Abstract
Cognitive impairment (CI), often examined with neuropsychological tests such as the Paced Auditory Serial Addition Test (PASAT), affects approximately 65% of multiple sclerosis (MS) patients. The P3b event-related potential (ERP), evoked when an infrequent target stimulus is presented, indexes cognitive function and is typically compared across subjects' scalp electroencephalography (EEG) data. However, the clustering of independent components (ICs) is superior to scalp-based EEG methods because it can accommodate the spatiotemporal overlap inherent in scalp EEG data. Event-related spectral perturbations (ERSPs; event-related mean power spectral changes) and inter-trial coherence (ITCs; event-related consistency of spectral phase) reveal a more comprehensive overview of EEG activity. Ninety-five subjects (56 MS patients, 39 controls) completed visual and auditory two-stimulus P3b event-related potential tasks and the PASAT. MS patients were also divided into CI and non-CI groups (n = 18 in each) based on PASAT scores. Data were recorded from 128-scalp EEG channels and 4 IC clusters in the visual, and 5 IC clusters in the auditory, modality were identified. In general, MS patients had significantly reduced ERSP theta power versus controls, and a similar pattern was observed for CI vs. non-CI MS patients. The ITC measures were also significantly different in the theta band for some clusters. The finding that MS patients had reduced P3b task-related theta power in both modalities is a reflection of compromised connectivity, likely due to demyelination, that may have disrupted early processes essential to P3b generation, such as orientating and signal detection. However, for posterior sources, MS patients had a greater decrease in alpha power, normally associated with enhanced cognitive function, which may reflect a compensatory mechanism in response to the compromised early cognitive processing.
Highlights
Cognitive impairment (CI) affects nearly 65% of multiple sclerosis (MS) patients and can occur in the absence of physical disability [1]
The Paced Auditory Serial Addition Test (PASAT) is influenced by practice effects [10], education, anxiety and physical ability which is usually impaired in MS [11], and objective and reliable cognitive electrophysiological measures, which are not affected by these factors, have potential to complement the assessment of CI in MS
PASAT raw scores were converted to demographically corrected scores (PASAT Z-scores: described in detail in [47]) and the MS patients were grouped into two extreme groups based on their PASAT Z-score: the MS patients with the highest PASAT Zscores or the non-cognitively impaired MS group, and the MS patients with the lowest PASAT Z-scores or the cognitively impaired MS group (CI MS) as detailed in Methods
Summary
Cognitive impairment (CI) affects nearly 65% of multiple sclerosis (MS) patients and can occur in the absence of physical disability [1]. Neuropsychological tests in the MS Functional Composite [7], such as the Paced Auditory Serial Addition Test (PASAT; a difficult test of attention and working memory [8,9]), are generally used to measure CI in MS. The PASAT is influenced by practice effects [10], education, anxiety and physical ability which is usually impaired in MS [11], and objective and reliable cognitive electrophysiological measures, which are not affected by these factors, have potential to complement the assessment of CI in MS
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