Tactile-'visual' acuity of the tongue in early blind individuals.

This study compares the 'tactile-visual' acuity of the tongue for 15 early blind participants with that of 24 age-matched and sex-matched sighted controls. Snellen's tumbling E test was used to assess 'visual' acuity using the tongue display unit. The tongue display unit is a sensory substitution device that converts a visual stimulus grabbed by a camera into electro-tactile pulses delivered to the tongue via a grid made out of electrodes. No overall significant difference was found in thresholds between early blind (1/206) and sighted control (1/237) participants. We found, however, a larger proportion of early blind in the two highest visual acuity categories (1/150 and 1/90). These results extend earlier findings that it is possible to measure visual acuity in the blind individuals using the tongue. Moreover, our data demonstrate that a subgroup of early blind participants is more efficient than controls in conveying visual information through the tongue.