Functional connectivity in the default network during resting state is preserved in a vegetative but not in a brain dead patient

Abstract

Recent studies on spontaneous fluctuations in the functional MRI blood oxygen level-dependent (BOLD) signal in awake healthy subjects showed the presence of coherent fluctuations among functionally defined neuroanatomical networks. However, the functional significance of these spontaneous BOLD fluctuations remains poorly understood. By means of 3 T functional MRI, we demonstrate absent cortico-thalamic BOLD functional connectivity (i.e. between posterior cingulate/precuneal cortex and medial thalamus), but preserved cortico-cortical connectivity within the default network in a case of vegetative state (VS) studied 2.5 years following cardio-respiratory arrest, as documented by extensive behavioral and paraclinical assessments. In the VS patient, as in age-matched controls, anticorrelations could also be observed between posterior cingulate/precuneus and a previously identified task-positive cortical network. Both correlations and anticorrelations were significantly reduced in VS as compared to controls. A similar approach in a brain dead patient did not show any such long-distance functional connectivity. We conclude that some slow coherent BOLD fluctuations previously identified in healthy awake human brain can be found in alive but unaware patients, and are thus unlikely to be uniquely due to ongoing modifications of conscious thoughts. Future studies are needed to give a full characterization of default network connectivity in the VS patients population.

Figure 1

Positive correlations with posterior cingulate/precuneus BOLD activity in healthy controls, brain dead and vegetative state patients. Results thresholded for display at uncorrected P < 0.01 in random effect group analysis in healthy controls, and at FDR‐corrected P value <0.05 in patients. MNI coordinates of sections are as follows: x = −10 mm for controls, −4 mm for BD patient and 2 mm for VS patient, y = −56 mm, z = 26 mm. Note that there is no residual long‐range functional connectivity in the brain dead patient. In the VS patient, despite residual functional connectivity within the default network, the observed correlations were significantly reduced compared to controls. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 2

Anticorrelations with posterior cingulate/precuneus in healthy controls and in VS patient. For display purposes, results are thresholded at uncorrected P < 0.01 in healthy controls, and at FDR‐corrected P < 0.05 in VS patient. MNI coordinates of sections are z = 60 mm, x = 62 mm and y = 20 mm. Note that the observed residual connectivity was significantly reduced in VS patient compared to controls. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]