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H Christiaan Stronks, Paulette Lieby, Daniel Parker, Janine Walker, Nick Barnes; The feasibility of vibration motors for a tactile display for the blind. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1455.
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We are developing a low-vision aid based on visual-to-tactile sensory substitution. In this study, we have determined the feasibility of vibration motors for use in a tactile display. A suitable motor should generate multiple discernible intensity levels in order to convey contrast information to the user.
Perceptual detection threshold and just-noticeable-difference (JND) for three different vibration motors (Precision Microdrives™, Ltd) were determined on the skin of the back of 7 healthy, normally-sighted subjects (3 females, 4 males; age range: 24-56). The motors differed in size (diameter 8, 10, and 12 mm), which in turn affected multiple parameters, including output energy (more for a larger motor) and acceleration time (longer for a larger motor). Vibrational stimuli were delivered in single 200-ms bursts. Perceptual threshold and JND, expressed as percentage of maximum duty cycle (%DC), were determined using adaptive methods based on a staircase procedure. The total number of available JNDs was estimated using Weber’s law. Overall effects of motor type on threshold and JND were tested by repeated measures ANOVA; differences between motors were tested for significance by Tukey’s multiple comparisons post hoc test.
The number of available JNDs was significantly different between motors (P< 0.05). The largest motor had the largest number of JNDs (mean: 19), followed by the middle (12), and the smallest motor (9). The number of JNDs was as high as 34 in one subject for the largest motor. Post hoc multiple comparisons showed a significant difference between the smallest and largest motor (P< 0.05). Perceptual thresholds differed markedly between motors (P< 0.001). The largest motor had a significantly lower threshold (15 %DC) than the middle motor (23 %DC), which in turn had a lower threshold than the smallest (26 %DC) (P< 0.01). In contrast, the Weber constant did not differ significantly between motors (P= 0.1).
We conclude that vibration motors are able to represent multiple intensity levels, making them suitable for use in a tactile display. The estimated average number of available JNDs increased with motor size, and one subject had nearly three dozen available JNDs when using the largest motor. The most important factor determining the total number of available JNDs was the detection threshold.
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