Tactile spatial acuity in childhood: effects of age and fingertip size

Abstract

Tactile acuity is known to decline with age in adults, possibly as the result of receptor loss, but less is understood about how tactile acuity changes during childhood. Previous research from our laboratory has shown that fingertip size influences tactile spatial acuity in young adults: those with larger fingers tend to have poorer acuity, possibly because mechanoreceptors are more sparsely distributed in larger fingers. We hypothesized that a similar relationship would hold among children. If so, children's tactile spatial acuity might be expected to worsen as their fingertips grow. However, concomitant CNS maturation might result in more efficient perceptual processing, counteracting the effect of fingertip growth on tactile acuity. To investigate, we conducted a cross-sectional study, testing 116 participants ranging in age from 6 to 16 years on a precision-controlled tactile grating orientation task. We measured each participant's grating orientation threshold on the dominant index finger, along with physical properties of the fingertip: surface area, volume, sweat pore spacing, and temperature. We found that, as in adults, children with larger fingertips (at a given age) had significantly poorer acuity, yet paradoxically acuity did not worsen significantly with age. We propose that finger growth during development results in a gradual decline in innervation density as receptive fields reposition to cover an expanding skin surface. At the same time, central maturation presumably enhances perceptual processing.

Figure 1. Bayesian adaptive procedure for threshold groove width estimation.

Sensory data for two participants (P) are shown in columns: Left panels: P1, female, age 15.9 years, fingertip surface area = 362.8 mm²; Right panels: P13, male, age 16.8 years, fingertip surface area = 519.9 mm². (A) Each participant's performance plot (+ = correct response, x = incorrect response) on a single testing block. Occasional trials in which the force sensor detected finger movement were automatically discarded (symbols not plotted) so as not to influence the Bayesian adaptive procedure. (B) Corresponding best-estimate psychometric functions. (C) PDFs over the threshold groove width. Note that, compared to P1, P13 has an upward-shifted performance plot, a rightward shifted psychometric function, and a rightward-shifted threshold PDF, indicative of poorer performance; given the participants' similar ages, this performance difference is likely due to the large difference between the participants' fingertip sizes. GW: groove width; Prob: probability.

Figure 2. GBF disqualification analysis and comparison between testing protocols.

(A) Proportion of participants in each age bracket for whom all four end-of-block GBFs exceeded the criterion value of 0.5. (B) Proportion of participants disqualified using the two testing protocols. (C) Average thresholds of qualifying participants on the two testing protocols (error bars: 1 SD).

Figure 3. Fingertip growth with age.

Six different fingertip metrics are plotted against age. (A) surface area, (B) volume, (C) temperature, (D) between-ridge sweat pore spacing, (E) within-ridge sweat pore spacing, (F) sweat pore density. Black lines: least-squared linear fits.

Figure 4. Tactile spatial acuity dependence on fingertip metrics.

(A) surface area, (B) volume, (C) temperature, (D) between-ridge sweat pore spacing, (E) within-ridge sweat pore spacing, (F) sweat pore density. Black curves: best-fit exponential functions from multiple regressions relating log threshold (dependent variable) to fingertip metric and age (independent variables).

Figure 5. Tactile spatial acuity from childhood into adulthood.

Data from the current study (filled circles) are plotted together with those from Peters et al. [11] (open circles). (A) and (B) show simple linear regressions. (A) threshold vs. age. (B) threshold vs. fingertip surface area. (C) and (D) show results of a multiple regression with both age and surface area as independent variables. (C) surface area-adjusted threshold vs. age. (D) age-adjusted threshold vs. surface area. In (C) and (D), thresholds were respectively adjusted to the mean surface area and mean age of the aggregate participant sample. Black solid curves in all panels: least-squared exponential fits. In (A) and (C), for plotting purposes only, we have omitted the data from the oldest participant, a 27.29 year-old from [11] (threshold 1.22 mm, area-adjusted threshold, 1.30 mm).