Effect of visual stimulation on quantitative light-touch assessment using the active von Frey filament test

Abstract

Quantitative assessment of light-touch sensation is essential in the evaluation of peripheral nerve disorders. Although the active von Frey filament test (AvF) provides objective and continuous tactile threshold measurements, the influence of testing conditions on AvF values remains unclear. This study aimed to evaluate whether visual stimulation affects tactile thresholds measured using AvF. Thirty-two healthy adult participants (24 men and 8 women) without comorbid disease were prospectively examined using AvF. The innervation zone of the median and the ulnar nerve (i.e., index and fifth finger volar pad, respectively) was examined using AvF with and without a visual stimulation. Five repeated measurements were obtained at each site, and the mean of the middle three measurements was used for analysis. The primary comparison used participant-level mean AvF thresholds; a mixed-effects model was additionally used to account for repeated finger-level measurements within participants. Participant-level mean AvF values during visual stimulation were significantly higher than those blindfolded (130.4 ± 51.4 mgf vs. 110.5 ± 38.6 mgf, Wilcoxon signed-rank test p = 0.0026). In mixed-effects model accounting for repeated finger-level measurements, visual stimulation increased AvF thresholds by 19.8 mgf (95% CI, 11.2 to 28.5 mgf, p < 0.001). No significant difference was observed between dominant and non-dominant hands during visual stimulation. Index finger measurements showed higher reliability than fifth finger measurements. Male participants tended to show higher AvF values than female participants, although the difference was not statistically significant at the participant level. Visual stimulation significantly increased AvF tactile detection thresholds, indicating decreased light-touch sensitivity during visual input. Therefore, visual input introduces a systematic source of measurement bias in quantitative tactile assessment. This effect reflects condition-dependent modulation of sensory measurement bias rather than true peripheral sensory dysfunction. Standardized control of visual input, such as blindfolding or eye closure, should therefore be considered an essential component of quantitative hand sensory testing protocols, particularly when subtle sensory changes or longitudinal comparisons are clinically significant.