Age Effects on Visuo-Haptic Length Discrimination: Evidence for Optimal Integration of Senses in Senior Adults

Demographic changes in most developed societies have fostered research on functional aging. While cognitive changes have been characterized elaborately, understanding of perceptual aging lacks behind. We investigated age effects on the mechanisms of how multiple sources of sensory information are merged into a common percept. We studied visuo-haptic integration in a length discrimination task. A total of 24 young (20-25 years) and 27 senior (69-77 years) adults compared standard stimuli to appropriate sets of comparison stimuli. Standard stimuli were explored under visual, haptic, or visuo-haptic conditions. The task procedure allowed introducing an intersensory conflict by anamorphic lenses. Comparison stimuli were exclusively explored haptically. We derived psychometric functions for each condition, determining points of subjective equality and discrimination thresholds. We notably evaluated visuo-haptic perception by different models of multisensory processing, i.e., the Maximum-Likelihood-Estimate model of optimal cue integration, a suboptimal integration model, and a cue switching model. Our results support robust visuo-haptic integration across the adult lifespan. We found suboptimal weighted averaging of sensory sources in young adults, however, senior adults exploited differential sensory reliabilities more efficiently to optimize thresholds. Indeed, evaluation of the MLE model indicates that young adults underweighted visual cues by more than 30%; in contrast, visual weights of senior adults deviated only by about 3% from predictions. We suggest that close to optimal multisensory integration might contribute to successful compensation for age-related sensory losses and provides a critical resource. Differentiation between multisensory integration during healthy aging and age-related pathological challenges on the sensory systems awaits further exploration.