A letter-recognition task to assess lingual tactile acuity

Purpose: This study evaluated a novel neurosensory test based on letter recognition developed for the assessment of spatial acuity of the tongue. Patients and Methods: An up-down tracking procedure was used to estimate the threshold height for the recognition of embossed letters of the alphabet examined by the tongue tip. The 48 stimuli consisted of acrylic strips, one side of which bore a letter (A, I, J, L, O, T, U, or W) of 3, 4, 5, 6, 7, or 8 mm in height. Twenty neurologically normal young adults were tested. Results: Stable estimates of the threshold height were obtained after delivery of only 15 trials. Threshold height averaged 5.1 mm (range, 3.7 to 6.6 mm) and was unaffected by gender (P > .88). Although the letters were identified correctly 54% of the time, on average, the tangibility of individual letters varied from 5% correct (for W) to 82% correct (for T). The letter W was never identified by 16 of 20 subjects; T, O, and UF were identified by all subjects. Analysis of errors confirmed that subjects relied on spatial cues to make the discriminations; 58% of the incorrect responses were made to 1 or 2 letters with spatial features similar to those of the stimulus letter. Conclusions: The threshold height for letter recognition can be obtained easily and rapidly, exhibits low among-subject variability, and reflects the capacity to extract and process spatial information. Letters of similar legibility on the tongue should be used to minimize underestimation of subjects' true acuities.