Default network connectivity reflects the level of consciousness in non-communicative brain-damaged patients

Abstract

The 'default network' is defined as a set of areas, encompassing posterior-cingulate/precuneus, anterior cingulate/mesiofrontal cortex and temporo-parietal junctions, that show more activity at rest than during attention-demanding tasks. Recent studies have shown that it is possible to reliably identify this network in the absence of any task, by resting state functional magnetic resonance imaging connectivity analyses in healthy volunteers. However, the functional significance of these spontaneous brain activity fluctuations remains unclear. The aim of this study was to test if the integrity of this resting-state connectivity pattern in the default network would differ in different pathological alterations of consciousness. Fourteen non-communicative brain-damaged patients and 14 healthy controls participated in the study. Connectivity was investigated using probabilistic independent component analysis, and an automated template-matching component selection approach. Connectivity in all default network areas was found to be negatively correlated with the degree of clinical consciousness impairment, ranging from healthy controls and locked-in syndrome to minimally conscious, vegetative then coma patients. Furthermore, precuneus connectivity was found to be significantly stronger in minimally conscious patients as compared with unconscious patients. Locked-in syndrome patient's default network connectivity was not significantly different from controls. Our results show that default network connectivity is decreased in severely brain-damaged patients, in proportion to their degree of consciousness impairment. Future prospective studies in a larger patient population are needed in order to evaluate the prognostic value of the presented methodology.

Figure 1

Default network identified in controls. Results are thresholded for display at whole brain false discovery rate corrected P < 0.01 and rendered on the mean T₁ structural image of the controls.

Figure 2

Default network connectivity correlates with the level of consciousness, ranging from healthy controls, to minimally conscious, vegetative then comatose patients. (A) Areas showing a linear correlation between default network connectivity and consciousness. Results are thresholded for display at uncorrected P < 0.05 and rendered on the mean T₁ structural image of the patients. (B) Mean Z-scores and 90% confidence interval for default network connectivity in PCC/precuneus, temporo-parietal junction, medial prefrontal cortex and parahippocampal gyrus across patient populations. Locked-in syndrome patient Z-scores are displayed for illustrative purposes as an additional red circles overlaid on control population data.

Figure 3

Brain areas within the default network connectivity which differentiate minimally conscious patients from unconscious patients. Results are thresholded for display at uncorrected P < 0.05 and rendered on the mean T₁ structural image of the patients.