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It is important to consider the effect of a previous experimental condition when analyzing resting-state functional connectivity magnetic resonance imaging (fcMRI) data. In this work, a simple sensory stimulation functional MRI (fMRI) experiment was conducted between two resting-state fcMRI acquisitions in anesthetized rats using a high-field small-animal MR scanner. Previous human studies have reported fcMRI network alteration by prior task/stimulus utilizing similar experimental paradigms. An anesthetized rat preparation was used to test whether brain regions with higher level functions are involved in post-task/stimulus fcMRI network alteration. We demonstrate significant fcMRI enhancement poststimulation in the sensory cortical, limbic, and insular brain regions in rats. These brain regions have been previously implicated in vigilance and anesthetic arousal networks. We tested their experimental paradigm in several inbred strains of rats with known phenotypic differences in anesthetic susceptibility and cerebral vascular function. Brown Norway (BN), Dahl Salt-Sensitive (SS), and consomic SSBN13 strains were tested. We have previously shown significant differences in blood oxygen level-dependent fMRI activity and fcMRI networks across these strains. Here we report statistically significant interstrain differences in regional fcMRI poststimulation enhancement. In the SS strain, poststimulation enhancement occurred in posterior sensory and limbic cortical brain regions. In the BN strain, poststimulation enhancement appeared in anterior cingulate and subcortical limbic brain regions. These results imply that a prior condition has a significant impact on fcMRI networks that depend on intersubject difference in genetics and physiology.

Research on neural correlates of consciousness has been conducted and carried out mostly from within two relatively autonomous paradigmatic traditions – studying the specific contents of conscious experience and their brain-process correlates and studying the level of consciousness. In the present paper we offer a theoretical integration suggesting that an emphasis has to be put on understanding the mechanisms of consciousness (and not a mere correlates) and in doing this, the two paradigmatic traditions must be combined. We argue that consciousness emerges as a result of interaction of brain mechanisms specialized for representing the specific contents of perception/cognition – the data -- and mechanisms specialized for regulating the level of activity of whatever data the content-carrying specific mechanisms happen to represent. Each of these mechanisms are necessary because without the contents there is no conscious experience and without the required level of activity the processed contents remain unconscious. Together the two mechanisms, when activated up to a necessary degree each, provide conditions sufficient for conscious experience to emerge. This proposal is related to pertinent experimental evidence.

States of consciousness have been associated with information integration in the brain as modulated by anesthesia and the ascending arousal system. The present study was designed to test the hypothesis that electrical stimulation of the oral part of the pontine reticular nucleus (PnO) can augment information integration in the cerebral cortex of anesthetized rats. Extracellular unit activity and local field potentials were recorded in freely moving animals from parietal association (PtA) and secondary visual (V2) cortices via chronically implanted microwire arrays at three levels of anesthesia produced by desflurane: 3.5, 4.5, and 6.0% (where 4.5% corresponds to that critical for the loss of consciousness). Information integration was characterized by integration (multiinformation) and interaction entropy, estimated from the statistical distribution of coincident spike patterns. PnO stimulation elicited electrocortical activation as indicated by the reductions in δ- and θ-band powers at the intermediate level of anesthesia. PnO stimulation augmented integration from 1.13 ± 0.03 to 6.12 ± 1.98 × 10(3) bits and interaction entropy from 0.44 ± 0.11 to 2.18 ± 0.72 × 10(3) bits; these changes were most consistent in the PtA at all desflurane concentrations. Stimulation of the retina with discrete light flashes after PnO stimulation elicited an additional 166 ± 25 and 92 ± 12% increase in interaction entropy in V2 during light and intermediate levels. The results suggest that the PnO may modulate spontaneous ongoing and sensory stimulus-related cortical information integration under anesthesia.

Loss of consciousness in anesthetized healthy participants and in patients with unresponsive wakefulness syndrome (UWS) is associated with substantial alterations of functional connectivity across large-scale brain networks. Yet, a prominent distinction between the two cases is that after anesthesia, brain connectivity and consciousness are spontaneously restored, whereas in patients with UWS this restoration fails to occur, but why? A possible explanation is that the self-organizing capability of the brain is compromised in patients with UWS but not in healthy participants undergoing anesthesia. According to the theory of self-organized criticality, many natural complex systems, including the brain, evolve spontaneously to a critical state wherein system behaviors display spatial and/or temporal scale-invariant characteristics. Here we tested the hypothesis that the scale-free property of brain network organization is in fact fundamentally different between anesthetized healthy participants and UWS patients. We introduced a novel, computationally efficient approach to determine anatomical-functional parcellation of the whole-brain network at increasingly finer spatial scales. We found that in healthy participants, scale-free distributions of node size and node degree were present across wakefulness, propofol sedation, and recovery, despite significant propofol-induced functional connectivity changes. In patients with UWS, the scale-free distribution of node degree was absent, reflecting a fundamental difference between the two groups in adaptive reconfiguration of functional interaction between network components. The maintenance of scale-invariance across propofol sedation in healthy participants suggests the presence of persistent, on-going self-organizing processes to a critical state – a capacity that is compromised in patients with UWS.

We examined the cerebral hyperemic response to halothane after treatment with bacterial lipopolysaccharide (LPS). To determine the involvement of inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX-2), we tested whether the effect of LPS on halothane-induced hyperemia was altered by pretreatment with the selective iNOS inhibitor, aminoguanidine (100 mg/ kg), COX-2 inhibitor, NS-398 (5 mg/kg), or enzyme expression inhibitor, dexamethasone (4 mg/kg). Further, we examined whether the administration of a nitric oxide donor, diethylamine NONOate, would change the cerebral hyperemic response of halothane. Sprague-Dawley rats were anesthetized with 0.5 minimum alveolar anesthetic concentration of halothane and artificially ventilated. Regional cerebrocortical blood flow (rCBF) was assessed by laser-Doppler flowmetry. LPS (1 mg/kg) was administered intracerebroventricularly; artificial cerebrospinal fluid was used in controls. Four hours after LPS infusion, iNOS and COX-2 messenger ribonucleic acid (mRNA) levels (reverse transcription-polymerase chain reaction) and enzyme activities (arginine-citrulline conversion and prostaglandin E 2 enzyme immunoassay) were significantly increased. LPS enhanced halothane-induced 3.9 and 1.6-fold increases in rCBF at 1.0 and 1.5 minimum alveolar concentration, respectively. Co-treatment with NS-398 attenuated, but aminoguanidine or dexamethasone abolished the effect of LPS on halothane-induced rCBF increase. Diethylamine NONOate mimicked the enhanced rCBF response to halothane. These results suggest that LPS augmented halothane-induced cerebrocortical hyperemia by induction of iNOS and COX-2.

This study tested the hypothesis that relaxation by guided imagery improves working-memory performance of healthy participants. 30 volunteers (both sexes, ages 17–56 years) were randomly assigned to one of three groups and administered the WAIS–III Letter-Number Sequencing Test before and after 10-min. treatment with guided imagery or popular music. The control group received no treatment. Groups' test scores were not different before treatment. The mean increased after relaxation by guided imagery but not after music or no treatment. This result supports the hypothesis that working-memory scores on the test are enhanced by guided imagery and implies that human information processing may be enhanced by prior relaxation.

Using magnetic resonance (MR) echo–planar imaging (EPI), we recently demonstrated the presence of low-frequency fluctuations (–3) within and across associated regions of the sensorimotor cortex. These fluctuations in MR signal intensity are believed to arise from fluctuations in capillary blood flow and oxygenation. A substantial overlap between the activation map generated by bilateral finger tapping and temporally-correlated voxels from the sensorimotor cortex obtained during rest was observed. In the work reported here, we investigated whether respiratory hypercapnia, which is known to suspend spontaneous oscillations in regional cerebral blood flow, influences these low-frequency fluctuations. The magnitude of low-frequency fluctuations was reversibly diminished during hypercapnia, resulting in a substantial decrease of the temporal correlation both within and across contralateral hemispheres of the sensorimotor cortex. After the breathing mixture was returned to ambient air, the magnitude and spatial extent of the temporal correlation of low-frequency fluctuations returned to normal. Results of this study support the hypothesis that low-frequency physiological fluctuations observed by MR in the human cortex and spontaneous flow oscillations observed in early studies by laser–Doppler flowmetry (LDF) in the cortex of the rat are identical and are secondary to fluctuations in neuronal activity.

The dose-dependent effects of anesthetics on brain functional connectivity are incompletely understood. Resting-state functional magnetic resonance imaging (rsfMRI) is widely used to assess the functional connectivity in humans and animals. Propofol is an anesthetic agent with desirable characteristics for functional neuroimaging in animals but its dose-dependent effects on rsfMRI functional connectivity have not been determined. Here we tested the hypothesis that brain functional connectivity undergoes specific changes in distinct neural networks at anesthetic depths associated with loss of consciousness. We acquired spontaneous blood oxygen level-dependent (BOLD) signals simultaneously with electroencephalographic (EEG) signals from rats under steady-state, intravenously administered propofol at increasing doses from light sedation to deep anesthesia (20, 40, 60, 80, and 100 mg/kg/h IV). Power spectra and burst suppression ratio were calculated from the EEG to verify anesthetic depth. Functional connectivity was determined from the whole brain correlation of BOLD data in regions of interest followed by a segmentation of the correlation maps into anatomically defined regional connectivity. We found that propofol produced multiphasic, dose dependent changes in functional connectivity of various cortical and subcortical networks. Cluster analysis predicted segregation of connectivity into two cortical and two subcortical clusters. In one cortical cluster (somatosensory and parietal), the early reduction in connectivity was followed by transient reversal; in the other cluster (sensory, motor and cingulate/retrosplenial), this rebound was absent. The connectivity of the subcortical cluster (brainstem, hippocampal and caudate) was strongly reduced, whereas that of another (hypothalamus, medial thalamus and n. basalis) did not. Subcortical connectivity increased again in deep anesthesia associated with EEG burst suppression. Regional correlation analysis confirmed the breakdown of connectivity within and between specific cortical and subcortical networks with deepening propofol anesthesia. Cortical connectivity was suppressed before subcortical connectivity at a critical propofol dose associated with loss of consciousness.

Long‐range gamma band EEG oscillations mediate information transmission between distant brain regions. Gamma band‐based coupling may not be restricted to cortex‐to‐cortex communication but may include extracortical parts of the visual system. The retinogram and visual event‐related evoked potentials exhibit time‐locked, forward propagating oscillations that are candidates of gamma oscillatory coupling between the retina and the visual cortex. In this study, we tested if this gamma coupling is present as indicated by the coherence of gamma‐range (70–200 Hz) oscillatory potentials (OPs) recorded simultaneously from the retina and the primary visual cortex in freely moving, adult rats. We found significant retino‐cortical OP coherence in a wide range of stimulus duration (0.01–1000 msec), stimulus intensity (800–5000 mcd/mm2), interstimulus interval (10–400 msec), and stimulus frequency (0.25–25 Hz). However, at low stimulus frequencies, the OPs were time‐locked, flickering light at 25 Hz entrained continuous OP coherence (steady‐state response, SSR). Our results suggest that the retina and the visual cortex exhibit oscillatory coupling at high‐gamma frequency with precise time locking and synchronization of information transfer from the retina to the visual cortex, similar to cortico‐cortical gamma coupling. The temporal fusion of retino‐cortical gamma coherence at stimulus rates of theater movies may explain the mechanism of the visual illusion of continuity. How visual perception depends on early transformations of ascending sensory information is incompletely understood. By simultaneous measurement of flash‐evoked potentials in the retina and the visual cortex in awake, freely moving rats, we demonstrate for the first time that time‐locked gamma oscillatory potentials exhibit stable retino‐cortical synchrony across a wide range of stimulus parameters and that the temporal continuity of coherence changes with stimulus frequency according to the expected change in the visual illusion of continuity.

Although autoregulation of cerebral blood flow is well established, the response of cerebral capillary circulation to reduced cerebral perfusion pressure (CPP) is unclear. The objective of this study was to determine whether red cell flow velocity in individual capillaries of the cerebral cortex is maintained during acute decreases in CPP. Microcirculation of the superficial parietal cerebral cortex of adult barbiturate-anesthetized artificially ventilated rats was visualized using a new design of closed-perfused cranial window and epifluorescent-intensified video microscopy. Fluorescein-isothiocyanate-labeled red blood cells (FRBC) injected intravenously were used as markers of capillary flow. CPP, defined as mean arterial pressure minus intracranial pressure, was reduced by controlled hemorrhage or by stepwise elevation of local intracranial pressure. The movement of FRBC in the parenchymal capillary network was video recorded at each pressure level, and FRBC velocity in each capillary was measured off-line with use of the dual-window digital cross-correlation technique. FRBC flux in the capillaries was measured by automated cell counting. FRBC velocity at normal perfusion pressure was 1.47 +/- 0.58 (SD) mm/s and changed little in the perfusion pressure range of 70-120 mmHg. The autoregulatory index in this pressure range was 0.0049 mm.s-1.mmHg-1. Opening of previously unperfused capillaries was not observed. FRBC flux correlated with FRBC velocity, but the latter was maintained in a narrower range than FRBC flux, suggesting a decrease in capillary diameter or hematocrit with decreasing perfusion pressure. The results suggest that flow autoregulation is associated with the maintenance of capillary flow velocity and that capillary recruitment does not contribute to flow autoregulation in the rat cerebral cortex.

The balance between excitation and inhibition is considered to be of significant importance for neural computation and cognitive function. Excitatory and inhibitory functional connectivity in intact cortical neuronal networks in wakefulness and graded levels of anesthesia has not been systematically investigated. We compared monosynaptic excitatory and inhibitory spike transmission probabilities using pairwise cross-correlogram analysis. Spikes were measured at 64 sites in the visual cortex of rats with chronically implanted microelectrode arrays during wakefulness and three levels of anesthesia produced by desflurane. Anesthesia decreased the number of active units, the number of functional connections, and the strength of excitatory connections. Connection probability (number of connections per number of active unit pairs) was unaffected until the deepest anesthesia level, at which a significant increase in the excitatory to inhibitory ratio of connection probabilities was observed. The results suggest that the excitatory-inhibitory balance is altered at an anesthetic depth associated with unconsciousness.

Background The nucleus basalis of Meynert of the basal forebrain has been implicated in the regulation of the state of consciousness across normal sleep-wake cycles. Its role in the modulation of general anesthesia was investigated. Methods Rats were chronically implanted with bilateral infusion cannulae in the nucleus basalis of Meynert and epidural electrodes to record the electroencephalogram in frontal and visual cortices. Animals were anesthetized with desflurane at a concentration required for the loss of righting reflex (4.6 ± 0.5%). Norepinephrine (17.8 nmol) or artificial cerebrospinal fluid was infused at 0.2 μl/min (1 μl total). Behavioral response to infusion was measured by scoring the orofacial, limb, and head movements, and postural changes. Results Behavioral responses were higher after norepinephrine (2.1 ± 1) than artificial cerebrospinal fluid (0.63 ± 0.8) infusion (P < 0.01, Student t test). Responses were brief (1-2 min), repetitive, and more frequent after norepinephrine infusion (P < 0.0001, chi-square test). Electroencephalogram delta power decreased after norepinephrine in frontal (70 ± 7%) but not in visual cortex (P < 0.05, Student t test). Simultaneously, electroencephalogram cross-approximate entropy between frontal and visual cortices increased from 3.17 ± 0.56 to 3.85 ± 0.29 after norepinephrine infusion (P < 0.01, Student t test). Behavioral activation was predictable by the decrease in frontal delta power (logistic regression, P < 0.05). Conclusions Norepinephrine infusion into the nucleus basalis of Meynert can modulate anesthetic depth presumably by ascending activation of the cortex. The transient nature of the responses suggests a similarity with microarousals normally observed during natural sleep, and may imply a mechanism for transient awareness under light anesthesia.

Information processing that subserves conscious cognitive functions is thought to involve recurrent signaling through feedforward and feedback loops among hierarchically arranged functional regions of the cerebral cortex. In the current issue of Consciousness and Cognition, Lee et al. report that loss of consciousness, as produced by a bolus injection of the general anesthetic propofol to human volunteers, was accompanied by a decrease in wide-band EEG feedback connectivity from frontal cortex to parietal cortex, confirming a prediction from previous experimental studies. Interestingly, frontoparietal feedback connectivity did not fully recover after the anesthetic effect wore off and the subjects first opened their eyes in response to a verbal command. Possible interpretations of the results and their implications with respect to the neural correlates of consciousness and unconsciousness are discussed.

Background The effect of inhalational anesthetics on sensory-evoked unit activity in the cerebral cortex has been controversial. Desflurane has desirable properties for in vivo neurophysiologic studies, but its effect on cortical neuronal activity and neuronal responsiveness is not known. The authors studied the effect of desflurane on resting and visual evoked unit activity in rat visual cortex in vivo. Methods Desflurane was administered to adult albino rats at steady-state concentrations at 2%, 4%, 6%, and 8%. Flashes from a light emitting diode were delivered to the left eye at 5-s intervals. Extracellular unit activity within the right visual cortex was recorded using a 49-electrode array. Individual units were identified using principal components analysis. Results At 2% desflurane, 578 active units were found. Of these, 75% increased their firing rate in response to flash. Most responses contained early (0-100 ms) and late (150-1000 ms) components. With increasing desflurane concentration, the number of units active at baseline decreased (-13%), the number of early-responding units increased (+31%), and number of late-responding units decreased (-15%). Simultaneously, baseline firing rate decreased (-77%), the early response was unchanged, and the late response decreased (-60%). Conclusions The results indicate that visual cortex neurons remain responsive to flash stimulation under desflurane anesthesia, but the long-latency component of their response is attenuated in a concentration-dependent manner. Suppression of the long-latency response may be related to a loss of corticocortical feedback and loss of consciousness.

The alpha-2-adrenoreceptor agonist, medetomidine, which exhibits dose-dependent sedative effects and is gaining acceptance in small-animal functional magnetic resonance imaging (fMRI), has been studied. Rats were examined on the bench using the classic tail-pinch method with three infusion sequences: 100 microg/kg/h, 300 microg/kg/h, or 100 microg/kg/h followed by 300 microg/kg/h. Stepping the infusion rate from 100 to 300 microg/kg/h after 2.5 h resulted in a prolonged period of approximately level sedation that cannot be achieved by a constant infusion of either 100 or 300 microg/kg/h. By stepping the infusion dosage, experiments as long as 6 h are possible. Functional MRI experiments were carried out on rats using a frequency dependent electrical stimulation protocol-namely, forepaw stimulation at 3, 5, 7, and 10 Hz. Each rat was studied for a four-hour period, divided into two equal portions. During the first portion, rats were started at a 100 microg/kg/h constant infusion. During the second portion, four secondary levels of infusion were used: 100, 150, 200, and 300 microg/kg/h. The fMRI response to stimulation frequency was used as an indirect measure of modulation of neuronal activity through pharmacological manipulation. The frequency response to stimulus was attenuated at the lower secondary infusion dosages 100 or 150 microg/kg/h but not at the higher secondary infusion dosages 200 or 300 microg/kg/h. Parallel experiments with the animal at rest were carried out using both electroencephalogram (EEG) and functional connectivity MRI (fcMRI) methods with consistent results. In the secondary infusion period using 300 microg/kg/h, resting-state functional connectivity is enhanced.

The fundamental thesis of this paper is that under general anesthesia, the brain preserves its reactivity to sensory stimulation, but the information conveyed by these stimuli is not integrated – therefore, it is not consciously perceived. This happens at anesthetic depths associated with behavioral unresponsiveness from which unconsciousness is inferred. A further hypothesis is that general anesthetic agents interfere with the integration of sensory information encoded in cortical neuronal activity in the gamma frequency band. A critical region whose connectivity with the rest of the brain may be disrupted in anesthesia is the posterior parietal association cortex. While it is true that most anesthetic agents utilize different cellular and molecular pathways and neuron groups to produce their pharmacological effects, a common systems level action that may lead to unconsciousness by all anesthetics is the disruption of cortical information processing. The plausibility of these hypotheses and supporting experimental evidence is discussed in the context of theoretical considerations for defining consciousness and inferring the presence or absence of consciousness in an anesthetized subject.

Objective To determine if ketamine attenuates postoperative delirium concomitant with an anti-inflammatory effect in patients undergoing cardiac surgery using cardiopulmonary bypass. Design A prospective randomized study. Setting A Veterans Affairs medical center. Participants Cardiac surgical patients. Interventions Patients at least 55 years of age randomly received placebo (0.9% saline, n = 29) or an intravenous bolus of ketamine (0.5 mg/kg intravenously, n = 29) during anesthetic induction in the presence of fentanyl and etomidate. Measurements and Main Results Delirium was assessed by using the Intensive Care Delirium Screening Checklist before and after surgery. Serum C-reactive protein concentrations were determined before and 1 day after surgery. The incidence of postoperative delirium was lower (p = 0.01, Fisher exact test) in patients receiving ketamine (3%) compared with placebo (31%). Postoperative C-reactive protein concentration was also lower (p < 0.05) in the ketamine-treated patients compared with the placebo-treated patients. The odds of developing postoperative delirium were greater for patients receiving placebo compared with ketamine treatment (odds ratio = 12.6; 95% confidence interval, 1.5-107.5; logistic regression). Conclusions After cardiac surgery using cardiopulmonary bypass, ketamine attenuates postoperative delirium concomitant with an anti-inflammatory effect.

Purpose To investigate the source of native low-frequency fluctuations (LFF) in functional MRI (fMRI) signal. Materials and Methods Phase analysis was performed on tissue-segmented fMRI data acquired at systematically varying sampling rates. Results LFF in fMRI signal were both native and aliased in origin. Scanner instability did not contribute to native or aliased LFF. Aliased LFF arose from cardiorespiratory processes and head motion. Native LFF did not arise from cardiorespiratory processes, but did so, at least in part, from head motion. Motion correction reduced native LFF, but did not eliminate them. The residual native LFF in motion-corrected fMRI data showed a systematic phase difference among different tissue structures. The native LFF in fMRI signals of cerebral blood vessels and CSF were synchronous, and preceded those of gray and white matter, indicating that the vascular fluctuations lead the metabolic fluctuations. Conclusion The primary physiologic source of native LFF in fMRI signal is vasomotion. J. Magn. Reson. Imaging 2008;27:891–897. © 2008 Wiley-Liss, Inc.

Anesthetic endpoints of unconsciousness and immobility result from agent effects on both brain and spinal cord that are difficult to separate during systemic administration. To investigate cerebral mechanism of anesthetic-induced unconsciousness, the authors studied behavioral and electrophysiologic effects of four anesthetics infused intracerebroventricularly to conscious rats. The authors aimed to produce progressively increasing anesthetic depths, indicated by electro-encephalographic synchronization and behavioral change.During anesthesia, rats were equipped with intracerebroventricular infusion catheters, hind-paw stimulation, and epidural electrodes to record the electroencephalogram from the somatosensory cortex. Silicone bolus was injected into the fourth ventricle to minimize drug distribution to the spinal cord. 60 min later, 50-min infusion of pentobarbital (6.0 mg/h), fentanyl (0.75 microg/h), propofol (3.0 mg/h), or midazolam (0.24 mg/h) was initiated. Vibrissal, olfactory, corneal, and tail-pinch responses were tested every 10 min.All agents depressed vibrissal, olfactory, and corneal responses; propofol and pentobarbital produced the strongest effect. All agents except propofol depressed tail-pinch response; fentanyl and pentobarbital produced the strongest effect. All agents except midazolam increased delta power. Pentobarbital enhanced theta power. All agents except fentanyl enhanced alpha and beta power. Pentobarbital and midazolam slightly increased, whereas fentanyl decreased, gamma power. Pentobarbital increased and midazolam decreased somatosensory evoked potential; these changes were small and apparently unrelated to behavior.Alpha and beta power increase may reflect sedative component of anesthesia. Simultaneous delta, alpha, and beta power increase may correlate with loss of consciousness. Theta and delta power increase may reflect surgical anesthesia. Opioid-induced gamma power decrease may reflect suppression of pain perception. Pentobarbital-, fentanyl-, and midazolam-induced immobility to noxious stimulation may be mediated supraspinally.

Consciousness presumes a set of integrated functions such as sensory processing, attention, and interpretation, and may depend upon both local and long-range phase synchronization of neuronal activity in cerebral cortex. Here we investigated whether volatile anesthetic isoflurane at concentrations that produce loss of consciousness (LOC) disrupts long-range anterio-posterior and local anterior synchronization of neuronal activity in the rat. In six rats, deep electrodes were chronically implanted in the primary visual cortex (V1) and in two areas of the motor cortex (M1 and M2) for recording of intracortical event-related potentials (ERP). Thirty discrete flashes were presented at random interstimulus intervals of 15-45 s, and ERPs were recorded at stepwise increasing isoflurane concentrations of 0-1.1%. Neuronal synchronization was estimated using wavelet coherence computed from the ERP data band-pass filtered at 5-50 Hz. We found that (1) in the waking state, long-range anterio-posterior coherence in 5-25 Hz and 25-50 Hz frequency bands was significantly higher than local anterior coherence; (2) anterio-posterior coherence in both 5-25 Hz and 26-50 Hz bands was significantly reduced by isoflurane in a concentration-dependent manner; (3) local anterior coherence was not affected by isoflurane at any of the concentrations studied. These findings suggest that a disruption of long-range anterio-posterior rather than local anterior synchronization of neuronal activity precedes the anesthetic-induced loss of consciousness.

To establish a model for functional hyperemia in the rat visual cortex, cortical blood flow responses to flash stimulation were measured with the laser Doppler flow (LDF) technique at various levels of halothane anesthesia. The concentration-dependent effect of halothane on arterial pressure and its consequent effect on the hyperemic response were also investigated. Using a stroboscopic light source, 10 flashes at 1 min intervals were delivered to the left eye of 12 Sprague–Dawley rats. LDF responses were measured bilaterally in the monocular primary visual cortex (V1M) at steady state halothane concentrations between 0.4 and 1.4%. In six rats, methoxamine (MX) was infused to prevent halothane-induced hypotension; the remaining rats did not receive MX. In all rats, LDF response to flash commenced within 1 s and peaked at 2.5 s in the contralateral V1M, but not in ipsilateral V1M. The maximum LDF response was 25% at 0.5% halothane and 12% at 1.4% halothane. In rats without MX infusion, mean arterial pressure (MAP) fell from 138 to 90 mmHg when halothane increased from 0.4 to 1.4%. MX infusion prevented the hypotension, but did not influence the LDF response, suggesting that the halothane's effect was direct rather than pressure-mediated. We demonstrate for the first time, a robust functional hyperemic response to discrete flash stimuli in the primary visual cortex of halothane-anesthetized albino rats that can be measured with LDF over a wide range of halothane concentrations and is not fully suppressed at surgical levels of halothane anesthesia.

We seek to understand neural correlates of anesthetic-induced unconsciousness. We hypothesize that cortical integration of sensory information may underlie conscious perception and may be disrupted by anesthetics. A critical role in frontal-posterior interactions has been proposed, and gamma (20-60 Hz) oscillations have also been assigned an essential role in consciousness. Here we investigated whether general anesthetics may interfere with the exchange of information encoded in gamma oscillations between frontal and posterior cortices. Bipolar electrodes for recording of event-related potentials (ERP) were chronically implanted in the primary visual cortex, parietal association and frontal association cortices of six rats. Sixty light flashes were presented every 5s, and ERPs were recorded at increasing concentrations of halothane or isoflurane (0-2%). Information exchange was estimated by transfer entropy, a novel measure of directional information transfer. Transfer entropy was calculated from 1-s wavelet-transformed ERPs. We found that (1) feedforward transfer entropy (FF-TE) and feedback transfer entropy (FB-TE) were balanced in conscious-sedated state; (2) anesthetics at concentrations producing unconsciousness augmented both FF-TE and FB-TE at 30 Hz but reduced them at 50 Hz; (3) reduction at 50 Hz was more pronounced for FB-TE, especially between frontal and posterior regions; (4) at high concentrations, both FF-TE and FB-TE at all frequencies were at or below conscious-sedated baseline. Our findings suggest that inhalational anesthetics preferentially impair frontal-posterior FB information transfer at high gamma frequencies consistent with the postulated role of frontal-posterior interactions in consciousness.

PURPOSE The objective of this study was to develop a technique to decouple the hemodynamic delay from the task-induced delay on a voxel-by-voxel basis in functional magnetic resonance imaging (fMRI) data sets. Prior fMRI studies have reported variability in temporal delays of up to several seconds among activated voxels. It is currently assumed that this variability primarily arises from differences in the onset of task-induced activation, although the precise source of these delays has not been well characterized. Here, we hypothesize that the total delay during task activation can be modeled as a combination of neuronal (caused by differences in onset of neuronal firing), vasomotor (caused by flow changes during activation), and transit (caused by differences in the red blood cell arrival time) delays. METHOD Subjects were scanned using a sequential dynamic susceptibility contrast (DSC) protocol during rest and fMRI of the motor cortex using a bilateral finger-tapping task. The total delay was determined using correlation coefficient analysis, whereas the intrinsic delay was determined from the DSC MRI. Subtraction of the transit delay from the corresponding total delay for each voxel yielded the task-induced delay. RESULTS In all subjects, a transit delay of 2.3 (+/-1.1) seconds and a task-induced delay of 0.7 (+/-0.6) second was observed between voxels, which is in good agreement with reports in the literature using other techniques. These results demonstrate the feasibility of the DSC MRI for separating the hemodynamic and task-induced delays in fMRI studies. CONCLUSION This approach has the potential to elucidate the temporal characteristics of the blood oxygenation level-dependent signal during fMRI as well as to further our understanding of the dynamics of the activation-induced signal in neuroimaging.

Background Despite intense investigation, the mechanism of isoflurane-induced cerebral hyperemia is unclear. The current study was designed to determine the contributions of neuronal nitric oxide synthase, prostaglandins, and epoxyeicosatrienoic acids to isoflurane-induced cerebral hyperemia. Methods Regional cerebral cortical blood flow was measured with laser Doppler flowmetry during stepwise increases of isoflurane from 0.0 to 1.2, 1.8, and 2.4 vol% end-tidal concentration in alpha-chloralose-urethane-anesthetized, C57BL/6 mice before and 45 min after administration of the neuronal nitric oxide synthase inhibitor 7-nitroindazole (7-NI, 40 mg/kg, intraperitoneal), the cyclooxygenase inhibitor indomethacin (INDO, 10 mg/kg, intravenous), and the cytochrome P450 epoxygenase inhibitor N-methylsulfonyl-6-(2-proparglyoxyphenyl)hexanoic acid (PPOH, 20 mg/kg, intravenous). Results Isoflurane increased regional cerebral cortical blood flow by 9 +/- 3, 46 +/- 21, and 101 +/- 26% (SD) at 1.2, 1.8, and 2.4 vol%, respectively. The increases in regional cerebral cortical blood flow were significantly (*P < 0.05) smaller after 7-NI (5 +/- 6, 29 +/- 19*, 68 +/- 15%*) or PPOH (4 +/- 8, 27 +/- 17*, 67 +/- 30%*), but not after administration of INDO (4 +/- 4, 33 +/- 18 [NS], 107 +/- 35% [NS]). The effect of combined treatment with 7-NI, PPOH, and INDO was not additive and was equal to that of either 7-NI or PPOH alone (5 +/- 5, 30 +/- 12*, 76 +/- 24%*). Chronic treatment of mice for 5 days with 7-NI (2 x 40 mg/kg, intraperitoneal) produced similar decreases in regional cerebral cortical blood flow as those seen with acute administration. Neither PPOH nor INDO conferred a significant additional block of the hyperemia in these animals. Conclusions Nitric oxide and epoxyeicosatrienoic acids contribute to isoflurane-induced hyperemia. However, only approximately one third of the cerebral hyperemic response to isoflurane is mediated by autacoids. The remaining part of this response appears to be mediated by a direct action of isoflurane on smooth muscle by some yet-unknown mechanism.

Blood oxygenation level dependent (BOLD) signal intensity (SI) and regional cerebral blood flow (CBF) during a 20-s apnea stimulus in awake humans and pentobarbital-anesthetized rats were measured to assess the usefulness of apnea in estimating cerebral vasodilatory capacity for functional MRI (fMRI) experiments. Rats were ventilated with either room air or 100% O2. While breathing room air, apnea for 20 s increased the BOLD SI in humans but decreased it in rats. However, in rats ventilated with 100% O2, BOLD SI increased upon apnea for 20 s. CBF measurements in rats using laser Doppler flowmetry (LDF) showed a 45% ± 8% increase during apnea with room air ventilation, and a 10% ± 3% increase with 100% O2. Arterial blood oxygen saturation fell from 96% ± 1% to 29% ± 5%, and cerebral tissue PO2 decreased from 15 ± 3 mmHg to 6 ± 2 mmHg by the end of 20-s apnea in rats breathing room air. However, with 100% O2 respiration, apnea produced no change in the arterial blood oxygen saturation, which remained at 99%, but increased tissue PO2 from 35 ± 9 mmHg to 39 ± 10 mmHg. From the results obtained in rats ventilated with room air, it is concluded that apnea induces hypoxia that results in a decrease in fMRI-BOLD signal. The signal decrease occurred despite an increase in PaCO2 and CBF. This BOLD response is the opposite of that observed in humans, who presumably do not develop hypoxia within the applied apnea period. These studies highlight the importance of the choice of ventilating gas mixture on the outcome of BOLD experiments during systemic perturbations. Magn Reson Med 47:864–870, 2002. © 2002 Wiley-Liss, Inc.

Many factors contribute to the severity of neuronal cell death and the functional outcome in stroke. We describe an embolic model of focal cerebral ischemia in the rat that does not require craniotomy and is compatible with continuous measurement of regional CBF using multichannel laser Doppler flow (LDF) technique. Either a 22 microliters (large lesion) or 11 microliters (small lesion) bolus of viscous silicone was injected cephalad into the internal carotid artery. Upon injection, LDF decreased abruptly, most severely in the parietal cortex (-74% +/- 5%) in the large lesion and in the occipital cortex (-69% +/- 10%) in the small lesion model. Over the first hour, post-embolization LDF improved in most areas (e.g. -48% +/- 9% parietal, large lesion) but declined in the small lesion group in the occipital region (-81% +/- 8%). CBF measured by [C]14-IAP autoradiography 1 h post-embolization in the large lesion model demonstrated near-hemispheric ischemia (70% of hemisphere) with sparing of cingulate cortex. Autoradiography demonstrated that ischemia in the small lesion was largely cortical. Light microscopy of brains embolized with 11 microliters of dyed silicone showed filling of pial vessels with no silicone in the Circle of Willis or parenchyma. No animals in the large lesion group survived 24 h. Thirteen of 15 animals in the small lesion group survived for two weeks with resolution of initial hemiplegia, ocular asymmetry and weight loss. Hematoxylin-eosin staining two weeks post-embolization showed signs of severe hypoxia and infarction. In conclusion, the intracarotid silicone embolization technique produces a titrable, reproducible permanent ischemic injury by blocking perfusion in the pial circulation, and is amenable to multisite monitoring with laser Doppler flowmetry. The smaller embolus produces cortical infarction with high rate of survival and neurological recovery.

Background The cerebral hyperemic effect of hemodilution is well known; however, its mechanism and potential modifying effect on the functional hyperemic response to neuronal activation are unclear. The authors investigated the effects of isovolemic hemodilution on vibrissal stimulation-induced changes in cerebrocortical laser Doppler flow and tissue oxygen tension in the rat. Methods The hyperemic response to whisker stimulation was assessed in the whisker barrel cortex of 12 rats anesthetized with chloralose-urethane before and after hemodilution. Graded, isovolemic hemodilution was performed by three repeated withdrawals of 3 ml blood with replacement of equal volume of 5% serum albumin. Measured systemic hematocrit values were 39.3 +/- 1.3% (control), 29.5 +/- 1.0%, 22.3 +/- 1.5%, and 17.0 +/- 1.6% (after the three hemodilution steps). Arterial blood pressure was maintained at control levels with an infusion of methoxamine. Unilateral whisker stimulation was performed with a mechanical actuator at 8 Hz, and 10 cycles of 10 s on-30 s off periods. In six control animals, shed blood was immediately reinfused, resulting in no change in hematocrit, and whisker stimulation was performed using the same timeline as in the other animals. In six additional experiments, resting cerebral cortical oxygen tension was measured using the phosphorescence quenching technique following the same hemodilution protocol. Results Graded hemodilution increased baseline laser Doppler flow by 5.5 +/- 0.9%, 13 +/- 1.6%, and 23.7 +/- 2.2%. Vibrissal stimulation transiently increased laser Doppler flow by 17.0 +/- 2.0%. The hyperemic response was unchanged after hemodilution and was identical to that seen in the control group in all conditions. Tissue oxygen tension increased slightly but significantly with hemodilution at a rate of 1.4 mmHg per 10% hematocrit change (r = 0.83). Mean arterial pressure, arterial oxygen tension, carbon dioxide tension, and pH were within normal limits in each experimental group and were not different from control during hemodilution. Conclusions The results suggest that an increase in baseline flow during hemodilution maintains cortical oxygen supply and consequently preserves the normal functional hyperemic response.

An automated correlation method is introduced to estimate erythrocyte velocity component of erythrocyte flux within the cerebral capillary network. Erythrocyte flux, defined as the number of red blood cells passing through a plane orthogonal to the axis of erythrocyte flow in a vessel per unit time, is considered to be the closest index of capillary flow. Introduced previously is the two-point cross-correlation method, a method whereby a video photometric analyzer captures the voltage produced from two electronic windows placed over a vessel of interest. In our new method, instead of using electronic windows, we use a CCD array, focused on a two- dimensional projection of the three-dimensional capillary structure. Simulations of this method yields accurate velocity measurements at a measured cell intensity of .2 standard deviations above mean noise values or cell counts fewer than 30 cells per minute for image sequences of 180 frames captured over a time interval of three seconds. We conclude that with proper reduction in the measured standard deviation of noise and by increasing the percentage of fluorscently labeled erythrocytes injected into the rat, the correlation timing method of estimating erythrocyte velocity is an accurate substitute for hand-measured velocity calculation.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The overall goal of this research is to better understand the mechanisms underlying the dysfunction of cerebral blood flow (CBF) autoregulation after an ischemic insult. Towards this purpose, a rat focal cerebral ischemia model has been developed that does not require craniotomy and would be compatible with continuous monitoring of regional CBF using multichannel laser Doppler flowmetry (LDF) throughout the ischemic period. The most common models of focal ischemia in rodents have utilized either ligation, coagulation or intraluminal obstruction of the middle cerebral artery.18,2,3,4,10 These approaches involve craniotomy or are performed in the supine position which is inconvenient for multichannel LDF recording. It is important to continuously monitor regional CBF in multiple brain areas because the pattern and the time course of CBF changes (hyperemia versus hypoperfusion) may be different in various brain regions and may thereby influence the extent and severity of eventual neuronal injury.

Spontaneous, low frequency (4–12 cpm) fluctuations, independent of the cardiac and respiratory cycles, in human and animal brains were first recorded with the O2 polarographic technique in the late 1950s. They were seen in NADH and cytochrome oxidase and associated with spontaneous vasomotion pial and large cerebral arteries. Renewed interest in spontaneous fluctuations was generated by studies with laser-Doppler flowmetry (LDF), reflectance oximetry and functional MRI. Spontaneous fluctuations were consistently produced when cerebral perfusion was challenged by systemic or local manipulations; the fluctuation amplitude reached 30–40% of the mean. The most potent stimuli are hypotension, hyperventilation, cerebral artery occlusion and cerebral vasoconstriction elicited, for example, by a nitric oxide synthase inhibitor but not by indomethacin. The fluctuations are suspended by CO2 and halothane at concentrations that produce hyperemia. Recently, spontaneous fluctuations were recorded by LDF microprobes in areas as small as 130 μm and by video-microscopy in single capillaries. The fluctuations were absent in severe, focally ischemic brain territories. The dependence of spontaneous fluctuations on intravascular pressure argues for the importance of a myogenic mechanism, however, neuronal modulation may also play a role. Coherence of small vessel vasomotion may be required for the emergence of regional flow fluctuations. There is a need to elucidate the spatial and frequency domains in which fluctuations are present under normal physiological conditions and those in which they may reflect brain injury and pathologies of diagnostic or prognostic value.

Inflammatory stimulation of leukocyte adhesion in cerebral ischemia/reperfusion may result in vascular endothelial dysfunction, compromised microvascular blood flow and tissue oxygenation and neuronal death. Ischemia facilitates rolling and adhesion of leukocytes in mesenteric venules (Perry and Granger, 1992), however, this has not been demonstrated in the cerebral microcirculation. Nitric oxide has antiadhesion properties for leukocytes in vivo (Kurose et al, 1994) and may modulate leukocyte adhesion during cerebral ischemia/reperfusion.

An intriguing characteristic of the ontogenic development of the cerebral vasculature is the rapid differentiation of the neonatal leptomeningeal vascular plexus into the mature, adult network form. The physiological and cellular mechanisms of this cerebrovascular remodeling process are unclear. The objective of this work was to determine and correlate changes in vascular density, network pattern and flow velocity in leptomeningeal microvessels of the rat during postnatal development in vivo. To this end, microvascular diameter, segment length, and vascular density of reconstructed leptomeningeal networks were measured from video-recordings of the microcirculation visualized through a cranial window in 0-15-day-old Sprague-Dawley rats. The velocity of erythrocytes in the microvessels was measured by frame to frame tracking of fluorescently labeled red blood cells. We found that surface vascular density (total vessel length per area), node density and segment density (object per area) decreased significantly by the second week after birth. Anastomosing vascular polygons, characteristic to newborn networks, became less numerous and larger in diameter during the postnatal 2-week period, indicating progressive rarefaction of the networks. Vessel diameter and red cell velocity showed transient increases at 1.5 weeks. The velocity/diameter ratio (V/D), an index of wall shear rate, increased by the age of 1.5 weeks and remained unchanged afterwards. There was a negative correlation between V/D and diameter at 1 week; this relationship was reversed to a positive correlation at 2 weeks. We conclude that postnatal remodeling of the leptomeningeal vascular network is associated with rarefaction and an adaptation of vessel caliber to wall shear rate. These changes may contribute to arterio-venous differentiation and redistribution of blood flow from the superficial to the intracortical vasculature in the developing brain.

Unilateral, transient (30, 60, and 120 minutes (min)) middle-cerebral-artery (MCA) occlusion was induced via transorbital craniotomy in 11 waking subhuman primates. Local cerebral blood flow (LCBF) was calculated from hydrogen clearance curves obtained through the use of intracerebral platinum microelectrodes. Unilateral MCA occlusion decreased LCBF in the territory of the ipsilateral MCA. Within minutes of the arterial occlusion all monkeys developed contralateral neurologic deficits that began disappearing three hours (h) after reopening the MCA. Regional ischemia, followed by 24 h of reperfusion, produced varying degrees of tissue vacuolation which correlated (r = 0.60, p less than 0.01, n = 49) with the percent reduction in LCBF multiplied by the occlusion time. Neurons were classified according to the structural features of their perikaryon. A plot of neuron types versus percent vacuolation suggested that normal neurons become increasingly scalloped under increasingly severe ischemic conditions. The number of scalloped neurons decreased precipitously in areas of marked sponginess coincident with the appearance of irreversibly damaged neurons. Local tissue edema values exceeding 30% correlated with irreversible injury to all neurons in the same area. Regional cerebral ischemia of increasing severity was acompanied by increasing numbers of lethally injured neurons.

Mechanisms of ischemic injury to the brain have been broadly classified into two large categories: (a) global, that is, involving the entire intracranial contents, and. (b) regional or focal, that is, confined to the territory of one or more blood vessels [16, 17].

This study examined the effects of blocking the formation of 20-hydroxyeicosatetraenoic acid (20-HETE) on the acute fall in cerebral blood flow after subarachnoid hemorrhage (SAH) in the rat. In vehicle-treated rats, regional cerebral blood flow (rCBF) measured with laser-Doppler flowmetry fell by 30% 10 min after the injection of 0.3 ml of arterial blood into the cisterna magna, and it remained at this level for 2 h. Pretreatment with inhibitors of the formation of 20-HETE, 17-octadecynoic acid (17-ODYA; 1.5 nmol intrathecally) and N-hydroxy- N′-(4-butyl-2-methylphenyl)formamidine (HET0016; 10 mg/kg iv), reduced the initial fall in rCBF by 40%, and rCBF fully recovered 1 h after induction of SAH. The concentration of 20-HETE in the cerebrospinal fluid rose from 12 ± 2 to 199 ± 17 ng/ml after SAH in vehicle-treated rats. 20-HETE levels averaged only 15 ± 11 and 39 ± 13 ng/ml in rats pretreated with 17-ODYA or HET0016, respectively. HET0016 selectively inhibited the formation of 20-HETE in rat renal microsomes with an IC50of 50 of 42, 125, and 100 nM, respectively. These results indicate that 20-HETE contributes to the acute fall in rCBF after SAH in rats.