Intrinsic Functional Connectivity Resembles Cortical Architecture at Various Levels of Isoflurane Anesthesia

Abstract Cortical single neuron activity and local field potential patterns change at different depths of general anesthesia. Here, we investigate the associated network level changes of functional connectivity. We recorded ongoing electrocorticographic (ECoG) activity from temporo-parieto-occipital cortex of 6 ferrets at various levels of isoflurane/nitrous oxide anesthesia and determined functional connectivity by computing amplitude envelope correlations. Through hierarchical clustering, we derived typical connectivity patterns corresponding to light, intermediate and deep anesthesia. Generally, amplitude correlation strength increased strongly with depth of anesthesia across all cortical areas and frequency bands. This was accompanied, at the deepest level, by the emergence of burst-suppression activity in the ECoG signal and a change of the spectrum of the amplitude envelope. Normalization of functional connectivity to the distribution of correlation coefficients showed that the topographical patterns remained similar across depths of anesthesia, reflecting the functional association of the underlying cortical areas. Thus, while strength and temporal properties of amplitude co-modulation vary depending on the activity of local neural circuits, their network-level interaction pattern is presumably most strongly determined by the underlying structural connectivity. amplitude correlations, anesthesia, ECoG, ICM, ongoing activity Introduction In the absence of specific tasks or stimuli the brain’s spontaneous, ongoing activity is structured on different spatiotemporal scales. Various measures of functional connectivity reflecting different modes of intrinsically generated coupling between different brain areas have been developed (Deco et al. 2011; Siegel et al. 2012; Engel et al. 2013). Among these intrinsic coupling modes (ICMs), amplitude envelope correlations (in the following termed “envelope ICMs”) represent the level of simultaneity of activity fluctuations between neural populations in 2 cortical areas at a particular carrier frequency (Engel et al. 2013). Amplitude envelopes are typically modulated at frequencies below 0.1 Hz and show low dependence on behavioral state (Leopold et al. 2003). In the awake state, high envelope ICMs are found between bilateral homologous areas (Nir et al. 2008; Hipp et al. 2012) as well as within functional areas of one hemisphere (He et al. 2008). The spatial specificity of the observed patterns can be increased by excluding effects of volume conduction through orthogonalization (see Methods; Hipp et al. 2012). Convolution of the amplitude envelope with the hemodynamic response function yields a close estimate of the functional magnetic resonance imaging (fMRI) blood-oxygen level dependent (BOLD) signal (Liu et al. 2011; Martin 2014). Accordingly, envelope ICMs across 2 brain areas observed with electrophysiological methods are closely linked to the correlations of their BOLD signals (He et al. 2008; Schölvinck et al. 2010; Keller et al. 2013). Thus, for instance, networks defined by envelope ICMs recorded with magnetoencephalography (MEG) show a high similarity to fMRI-resting networks (de Pasquale et al. 2010; Hipp et al. 2012). Ongoing activity is severely altered during general anesthesia, that is, the reversible loss of consciousness induced through the administration of hypnotic agents. Patterns of cortical activity under general anesthesia change with the level of the hypnotic agent. After loss of consciousness, the EEG first shows continuous patterns with increased delta- and alpha- and decreased beta power (Brown et al. 2010). In deeper anesthesia, brain activity changes to burst suppression, which is characterized by an isoelectric line interrupted by bursts of activity (Llinás and Steriade 2006; Brown et al. 2010). The frequency of bursts decreases with deepening anesthesia, until complete suppression is reached (Brown et al. 2010). Network-level changes caused by anesthesia have been examined using various recording techniques and analysis approaches. Studies focusing on the awake-anesthetized transition have suggested that loss of consciousness is associated with global changes in functional connectivity (Cimenser et al. 2011) and distinct alterations of phase-amplitude coupling (Lewis et al. 2012; Mukamel et al. 2014). Large-scale envelope ICMs under different anesthesia depths have been studied using fMRI. While some of these studies observe an increase of connectivity with deepening anesthesia (Liu et al. 2013), others report a breakdown of coupling (Bettinardi et al. 2015). Micro-scale networks on the cellular level have been characterized using extra- and intracellular microelectrode recordings under different concentrations of anesthetic agents (Llinás and Steriade 2006). Nevertheless, little is known about the corresponding connectivity patterns in mesoscale cortical networks consisting of spatially and functionally related cortical areas. Here, we characterize envelope ICMs in visual, auditory and parietal areas of the ferret at different levels of isoflurane anesthesia and a constant nitrous oxide administration. We recorded local field potentials from the cortical surface using a custom-built 64-contact electrocorticographic (ECoG) array. Unlike EEG, these recordings are not affected by the distortion of the electrical field in space and frequency by the surrounding tissues (Buzsáki et al. 2012). The isoflurane concentration was varied between 0.4% and 1.6% in steps of 0.2%. For each anesthesia level, envelope ICMs were quantified in different frequency bands using orthogonalized amplitude correlation (Hipp et al. 2012). Similar coupling patterns were visible at different isoflurane concentrations across animals, reflecting inter-individual differences in drug sensitivity. We therefore applied hierarchical clustering to all connectivity matrices of each frequency band and identified similar coupling-defined network states across animals. The resulting 3 clusters corresponded to light, medium, and deep anesthesia. Overall, the strength of envelope ICMs increased across anesthesia depths. Normalized connectivity patterns, however, remained similar across anesthesia depths, resembling in their topology known patterns of structural connectivity in the cortex. Methods Data presented in this study were collected from 6 adult female ferrets (Mustela putorius). All experiments were approved by the independent Hamburg state authority for animal welfare (BUG-Hamburg) and were performed in accordance with the guidelines of the German Animal Protection Law. ECoG-Array We employed a micro-ECoG-array co-developed with the University of Freiburg (ECoG-array; IMTEK, Freiburg) covering a large portion of the posterior, parietal, and temporal surface of the left ferret brain hemisphere (Rubehn et al. 2009). The probe was designed to record from posterior early and higher visual areas, auditory, somatosenso (...truncated)