Disorder of consciousness and N400 ERP measures in response to a semantic task

Background: Disorders of consciousness (DOC) were amply studied in the recent years. In the present research electrophysiological measures (ERPs) were used to verify the preservation of semantic linguistic processes in vegetative states (VS) and minimal consciousness state (MCS). Materials and methods: Eighteen patients classified as VS or MCS and twenty controls were submitted to a semantic associative task congruous or incongruous word sequences (auditory stimuli). Each auditory sequence was composed of four words that were presented to the subjects, and it had a congruous (semantic related words) or an incongruous (semantic unrelated words) final word based on its semantic content. Results: Two sets of analysis were conducted: the first was finalized to compare the general patient category with the control group. The second was aimed to compare VS with MCS sub- category each other. Two dependent measures were used, respectively the peak amplitude and the peak latency. The ERPs were entered into a four-way repeated measure ANOVA. Three main results were found in the present research. A first main result was that a morphologically similar N400, peaking at about 410 msec. post-stimulus, more frontally distributed 186 and higher for incongruous condition, was found for patients and controls. However, the comparison between patients’ and controls’ performance revealed a significant difference for N400 latency. In fact it was found a delayed peak for patients in incongruous condition in comparison with control subjects. Specifically this temporal delay was observed within the frontal sites. Third MCS and mainly VS diagnosis was not accompanied by the abolition or reduction of ERP N400 component. Conclusions: Thus, the current diagnostic criteria for DOC, mainly based only on observing patient behavior, may have prevent to deeply comprehend the real cognitive potentialities of this patient group and to correctly classify different consciousness profiles. In some cases misdiagnosis occurs for patients who are considered in VS and that instead could be more correctly classified as minimally conscious, by adopting ERP measures (and specifically N400 index). This marker highlights the ability to evaluate intent, semantic associations, and meaning which are critical components of awareness.