Intact embodiment during perspective-taking in older adults is not affected by focal tDCS

Embodied processing is crucial for visual perspective taking (VPT), with evidence from non-invasive transcranial direct current stimulation (tDCS) suggesting a causal role of the right temporoparietal junction (rTPJ). However, it is not known whether such embodied factors are maintained in older adults or whether rTPJ-tDCS has comparable effects in advanced age. We employed a balanced and sham-tDCS controlled, double-blinded, cross-over design, including two randomized experimental groups of healthy older adults, receiving focal tDCS over either the rTPJ (n = 30), or a control region in the dorsomedial prefrontal cortex (dmPFC, n = 30). A healthy young control group (n = 30, not receiving tDCS) was included to investigate potential changes in embodied processing in older adults. All groups completed neuropsychological baseline testing and an experimental VPT paradigm, in which perspective-taking (requiring embodied rotation) and perspective-tracking (line-of-sight judgements) were assessed. Structural magnetic resonance imaging data were acquired to conduct individualized current flow simulations, aimed at identifying potential changes in neurophysiological effects of tDCS in older adults. Older adults responded slower across perspective tracking and perspective taking tasks but showed comparable embodied effects of body posture and angle of rotation during perspective taking. Contrary to previous results in younger adults that demonstrated regionally and task-specific effects of focal rTPJ-tDCS, no stimulation effects on embodied processing were found in older adults. Electrical field simulations suggested focal current delivery in both age-groups but also significantly reduced current strength in the target regions for tDCS in older adults. Older adults are as embodied as young adults during perspective taking. However, tDCS administered to the rTPJ or dmPFC had no effect, which may be explained by reduced current delivery to the target regions due to age-associated changes in skull and brain anatomy and/or functional brain reorganization. Our results are in line with previous studies suggesting that tDCS effects obtained in young participants may not translate directly to advanced age. Future studies could address this by using individualized modelling approaches aimed at adjusting current dose for (older) study participants and pre-stimulation functional imaging involving VPT tasks-of-interest, to identify optimized target regions for tDCS.