Sequential effects of propofol on functional brain activation induced by auditory language processing: an event-related functional magnetic resonance imaging study

Comparison of fMRI language laterality with and without sedation in pediatric epilepsy

Functional MRI is an essential component of presurgical language mapping. In clinical settings, young children may be sedated for the MRI with the functional stimuli presented passively. Research has found that sedation changes language activation in healthy adults and children. However, there is limited research comparing sedated and unsedated functional MRI in pediatric epilepsy patients. We compared language activation patterns in children with epilepsy who received sedation for functional MRI to the ones who did not. We retrospectively identified the patients with focal epilepsy who underwent presurgical functional MRI including Auditory Descriptive Decision Task at Boston Children's Hospital from 2014 to 2022. Patients were divided into sedated and awake groups, based on their sedation status during functional MRI. Auditory Descriptive Decision Task stimuli were presented passively to the sedated group per clinical protocol. We extracted language activation maps contrasted against a control task (reverse speech) in the Frontal and Temporal language regions and calculated separate language laterality indexes for each region. We considered positive laterality indexes as left dominant, negative laterality indexes as right dominant, and absolute laterality indexes <0.2 as bilateral. We defined 2 language patterns: typical (i.e., primarily left-sided) and atypical. Typical pattern required at least one left dominant region (either frontal or temporal) and no right dominant region. We then compared the language patterns between the sedated and awake groups. Seventy patients met the inclusion criteria, 25 sedated, and 45 awake. Using the Auditory Descriptive Decision Task paradigm, when adjusted for age, handedness, gender, and laterality of lesion in a weighted logistic regression model, the odds of the atypical pattern were 13.2 times higher in the sedated group compared to the awake group (Confidence Interval: 2.55-68.41, p-value < 0.01). Sedation may alter language activation patterns in pediatric epilepsy patients. Language patterns on sedated functional MRI with passive tasks may not represent language networks during wakefulness, sedation may differentially suppress some networks, or require a different task or method of analysis to capture the awake language network. Given the critical surgical implication of these findings, additional studies are needed to better understand how sedation impacts the functional MRI blood oxygenation level-dependent signal. Consistent with current practice, sedated functional MRI should be interpreted with greater caution and requires additional validation as well as research on post-surgical language outcomes.

The effects of propofol anaesthesia on molecular-enriched networks during resting-state and naturalistic listening

Placing a patient in a state of anaesthesia is crucial for modern surgical practice. However, the mechanisms by which anaesthetic drugs, such as propofol, impart their effects on consciousness remain poorly understood. Propofol potentiates GABAergic transmission, which purportedly has direct actions on cortex as well as indirect actions via ascending neuromodulatory systems. Functional imaging studies to date have been limited in their ability to unravel how these effects on neurotransmission impact the system-level dynamics of the brain. Here, we leveraged advances in multi-modal imaging, Receptor-Enriched Analysis of functional Connectivity by Targets (REACT), to investigate how different levels of propofol-induced sedation alter neurotransmission-related functional connectivity (FC), both at rest and when individuals are exposed to naturalistic auditory stimulation. Propofol increased GABA-A- and noradrenaline transporter-enriched FC within occipital and somatosensory regions respectively. Additionally, during auditory stimulation, the network related to the dopamine transporter showed reduced FC within bilateral regions of temporal and mid/posterior cingulate cortices, with the right temporal cluster showing an interaction between auditory stimulation and level of consciousness. In bringing together these micro- and macro-scale systems, we provide support for both direct GABAergic and indirect noradrenergic and dopaminergic-related network changes under propofol sedation. Further, we delineate a cognition-related reconfiguration of the dopaminergic network, highlighting the utility of REACT to explore the molecular substrates of consciousness and cognition.

Propofol anesthesia concentration rather than abrupt behavioral unresponsiveness linearly degrades responses in the rat primary auditory cortex

Despite extensive knowledge of its molecular and cellular effects, how anesthesia affects sensory processing remains poorly understood. In particular, it remains unclear whether anesthesia modestly or robustly degrades activity in primary sensory regions, and whether such changes are linked to anesthesia drug concentration versus behavioral unresponsiveness, which are typically confounded. Here, we used slow gradual intravenous propofol anesthesia induction together with auditory stimulation and intermittent assessment of behavioral responsiveness while recording epidural electroencephalogram, and neuronal spiking activity in primary auditory cortex (PAC) of eight rats. We found that all main components of neuronal activity including spontaneous firing rates, onset response magnitudes, onset response latencies, postonset neuronal silence duration, late-locking to 40 Hz click-trains, and offset responses, gradually changed in a dose-dependent manner with increasing anesthesia levels without showing abrupt shifts around loss of righting reflex or other time-points. Thus, the dominant factor affecting PAC responses is the anesthesia drug concentration rather than any sudden, dichotomous behavioral state changes. Our findings explain a wide array of seemingly conflicting results in the literature that, depending on the precise definition of wakefulness (vigilant vs. drowsy) and anesthesia (light vs. deep/surgical), report a spectrum of effects in primary regions ranging from minimal to dramatic differences.

Selective corticocortical connectivity suppression during propofol-induced anesthesia in healthy volunteers

We comprehensively studied directional feedback and feedforward connectivity to explore potential connectivity changes that underlie propofol-induced deep sedation. We further investigated the corticocortical connectivity patterns within and between hemispheres. Sixty-channel electroencephalographic data were collected from 19 healthy volunteers in a resting wakefulness state and propofol-induced deep unconsciousness state defined by a bispectral index value of 40. A source analysis was employed to locate cortical activity. The Desikan-Killiany atlas was used to partition cortices, and directional functional connectivity was assessed by normalized symbolic transfer entropy between higher-order (prefrontal and frontal) and lower-order (auditory, sensorimotor and visual) cortices and between hot-spot frontal and parietal cortices. We found that propofol significantly suppressed feedforward connectivity from the left parietal to right frontal cortex and bidirectional connectivity between the left frontal and left parietal cortex, between the frontal and auditory cortex, and between the frontal and sensorimotor cortex. However, there were no significant changes in either feedforward or feedback connectivity between the prefrontal and all the lower-order cortices and between the frontal and visual cortices or in feedback connectivity from the frontal to parietal cortex. Propofol anesthetic selectively decreased the unidirectional interaction between higher-order frontoparietal cortices and bidirectional interactions between the higher-order frontal cortex and lower-order auditory and sensorimotor cortices, which indicated that both feedback and feedforward connectivity were suppressed under propofol-induced deep sedation. Our findings provide critical insights into the connectivity changes underlying the top-down mechanism of propofol anesthesia at deep sedation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11571-021-09775-x.

Cortical Responses to Vowel Sequences in Awake and Anesthetized States: A Human Intracranial Electrophysiology Study

Elucidating neural signatures of sensory processing across consciousness states is a major focus in neuroscience. Noninvasive human studies using the general anesthetic propofol reveal differential effects on auditory cortical activity, with a greater impact on nonprimary and auditory-related areas than primary auditory cortex. This study used intracranial electroencephalography to examine cortical responses to vowel sequences during induction of general anesthesia with propofol. Subjects were adult neurosurgical patients with intracranial electrodes placed to identify epileptic foci. Data were collected before electrode removal surgery. Stimuli were vowel sequences presented in a target detection task during awake, sedated, and unresponsive states. Averaged evoked potentials (AEPs) and high gamma (70-150 Hz) power were measured in auditory, auditory-related, and prefrontal cortex. In the awake state, AEPs were found throughout studied brain areas; high gamma activity was limited to canonical auditory cortex. Sedation led to a decrease in AEP magnitude. Upon LOC, there was a decrease in the superior temporal gyrus and adjacent auditory-related cortex and a further decrease in AEP magnitude in core auditory cortex, changes in the temporal structure and increased trial-to-trial variability of responses. The findings identify putative biomarkers of LOC and serve as a foundation for future investigations of altered sensory processing.

Comparison of anaesthetic- and seizure-induced states of unconsciousness: a narrative review

In order to understand general anaesthesia and certain seizures, a fundamental understanding of the neurobiology of unconsciousness is needed. This review article explores similarities in neuronal and network changes during general anaesthesia and seizure-induced unconsciousness. Both seizures and anaesthetics cause disruption in similar anatomical structures that presumably lead to impaired consciousness. Despite differences in behaviour and mechanisms, both of these conditions are associated with disruption of the functionality of subcortical structures that mediate neuronal activity in the frontoparietal cortex. These areas are all likely to be involved in maintaining normal consciousness. An assessment of the similarities in the brain network disruptions with certain seizures and general anaesthesia might provide fresh insights into the mechanisms of the alterations of consciousness seen in these particular unconscious states, allowing for innovative therapies for seizures and the development of anaesthetic approaches targeting specific networks.

A Narrative Review on Clinical Applications of fNIRS

Functional near-infrared spectroscopy (fNIRS) is a relatively new imaging modality in the functional neuroimaging research arena. The fNIRS modality non-invasively investigates the change of blood oxygenation level in the human brain utilizing the transillumination technique. In the last two decades, the interest in this modality is gradually evolving for its real-time monitoring, relatively low-cost, radiation-less environment, portability, patient-friendliness, etc. Including brain-computer interface and functional neuroimaging research, this technique has some important application of clinical perspectives such as Alzheimer's disease, schizophrenia, dyslexia, Parkinson's disease, childhood disorders, post-neurosurgery dysfunction, attention, functional connectivity, and many more can be diagnosed as well as in some form of assistive modality in clinical approaches. Regarding the issue, this review article presents the current scopes of fNIRS in medical assistance, clinical decision making, and future perspectives. This article also covers a short history of fNIRS, fundamental theories, and significant outcomes reported by a number of scholarly articles. Since this review article is hopefully the first one that comprehensively explores the potential scopes of the fNIRS in a clinical perspective, we hope it will be helpful for the researchers, physicians, practitioners, current students of the functional neuroimaging field, and the related personnel for their further studies and applications.

Anesthesia-induced loss of consciousness disrupts auditory responses beyond primary cortex

Despite its ubiquitous use in medicine, and extensive knowledge of its molecular and cellular effects, how anesthesia induces loss of consciousness (LOC) and affects sensory processing remains poorly understood. Specifically, it is unclear whether anesthesia primarily disrupts thalamocortical relay or intercortical signaling. Here we recorded intracranial electroencephalogram (iEEG), local field potentials (LFPs), and single-unit activity in patients during wakefulness and light anesthesia. Propofol infusion was gradually increased while auditory stimuli were presented and patients responded to a target stimulus until they became unresponsive. We found widespread iEEG responses in association cortices during wakefulness, which were attenuated and restricted to auditory regions upon LOC. Neuronal spiking and LFP responses in primary auditory cortex (PAC) persisted after LOC, while responses in higher-order auditory regions were variable, with neuronal spiking largely attenuated. Gamma power induced by word stimuli increased after LOC while its frequency profile slowed, thus differing from local spiking activity. In summary, anesthesia-induced LOC disrupts auditory processing in association cortices while relatively sparing responses in PAC, opening new avenues for future research into mechanisms of LOC and the design of anesthetic monitoring devices.

Functional diversity of brain networks supports consciousness and verbal intelligence

How are the myriad stimuli arriving at our senses transformed into conscious thought? To address this question, in a series of studies, we asked whether a common mechanism underlies loss of information processing in unconscious states across different conditions, which could shed light on the brain mechanisms of conscious cognition. With a novel approach, we brought together for the first time, data from the same paradigm-a highly engaging auditory-only narrative-in three independent domains: anesthesia-induced unconsciousness, unconsciousness after brain injury, and individual differences in intellectual abilities during conscious cognition. During external stimulation in the unconscious state, the functional differentiation between the auditory and fronto-parietal systems decreased significantly relatively to the conscious state. Conversely, we found that stronger functional differentiation between these systems in response to external stimulation predicted higher intellectual abilities during conscious cognition, in particular higher verbal acuity scores in independent cognitive testing battery. These convergent findings suggest that the responsivity of sensory and higher-order brain systems to external stimulation, especially through the diversification of their functional responses is an essential feature of conscious cognition and verbal intelligence.

Auditory Predictive Coding across Awareness States under Anesthesia: An Intracranial Electrophysiology Study

The systems-level mechanisms underlying loss of consciousness (LOC) under anesthesia remain unclear. General anesthetics suppress sensory responses within higher-order cortex and feedback connections, both critical elements of predictive coding hypotheses of conscious perception. Responses to auditory novelty may offer promise as biomarkers for consciousness. This study examined anesthesia-induced changes in auditory novelty responses over short (local deviant [LD]) and long (global deviant [GD]) time scales, envisioned to engage preattentive and conscious levels of processing, respectively. Electrocorticographic recordings were obtained in human neurosurgical patients (3 male, 3 female) from four hierarchical processing levels: core auditory cortex, non-core auditory cortex, auditory-related, and PFC. Stimuli were vowel patterns incorporating deviants within and across stimuli (LD and GD). Subjects were presented with stimuli while awake, and during sedation (responsive) and following LOC (unresponsive) under propofol anesthesia. LD and GD effects were assayed as the averaged evoked potential and high gamma (70-150 Hz) activity. In the awake state, LD and GD effects were present in all recorded regions, with averaged evoked potential effects more broadly distributed than high gamma activity. Under sedation, LD effects were preserved in all regions, except PFC. LOC was accompanied by loss of LD effects outside of auditory cortex. By contrast, GD effects were markedly suppressed under sedation in all regions and were absent following LOC. Thus, although the presence of GD effects is indicative of being awake, its absence is not indicative of LOC. Loss of LD effects in higher-order cortical areas may constitute an alternative biomarker of LOC.SIGNIFICANCE STATEMENT Development of a biomarker that indexes changes in the brain upon loss of consciousness (LOC) under general anesthesia has broad implications for elucidating the neural basis of awareness and clinical relevance to mechanisms of sleep, coma, and disorders of consciousness. Using intracranial recordings from neurosurgery patients, we investigated changes in the activation of cortical networks involved in auditory novelty detection over short (local deviance) and long (global deviance) time scales associated with sedation and LOC under propofol anesthesia. Our results indicate that, whereas the presence of global deviance effects can index awareness, their loss cannot serve as a biomarker for LOC. The dramatic reduction of local deviance effects in areas beyond auditory cortex may constitute an alternative biomarker of LOC.

Spoken words are processed during dexmedetomidine-induced unresponsiveness

BACKGROUND

Studying the effects of anaesthetic drugs on the processing of semantic stimuli could yield insights into how brain functions change in the transition from wakefulness to unresponsiveness. Here, we explored the N400 event-related potential during dexmedetomidine- and propofol-induced unresponsiveness.

METHODS

Forty-seven healthy subjects were randomised to receive either dexmedetomidine (n=23) or propofol (n=24) in this open-label parallel-group study. Loss of responsiveness was achieved by stepwise increments of pseudo-steady-state plasma concentrations, and presumed loss of consciousness was induced using 1.5 times the concentration required for loss of responsiveness. Pre-recorded spoken sentences ending either with an expected (congruous) or an unexpected (incongruous) word were presented during unresponsiveness. The resulting electroencephalogram data were analysed for the presence of the N400 component, and for the N400 effect defined as the difference between the N400 components elicited by congruous and incongruous stimuli, in the time window 300-600 ms post-stimulus. Recognition of the presented stimuli was tested after recovery of responsiveness.

RESULTS

The N400 effect was not observed during dexmedetomidine- or propofol-induced unresponsiveness. The N400 component, however, persisted during dexmedetomidine administration. The N400 component elicited by congruous stimuli during unresponsiveness in the dexmedetomidine group resembled the large component evoked by incongruous stimuli at the awake baseline. After recovery, no recognition of the stimuli heard during unresponsiveness occurred.

CONCLUSIONS

Dexmedetomidine and propofol disrupt the discrimination of congruous and incongruous spoken sentences, and recognition memory at loss of responsiveness. However, the processing of words is partially preserved during dexmedetomidine-induced unresponsiveness.

CLINICAL TRIAL REGISTRATION

NCT01889004.

Investigation of data-driven optical neuromonitoring approach during general anesthesia with sevoflurane

Anesthesia monitoring currently needs a reliable method to evaluate the effects of the anesthetics on its primary target, the brain. This study focuses on investigating the clinical usability of a functional near-infrared spectroscopy (fNIRS)-derived machine learning classifier to perform automated and real-time classification of maintenance and emergence states during sevoflurane anesthesia. For 19 surgical procedures, we examine the entire continuum of the maintenance-transition-emergence phases and evaluate the predictive capability of a support vector machine (SVM) classifier during these phases. We demonstrate the robustness of the predictions made by the SVM classifier and compare its performance with that of minimum alveolar concentration (MAC) and bispectral (BIS) index-based predictions. The fNIRS-SVM investigated in this study provides evidence to the usability of the fNIRS signal for anesthesia monitoring. The method presented enables classification of the signal as maintenance or emergence automatically as well as in real-time with high accuracy, sensitivity, and specificity. The features local mean HbTotal, std [Formula: see text], local min Hb and [Formula: see text], and range Hb and [Formula: see text] were found to be robust biomarkers of this binary classification task. Furthermore, fNIRS-SVM was capable of identifying emergence before movement in a larger number of patients than BIS and MAC.

Dynamics of Propofol-Induced Loss of Consciousness Across Primate Neocortex

The precise neural mechanisms underlying transitions between consciousness and anesthetic-induced unconsciousness remain unclear. Here, we studied intracortical neuronal dynamics leading to propofol-induced unconsciousness by recording single-neuron activity and local field potentials directly in the functionally interconnecting somatosensory (S1) and frontal ventral premotor (PMv) network during a gradual behavioral transition from full alertness to loss of consciousness (LOC) and on through a deeper anesthetic level. Macaque monkeys were trained for a behavioral task designed to determine the trial-by-trial alertness and neuronal response to tactile and auditory stimulation. We show that disruption of coherent beta oscillations between S1 and PMv preceded, but did not coincide with, the LOC. LOC appeared to correspond to pronounced but brief gamma-/high-beta-band oscillations (lasting ∼3 min) in PMv, followed by a gamma peak in S1. We also demonstrate that the slow oscillations appeared after LOC in S1 and then in PMv after a delay, together suggesting that neuronal dynamics are very different across S1 versus PMv during LOC. Finally, neurons in both S1 and PMv transition from responding to bimodal (tactile and auditory) stimulation before LOC to only tactile modality during unconsciousness, consistent with an inhibition of multisensory integration in this network. Our results show that propofol-induced LOC is accompanied by spatiotemporally distinct oscillatory neuronal dynamics across the somatosensory and premotor network and suggest that a transitional state from wakefulness to unconsciousness is not a continuous process, but rather a series of discrete neural changes.

SIGNIFICANCE STATEMENT

How information is processed by the brain during awake and anesthetized states and, crucially, during the transition is not clearly understood. We demonstrate that neuronal dynamics are very different within an interconnecting cortical network (primary somatosensory and frontal premotor area) during the loss of consciousness (LOC) induced by propofol in nonhuman primates. Coherent beta oscillations between these regions are disrupted before LOC. Pronounced but brief gamma-band oscillations appear to correspond to LOC. In addition, neurons in both of these cortices transition from responding to both tactile and auditory stimulation before LOC to only tactile modality during unconsciousness. We demonstrate that propofol-induced LOC is accompanied by spatiotemporally distinctive neuronal dynamics in this network with concurrent changes in multisensory processing.

Disconnecting Consciousness: Is There a Common Anesthetic End Point?

A quest for a systems-level neuroscientific basis of anesthetic-induced loss and return of consciousness has been in the forefront of research for the past 2 decades. Recent advances toward the discovery of underlying mechanisms have been achieved using experimental electrophysiology, multichannel electroencephalography, magnetoencephalography, and functional magnetic resonance imaging. By the careful dosing of various volatile and IV anesthetic agents to the level of behavioral unresponsiveness, both specific and common changes in functional and effective connectivity across large-scale brain networks have been discovered and interpreted in the context of how the synthesis of neural information might be affected during anesthesia. The results of most investigations to date converge toward the conclusion that a common neural correlate of anesthetic-induced unresponsiveness is a consistent depression or functional disconnection of lateral frontoparietal networks, which are thought to be critical for consciousness of the environment. A reduction in the repertoire of brain states may contribute to the anesthetic disruption of large-scale information integration leading to unconsciousness. In future investigations, a systematic delineation of connectivity changes with multiple anesthetics using the same experimental design, and the same analytical method will be desirable. The critical neural events that account for the transition between responsive and unresponsive states should be assessed at similar anesthetic doses just below and above the loss or return of responsiveness. There will also be a need to identify a robust, sensitive, and reliable measure of information transfer. Ultimately, finding a behavior-independent measure of subjective experience that can track covert cognition in unresponsive subjects and a delineation of causal factors versus correlated events will be essential to understand the neuronal basis of human consciousness and unconsciousness.

Electrocorticographic delineation of human auditory cortical fields based on effects of propofol anesthesia

The functional organization of human auditory cortex remains incompletely characterized. While the posteromedial two thirds of Heschl's gyrus (HG) is generally considered to be part of core auditory cortex, additional subdivisions of HG remain speculative. To further delineate the hierarchical organization of human auditory cortex, we investigated regional heterogeneity in the modulation of auditory cortical responses under varying depths of anesthesia induced by propofol. Non-invasive studies have shown that propofol differentially affects auditory cortical activity, with a greater impact on non-core areas. Subjects were neurosurgical patients undergoing removal of intracranial electrodes placed to identify epileptic foci. Stimuli were 50Hz click trains, presented continuously during an awake baseline period, and subsequently, while propofol infusion was incrementally titrated to induce general anesthesia. Electrocorticographic recordings were made with depth electrodes implanted in HG and subdural grid electrodes implanted over superior temporal gyrus (STG). Depth of anesthesia was monitored using spectral entropy. Averaged evoked potentials (AEPs), frequency-following responses (FFRs) and high gamma (70-150Hz) event-related band power were used to characterize auditory cortical activity. Based on the changes in AEPs and FFRs during the induction of anesthesia, posteromedial HG could be divided into two subdivisions. In the most posteromedial aspect of the gyrus, the earliest AEP deflections were preserved and FFRs increased during induction. In contrast, the remainder of the posteromedial HG exhibited attenuation of both the AEP and the FFR. The anterolateral HG exhibited weaker activation characterized by broad, low-voltage AEPs and the absence of FFRs. Lateral STG exhibited limited activation by click trains, and FFRs there diminished during induction. Sustained high gamma activity was attenuated in the most posteromedial portion of HG, and was absent in all other regions. These differential patterns of auditory cortical activity during the induction of anesthesia may serve as useful physiological markers for field delineation. In this study, the posteromedial HG could be parcellated into at least two subdivisions. Preservation of the earliest AEP deflections and FFRs in the posteromedial HG likely reflects the persistence of feedforward synaptic activity generated by inputs from subcortical auditory pathways, including the medial geniculate nucleus.

Investigation of optical neuro-monitoring technique for detection of maintenance and emergence states during general anesthesia

The American Society of Anesthesiologist recommends peripheral physiological monitoring during general anesthesia, which offers no information regarding the effects of anesthetics on the brain. Since no "gold standard" method exists for this evaluation, such a technique is needed to ensure patient comfort, procedure quality and safety. In this study we investigated functional near infrared spectroscopy (fNIRS) as possible monitor of anesthetic effects on the prefrontal cortex. Anesthetic drugs, such as sevoflurane, suppress the cerebral metabolism and alter the cerebral blood flow. We hypothesize that fNIRS derived features carry information on the effects of anesthetics on the prefrontal cortex (PFC) that can be used for the classification of the anesthetized state. In this study, patients were continuously monitored using fNIRS, BIS and standard monitoring during surgical procedures under sevoflurane general anesthesia. Maintenance and emergence states were identified and fNIRS features were identified and compared between states. Linear and non-linear machine learning algorithms were investigated as methods for the classification of maintenance/emergence. The results show that changes in oxygenated (HbO₂) and deoxygenated hemoglobin (HHb) concentration and blood volume measured by fNIRS were associated with the transition between maintenance and emergence that occurs as a result of sevoflurane washout. We observed that during maintenance the signal is relatively more stable than during emergence. Maintenance and emergence states were classified with 94.7% accuracy with a non-linear model using the locally derived mean total hemoglobin, standard deviation of HbO₂, minimum and range of HbO₂ and HHb as features. These features were found to be correlated with the effects of sevoflurane and to carry information that allows real time and automatic classification of the anesthetized state with high accuracy.

Cerebral responses to local and global auditory novelty under general anesthesia

Primate brains can detect a variety of unexpected deviations in auditory sequences. The local-global paradigm dissociates two hierarchical levels of auditory predictive coding by examining the brain responses to first-order (local) and second-order (global) sequence violations. Using the macaque model, we previously demonstrated that, in the awake state, local violations cause focal auditory responses while global violations activate a brain circuit comprising prefrontal, parietal and cingulate cortices. Here we used the same local-global auditory paradigm to clarify the encoding of the hierarchical auditory regularities in anesthetized monkeys and compared their brain responses to those obtained in the awake state as measured with fMRI. Both, propofol, a GABAA-agonist, and ketamine, an NMDA-antagonist, left intact or even enhanced the cortical response to auditory inputs. The local effect vanished during propofol anesthesia and shifted spatially during ketamine anesthesia compared with wakefulness. Under increasing levels of propofol, we observed a progressive disorganization of the global effect in prefrontal, parietal and cingulate cortices and its complete suppression under ketamine anesthesia. Anesthesia also suppressed thalamic activations to the global effect. These results suggest that anesthesia preserves initial auditory processing, but disturbs both short-term and long-term auditory predictive coding mechanisms. The disorganization of auditory novelty processing under anesthesia relates to a loss of thalamic responses to novelty and to a disruption of higher-order functional cortical networks in parietal, prefrontal and cingular cortices.

Auditory functional magnetic resonance in awake (nonsedated) and propofol-sedated children

BACKGROUND

Functional Magnetic Resonance Imaging (fMRI) is often used in preoperative assessment before epilepsy surgery, tumor or cavernous malformation resection, or cochlear implantation. As it requires complete immobility, sedation is needed for uncooperative patients.

OBJECTIVE

The aim of this study was to compare the fMRI cortical activation pattern after auditory stimuli in propofol-sedated 5- to 8-year-old children with that of similarly aged nonsedated children.

METHODS

When possible, children underwent MRI without sedation, otherwise it was induced with i.v. propofol 2 mg·kg(-1) and maintained with i.v. propofol 4-5 mg·kg(-1) ·h(-1) . Following diagnostic MRI, fMRi was carried out, randomly alternating two passive listening tasks (a fairy-tale and nonsense syllables).

RESULTS

We studied 14 awake and 15 sedated children. During the fairy-tale task, the nonsedated children's blood-oxygen-level-dependent (BOLD) signal was bilaterally present in the posterior superior temporal gyrus (STG), Wernicke's area, and Broca's area. Sedated children showed similar activation, with lesser extension to Wernicke's area, and no activation in Broca's area. During the syllable task, the nonsedated children's BOLD signal was bilaterally observed in the STG and Wernicke's area, in Broca's area with leftward asymmetry, and in the premotor area. In sedated children, cortical activation was present in the STG, but not in the frontal lobes. BOLD signal change areas in sedated children were less extended than in nonsedated children during both the fairy-tale and syllable tasks. Modeling the temporal derivative during both the fairy-tale and syllable tasks, nonsedated children showed no response while sedated children did.

CONCLUSIONS

After auditory stimuli, propofol-sedated 5- to 8-year-old children exhibit an fMRI cortical activation pattern which is different from that in similarly aged nonsedated children.

Neural selectivity for communicative auditory signals in Phelan-McDermid syndrome

BACKGROUND

Phelan-McDermid syndrome (PMS), a neurodevelopmental disorder caused by deletion or mutation in the SHANK3 gene, is one of the more common single-locus causes of autism spectrum disorder (ASD). PMS is characterized by global developmental delay, hypotonia, delayed or absent speech, increased risk of seizures, and minor dysmorphic features. Impairments in language and communication are one of the most consistent characteristics of PMS. Although there is considerable overlap in the social communicative deficits associated with PMS and ASD, there is a dearth of data on underlying abnormalities at the level of neural systems in PMS. No controlled neuroimaging studies of PMS have been reported to date. The goal of this study was to examine the neural circuitry supporting the perception of auditory communicative signals in children with PMS as compared to idiopathic ASD (iASD).

METHODS

Eleven children with PMS and nine comparison children with iASD were scanned using functional magnetic resonance imaging (fMRI) under light sedation. The fMRI paradigm was a previously validated passive auditory task, which presented communicative (e.g., speech, sounds of agreement, disgust) and non-communicative vocalizations (e.g., sneezing, coughing, yawning).

RESULTS

Previous research has shown that the superior temporal gyrus (STG) responds selectively to communicative vocal signals in typically developing children and adults. Here, selective activity for communicative relative to non-communicative vocalizations was detected in the right STG in the PMS group, but not in the iASD group. The PMS group also showed preferential activity for communicative vocalizations in a range of other brain regions associated with social cognition, such as the medial prefrontal cortex (MPFC), insula, and inferior frontal gyrus. Interestingly, better orienting toward social sounds was positively correlated with selective activity in the STG and other "social brain" regions, including the MPFC, in the PMS group. Finally, selective MPFC activity for communicative sounds was associated with receptive language level in the PMS group and expressive language in the iASD group.

CONCLUSIONS

Despite shared behavioral features, children with PMS differed from children with iASD in their neural response to communicative vocal sounds and showed relative strengths in this area. Furthermore, the relationship between clinical characteristics and neural selectivity also differed between the two groups, suggesting that shared ASD features may partially reflect different neurofunctional abnormalities due to differing etiologies.

Evidence of Resting-state Activity in Propofol-anesthetized Patients with Intracranial Tumors

RATIONALE AND OBJECTIVES

Resting-state (RS) networks, revealed by functional magnetic resonance imaging (fMRI) studies in healthy volunteers, have never been evaluated in anesthetized patients with brain tumors. Our purpose was to examine the presence of residual brain activity on the auditory network during propofol-induced loss of consciousness in patients with brain tumors.

MATERIALS AND METHODS

Twenty subjects with intracranial masses were prospectively studied by means of intraoperative RS-fMRI acquisitions before any craniectomy. After performing single-subject independent component analysis, spatial maps and time courses were assigned to an auditory RS network template from the literature and compared via spatial regression coefficients.

RESULTS

All fMRI data were of sufficient quality for further postprocessing. In all but two patients, the RS functional activity of the auditory network could be successfully mapped. In almost all patients, contralateral activation of the auditory network was present. No significant difference was found between the mean distance of the RS activity clusters and the lesion periphery for tumors located in the temporal gyri vs. those in other brain regions. The spatial deviation between the activated cluster in our experiment and the template was significantly (P = 0.04) higher in patients with tumors located in the temporal gyri than in patients with tumors located in other regions.

CONCLUSIONS

Propofol-induced anesthesia in patients with intracranial lesions does not alter the blood-oxygenation level-depended signal, and independent component analysis of intraoperative RS-fMRI may allow assessment of the auditory network in a clinical setting.

Near-infrared spectroscopy for the evaluation of anesthetic depth

The standard-of-care guidelines published by the American Society of Anesthesiologists (ASA) recommend monitoring of pulse oximetry, blood pressure, heart rate, and end tidal CO2 during the use of anesthesia and sedation. This information can help to identify adverse events that may occur during procedures. However, these parameters are not specific to the effects of anesthetics or sedatives, and therefore they offer little, to no, real time information regarding the effects of those agents and do not give the clinician the lead-time necessary to prevent patient "awareness." Since no "gold-standard" method is available to continuously, reliably, and effectively monitor the effects of sedatives and anesthetics, such a method is greatly needed. Investigation of the use of functional near-infrared spectroscopy (fNIRS) as a method for anesthesia or sedation monitoring and for the assessment of the effects of various anesthetic drugs on cerebral oxygenation has started to be conducted. The objective of this paper is to provide a thorough review of the currently available published scientific studies regarding the use of fNIRS in the fields of anesthesia and sedation monitoring, comment on their findings, and discuss the future work required for the translation of this technology to the clinical setting.

Memory formation during anaesthesia: plausibility of a neurophysiological basis

As opposed to conscious, personally relevant (explicit) memories that we can recall at will, implicit (unconscious) memories are prototypical of 'hidden' memory; memories that exist, but that we do not know we possess. Nevertheless, our behaviour can be affected by these memories; in fact, these memories allow us to function in an ever-changing world. It is still unclear from behavioural studies whether similar memories can be formed during anaesthesia. Thus, a relevant question is whether implicit memory formation is a realistic possibility during anaesthesia, considering the underlying neurophysiology. A different conceptualization of memory taxonomy is presented, the serial parallel independent model of Tulving, which focuses on dynamic information processing with interactions among different memory systems rather than static classification of different types of memories. The neurophysiological basis for subliminal information processing is considered in the context of brain function as embodied in network interactions. Function of sensory cortices and thalamic activity during anaesthesia are reviewed. The role of sensory and perisensory cortices, in particular the auditory cortex, in support of memory function is discussed. Although improbable, with the current knowledge of neurophysiology one cannot rule out the possibility of memory formation during anaesthesia.

Neural correlates of successful semantic processing during propofol sedation

Sedation has a graded effect on brain responses to auditory stimuli: perceptual processing persists at sedation levels that attenuate more complex processing. We used fMRI in healthy volunteers sedated with propofol to assess changes in neural responses to spoken stimuli. Volunteers were scanned awake, sedated, and during recovery, while making perceptual or semantic decisions about nonspeech sounds or spoken words respectively. Sedation caused increased error rates and response times, and differentially affected responses to words in the left inferior frontal gyrus (LIFG) and the left inferior temporal gyrus (LITG). Activity in LIFG regions putatively associated with semantic processing, was significantly reduced by sedation despite sedated volunteers continuing to make accurate semantic decisions. Instead, LITG activity was preserved for words greater than nonspeech sounds and may therefore be associated with persistent semantic processing during the deepest levels of sedation. These results suggest functionally distinct contributions of frontal and temporal regions to semantic decision making. These results have implications for functional imaging studies of language, for understanding mechanisms of impaired speech comprehension in postoperative patients with residual levels of anesthetic, and may contribute to the development of frameworks against which EEG based monitors could be calibrated to detect awareness under anesthesia.

Reduced cerebral oxygen-carbohydrate index during endotracheal intubation in vascular surgical patients

Brain activation reduces balance between cerebral consumption of oxygen versus carbohydrate as expressed by the so-called cerebral oxygen-carbohydrate-index (OCI). We evaluated whether preparation for surgery, anaesthesia including tracheal intubation and surgery affect OCI. In patients undergoing aortic surgery, arterial to internal jugular venous (a-v) concentration differences for oxygen versus lactate and glucose were determined from before anaesthesia to when the patient left the recovery room. Intravenous anaesthesia was supplemented with thoracic epidural anaesthesia for open aortic surgery (n = 5) and infiltration with bupivacaine for endovascular procedures (n = 14). The a-v difference for O2 decreased throughout anaesthesia and in the recovery room (1.6 ± 1.9 versus 3.2 ± 0.8 mmol l(-1), mean ± SD), and while a-v glucose decreased during surgery and into the recovery (0.4 ± 0.2 versus 0.7 ± 0.2 mmol l(-1) , P<0.05), a-v lactate did not change significantly (0.03 ± 0.16 versus -0.03 ± 0.09 mmol l(-1)). Thus, OCI decreased from 5.2 ± 1.8 before induction of anaesthesia to 3.2 ± 1.0 following tracheal intubation (P<0.05) because of the decrease in a-v O2 with a recovery for OCI to 4.6 ± 1.4 during surgery and to 5.6 ± 1.7 in the recovery room. In conclusion, preparation for surgery and tracheal intubation decrease OCI that recovers during surgery under the influence of sensory blockade.

Cerebral mechanisms of general anesthesia

How does general anesthesia (GA) work? Anesthetics are pharmacological agents that target specific central nervous system receptors. Once they bind to their brain receptors, anesthetics modulate remote brain areas and end up interfering with global neuronal networks, leading to a controlled and reversible loss of consciousness. This remarkable manipulation of consciousness allows millions of people every year to undergo surgery safely most of the time. However, despite all the progress that has been made, we still lack a clear and comprehensive insight into the specific neurophysiological mechanisms of GA, from the molecular level to the global brain propagation. During the last decade, the exponential progress in neuroscience and neuro-imaging led to a significant step in the understanding of the neural correlates of consciousness, with direct consequences for clinical anesthesia. Far from shutting down all brain activity, anesthetics lead to a shift in the brain state to a distinct, highly specific and complex state, which is being increasingly characterized by modern neuro-imaging techniques. There are several clinical consequences and challenges that are arising from the current efforts to dissect GA mechanisms: the improvement of anesthetic depth monitoring, the characterization and avoidance of intra-operative awareness and post-anesthesia cognitive disorders, and the development of future generations of anesthetics.

Functional near-infrared spectroscopy for the measurement of propofol effects in conscious sedation during outpatient elective colonoscopy

Endoscopic procedures performed in the United States routinely involve the use of conscious sedation as standard of care. The use of sedation reduces patient discomfort and anxiety while improving the technical quality of the procedure, and as a result, over 98% of clinicians have adopted the practice. The tremendous benefits of sedation are offset by heightened costs, increased patient discharge time, and cardiopulmonary complication risks. The inherent liabilities of putting patients under sedation have necessitated a large number of physiological monitoring systems in order to ensure patient comfort and safety. Currently American Society of Anesthesiologist (ASA) guidelines recommend monitoring of pulse oximetry, blood pressure, heart rate, and end-tidal CO2; although important safeguards, these physiological measurements do not allow for the reliable assessment of patient sedation. Proper monitoring of patient state ensures procedure quality and patient safety; however no "gold-standard" is available to determine the depth of sedation which is comparable to the anesthesiologist's professional judgment. Developments in functional near-infrared spectroscopy (fNIRS) over the past two decades have introduced cost-effective, portable, and non-invasive neuroimaging tools which measure cortical hemodynamic activity as a correlate of neural functions. Anesthetic drugs, such as propofol, operate by suppressing cerebral metabolism. fNIRS imaging methods have the ability to detect these drug related effects as well as neuronal activity through the measurement of local cerebral hemodynamic changes. In the present study, 41 patients were continuously monitored using fNIRS while undergoing outpatient elective colonoscopy with propofol sedation. The preliminary results indicated that oxygenated hemoglobin changes in the dorsolateral prefrontal cortex, as assessed by fNIRS were correlated with changes in response to bolus infusions of propofol, whereas other standard physiological measures were not significantly associated.

Quantitative fMRI and oxidative neuroenergetics

The discovery of functional magnetic resonance imaging (fMRI) has greatly impacted neuroscience. The blood oxygenation level-dependent (BOLD) signal, using deoxyhemoglobin as an endogenous paramagnetic contrast agent, exposes regions of interest in task-based and resting-state paradigms. However the BOLD contrast is at best a partial measure of neuronal activity, because the functional maps obtained by differencing or correlations ignore the total neuronal activity in the baseline state. Here we describe how studies of brain energy metabolism at Yale, especially with (13)C magnetic resonance spectroscopy and related techniques, contributed to development of quantitative functional brain imaging with fMRI by providing a reliable measurement of baseline energy. This narrative takes us on a journey, from molecules to mind, with illuminating insights about neuronal-glial activities in relation to energy demand of synaptic activity. These results, along with key contributions from laboratories worldwide, comprise the energetic basis for quantitative interpretation of fMRI data.

Neural systems for speech and song in autism

Despite language disabilities in autism, music abilities are frequently preserved. Paradoxically, brain regions associated with these functions typically overlap, enabling investigation of neural organization supporting speech and song in autism. Neural systems sensitive to speech and song were compared in low-functioning autistic and age-matched control children using passive auditory stimulation during functional magnetic resonance and diffusion tensor imaging. Activation in left inferior frontal gyrus was reduced in autistic children relative to controls during speech stimulation, but was greater than controls during song stimulation. Functional connectivity for song relative to speech was also increased between left inferior frontal gyrus and superior temporal gyrus in autism, and large-scale connectivity showed increased frontal–posterior connections. Although fractional anisotropy of the left arcuate fasciculus was decreased in autistic children relative to controls, structural terminations of the arcuate fasciculus in inferior frontal gyrus were indistinguishable between autistic and control groups. Fractional anisotropy correlated with activity in left inferior frontal gyrus for both speech and song conditions. Together, these findings indicate that in autism, functional systems that process speech and song were more effectively engaged for song than for speech and projections of structural pathways associated with these functions were not distinguishable from controls.

Propofol disrupts functional interactions between sensory and high-order processing of auditory verbal memory

Current theories suggest that disrupting cortical information integration may account for the mechanism of general anesthesia in suppressing consciousness. Human cognitive operations take place in hierarchically structured neural organizations in the brain. The process of low-order neural representation of sensory stimuli becoming integrated in high-order cortices is also known as cognitive binding. Combining neuroimaging, cognitive neuroscience, and anesthetic manipulation, we examined how cognitive networks involved in auditory verbal memory are maintained in wakefulness, disrupted in propofol-induced deep sedation, and re-established in recovery. Inspired by the notion of cognitive binding, an functional magnetic resonance imaging-guided connectivity analysis was utilized to assess the integrity of functional interactions within and between different levels of the task-defined brain regions. Task-related responses persisted in the primary auditory cortex (PAC), but vanished in the inferior frontal gyrus (IFG) and premotor areas in deep sedation. For connectivity analysis, seed regions representing sensory and high-order processing of the memory task were identified in the PAC and IFG. Propofol disrupted connections from the PAC seed to the frontal regions and thalamus, but not the connections from the IFG seed to a set of widely distributed brain regions in the temporal, frontal, and parietal lobes (with exception of the PAC). These later regions have been implicated in mediating verbal comprehension and memory. These results suggest that propofol disrupts cognition by blocking the projection of sensory information to high-order processing networks and thus preventing information integration. Such findings contribute to our understanding of anesthetic mechanisms as related to information and integration in the brain.

Functional neuroimaging to characterize visual system development in children with retinoblastoma

PURPOSE

To use functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) to investigate visual system development in children being treated for retinoblastoma.

METHODS

Informed consent was obtained for all participants (N = 42) in this institutional review board-approved study. Participants were imaged with a 1.5-T scanner while under propofol sedation. Diagnostic brain and orbital imaging was followed by investigational functional neuroimaging, which included fMRI during photic stimulation through closed eyelids, to measure functional activation in the visual cortex, and DTI, to evaluate diffusion parameters of white matter tracts in the corpus callosum and the periventricular optic radiations. Analysis included 115 examinations of 39 patients with a median age of 16.4 months and age range from 1.5 to 101.5 months at first evaluation.

RESULTS

The blood oxygen level-dependent signal was predominantly negative and located in the anterior visual cortex. Activation was affected by tumor lateralization (unilateral or bilateral), macular involvement, and retinal detachment. Patients who had undergone unilateral enucleation showed cortical dominance corresponding to the projection from the nasal hemiretina in the unaffected eye. Diffusion parameters followed a normal developmental trajectory in the optic radiations and corpus callosum, but variability was greater in the splenium than in the genu of the corpus callosum.

CONCLUSIONS

Longitudinal functional neuroimaging demonstrated important effects of disease and treatment. Therefore, fMRI and DTI may be useful for characterizing the impact of retinoblastoma on the developing visual system and improving the prediction of visual outcome in survivors.

Clinical functional MRI of the language domain in children with epilepsy

Functional MRI (fMRI) for the assessment of language functions is increasingly used in the diagnostic workup of patients with epilepsy. Termed "clinical fMRI," such an approach is also feasible in children who may display specific patterns of language reorganization. This study was aimed at assessing language reorganization in pediatric epilepsy patients, using fMRI. We studied 26 pediatric epilepsy patients (median age, 13.05 years; range, 5.6-18.7 years) and 23 healthy control children (median age, 9.37 years; range, 6.2-15.4 years), using two child-friendly fMRI tasks and adapted data-processing streams. Overall, 81 functional series could be analyzed. Reorganization seemed to occur primarily in homotopic regions in the contralateral hemisphere, but lateralization in the frontal as well as in the temporal lobes was significantly different between patients and controls. The likelihood to find atypical language organization was significantly higher in patients. Additionally, we found significantly stronger activation in the healthy controls in a primarily passive task, suggesting a systematic confounding influence of antiepileptic medication. The presence of a focal cortical dysplasia was significantly associated with atypical language lateralization. We conclude that important confounds need to be considered and that the pattern of language reorganization may be distinct from the patterns seen in later-onset epilepsy.

Functional connectivity and alterations in baseline brain state in humans

This work examines the influence of changes in baseline activity on the intrinsic functional connectivity fMRI (fc-fMRI) in humans. Baseline brain activity was altered by inducing anesthesia (sevoflurane end-tidal concentration 1%) in human volunteers and fc-fMRI maps between the pre-anesthetized and anesthetized conditions were compared across different brain networks. We particularly focused on low-level sensory areas (primary somatosensory, visual, and auditory cortices), the thalamus, and pain (insula), memory (hippocampus) circuits, and the default mode network (DMN), the latter three to examine higher-order brain regions. The results indicate that, while fc-fMRI patterns did not significantly differ (p<0.005; 20-voxel cluster threshold) in sensory cortex and in the DMN between the pre- and anesthetized conditions, fc-fMRI in high-order cognitive regions (i.e. memory and pain circuits) was significantly altered by anesthesia. These findings provide further evidence that fc-fMRI reflects intrinsic brain properties, while also demonstrating that 0.5 MAC sevoflurane anesthesia preferentially modulates higher-order connections.

Baseline brain energy supports the state of consciousness

An individual, human or animal, is defined to be in a conscious state empirically by the behavioral ability to respond meaningfully to stimuli, whereas the loss of consciousness is defined by unresponsiveness. PET measurements of glucose or oxygen consumption show a widespread approximately 45% reduction in cerebral energy consumption with anesthesia-induced loss of consciousness. Because baseline brain energy consumption has been shown by (13)C magnetic resonance spectroscopy to be almost exclusively dedicated to neuronal signaling, we propose that the high level of brain energy is a necessary property of the conscious state. Two additional neuronal properties of the conscious state change with anesthesia. The delocalized fMRI activity patterns in rat brain during sensory stimulation at a higher energy state (close to the awake) collapse to a contralateral somatosensory response at lower energy state (deep anesthesia). Firing rates of an ensemble of neurons in the rat somatosensory cortex shift from the gamma-band range (20-40 Hz) at higher energy state to <10 Hz at lower energy state. With the conscious state defined by the individual's behavior and maintained by high cerebral energy, measurable properties of that state are the widespread fMRI patterns and high frequency neuronal activity, both of which support the extensive interregional communication characteristic of consciousness. This usage of high brain energies when the person is in the "state" of consciousness differs from most studies, which attend the smaller energy increments observed during the stimulations that form the "contents" of that state.

The fMRI signal, slow cortical potential and consciousness

As functional magnetic resonance imaging (fMRI) has become a driving force in cognitive neuroscience, it is crucial to understand the neural basis of the fMRI signal. Here, we discuss a novel neurophysiological correlate of the fMRI signal, the slow cortical potential (SCP), which also seems to modulate the power of higher-frequency activity, the more established neurophysiological correlate of the fMRI signal. We further propose a hypothesis for the involvement of the SCP in the emergence of consciousness, and review existing data that lend support to our proposal. This hypothesis, unlike several previous theories of consciousness, is firmly rooted in physiology and as such is entirely amenable to empirical testing.

Simultaneous electroencephalography and functional magnetic resonance imaging of general anesthesia

It has been long appreciated that anesthetic drugs induce stereotyped changes in electroencephalogram (EEG), but the relationships between the EEG and underlying brain function remain poorly understood. Functional imaging methods including positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), have become important tools for studying how anesthetic drugs act in the human brain to induce the state of general anesthesia. To date, no investigation has combined functional MRI with EEG to study general anesthesia. We report here a paradigm for conducting combined fMRI and EEG studies of human subjects under general anesthesia. We discuss the several technical and safety problems that must be solved to undertake this type of multimodal functional imaging and show combined recordings from a human subject. Combined fMRI and EEG exploits simultaneously the high spatial resolution of fMRI and the high temporal resolution of EEG. In addition, combined fMRI and EEG offers a direct way to relate established EEG patterns induced by general anesthesia to changes in neural activity in specific brain regions as measured by changes in fMRI blood oxygen level dependent (BOLD) signals.

Spatial nonuniformity of the resting CBF and BOLD responses to sevoflurane: in vivo study of normal human subjects with magnetic resonance imaging

Pulsed arterial spin labeling magnetic resonance imaging (MRI) was performed to investigate the local coupling between resting regional cerebral blood flow (rCBF) and BOLD (blood oxygen level dependent) signal changes in 22 normal human subjects during the administration of 0.25 MAC (minimum alveolar concentration) sevoflurane. Two states were compared with subjects at rest: anesthesia and no-anesthesia. Regions of both significantly increased and decreased resting-state rCBF were observed. Increases were limited primarily to subcortical structures and insula, whereas, decreases were observed primarily in neocortical regions. No significant change was found in global CBF (gCBF). By simultaneously measuring rCBF and BOLD, region-specific anesthetic effects on the coupling between rCBF and BOLD were identified. Multiple comparisons of the agent-induced rCBF and BOLD changes demonstrated significant (P < 0.05) spatial variability in rCBF-BOLD coupling. The slope of the linear regression line for AC, where rCBF was increased by sevoflurane, was markedly smaller than the slope for those ROIs where rCBF was decreased by sevoflurane, indicating a bigger change in BOLD per unit change in rCBF in regions where rCBF was increased by sevoflurane. These results suggest that it would be inaccurate to use a global quantitative model to describe coupling across all brain regions and in all anesthesia conditions. The observed spatial nonuniformity of rCBF and BOLD signal changes suggests that any interpretation of BOLD fMRI data in the presence of an anesthetic requires consideration of these insights.

fMRI reveals cognitive and emotional processing in a long-term comatose patient

We report on a 41-year old woman with prolonged comatose unresponsiveness following traumatic head injury. Structural MRI showed bilateral midbrain damage and ventriculomegalia. Functional MRI revealed robust cortical responses to visual, auditory and tactile stimulation. Speech stimuli moreover consistently elicited activation in Broca's and Wernicke's areas. Familiar speakers and direct addressing evoked significantly stronger amygdala activation than unfamiliar speakers and neutral phrases. This study hence demonstrates the potential of functional neuroimaging in the investigation of residual higher cortical functions in unresponsive comatose patients.

Silent and continuous fMRI scanning differentially modulate activation in an auditory language comprehension task

Sparse temporal acquisition schemes have been adopted to investigate the neural correlates of human audition using blood-oxygen-level dependent (BOLD) based functional magnetic resonance imaging (fMRI) devoid of ambient confounding acoustic scanner noise. These schemes have previously been extended to clustered-sparse temporal acquisition designs which record several subsequent BOLD contrast images in rapid succession in order to enhance temporal sampling efficiency. In the present study we demonstrate that an event-related task design can effectively be combined with a clustered temporal acquisition technique in an auditory language comprehension task. The same fifteen volunteers performed two separate auditory runs which either applied customary fMRI acquisition (CA) composed of continuous scanner noise or "silent" fMRI built on a clustered temporal acquisition (CTA) protocol. In accord with our hypothesis, the CTA scheme relative to the CA protocol is accompanied by significantly stronger functional responses along the entire superior temporal plane. By contrast, the bilateral insulae engage more strongly during continuous scanning. A post-hoc region-of-interest analysis reveals cortical activation in subportions of the supratemporal plane which varies as a function of acquisition protocol. The middle part of the supratemporal plane shows a rightward asymmetry only for the CTA scheme while the posterior supratemporal plane exposes a significantly stronger leftward asymmetry during the CTA. Our findings implicate that silent fMRI is advantageous when it comes to the exploration of auditory and speech functions residing in the supratemporal plane.

Functions of the left and right posterior temporal lobes during segmental and suprasegmental speech perception

This manuscript reviews evidence from neuroimaging studies on elementary processes of speech perception and their implications for our understanding of the brain-speech relationship. Essentially, differential preferences of the left and right auditory-related cortex for rapidly and slowly changing acoustic cues that constitute (sub)segmental and suprasegmental parameters, e. g. phonemes, prosody, and rhythm. The adopted parameter-based research approach takes the early stages of speech perception as being of fundamental relevance for simple as well as complex language functions. The current state of knowledge necessitates an extensive revision of the classical neurologically oriented model of language processing that was aimed at identifying the neural correlates of linguistic components (e. g. phonology, syntax and semantics) more than at substantiating the importance of (supra)segmental information during speech perception.

Energetics of neuronal signaling and fMRI activity

Energetics of resting and evoked fMRI signals were related to localized ensemble firing rates (nu) measured by electrophysiology in rats. Two different unstimulated, or baseline, states were established by anesthesia. Halothane and alpha-chloralose established baseline states of high and low energy, respectively, in which forepaw stimulation excited the contralateral primary somatosensory cortex (S1). With alpha-chloralose, forepaw stimulation induced strong and reproducible fMRI activations in the contralateral S1, where the ensemble firing was dominated by slow signaling neurons (SSN; nu range of 1-13 Hz). Under halothane, weaker and less reproducible fMRI activations were observed in the contralateral S1 and elsewhere in the cortex, but ensemble activity in S1 was dominated by rapid signaling neurons (RSN; nu range of 13-40 Hz). For both baseline states, the RSN activity (i.e., higher frequencies, including the gamma band) did not vary upon stimulation, whereas the SSN activity (i.e., alpha band and lower frequencies) did change. In the high energy baseline state, a large majority of total oxidative energy [cerebral metabolic rate of oxygen consumption (CMR(O2))] was devoted to RSN activity, whereas in the low energy baseline state, it was roughly divided between SSN and RSN activities. We hypothesize that in the high energy baseline state, the evoked changes in fMRI activation in areas beyond S1 are supported by rich intracortical interactions represented by RSN. We discuss implications for interpreting fMRI data where stimulus-specific DeltaCMR(O2) is generally small compared with baseline CMR(O2).

Dissociating speech perception and comprehension at reduced levels of awareness

We used functional MRI and the anesthetic agent propofol to assess the relationship among neural responses to speech, successful comprehension, and conscious awareness. Volunteers were scanned while listening to sentences containing ambiguous words, matched sentences without ambiguous words, and signal-correlated noise (SCN). During three scanning sessions, participants were nonsedated (awake), lightly sedated (a slowed response to conversation), and deeply sedated (no conversational response, rousable by loud command). Bilateral temporal-lobe responses for sentences compared with signal-correlated noise were observed at all three levels of sedation, although prefrontal and premotor responses to speech were absent at the deepest level of sedation. Additional inferior frontal and posterior temporal responses to ambiguous sentences provide a neural correlate of semantic processes critical for comprehending sentences containing ambiguous words. However, this additional response was absent during light sedation, suggesting a marked impairment of sentence comprehension. A significant decline in postscan recognition memory for sentences also suggests that sedation impaired encoding of sentences into memory, with left inferior frontal and temporal lobe responses during light sedation predicting subsequent recognition memory. These findings suggest a graded degradation of cognitive function in response to sedation such that "higher-level" semantic and mnemonic processes can be impaired at relatively low levels of sedation, whereas perceptual processing of speech remains resilient even during deep sedation. These results have important implications for understanding the relationship between speech comprehension and awareness in the healthy brain in patients receiving sedation and in patients with disorders of consciousness.

Mechanisms of anesthetic actions and the brain

The neural mechanisms behind anesthetic-induced behavioral changes such as loss of consciousness, amnesia, and analgesia, are insufficiently understood, though general anesthesia has been of tremendous importance for the development of medicine. In this review, I summarize what is currently known about general anesthetic actions at different organizational levels and discuss current and future research, using systems neuroscience approaches such as functional neuroimaging and quantitative electrophysiology to understand anesthesia actions at the integrated brain level.