Older adults rely on somatosensory information from the effector limb in the planning of discrete movements to somatosensory cues

Abstract

While younger and older adults can perform upper-limb reaches to spatial targets with comparable endpoint accuracy (i.e., Helsen et al., 2016; Goodman et al., 2020), movement planning (i.e., reaction time) is significantly longer in older versus younger adults (e.g., Pohl et al., 1996; Goodman et al., 2020). Critically relevant to the current study, age-related differences in reaction time are even greater when older adults plan movement towards somatosensory versus visual or bimodal targets in the absence of vision of the moving limb (e.g., Goodman et al., 2020). One proposed explanation of these lengthened reaction times to somatosensory targets is that older adults may be experiencing challenges in implementing sensorimotor transformations when planning discrete movements of their unseen limb. To test this idea and assess the contributions of somatosensory information to these motor planning processes, tendon vibration was applied to the muscles of the effector limb between reaching movements made towards visual, somatosensory, or bimodal targets. The results revealed that older adults show the greatest increases in reaction times when vibration was applied during the preparation of movements to somatosensory targets. Further, both older and younger adults exhibited decreased movement endpoint precision when tendon vibration was applied. However, only older adults showed significantly lower movement endpoint precision due to tendon vibration when making movements to somatosensory targets, versus both visual and bimodal targets. These results corroborate previous evidence that older adults have difficulties planning upper-limb movements to somatosensory targets. As well, these results yielded novel evidence that such motor planning processes in older adult rely on somatosensory cues from the effector limb.