Age related changes in skin sensitivity assessed with smartphone vibration testing

Abstract

The capacity to perceive tactile input at the fingertips, referred to as tactile sensitivity, is known to diminish with age due to regressive changes to mechanoreceptor density and morphology. Sensitivity is measured as perceptual responses to stimuli of varying intensity. Contrary to traditional sensitivity monitoring instruments, smartphones are uniquely suited for remote assessment and have shown to deliver highly calibrated stimuli along a broad spectrum of intensity, which may improve test reliability. The aim of this study was to evaluate a vibration-emitting smartphone application, the Vibratus App, as a mode of estimating tactile sensory thresholds in the aging adult. The peripheral nerve function of 40 neurologically healthy volunteers (ages 18-71) was measured using monofilaments, a 128-Hz tuning fork, the Vibratus App, and nerve conduction studies (NCS). Between group differences were analyzed to determine each measurement's sensitivity to age. Spearman correlation coefficients depicted the associative strength between hand-held measurements and sensory nerve action potential (SNAP) amplitude. Inter-rater reliability of traditional instruments and the software-operated smartphone were assessed by intraclass correlation coefficient (ICC_2,k). Measurements taken with Vibratus App were significantly different between age groups (p < 0.001). The inter-rater reliability of monofilament, smartphone vibration, and tuning fork testing was moderate to good (ICC_2,k = 0.65, 0.69, and 0.79, respectively). The findings of this study support further investigation of smartphones as sensitivity monitoring devices for at home monitoring of skin sensitivity.

Keywords: Aging; Perception; Smartphone; Tactile sensitivity; Vibrotactile.

Figure 1

Routine neurological assessments of peripheral nerve function. (A) Nerve conduction studies, performed by sending electrical impulses to the median nerve and recording the latency and amplitude of compound action potential at the finger. (B) Timed tuning fork testing, performed by applying an activated 128-Hz tuning fork to the finger and asking participants to indicate when they no longer feel vibration. (C) Semmes–Weinstein monofilament testing, performed by applying monofilaments of varying stiffness to the finger and asking participants to indicate whether they can or cannot feel the stimulus.

Figure 2

Schematic representation of 4-2-1 staircase procedure. The psychophysical paradigm begins mid-way between the least and most intense stimuli. Stimulus intensity changes intensity in 4-step increments with each correct response initially, reversing intensity in 2-step increments at the first incorrect response, and reversing intensity in 1-step increments at each change in response thereafter. The mean of 1-step reversals was used to estimate threshold intensity.

Figure 3

Vibrotactile detection threshold estimation using the Vibratus App. App screenshots from left to right depict the sequence of events occurring on a single trial. Each trials begins when the user places their fingertip or other skin surface onto the screen (First screen). Following this, two sequentially presented intervals denoted “1” and “2” are provided. Vibration is applied randomly during one of these two intervals, and the user must then select which interval (“1” or “2”) contained the vibration by pressing buttons on the screen (fourth screen). This is a two-alternative forced choice (2AFC) trial structure. Based on whether the user responds correctly on each trial, the amplitude of vibration is stepped up or down using a 4-2-1 staircase algorithm. Vibrotactile detection is estimated as the lowest detectable vibratory “intensity” ranging from 0.05 to 1 (fifth screen).

Figure 4

The Vibratus App, as well as monofilaments and nerve conduction studies, provides sensitive estimates of age-related changes to tactile sensitivity. (A–D) Bar graphs showing raw median values (± SD) of perceptual thresholds (A–C) and sensory nerve action potential amplitude (D) between young and older adults.

Figure 5

Measurements taken with monofilaments showed a strong association with sensory nerve action potential amplitude, whereas the Vibratus App and tuning fork testing did not. (A–C) Scatter plots showing the association between age-related decline of sensitivity and sensory nerve action potential amplitude.