Cortical source of blink-related delta oscillations and their correlation with levels of consciousness

Abstract

Recently, blink-related delta oscillations (delta BROs) have been observed in healthy subjects during spontaneous blinking at rest. Delta BROs have been linked with continuous gathering of information from the surrounding environment, which is classically attributed to the precuneus. Furthermore, fMRI studies have shown that precuneal activity is reduced or missing when consciousness is low or absent. We therefore hypothesized that the source of delta BROs in healthy subjects could be located in the precuneus and that delta BROs could be absent or reduced in patients with disorders of consciousness (DOC). To test these hypotheses, electroencephalographic (EEG) activity at rest was recorded in 12 healthy controls and nine patients with DOC (four vegetative states, and five minimally conscious states). Three-second-lasting EEG epochs centred on each blink instance were analyzed in both time- (BROs) and frequency domains (event-related spectral perturbation or ERSP and intertrial coherence or ITC). Cortical sources of the maximum blink-related delta power, corresponding to the positive peak of the delta BROs, were estimated by standardized Low Resolution Electromagnetic Tomography. In control subjects, as expected, the source of delta BROs was located in the precuneus, whereas in DOC patients, delta BROs were not recognizable and no precuneal localization was possible. Furthermore, we observed a direct relationship between spectral indexes and levels of cognitive functioning in all subjects participating in the study. This reinforces the hypothesis that delta BROs reflect neural processes linked with awareness of the self and of the environment.

Keywords: blinking; consciousness; delta rhythm; event-related oscillations EROs; event-related potentials ERPs; precuneus.

Figure 1

(A) Average blink‐related ERPs in all channel locations for control subjects, after ICA‐artifact rejection. Time ranges from −1500 ms to +1500 ms, frequency from 0.5 Hz to 45 Hz. (B) Average blink‐related ERP in channel Pz in the delta band (0.5–3.0 Hz) for VS (dashed gray line), MCS (dashed black line), and control (solid black line) subjects.

Figure 2

(A) Grand average current densities for the delta‐band blink‐related ERP computed in correspondence to the peak in delta power (t = +300 ms) for control subjects, with three‐dimensional rendering for the left and right hemispheres. (B and C) Sagittal view of single‐subject current densities for the delta‐band blink‐related ERP computed in correspondence to the peak in delta power (t = +300 ms) for, respectively, VS (panel B), and MCS (panel C) subjects. Sagittal planes are computed in correspondence with the maximum current density. All current density values have been normalized in the 0‐max current range.

Figure 3

(A) ERSPs for VS, MCS and control subjects, in the −1000 to +1000 ms and 1–45 Hz range, with significant differences between the two groups (P < 0.05) highlighted via a bootstrap technique. (B) Left: box plots showing average ERSP in the time‐window from +250 and +350 ms within the ROI (delta range) for VS, MCS, and control subjects. Right: average ERSP in the time‐window from +250 and +350 ms within the ROI (delta range) as a function of LCFS score. Regression (linear model fitting) showed a significant value P < 0.01, with r ² = 0.32. (C) ITCs for healthy controls and subjects with DOC, in the −1000 to +1000 ms and 1–45 Hz range, with significant differences between the two groups (P < 0.05) highlighted via a bootstrap technique. (D) Left: box plots showing average ITC in the time‐window from +250 and +350 ms within the ROI (delta range) for VS, MCS, and control subjects. Right: average ITC in the time‐window from +250 and +350 ms within the ROI (delta range) as a function of LCFS score. Regression (linear model fitting) showed a significant value P < 0.01, with r ² = 0.44. Asterisks in panels B and D mark significant differences between groups (P < 0.05, One‐Way ANOVA, and post hoc Tukey's test).