Spontaneous fluctuations in the blood-oxygen-level-dependent (BOLD) signals demonstrate consistent temporal correlations within large-scale brain networks associated with different functions. The neurophysiological correlates of this phenomenon remain elusive. Here, we show in humans that the slow cortical potentials recorded by electrocorticography demonstrate a correlation structure similar to that of spontaneous BOLD fluctuations across wakefulness, slow-wave sleep, and rapid-eye-movement sleep. Gamma frequency power also showed a similar correlation structure but only during wakefulness and rapid-eye-movement sleep. Our results provide an important bridge between the largescale brain networks readily revealed by spontaneous BOLD signals and their underlying neurophysiology.electrocorticography ͉ fMRI ͉ functional connectivity ͉ human ͉ sleep S pontaneous slow (Ͻ0.1 Hz) fluctuations in the blood-oxygenlevel-dependent (BOLD) signals of functional magnetic resonance imaging (fMRI) appear to reflect a fundamental aspect of the brain's organization (1, 2). These fluctuations are temporally covariant within large-scale functional brain networks, such as those associated with sensorimotor (1), language (3), attention (4), and executive (5) functions as well as the ''default network'' (6). These covariant relations (i.e., correlation structures) of spontaneous BOLD signals exist during restful waking (1, 3-6), task performance (3, 7), sleep (8), and even general anesthesia (2). Furthermore, their integrity appears to be essential to normal brain function (7). However, in contrast to evoked BOLD responses (9-12), the electrophysiological basis of these spontaneous covariant BOLD fluctuations is unknown. Here, we investigated this question in five patients with intractable epilepsy undergoing evaluation with surgically implanted grids of subdural electrodes. Each patient underwent about a week of continuous videomonitored electrocorticography (ECoG) for the purpose of determining the epileptic focus before surgical resection.The present analyses were based on ECoG data recorded in three distinct arousal states: (i) extended awake periods during which patients were in bed or seated, typically watching TV, eating, or engaged in social interactions; (ii) slow-wave sleep (SWS); and (iii) rapid-eye-movement (REM) sleep. Representative ECoG data are shown in supporting information (SI) Fig. S1. Resting-state (maintaining visual fixation) BOLD fMRI was acquired in a separate session either before or after surgical intervention. Patient information and data details are included in Table S1. In what follows, we present analyses using four different strategies to compare the correlation structures of BOLD and ECoG signals.
Results
Correlation Structures of Spontaneous BOLD Signal and Slow CorticalPotential. In the first three analyses, we focused on the sensorimotor network, because the ECoG electrodes provided adequate coverage of the sensorimotor network in all presently studied patients but much poorer coverage of the other ...