Comparative tactile sensitivity of the fingertip and apical tongue using complex and pure tactile tasks

Both the tongue and fingertip are highly tactile tissues relevant in texture perception, but work comparing relative sensitivity to elucidate potential differences in stimulus processing is limited. Presently, the acuity of the tongue and fingertip were compared using a series of tactile acuity tasks. We hypothesized the tongue would show superior acuity regardless of stimuli due to an absent epidermal barrier and its involvement in many high-sensitivity behaviors (e.g. eating, speaking). Acuity was determined using three different tests, two "purely-tactile" just noticeable difference (JND) tasks (punctate pressure and roughness sensitivity) and a more-complex, stereognostic letter-recognition task to evaluate point-and-edge sensitivity. JNDs were determined using the forced-choice staircase method for the punctate deformation force of a monofilament (F;0.0044-0.010 g) and the surface roughness of stainless steel coupons (Ra; 0.177-0.465 μm) in populations of 30 and 31 individuals, respectively. Point-and-edge sensitivity was assessed by determining the letter recognition threshold (RT) based on height (h;1.5-8.0 mm) in an additional 28 individuals using a modified staircase method. While subjects had significantly lower JNDs with their tongues for both "purely-tactile" tasks (punctate: 0.0017 ± 0.0001 g vs. 0.0023 ± 0.0002 g (fingertip), p = .018; roughness: 0.039 ± 0.004 μm vs. 0.112 ± 0.020 μm (fingertip), p < .001), subjects had significantly higher RTs with their tongues for the letter identification task (3.98 ± 0.84 mm vs. 4.54 ± 1.41 mm (fingertip), p = .0417). The latter difference is likely attributable to the more complex nature of the RT task and the finger's frequent involvement in object recognition. Binomial statistics (p = 1/2, α = 0.05) showed a significant number of subjects were better at the roughness task with their tongues (p = .021); however, a significant majority were better at the letter identification task with their fingers (p = .049); no significant difference was found for the punctate pressure task. While data appear to suggest the tongue is more sensitive to exclusively tactile stimuli, further study of other "pure-tactile" sensations should help clarify the contradictory results of the RT task.