Date Published: January 18, 2017
Publisher: Public Library of Science
Author(s): Chesney E. Craig, Michail Doumas, Andrea Antal.
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conditions delayed the increase in sway amplitude witnessed between blocks one and two until stimulation was no longer active.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.
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 , 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. Previous experimental evidence suggests that sensory reweighting is slower in older adults [2–4]. For example, 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 , especially within the first trial. Although 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.
The aim of the current study was to assess whether anodal tDCS of the cerebellum or M1 could affect postural stability in young or older adults. This was tested in a double-blind, sham-controlled design in which postural stability was assessed using a sway-referencing paradigm, which aimed to minimise baseline age differences in postural sway. Effects of stimulation on postural control were minimal and varied greatly between the postural measures, age groups and visual conditions, and there was no effect of stimulation on corticospinal excitability. The minimal effects of stimulation on postural measures were only observed in the eyes open visual condition. For older adults, this was manifested in the sway amplitude measure, as both stimulation conditions (cerebellar and M1) delayed the increase in sway amplitude witnessed between baseline and during ‘stimulation’ (DC) in the sham condition, until the post0 block. This suggests that online anodal stimulation over the cerebellum or M1 prevented the increase in sway amplitude witnessed between the first and second block in older adults, delaying this response until stimulation was no longer active. In contrast, in young adults, minimal effects of stimulation were shown only in the MPF measure, once stimulation was no longer active (off-line), as young adults showed significantly higher MPF in the post30 block in the cerebellar stimulation condition compared to the sham condition. This suggests that cerebellar stimulation deters the typical decline in sway frequency witnessed between post0 and post30 in young adults, thus interrupting the postural practice effect witnessed in these final blocks.