Abstract
BackgroundFor nearly four decades, the N400 has been an important brainwave marker of semantic processing. It can be recorded non-invasively from the scalp using electrical and/or magnetic sensors, but largely within the restricted domain of research laboratories specialized to run specific N400 experiments. However, there is increasing evidence of significant clinical utility for the N400 in neurological evaluation, particularly at the individual level. To enable clinical applications, we recently reported a rapid evaluation framework known as “brain vital signs” that successfully incorporated the N400 response as one of the core components for cognitive function evaluation. The current study characterized the rapidly evoked N400 response to demonstrate that it shares consistent features with traditional N400 responses acquired in research laboratory settings—thereby enabling its translation into brain vital signs applications.MethodsData were collected from 17 healthy individuals using magnetoencephalography (MEG) and electroencephalography (EEG), with analysis of sensor-level effects as well as evaluation of brain sources. Individual-level N400 responses were classified using machine learning to determine the percentage of participants in whom the response was successfully detected.ResultsThe N400 response was observed in both M/EEG modalities showing significant differences to incongruent versus congruent condition in the expected time range (p < 0.05). Also as expected, N400-related brain activity was observed in the temporal and inferior frontal cortical regions, with typical left-hemispheric asymmetry. Classification robustly confirmed the N400 effect at the individual level with high accuracy (89%), sensitivity (0.88) and specificity (0.90).ConclusionThe brain vital sign N400 characteristics were highly consistent with features of the previously reported N400 responses acquired using traditional laboratory-based experiments. These results provide important evidence supporting clinical translation of the rapidly acquired N400 response as a potential tool for assessments of higher cognitive functions.
Highlights
IntroductionThe N400 has been an important brainwave marker of semantic processing
For nearly four decades, the N400 has been an important brainwave marker of semantic processing
The N400 response was first described when Kutas and Hillyard presented participants with visual sentences that either had a semantically related or semantically unrelated ending [8]. It was observed as a negative deflection of the incongruent relative to congruent condition waveforms which peaked at approximately 400 ms latency following stimulus presentation, and the authors suggested that this differential was a neural marker of semantic language processing
Summary
The N400 has been an important brainwave marker of semantic processing. The N400 response was first described when Kutas and Hillyard presented participants with visual sentences that either had a semantically related (i.e. congruent) or semantically unrelated (i.e. incongruent) ending [8] It was observed as a negative deflection of the incongruent relative to congruent condition waveforms which peaked at approximately 400 ms latency following stimulus presentation, and the authors suggested that this differential was a neural marker of semantic language processing. Others have demonstrated overlapping features in the temporal and spatial characteristics of the N400 response when elicited using language- as well as non-languagebased stimuli [17], with the spectral content in particular demonstrating potential in distinguishing between different neural processes [19]. One of the key spectral features of the N400 response has been shown to be a reduction in beta band oscillations when processing incongruent relative to congruent stimuli in semantic language paradigms [20]
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