Older adults sacrifice response speed to preserve multisensory integration performance

Ageing is thought to impact multisensory integration, but the underlying computational mechanisms are poorly understood. Signals that come from a common source should be integrated, weighted by their reliability; those that do not share a source should be segregated. Combining psychophysics and Bayesian modelling we investigated how ageing affects audiovisual integration of spatial signals. Under unspeeded conditions, older and younger adults were comparable in their localisation responses and common-source judgements. Likewise, the Bayesian Causal Inference model fitted to observers9 response choices indicated that sensory reliabilities and arbitration between integration and segregation were similar across age groups. However, under speeded conditions, older observers were slower at localising sounds, particularly when they were paired with spatially incongruent visual signals. A Bayesian evidence accumulation model, fitted jointly to response choices and response times, revealed that older observers accumulate noisier auditory representations for longer, set higher decisional thresholds, and have impaired motor speeds. Thus, modelling the within-trial dynamics of multisensory evidence accumulation reveals that older observers preserve audiovisual localisation performance by sacrificing response speed.