate of the participant's threshold.
Figure 3.
Figure 3.
Posterior PDFs for thresholds of a single CBPR participant (top panels) and a single sighted participant (bottom panels) on all six tasks. Left, SV (solid lines) and AD (dashed lines). Middle, BM. Right, FM. In middle and right panels, the solid line indicates 100 ms target–masker delay, and the dashed line indicates 50 ms target–masker delay. Note that the estimated target detection thresholds at 50 ms delay are higher than those that at 100 ms delay; target detection thresholds under forward masking are higher than under backward masking; and the thresholds of the sighted participant on each task are higher than those of the CBPR participant.
Figure 4.
Figure 4.
Mean performance of the five participant groups on the nonmasking tasks. A, SV. B, AD. Error bars indicate ±1 SE. S, Sighted.
Figure 5.
Figure 5.
Mean masking performance metric of the five participant groups. A–D, Backward masking, 100 ms, 50 ms; forward masking, 100 ms, 50 ms. Error bars indicate ±1 SE. For masked target detection thresholds in micrometers, see supplemental Figure 3 (available at www.jneurosci.org as supplemental material).
Figure 6.
Figure 6.
Mean masking decay time constants of the five participant groups. A, Backward masking decay time constant. B, Forward masking decay time constant. Error bars indicate ±1 SE.
Figure 7.
Figure 7.
Mean time taken to read the Braille test passage for each of the proficient Braille reader groups. Error bars indicate ±1 SE.
Figure 8.
Figure 8.
Masking task performance metric of proficient Braille readers plotted against time required to read the Braille test passage. A–D, Backward masking, 100 ms, 50 ms; forward masking, 100 ms, 50 ms.
Figure 9.
Figure 9.
Estimated COA during Braille reading plotted against masking decay time constants. A, Backward masking decay time constant. B, Forward masking decay time constant. For breakdown by proficient reader group, see supplemental Figure 5 (available at www.jneurosci.org as supplemental material).

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