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H. Christiaan Stronks, Daniel John Parker, Nick M Barnes; Tactile acuity determined with vibration motors for use in a sensory substitution device for the blind. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2920.
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We aim to construct a back-worn tactile substitution device for the blind that redirects camera images to a vibrotactile display. To establish a feasible between-motor distance we have determined the vibrotactile acuity on the lower back. In addition, we have established what the impact of experience is on tactile acuity using an active learning procedure.
This study included 15 healthy subjects. Vibrational stimuli (100-ms stimulations) were delivered by coin motors (Precision Microdrives, LLC) with a diameter of 12 mm (Stronks et al., Artif Organs. in press). The motors were mounted on a carrier of urethane at various between-motor spacings (Fig. 1). Tactile acuities were determined with an adaptive, 2-alternative forced choice task using various offsets (0 - 200 ms) between two successive motor activations (n= 12). Training effects (n= 3) were tested at a stimulation offset of 0 ms. Subjects received 30 minutes of practice with feedback after each trial. Acuity tests were done before, during and after training.
Average tactile acuities varied from 36 to 63 mm center-to-center, dependent on stimulus offset. The least favorable acuities were found when the vibrational stimuli were presented simultaneously (offset 0 msec). Acuities at the longest offset tested (200 ms) significantly differed from those at 0 ms and were 41 mm on average (RM ANOVA and Tukey’s post hoc test; P< 0.05). Acuities at intermediate offsets (50 - 175 ms) did not differ significantly from those obtained at 0 and 200 ms. The correlation between acuity and offset time approached significance (linear regression and F-test, P= 0.07). After practice, two subjects improved by 30% and 90%, respectively, while one subject performed 10% worse. The average improvement was 35%.
Based on the regression analysis, the interpolated tactile acuity at 200 ms stimulus offset was 42 mm center-to-center. Preliminary results showed that subjects improved after practice. This suggests that after training motor spacings of 32 mm may be effective for use in a vibrotactile display, provided that adjacent motors are stimulated with an offset of 200 ms.
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