Abstract
Background: Impaired motor vigor (MV) is a critical aspect of Parkinson's disease (PD) pathophysiology. While MV is predominantly encoded in the basal ganglia, deriving (cortical) EEG measures of MV may provide valuable targets for modulation via galvanic vestibular stimulation (GVS).Objective: To find EEG features predictive of MV and examine the effects of high-frequency GVS.Methods: Data were collected from 20 healthy control (HC) and 18 PD adults performing 30 trials total of a squeeze bulb task with sham or multi-sine (50–100 Hz “GVS1” or 100–150 Hz “GVS2”) stimuli. For each trial, we determined the time to reach maximum force after a “Go” signal, defined MV as the inverse of this time, and used the EEG data 1-sec prior to this time for prediction. We utilized 53 standard EEG features, including relative spectral power, harmonic parameters, and amplitude and phase of bispectrum corresponding to standard EEG bands from each of 27 EEG channels. We then used LASSO regression to select a sparse set of features to predict MV. The regression weights were examined, and separate band-specific models were developed by including only band-specific features (Delta, Theta, Alpha-low, Alpha-high, Beta, Gamma). The correlation between MV prediction and measured MV was used to assess model performance.Results: Models utilizing broadband EEG features were capable of accurately predicting MV (controls: 75%, PD: 81% of the variance). In controls, all EEG bands performed roughly equally in predicting MV, while in the PD group, the model using only beta band features did not predict MV well compared to other bands. Despite having minimal effects on the EEG feature values themselves, both GVS stimuli had significant effects on MV and profound effects on MV predictability via the EEG. With the GVS1 stimulus, beta-band activity in PD subjects became more closely associated with MV compared to the sham condition. With GVS2 stimulus, MV could no longer be accurately predicted from the EEG.Conclusions: EEG features can be a proxy for MV. However, GVS stimuli have profound effects on the relationship between EEG and MV, possibly via direct vestibulo-basal ganglia connections not measurable by the EEG.
Highlights
The complex ways neural activity encodes motor actions and how this can be modulated is an area of active research
We found a significant main effect of Stimulus type, F(2, 234) = 103.34, p < 0.001, ηp2 = 0.47, such that average performance of predicting motor vigor (MV) in the sham condition (M = 0.54, SD = 0.12) was significantly higher than that in GVS1(M = 0.26, SD = 0.11), t(39) = 9.56, p < 0.001, which in turn was significantly higher than GVS2 (M = 0.13, SD = 0.15), t(39) =4.33, p < 0.001
Using a comprehensive set of EEG features time-locked to reaching the maximum force, we could predict MV with an accuracy of around 75% in HC and around 81% in Parkinson’s disease (PD)
Summary
The complex ways neural activity encodes motor actions and how this can be modulated is an area of active research. Oscillatory coupling of neural activity between the motor cortex and the basal ganglia is normally required for the execution of voluntary movements [3]. Altered oscillatory/behavioral relations in Parkinson’s may lead to functional deficiencies such as impaired MV, defined as the ability to execute component movements over a range of speeds, amplitudes, and frequencies [7]. Previous findings suggest that the quality and components of voluntary movement (e.g., velocity, accuracy, energy consumption, endpoint variability, etc.) are modulated by the action execution time if the time carries a cost [8, 9]. Dopaminergic striatal activity is likely involved in value-based behavioral activation and invigoration, and a recent model of dopaminergic function suggests the dorsolateral motor striatum estimates how worthwhile it is to expend effort for the energy costs of moving [13]. While MV is predominantly encoded in the basal ganglia, deriving (cortical) EEG measures of MV may provide valuable targets for modulation via galvanic vestibular stimulation (GVS)
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