Elsevier

Neuroscience

Volume 170, Issue 2, 13 October 2010, Pages 599-609
Neuroscience

Cognitive, Behavioral, and Systems Neuroscience
Research Paper
Perturbation-evoked cortical activity reflects both the context and consequence of postural instability

https://doi.org/10.1016/j.neuroscience.2010.07.008Get rights and content

Abstract

The cerebral cortex may play a role in the control of compensatory balance reactions by optimizing these responses to suit the task conditions and/or to stimulus (i.e. perturbation) characteristics. These possible contributions appear to be reflected by pre-perturbation and post-perturbation cortical activity. While studies have explored the characteristics and possible meaning of these different events (pre- vs. post-) there is little insight into the possible association between them. The purpose of this study was to explore whether pre- and post-perturbation cortical events are associated or whether they reflect different control processes linked to the control of balance. Twelve participants were presented temporally-predictable postural perturbations under four test conditions. The Block/Random tasks were designed to assess modifiability in CNS gain prior to instability, while the Unconstrained/Constrained tasks assessed responsiveness to the magnitude of instability. Perturbations were evoked by releasing a cable which held the participant in a forward lean position. The magnitude of pre-perturbation cortical activity scaled to perturbation amplitude when the magnitude of the perturbation was predictable [F(3,11)=2.906, P<0.05]. The amplitude of pre-perturbation cortical activity was large when the size of the forthcoming perturbation was unknown (13.8±7.9, 11.4±9.9, 16.9±9.3, and 16.1±10.6 μV for the Block Unconstrained and Constrained and Random Unconstrained and Constrained, respectively). In addition, N1 amplitude scaled to perturbation amplitude regardless of whether the size of the forthcoming perturbation was known (30.1±17.7, 11.4±7.1, 30.9±18.4, 12.4±6.1 μV). This is the first work to examine modifiability in the pre-perturbation cortical activity related to postural set alterations. The cerebral cortex differentially processes independent components prior to and following postural instability to generate compensatory responses linked to the conditions under which instability is experienced.

Section snippets

Subjects

Twelve subjects (six male, 29.3±6.4 years, 172.1±9.9 cm, 71.6±15.9 kg) agreed to participate in the study. All subjects were free of neuromuscular disorders and each provided written, informed consent prior to the onset of the study. The study was conducted with approval from the Research Ethics Board at the Toronto Rehabilitation Institute.

Electroencephalography

Electroencephalographic (EEG) signals were obtained using a 32 channel electrode cap (Quik-Cap, Neuroscan, El Paso, TX, USA) based on the International

Results

Following data analysis and subsequent removal of individual trials due to ocular artifact or anticipatory postural responses, a similar number of trials [F(3,11)=1.437, P>0.05] per subject for each of the task conditions was included in the statistical analysis (20±4, 20±4, 18±1 and 20±3 Block Unconstrained and Constrained, Random Unconstrained and Constrained respectively). Fig. 2 is a representative depiction of the EEG, EMG, and COP plots for a single participant, averaged across all trials.

Discussion

This study set out to determine the extent to which alterations in postural set modified the spatio-temporal characteristics of cortical potentials evoked by instability. Specifically, the current study explored the association between pre-perturbation (context) and post-perturbation (consequence) cortical activity associated with perturbations to upright stability. The main findings of this study were: (1) the magnitude of pre-perturbation cortical activity was scaled to perturbation amplitude

Conclusion

In summary, this study has demonstrated that pre-perturbation cortical activity tends to vary with the anticipated amplitude of perturbation. If the amplitude of imminent instability is unpredictable, the CNS sets a “default” setting to an increase in gain. These characteristics of cortical activity parallel previous observations based on muscle activity and provide additional support for the hypothesis that slow-wave potentials generated in advance of imminent instability are indicators of

Acknowledgments

The authors wish to thank H Cheung and R Lee for assistance with data collection. This work is supported by funding from the Heart and Stroke Foundation of Canada (GM, SB), the Canadian Stroke Network (SB), and the Natural Science and Engineering Research Council (WEM). We acknowledge the support of the Toronto Rehabilitation Institute who receives funding under the Provincial Rehabilitation Research Program from the Ministry of Health and Long Term Care in Ontario.

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