Visuotactile interaction even in far sagittal space in older adults with decreased gait and balance functions.

Spatial proximity of signals from different sensory modalities is known to be a crucial factor in facilitating efficient multisensory processing in young adults. However, recent studies have demonstrated that older adults exhibit strong visuotactile interactions even when the visual stimuli were presented in a spatially disparate position from a tactile stimulus. This suggests that visuotactile peripersonal space differs between older and younger adults. In the present study, we investigated to what extent peripersonal space expands in the sagittal direction and whether this expansion was linked to the decline in sensorimotor functions in older adults. Vibrotactile stimuli were delivered either to the left or right index finger, while visual stimuli were presented at a distance of 5 cm (near), 37.5 cm (middle), or 70 cm (far) from each finger. The participants had to respond rapidly to a randomized sequence of unimodal (visual or tactile) and simultaneous visuotactile targets (i.e., a redundant target paradigm). Sensorimotor functions were independently assessed by the Timed Up and Go (TUG) and postural stability tests. Results showed that reaction times to the visuotactile bimodal stimuli were significantly faster than those to the unimodal stimuli, irrespective of age group [younger adults: 22.0 ± 0.6 years, older adults: 75.0 ± 3.3 years (mean ± SD)] and target distance. Of importance, a race model analysis revealed that the co-activation model (i.e., visuotactile multisensory integrative process) is supported in the far condition especially for older adults with relatively poor performance on the TUG or postural stability tests. These results suggest that aging can change visuotactile peripersonal space and that it may be closely linked to declines in sensorimotor functions related to gait and balance in older adults.