Characterization of Artifacts Produced by Gel Displacement on Non-invasive Brain-Machine Interfaces during Ambulation

Abstract

So far, Brain-Machine Interfaces (BMIs) have been mainly used to study brain potentials during movement-free conditions. Recently, due to the emerging concern of improving rehabilitation therapies, these systems are also being used during gait experiments. Under this new condition, the evaluation of motion artifacts has become a critical point to assure the validity of the results obtained. Due to the high signal to noise ratio provided, the use of wet electrodes is a widely accepted technic to acquire electroencephalographic (EEG signals). To perform these recordings it is necessary to apply a conductive gel between the scalp and the electrodes. This work is focused on the study of gel displacements produced during ambulation and how they affect the amplitude of EEG signals. Data recorded during three ambulation conditions (gait training) and one movement-free condition (BMI motor imagery task) are compared to perform this study. Two phenomenons, manifested as unusual increases of the signals' amplitude, have been identified and characterized during this work. Results suggest that they are caused by abrupt changes on the conductivity between the electrode and the scalp due to gel displacement produced during ambulation and head movements. These artifacts significantly increase the Power Spectral Density (PSD) of EEG recordings at all frequencies from 5 to 90 Hz, corresponding to the main bandwidth of electrocortical potentials. They should be taken into consideration before performing EEG recordings in order to asses the correct gel allocation and to avoid the use of electrodes on certain scalp areas depending on the experimental conditions.

Corticomuscular control of walking in older people and people with Parkinson’s disease

“…Finally, mobile EEG is susceptible to movement artefacts and caution should hence be exerted when interpreting phasic changes in EEG activity (Gwin et al 2010, Castermans et al 2014, Costa et al 2016, Nathan & Contreras-Vidal 2016, Oliveira et al 2016, Peterson & Ferris 2018a. In this study, we applied multiple strategies to minimize the effects of motion artefacts according to standards currently established in this research field.…”

Estimation of Neuromuscular Primitives from EEG Slow Cortical Potentials in Incomplete Spinal Cord Injury Individuals for a New Class of Brain-Machine Interfaces

“…Another possible reason could be related to the presence of motion artifacts affecting the global activity of the whole cortex. Motion artifacts are a key limitation in the application of BMIs under ambulatory conditions, particularly during gait rehabilitation procedures (Costa et al, 2016 ). This fact could also explain why all the configurations showed a similar behavior in subjects P03 and P04.…”

“…Mechanical artifacts strongly affect the EEG low-frequency range during cyclic motion activities. This is suggested to directly influence the reliability of these decoders (Castermans et al, 2014 ; Costa et al, 2016 ). Moreover, other studies show that performance is not statistically different from chance levels due to the inherent properties of linear regression (Antelis et al, 2013 ).…”

Dynamics of corticospinal motor control during overground and treadmill walking in humans

“…Ambulatory EEG is susceptible to movement artefacts and caution should hence be exerted when interpreting phasic changes in EEG activity (Castermans et al 2014; Costa et al 2016; Gwin et al 2010; Kline et al 2015; Nathan and Contreras-Vidal 2016; Oliveira et al 2016; Snyder et al 2015). In this study, we applied multiple strategies to mitigate the effects of motion artefacts: We excluded gait cycles with excessive artefacts, we performed ICA, and we calculated a bipolar derivative EEG signal, which removes further shared artefactual activity across channels similar to a re-referencing approach (Snyder et al 2015).…”