Longitudinal Analysis of the Connectivity and Complexity of Complete Locked-in Syndrome Patients Electroencephalographic signal

Objectives: The aim of this investigation is to study development of the functional connectivity and the complexity of the electroencephalogram (EEG) recorded from completely locked-in patients (CLIS) across two consecutive years. CLIS patients lose their ability to voluntary control their muscles but maintain cognitive abilities, although the extant is unknown. The goal is to investigate the effect of time on brain signals connectivity and complexity in the completely locked-in state over time. Methods: EEG data were recorded from three CLIS patients, identified by the numbers 2, 5 and 7. Each of the patients satisfy the following conditions: at least two years of available recordings, specifically signals from channels FC5 and FC6. This results in two years data for each of the patients. Since the recording channels were not consistent over the years, the choice of the channels was made on the basis that they were common to the most of these patients for more than a year, for comparability purposes. The connectivity measures were evaluated using the imaginary part of the coherence (iCOH) and the weighted symbolic mutual information (wSMI) on the recorded data. Using only the imaginary part of the coherence allows to reduce the effects of volume conduction. On the other hand, wSMI values estimate the information shared between two EEG channels. In both method, a higher value suggests an increase of the functional connectivity between two channels or two brain regions. The complexity was estimated using the Poincaré index, computed as the ratio between the short-term variability and the long-term variability of the Poincaré plot. A high Poincaré index value suggests an activated cortex. Results: Imaginary coherence and wSMI values showed high variability from day to day the first year for Patients 2 and 5. The overall connectivity values slightly decrease from the first to the second year for both patients. Similarly, for both patients, slight increases of the Poincaré index values are observed from the first to the second year. As for Patient 7, coherence values were in the same range for both years as opposed to the wSMI and Poincaré index values, which showed substantial differences. Conclusion: Patients 2 and 5 showed similar results, indicating a possible degradation of the functional connectivity between the chosen channels but an increased activity at the single channel level, on the contrary of Patient 7. The decrease in the connectivity values suggests a progressive decrease of communication between the channels located in the motor area, correlated with the loss of residual invisible motor function.