Gamma-Band Modulation and Coherence in the EEG by Involuntary Eye Movements in Patients in Unresponsive Wakefulness Syndrome

Gamma power and coherence in the electroencephalogram increase in healthy individuals in association with voluntary eye movements, saccades. Patients with unresponsive wakefulness syndrome show repetitive involuntary eye movements that are similar to saccades but progress at a much lower speed. In the present study, we explored the changes in gamma power and coherence related to these eye movements and investigated whether any relationship to the patients' clinical status could be found that would indicate first neurophysiological signs of recovery. To this end, we assessed the clinical status and registered classical scalp electroencephalography with 19 surface electrodes and electro-oculogram of 45 consecutive patients at admission and at 4 weekly intervals. Slow gamma activity (in the frequency range of 37-40 Hz) was analyzed before, during, and after eye movements (pre, -intra and post-eye movement) by means of "continuous wavelet transform." We graded recovery using clinical behavioral scales, taking into account the variables, age, gender, recovery (yes or no), as well as the patients diagnoses (traumatic brain injury, hypoxia, hemorrhage, infection). Statistical evaluation was performed using DataLab, R, and Kruskal-Wallis methods. Based on the clinical status, we distinguished between recovering and chronic groups of patients. In comparison with the chronic group, the recovering group showed significantly higher gamma power over the posterior electrodes and significant higher values of coherence in the gamma-band activity during the presaccadic period of eye movements. We suggest that our findings on the onset of involuntary eye movements in the recovering group of patients with unresponsive wakefulness syndrome indicates a first neurophysiological sign of favorable prognosis.