Information processing in severe disorders of consciousness: Vegetative state and minimally conscious state☆
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Fig. 1
Typical examples of the EEG frequency spectrum in 4 PVS patients two of which belonged to the Control group 1 (CG1, dashed lines), and two to the Main group 1 (MG1, solid lines). Pz lead. The spectral analysis is based on a 131.07 s time epoch (32758 points) of rest EEG. Hanning window was used. Each data point stands for a frequency range of 1 Hz, each tick on the X-axis, for 5 Hz. Asterisks indicate a relative increase of the power in the theta band in MG1 patients. This increase was lacking in all CG1 patients.
Fig. 2
Examples of various ERP phenomena in patients with severe brain damage. (A): Primary cortical components N1–P2—however, without any differentiation between frequent and rare stimuli, in a male patient, 51, anoxic brain injury. (B): Apart of the primary complex, a delayed P3 to rare stimuli in a female, 29, subarachnoidal hemorrhage. (C): A long-latency, significant (P=0.014) positive deflection (‘P600’) to the counted word category (animals) in the semantic oddball, a female, 19, PVS after a post-operative intraventricular hemorrhage. (D): An N400 in the Word pair experiment to semantically unrelated words, in contrast to a positivity to related words, in a male, 42, head injury. (E): An N400 to incongruent end words in the Sentence experiment, in a male, 61, MCS following bilateral infarcts in basal ganglia. For simplicity only one lead (Cz) is presented. Ticks on the amplitude axis indicate: 5 μV on A and B, 2 μV on C, D, and E. Ticks on the time axis indicate 200 ms in all graphics. The negativity is plotted upwards. More examples of ERP waveforms in PVS and MCS patients can be obtained from the corresponding author.
Fig. 3
ERPs of a female PVS patient (64 years, 7 months following brain anoxia) with a pronounced diffuse delta-activity in the rest EEG (CG1). Surprisingly, there was a significant MMN in the MMN experiment with two pure tones (left; P=0.017), a highly-significant (P=0.001) N1 in the oddball with two vowels, and an apparent P3-like deflection which, however, did not reach significance (P=0.12).
Fig. 4
A distinct and statistically significant (P<0.01) P3 in Oddball II despite the lack of N1 and P2 components, in an MCS patient (male, 55), 4 months after an anoxic brain injury.
Fig. 5
Left panel: An example of a high-amplitude MMN to musical tones in a patient with a diagnosis MCS following head injury. The patient was discharged 3 months later with a considerable improvement. Right panel: Percentage of clinical improvement in patients with present versus absent MMN responses.
Abstract
Objective
To study the presence of electrophysiological indicators of remaining cortical functions in patients with persistent vegetative state (PVS) and minimally conscious state (MCS). Previous electrophysiological and PET data indicated that some PVS patients have partially intact cortical processing functions. However, it remains unclear whether the reported patients were representative for PVS population or just some exceptional cases.
Methods: Event-related brain responses to stimuli of different complexity levels, recorded in 98 patients with extremely severe diffuse brain injuries, 50 of which in PVS. Four main indicators of cortical functions were: (i) N1–P2 complex as an index of simple, undifferentiated cortical processing; (ii) mismatch negativity as an index of pre-attentive, probably unconscious, cortical orientation; (iii) P3 wave as an index of deep cortical analysis of physical stimuli, and (iv) brain responses to semantic stimuli.
Results
Cortical responses were found in all PVS patients with a background EEG activity >4 Hz. All responses investigated, including those to semantic stimuli that indicated comprehension of meaning, occurred significantly above chance, though less frequently than in patients with severe brain injuries who were conscious.
Conclusions
Cortical responses were lacking in most patients with severe EEG slowing (<4 Hz). Follow-up data revealed that the presence of a mismatch negativity, a short disease duration, and the traumatic etiology were related to a better outcome.
Significance
The data show that in a subpopulation of PVS patients with preserved thalamocortical feedback connections, remaining cortical information processing is a consistent finding and may even involve semantic levels of processing.
Keywords:
Consciousness, Event-related potentials, Minimally conscious state, P300, Vegetative stateTo access this article, please choose from the options below
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☆Supported by the German Research Society (Deutsche Forschungsgemeinschaft, DFG), SFB 550. We thank anonymous reviewers for their substantial contribution to the improvement of this manuscript. Portions of the data were reported on the 4th International Congress on Coma and Death, Habana, Cuba, March 2004, and on the Seminar of the Reuth Rehabilitation Center in Tel Aviv, Israel, May 2004.
