Abstract
We investigated whether stimulating the cerebellum and primary motor cortex (M1) using transcranial direct current stimulation (tDCS) could affect postural control in young and older adults. tDCS was employed using a double-blind, sham-controlled design, in which young (aged 18–35) and older adults (aged 65+) were assessed over three sessions, one for each stimulatory condition–M1, cerebellar and sham. The effect of tDCS on postural control was assessed using a sway-referencing paradigm, which induced platform rotations in proportion to the participant’s body sway, thus assessing sensory reweighting processes. Task difficulty was manipulated so that young adults experienced a support surface that was twice as compliant as that of older adults, in order to minimise baseline age differences in postural sway. Effects of tDCS on postural control were assessed during, immediately after and 30 minutes after tDCS. Additionally, the effect of tDCS on corticospinal excitability was measured by evaluating motor evoked potentials using transcranial magnetic stimulation immediately after and 30 minutes after tDCS. Minimal effects of tDCS on postural control were found in the eyes open condition only, and this was dependent on the measure assessed and age group. For young adults, stimulation had only offline effects, as cerebellar stimulation showed higher mean power frequency (MPF) of sway 30 minutes after stimulation. For older adults, both stimulation conditions delayed the increase in sway amplitude witnessed between blocks one and two until stimulation was no longer active. In conclusion, despite tDCS’ growing popularity, we would caution researchers to consider carefully the type of measures assessed and the groups targeted in tDCS studies of postural control.
Highlights
Postural control is an adaptive sensorimotor process involving constant integration of sensory information from three channels; visual, somatosensory and vestibular
Further exploration using simple planned contrasts, with an alpha value corrected for multiple comparisons to 0.013, revealed a significant difference in path length between pre-test and every other block–direct current (DC) Fð1; 30Þ 1⁄4 21:49; p < :001; Z2p 1⁄4 :42, post0 Fð1; 30Þ 1⁄4 43:14; p < :001; Z2p 1⁄4 :59, and post30 Fð1; 30Þ 1⁄4 57:35; p < :001; Z2p 1⁄4
This was manifested in the sway amplitude measure, as both stimulation conditions delayed the increase in sway amplitude witnessed between baseline and during ‘stimulation’ (DC) in the sham condition, until the post0
Summary
Postural control is an adaptive sensorimotor process involving constant integration of sensory information from three channels; visual, somatosensory (proprioceptive) and vestibular. Information from these channels is integrated using a sensory reweighting process [1], under which the weight of each channel is determined by the channel’s relative reliability, in order to obtain the most accurate percept of the current postural state. Older adults are more likely to fall if they experience a conflict in any channel (visual or proprioceptive) of sensory information compared to young adults [2], especially within the first trial. Sensory reweighting mechanisms have been highlighted as a contributor to the high prevalence of falls in older adults [5,6], the literature on this issue is limited, especially in terms of the neural mechanisms underlying this process. The present study aimed to investigate the role of the cerebellum and the primary motor cortex in sensory reweighting in young and older adults’ postural control and whether brain stimulation over these areas could affect this process
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