Mechanisms of tactile sensory deterioration amongst the elderly

Abstract

It is known that roughness-smoothness, hardness-softness, stickiness-slipperiness and warm-cold are predominant perceptual dimensions in macro-, micro- and nano- texture perception. However, it is not clear to what extent active tactile texture discrimination remains intact with age. The general decrease in tactile ability induces physical and emotional dysfunction in elderly, and has increasing significance for an aging population. We report a method to quantify tactile acuity based on blinded active exploration of systematically varying micro-textured surfaces and a same-different paradigm. It reveals that elderly participants show significantly reduced fine texture discrimination ability. The elderly group also displays statistically lower finger friction coefficient, moisture and elasticity, suggesting a link. However, a subpopulation of the elderly retains discrimination ability irrespective of cutaneous condition and this can be related to a higher density of somatosensory receptors on the finger pads. Skin tribology is thus not the primary reason for decline of tactile discrimination with age. The remediation of cutaneous properties through rehydration, however leads to a significantly improved tactile acuity. This indicates unambiguously that neurological tactile loss can be temporarily compensated by restoring the cutaneous contact mechanics. Such mechanical restoration of tactile ability has the potential to increase the quality of life in elderly.

Figure 1

Tactile discrimination ability for the two groups. (a) Proportion of correct answers in identifying whether the stimulus was different to the reference sample Ref100 (100 µm in wrinkle wavelength). The difficulty of the task clearly increases for both groups as the wavelength of the stimulus texture approaches that of the reference. The “break point” (defined as when the success rate falls below 80%) is seen for S20 (20 µm) for the elderly group and for S60 (60 µm) for the young group (N = 30 for both groups). (b) Box plot of the means from each participant in the two groups (N = 2 × 30), showing the tactile discrimination ability of S20 versus Ref100 as well as the correct responses for Ref100 versus itself (meaning “same” as the correct answer). Since the young and the elderly groups have a very similar proportion of correct responses for the latter task, the tendency to guess “different” is the same for both groups when the task is perceived as difficult.

Figure 2

Not all elderly subjects show a decrease in tactile discrimination ability. Average correct responses for S20 of all 60 subjects, showing that 13/30 from the elderly group perform at the same level as the young. The criterion of 80% correct responses is used to separate high and low performers. Age is plotted on the abscissa as a convenient means to separate the two groups.

Figure 3

The groups display significantly different bio-mechanical and bio-tribological values. The elderly group display (a) lower finger hydration (b) lower finger elasticity and (c) lower tactile friction coefficients obtained by the continuous recording of friction force and applied load upon interaction using a (d) ForceBoard.

Figure 4

Biomechanical properties and tactile ability. (a) Scatter plot showing the correct responses on S20 (perceiving S20 as different from Ref100). There is improvement potential for the subjects below 80% correct responses. (b) Finger elasticity appears to increase with increasing hydration.

Figure 5

Improvement in tactile discrimination ability with increased finger hydration. Data is shown only for the low performing elderly group (a) Improvement in correct responses measured 30 min after application of humectants containing 5% and 7% glycerol (applied on two different days). (b) As expected, both humectants increase the finger hydration level. (c) Finger elasticity is analogously increased. (d) Greater differences in tactile friction between S20 and Ref100 in the treated finger state that could explain the increase in tactile discrimination ability based on greater differences in sticky/slippery feel. Note that when humectant was applied, the physical parameters were measured both before and after. Thus there are “untreated” data which are nonetheless labelled as (5%) and (7%) according to the respective, subsequent treatment.