Central thalamic contributions to arousal regulation and neurological disorders of consciousness

Hippocampal-prefrontal circuit and disrupted functional connectivity in psychiatric and neurodegenerative disorders

In rodents, the hippocampus has been studied extensively as part of a brain system responsible for learning and memory, and the prefrontal cortex (PFC) participates in numerous cognitive functions including working memory, flexibility, decision making, and rewarding learning. The neuronal projections from the hippocampus, either directly or indirectly, to the PFC, referred to as the hippocampal-prefrontal cortex (Hip-PFC) circuit, play a critical role in cognitive and emotional regulation and memory consolidation. Although in certain psychiatric and neurodegenerative diseases, structural connectivity viewed by imaging techniques has been consistently found to be associated with clinical phenotype and disease severity, the focus has moved towards the investigation of connectivity correlates of molecular pathology and coupling of oscillation. Moreover, functional and structural connectivity measures have been emerging as potential intermediate biomarkers for neuronal disorders. In this review, we summarize progress on the anatomic, molecular, and electrophysiological characters of the Hip-PFC circuit in cognition and emotion processes with an emphasis on oscillation and functional connectivity, revealing a disrupted Hip-PFC connectivity and electrical activity in psychiatric and neurodegenerative disorders as a promising candidate of neural marker for neuronal disorders.

How acute total sleep loss affects the attending brain: a meta-analysis of neuroimaging studies

STUDY OBJECTIVES

Attention is a cognitive domain that can be severely affected by sleep deprivation. Previous neuroimaging studies have used different attention paradigms and reported both increased and reduced brain activation after sleep deprivation. However, due to large variability in sleep deprivation protocols, task paradigms, experimental designs, characteristics of subject populations, and imaging techniques, there is no consensus regarding the effects of sleep loss on the attending brain. The aim of this meta-analysis was to identify brain activations that are commonly altered by acute total sleep deprivation across different attention tasks.

DESIGN

Coordinate-based meta-analysis of neuroimaging studies of performance on attention tasks during experimental sleep deprivation.

METHODS

The current version of the activation likelihood estimation (ALE) approach was used for meta-analysis. The authors searched published articles and identified 11 sleep deprivation neuroimaging studies using different attention tasks with a total of 185 participants, equaling 81 foci for ALE analysis.

RESULTS

The meta-analysis revealed significantly reduced brain activation in multiple regions following sleep deprivation compared to rested wakefulness, including bilateral intraparietal sulcus, bilateral insula, right prefrontal cortex, medial frontal cortex, and right parahippocampal gyrus. Increased activation was found only in bilateral thalamus after sleep deprivation compared to rested wakefulness.

CONCLUSION

Acute total sleep deprivation decreases brain activation in the fronto-parietal attention network (prefrontal cortex and intraparietal sulcus) and in the salience network (insula and medial frontal cortex). Increased thalamic activation after sleep deprivation may reflect a complex interaction between the de-arousing effects of sleep loss and the arousing effects of task performance on thalamic activity.

The neurobiology of thalamic amnesia: Contributions of medial thalamus and prefrontal cortex to delayed conditional discrimination

Although medial thalamus is well established as a site of pathology associated with global amnesia, there is uncertainty about which structures are critical and how they affect memory function. Evidence from human and animal research suggests that damage to the mammillothalamic tract and the anterior, mediodorsal (MD), midline (M), and intralaminar (IL) nuclei contribute to different signs of thalamic amnesia. Here we focus on MD and the adjacent M and IL nuclei, structures identified in animal studies as critical nodes in prefrontal cortex (PFC)-related pathways that are necessary for delayed conditional discrimination. Recordings of PFC neurons in rats performing a dynamic delayed non-matching-to position (DNMTP) task revealed discrete populations encoding information related to planning, execution, and outcome of DNMTP-related actions and delay-related activity signaling previous reinforcement. Parallel studies recording the activity of MD and IL neurons and examining the effects of unilateral thalamic inactivation on the responses of PFC neurons demonstrated a close coupling of central thalamic and PFC neurons responding to diverse aspects of DNMTP and provide evidence that thalamus interacts with PFC neurons to give rise to complex goal-directed behavior exemplified by the DNMTP task.

Altered activity in the central medial thalamus precedes changes in the neocortex during transitions into both sleep and propofol anesthesia

How general anesthetics cause loss of consciousness is unknown. Some evidence points toward effects on the neocortex causing "top-down" inhibition, whereas other findings suggest that these drugs act via subcortical mechanisms, possibly selectively stimulating networks promoting natural sleep. To determine whether some neuronal circuits are affected before others, we used Morlet wavelet analysis to obtain high temporal resolution in the time-varying power spectra of local field potentials recorded simultaneously in discrete brain regions at natural sleep onset and during anesthetic-induced loss of righting reflex in rats. Although we observed changes in the local field potentials that were anesthetic-specific, there were some common changes in high-frequency (20-40 Hz) oscillations (reductions in frequency and increases in power) that could be detected at, or before, sleep onset and anesthetic-induced loss of righting reflex. For propofol and natural sleep, these changes occur first in the thalamus before changes could be detected in the neocortex. With dexmedetomidine, the changes occurred simultaneously in the thalamus and neocortex. In addition, the phase relationships between the low-frequency (1-4 Hz) oscillations in thalamic nuclei and neocortical areas are essentially the same for natural sleep and following dexmedetomidine administration, but a sudden change in phase, attributable to an effect in the central medial thalamus, occurs at the point of dexmedetomidine loss of righting reflex. Our data are consistent with the central medial thalamus acting as a key hub through which general anesthesia and natural sleep are initiated.

Neural correlates of consciousness: what we know and what we have to learn!

Consciousness is a multifaceted concept with two major components: awareness of environment and of self (i.e., the content of consciousness) and wakefulness (i.e., the level of consciousness). Medically speaking, consciousness is the state of the patient's awareness of self and environment and his responsiveness to external stimulation and inner need. A basic understanding of consciousness and its neural correlates is of major importance for all clinicians, especially those involved with patients suffering from altered states of consciousness. To this end, in this review it is shown that consciousness is dependent on the brainstem and thalamus for arousal; that basic cognition is supported by recurrent electrical activity between the cortex and the thalamus at gamma band frequencies; and that some kind of working memory must, at least fleetingly, be present for awareness to occur. New advances in neuroimaging studies are also presented in order to better understand and demonstrate the neurophysiological basis of consciousness. In particular, recent functional magnetic resonance imaging studies have offered the possibility to measure directly and non-invasively normal and severely brain damaged subjects' brain activity, whilst diffusion tensor imaging studies have allowed evaluating white matter integrity in normal subjects and patients with disorder of consciousness.

Thalamo-frontal connectivity mediates top-down cognitive functions in disorders of consciousness

OBJECTIVE

We employed functional MRI (fMRI) to assess whether (1) patients with disorders of consciousness (DOC) retain the ability to willfully engage in top-down processing and (2) what neurophysiologic factors distinguish patients who can demonstrate this ability from patients who cannot.

METHODS

Sixteen volunteers, 8 patients in vegetative state (VS), 16 minimally conscious patients (MCS), and 4 exit from MCS (eMCS) patients were enrolled in a prospective cross-sectional fMRI study. Participants performed a target detection task in which they counted the number of times a (changing) target word was presented amidst a set of distractors.

RESULTS

Three of 8 patients diagnosed as being in a VS exhibited significant activations in response to the task, thereby demonstrating a state of consciousness. Differential activations across tasks were also observed in 6 MCS patients and 1 eMCS patient. A psycho-physiologic interaction analysis revealed that the main factor distinguishing patients who responded to the task from those who did not was a greater connectivity between the anterior section of thalamus and prefrontal cortex.

CONCLUSIONS

In our sample of patients, the dissociation between overt behavior observable in clinical assessments and residual cognitive faculties is prevalent among DOC patients (37%). A substantial number of patients, including some diagnosed with VS, can demonstrate willful engagement in top-down cognition. While neuroimaging data are not the same as observable behavior, this suggests that the mental status of some VS patients exceeds what can be appreciated clinically. Furthermore, thalamo-frontal circuits might be crucial to sustaining top-down functions.

The functional connectivity of intralaminar thalamic nuclei in the human basal ganglia

Projections of the centromedian-parafasicularis neurons of the intralaminar thalamus are major inputs of the striatum. Their functional role in the activity of human basal ganglia (BG) is not well known. The aim of this work was to study the functional connectivity of intralaminar thalamic nuclei with other BG by using the correlations of the BOLD signal recorded during "resting" and a motor task. Intralaminar nuclei showed a marked functional connectivity with all the tested BG, which was observed during "resting" and did not change with the motor task. As regards the intralaminar nuclei, BG connectivity was much lower for the medial dorsal nucleus (a thalamic nucleus bordering the intralaminar nuclei) and for the default mode network (although intralaminar nuclei showed a negative correlation with the default mode network). After the "regression" of intralaminar nuclei activity (partial correlation), the functional connectivity of the caudate and putamen nuclei with other BG decreased (but not with the primary sensorimotor cortex). Present data provide evidence that intralaminar nuclei are not only critical for striatal activity but also for the global performance of human BG, an action involving subcortical BG loops more than cortico-subcortical loops. The high correlation found between BG suggest that, similarly to that reported in other brain centers, the very-slow frequency fluctuations are relevant for the functional activity of these centers.

The nonspecific thalamus: A place in a wedding bed for making memories last?

We summarize anatomical, electrophysiological and behavioral evidence that the rostral intralaminar (ILN) and the reuniens and rhomboid (ReRh) nuclei that belong to the nonspecific thalamus, might be part of a hippocampo-cortico-thalamic network underlying consolidation of enduring declarative(-like) memories at systems level. The first part of this review describes the anatomical and functional organization of these thalamic nuclei. The second part presents the theoretical models supporting the active systems-level consolidation, a process that relies upon sleep specific field-potential oscillations occurring during both slow-wave sleep (SWS) and rapid eye movement (REM) sleep. The last part presents data in the rat showing that the lesion of the rostral ILN or of the ReRh specifically hinders the formation of remote spatial memories without affecting task acquisition or retrieval of a recent memory. These results showing a critical role of the ILN and ReRh nuclei in the transformation of a recent memory into a remote one are discussed in the context of their control of cortical arousal (ARAS) and of thalamo-cortico-thalamic synchronization.

Preservation of electroencephalographic organization in patients with impaired consciousness and imaging-based evidence of command-following

OBJECTIVE

Standard clinical characterization of patients with disorders of consciousness (DOC) relies on observation of motor output and may therefore lead to the misdiagnosis of vegetative state or minimally conscious state in patients with preserved cognition. We used conventional electroencephalographic (EEG) measures to assess a cohort of DOC patients with and without functional magnetic resonance imaging (fMRI)-based evidence of command-following, and correlated the findings with standard clinical behavioral evaluation and brain metabolic activity.

METHODS

We enrolled 44 patients with severe brain injury. Behavioral diagnosis was established using standardized clinical assessments. Long-term EEG recordings were analyzed to determine wakeful background organization and presence of elements of sleep architecture. A subset of patients had fMRI testing of command-following using motor imagery paradigms (26 patients) and resting brain metabolism measurement using (18) fluorodeoxyglucose positron emission tomography (31 patients).

RESULTS

All 4 patients with fMRI evidence of covert command-following consistently demonstrated well-organized EEG background during wakefulness, spindling activity during sleep, and relative preservation of cortical metabolic activity. In the entire cohort, EEG organization and overall brain metabolism showed no significant association with bedside behavioral testing, except in a few cases when EEG was severely abnormal.

INTERPRETATION

These findings suggest that conventional EEG is a simple strategy that complements behavioral and imaging characterization of DOC patients. Preservation of specific EEG features may be used to assess the likelihood of unrecognized cognitive abilities in severely brain-injured patients with very limited or no motor responses.

Neuromodulation of the conscious state following severe brain injuries

Disorders of consciousness (DOC) following severe structural brain injuries globally affect the conscious state and the expression of goal-directed behaviors. In some subjects, neuromodulation with medications or electrical stimulation can markedly improve the impaired conscious state present in DOC. We briefly review recent studies and provide an organizing framework for considering the apparently widely disparate collection of medications and approaches that may modulate the conscious state in subjects with DOC. We focus on neuromodulation of the anterior forebrain mesocircuit in DOC and briefly compare mechanisms supporting recovery from structural brain injuries to those underlying facilitated emergence from unconsciousness produced by anesthesia. We derive some general principles for approaching the problem of restoration of consciousness after severe structural brain injuries, and suggest directions for future research.

Thalamic Involvement in Fluctuating Cognition in Dementia with Lewy Bodies: Magnetic Resonance Evidences

Dementia with Lewy bodies (DLB) is characterized by fluctuation in cognition and attention. Thalamocortical connectivity and integrity of thalami are central to attentional function. We hypothesize that DLB patients with marked and frequent fluctuating cognition (flCog) have a loss of thalamocortical connectivity, an intrinsic disruption to thalamic structure and imbalances in thalamic neurotransmitter levels. To test this, magnetic resonance imaging (MRI), diffusion tensor imaging (DTI) and proton MR spectroscopy on thalami were performed on 16 DLB, 16 Alzheimer's disease (AD) and 13 healthy subjects. MRI and DTI were combined to subdivide thalami according to their cortical connectivity and to investigate microstructural changes in connectivity-defined thalamic regions. Compared with controls, lower N-acetyl-aspartate/total creatine (NAA/tCr) and higher total choline/total creatine (tCho/tCr) values were observed within thalami of DLB patients. tCho/tCr increase was found within right thalamus of DLB patients as compared with AD. This increase correlated with severity and frequency of flCog. As compared with controls, DLB patients showed bilateral damage within thalamic regions projecting to prefrontal and parieto-occipital cortices, whereas AD patients showed bilateral alteration within thalamic region projecting to temporal cortex. We posit that microstructural thalamic damage and cholinergic imbalance may be central to the etiology of flCog in DLB.

Temporally-patterned deep brain stimulation in a mouse model of multiple traumatic brain injury

We report that mice with closed-head multiple traumatic brain injury (TBI) show a decrease in the motoric aspects of generalized arousal, as measured by automated, quantitative behavioral assays. Further, we found that temporally-patterned deep brain stimulation (DBS) can increase generalized arousal and spontaneous motor activity in this mouse model of TBI. This arousal increase is input-pattern-dependent, as changing the temporal pattern of DBS can modulate its effect on motor activity. Finally, an extensive examination of mouse behavioral capacities, looking for deficits in this model of TBI, suggest that the strongest effects of TBI in this model are found in the initiation of any kind of movement.

Traumatic axonal injury: the prognostic value of lesion load in corpus callosum, brain stem, and thalamus in different magnetic resonance imaging sequences

The aim of this study was to explore the prognostic value of visible traumatic axonal injury (TAI) loads in different MRI sequences from the early phase after adjusting for established prognostic factors. Likewise, we sought to explore the prognostic role of early apparent diffusion coefficient (ADC) values in normal-appearing corpus callosum. In this prospective study, 128 patients (mean age, 33.9 years; range, 11-69) with moderate (n = 64) and severe traumatic brain injury (TBI) were examined with MRI at a median of 8 days (range, 0-28) postinjury. TAI lesions in fluid-attenuated inversion recovery (FLAIR), diffusion-weighted imaging (DWI), and T2*-weighted gradient echo (T2*GRE) sequences were counted and FLAIR lesion volumes estimated. In patients and 47 healthy controls, mean ADC values were computed in 10 regions of interests in the normal-appearing corpus callosum. Outcome measure was the Glasgow Outcome Scale-Extended (GOS-E) at 12 months. In patients with severe TBI, number of DWI lesions and volume of FLAIR lesions in the corpus callosum, brain stem, and thalamus predicted outcome in analyses with adjustment for age, Glasgow Coma Scale score, and pupillary dilation (odds ratio, 1.3-6.9; p = <0.001-0.017). The addition of Rotterdam CT score and DWI lesions in the corpus callosum yielded the highest R2 (0.24), compared to all other MRI variables, including brain stem lesions. For patients with moderate TBI only the number of cortical contusions (p = 0.089) and Rotterdam CT score (p = 0.065) tended to predict outcome. Numbers of T2*GRE lesions did not affect outcome. Mean ADC values in the normal-appearing corpus callosum did not differ from controls. In conclusion, the loads of visible TAI lesions in the corpus callosum, brain stem, and thalamus in DWI and FLAIR were independent prognostic factors in patients with severe TBI. DWI lesions in the corpus callosum were the most important predictive MRI variable. Interestingly, number of cortical contusions in MRI and CT findings seemed more important for patients with moderate TBI.

Selective impairment of attentional networks of alerting in Wilson's disease

Wilson's disease (WD) is typically affected by attention, which is one of the cognitive domains. The Attention Network Test (ANT) was developed to measure the functioning of the following three individual attentional networks: orienting, alerting, and executive control. The ANT has been used in a variety of neuropsychiatric conditions; however, it has not been used in WD. The aim of this study was to investigate the attentional function of WD patients, and 35 patients with early and moderate neurological WD, as well as 35 gender-, age-, and education-matched healthy controls performed the ANT. Remarkable differences between the patients and healthy controls were observed in the alerting network (p = 0.007) in contrast the differences in the orienting (p = 0.729) and executive control (p = 0.888) networks of visual attention. The mean reaction time in the ANT was significantly longer in the WD patients than in the controls (p<0.001, 0.001). In the WD patients, there was an effect specifically on the alerting domain of the attention network, whereas the orienting and executive control domains were not affected.

Recovery of consciousness is mediated by a network of discrete metastable activity states

It is not clear how, after a large perturbation, the brain explores the vast space of potential neuronal activity states to recover those compatible with consciousness. Here, we analyze recovery from pharmacologically induced coma to show that neuronal activity en route to consciousness is confined to a low-dimensional subspace. In this subspace, neuronal activity forms discrete metastable states persistent on the scale of minutes. The network of transitions that links these metastable states is structured such that some states form hubs that connect groups of otherwise disconnected states. Although many paths through the network are possible, to ultimately enter the activity state compatible with consciousness, the brain must first pass through these hubs in an orderly fashion. This organization of metastable states, along with dramatic dimensionality reduction, significantly simplifies the task of sampling the parameter space to recover the state consistent with wakefulness on a physiologically relevant timescale.

Speculations on the Evolution of Awareness

The “attention schema” theory provides one possible account of the biological basis of consciousness, tracing the evolution of awareness through steps from the advent of selective signal enhancement about half a billion years ago to the top–down control of attention, to an internal model of attention (which allows a brain, for the first time, to attribute to itself that it has a mind that is aware of something), to the ability to attribute awareness to other beings, and from there to the human attribution of a rich spirit world surrounding us. Humans have been known to attribute awareness to plants, rocks, rivers, empty space, and the universe as a whole. Deities, ghosts, souls—the spirit world swirling around us is arguably the exuberant attribution of awareness.

Intralaminar and medial thalamic influence on cortical synchrony, information transmission and cognition

The intralaminar and medial thalamic nuclei are part of the higher-order thalamus, which receives little sensory input, and instead forms extensive cortico-thalamo-cortical pathways. The large mediodorsal thalamic nucleus predominantly connects with the prefrontal cortex, the adjacent intralaminar nuclei connect with fronto-parietal cortex, and the midline thalamic nuclei connect with medial prefrontal cortex and medial temporal lobe. Taking into account this connectivity pattern, it is not surprising that the intralaminar and medial thalamus has been implicated in a variety of cognitive functions, including memory processing, attention and orienting, as well as reward-based behavior. This review addresses how the intralaminar and medial thalamus may regulate information transmission in cortical circuits. A key neural mechanism may involve intralaminar and medial thalamic neurons modulating the degree of synchrony between different groups of cortical neurons according to behavioral demands. Such a thalamic-mediated synchronization mechanism may give rise to large-scale integration of information across multiple cortical circuits, consequently influencing the level of arousal and consciousness. Overall, the growing evidence supports a general role for the higher-order thalamus in the control of cortical information transmission and cognitive processing.

Regional cerebral metabolic patterns demonstrate the role of anterior forebrain mesocircuit dysfunction in the severely injured brain

Although disorders of consciousness (DOCs) demonstrate widely varying clinical presentations and patterns of structural injury, global down-regulation and bilateral reductions in metabolism of the thalamus and frontoparietal network are consistent findings. We test the hypothesis that global reductions of background synaptic activity in DOCs will associate with changes in the pattern of metabolic activity in the central thalamus and globus pallidus. We compared 32 [(18)F]fluorodeoxyglucose PETs obtained from severely brain-injured patients (BIs) and 10 normal volunteers (NVs). We defined components of the anterior forebrain mesocircuit on high-resolution T1-MRI (ventral, associative, and sensorimotor striatum; globus pallidus; central thalamus and noncentral thalamus). Metabolic profiles for BI and NV demonstrated distinct changes in the pattern of uptake: ventral and association striatum (but not sensorimotor) were significantly reduced relative to global mean uptake after BI; a relative increase in globus pallidus metabolism was evident in BI subjects who also showed a relative reduction of metabolism in the central thalamus. The reversal of globus pallidus and central thalamus profiles across BIs and NVs supports the mesocircuit hypothesis that broad functional (or anatomic) deafferentation may combine to reduce central thalamus activity and release globus pallidus activity in DOCs. In addition, BI subjects showed broad frontoparietal metabolic down-regulation consistent with prior studies supporting the link between central thalamic/pallidal metabolism and down-regulation of the frontoparietal network. Recovery of left hemisphere frontoparietal metabolic activity was further associated with command following.

Possible role of GABAergic depolarization in neocortical neurons in generating hyperexcitatory behaviors during emergence from sevoflurane anesthesia in the rat

Hyperexcitatory behaviors occurring after sevoflurane anesthesia are of serious clinical concern, but the underlying mechanism is unknown. These behaviors may result from the potentiation by sevoflurane of GABAergic depolarization/excitation in neocortical neurons, cells implicated in the genesis of consciousness and arousal. The current study sought to provide evidence for this hypothesis with rats, the neocortical neurons of which are known to respond to GABA (γ-aminobutyric acid) with depolarization/excitation at early stages of development (i.e., until the second postnatal week) and with hyperpolarization/inhibition during adulthood. Employing behavioral tests and electrophysiological recordings in neocortical slice preparations, we found: (1) sevoflurane produced PAHBs (post-anesthetic hyperexcitatory behaviors) in postnatal day (P)1–15 rats, whereas it failed to elicit PAHBs in P16 or older rats; (2) GABAergic PSPs (postsynaptic potentials) were depolarizing/excitatory in the neocortical neurons of P5 and P10 rats, whereas mostly hyperpolarizing/inhibitory in the cells of adult rats; (3) at P14–15, <50% of rats had PAHBs and, in general, the cells of the animals with PAHBs exhibited strongly depolarizing GABAergic PSPs, whereas those without PAHBs showed hyperpolarizing or weakly depolarizing GABAergic PSPs; (4) bumetanide [inhibitor of the Cl⁻ importer NKCC (Na⁺–K⁺–2Cl⁻ cotransporter)] treatment at P5 suppressed PAHBs and depolarizing GABAergic responses; and (5) sevoflurane at 1% (i.e., concentration <1 minimum alveolar concentration) potentiated depolarizing GABAergic PSPs in the neurons of P5 and P10 rats and of P14–15 animals with PAHBs, evoking action potentials in ≥50% of these cells. On the basis of these results, we conclude that sevoflurane may produce PAHBs by potentiating GABAergic depolarization/excitation in neocortical neurons.

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.

Centromedian-parafascicular deep brain stimulation induces differential functional inhibition of the motor, associative, and limbic circuits in large animals

BACKGROUND

Deep brain stimulation (DBS) of the centromedian-parafascicular (CM-Pf) thalamic nuclei has been considered an option for treating Tourette syndrome. Using a large animal DBS model, this study was designed to explore the network effects of CM-Pf DBS.

METHODS

The combination of DBS and functional magnetic resonance imaging is a powerful means of tracing brain circuitry and testing the modulatory effects of electrical stimulation on a neuronal network in vivo. With a within-subjects design, we tested the proportional effects of CM and Pf DBS by manipulating current spread and varying stimulation contacts in healthy pigs (n = 5).

RESULTS

Our results suggests that CM-Pf DBS has an inhibitory modulating effect in areas that have been suggested as contributing to impaired sensory-motor and emotional processing. The results also help to define the differential neural circuitry effects of the CM and Pf with evidence of prominent sensorimotor/associative effects for CM DBS and prominent limbic/associative effects for Pf DBS.

CONCLUSIONS

Our results support the notion that stimulation of deep brain structures, such as the CM-Pf, modulates multiple networks with cortical effects. The networks affected by CM-Pf stimulation in this study reinforce the conceptualization of Tourette syndrome as a condition with psychiatric and motor symptoms and of CM-Pf DBS as a potentially effective tool for treating both types of symptoms.

Thalamus, brainstem and salience network connectivity changes during propofol-induced sedation and unconsciousness

In this functional magnetic resonance imaging study, we examined the effect of mild propofol sedation and propofol-induced unconsciousness on resting state brain connectivity, using graph analysis based on independent component analysis and a classical seed-based analysis. Contrary to previous propofol research, which mainly emphasized the importance of connectivity in the default mode network (DMN) and external control network (ECN), we focused on the salience network, thalamus, and brainstem. The importance of these brain regions in brain arousal and organization merits a more detailed examination of their connectivity response to propofol. We found that the salience network disintegrated during propofol-induced unconsciousness. The thalamus decreased connectivity with the DMN, ECN, and salience network, while increasing connectivity with sensorimotor and auditory/insular cortices. Brainstem regions disconnected from the DMN with unconsciousness, while the pontine tegmental area increased connectivity with the insulae during mild sedation. These findings illustrate that loss of consciousness is associated with a wide variety of decreases and increases of both cortical and subcortical connectivity. It furthermore stresses the necessity of also examining resting state connectivity in networks representing arousal, not only those associated with awareness.

Slow-wave activity saturation and thalamocortical isolation during propofol anesthesia in humans

The altered state of consciousness produced by general anesthetics is associated with a variety of changes in the brain's electrical activity. Under hyperpolarizing influences such as anesthetic drugs, cortical neurons oscillate at ~1 Hz, which is measurable as slow waves in the electroencephalogram (EEG). We have administered propofol anesthesia to 16 subjects and found that, after they had lost behavioral responsiveness (response to standard sensory stimuli), each individual's EEG slow-wave activity (SWA) rose to saturation and then remained constant despite increasing drug concentrations. We then simultaneously collected functional magnetic resonance imaging and EEG data in 12 of these subjects during propofol administration and sensory stimulation. During the transition to SWA saturation, the thalamocortical system became isolated from sensory stimuli, whereas internal thalamocortical exchange persisted. Rather, an alternative and more fundamental cortical network (which includes the precuneus) responded to all sensory stimulation. We conclude that SWA saturation is a potential individualized indicator of perception loss that could prove useful for monitoring depth of anesthesia and studying altered states of consciousness.

Thalamic atrophy in antero-medial and dorsal nuclei correlates with six-month outcome after severe brain injury

The primary and secondary damage to neural tissue inflicted by traumatic brain injury is a leading cause of death and disability. The secondary processes, in particular, are of great clinical interest because of their potential susceptibility to intervention. We address the dynamics of tissue degeneration in cortico-subcortical circuits after severe brain injury by assessing volume change in individual thalamic nuclei over the first six-months post-injury in a sample of 25 moderate to severe traumatic brain injury patients. Using tensor-based morphometry, we observed significant localized thalamic atrophy over the six-month period in antero-dorsal limbic nuclei as well as in medio-dorsal association nuclei. Importantly, the degree of atrophy in these nuclei was predictive, even after controlling for full-brain volume change, of behavioral outcome at six-months post-injury. Furthermore, employing a data-driven decision tree model, we found that physiological measures, namely the extent of atrophy in the anterior thalamic nucleus, were the most predictive variables of whether patients had regained consciousness by six-months, followed by behavioral measures. Overall, these findings suggest that the secondary non-mechanical degenerative processes triggered by severe brain injury are still ongoing after the first week post-trauma and target specifically antero-medial and dorsal thalamic nuclei. This result therefore offers a potential window of intervention, and a specific target region, in agreement with the view that specific cortico-thalamo-cortical circuits are crucial to the maintenance of large-scale network neural activity and thereby the restoration of cognitive function after severe brain injury.

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Performed acute and chronic structural MRI in 25 severe TBI patients

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Tensor brain morphometry (TBM) shows localized thalamic acute-to-chronic atrophy.

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Anterior, medio- and lateral-dorsal nuclei are the most significant.

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Atrophy in these nuclei predicts 6-month outcome scores (GOSe).

The evolutionary and genetic origins of consciousness in the Cambrian Period over 500 million years ago

Vertebrates evolved in the Cambrian Period before 520 million years ago, but we do not know when or how consciousness arose in the history of the vertebrate brain. Here we propose multiple levels of isomorphic or somatotopic neural representations as an objective marker for sensory consciousness. All extant vertebrates have these, so we deduce that consciousness extends back to the group's origin. The first conscious sense may have been vision. Then vision, coupled with additional sensory systems derived from ectodermal placodes and neural crest, transformed primitive reflexive systems into image forming brains that map and perceive the external world and the body's interior. We posit that the minimum requirement for sensory consciousness and qualia is a brain including a forebrain (but not necessarily a developed cerebral cortex/pallium), midbrain, and hindbrain. This brain must also have (1) hierarchical systems of intercommunicating, isomorphically organized, processing nuclei that extensively integrate the different senses into representations that emerge in upper levels of the neural hierarchy; and (2) a widespread reticular formation that integrates the sensory inputs and contributes to attention, awareness, and neural synchronization. We propose a two-step evolutionary history, in which the optic tectum was the original center of multi-sensory conscious perception (as in fish and amphibians: step 1), followed by a gradual shift of this center to the dorsal pallium or its cerebral cortex (in mammals, reptiles, birds: step 2). We address objections to the hypothesis and call for more studies of fish and amphibians. In our view, the lamprey has all the neural requisites and is likely the simplest extant vertebrate with sensory consciousness and qualia. Genes that pattern the proposed elements of consciousness (isomorphism, neural crest, placodes) have been identified in all vertebrates. Thus, consciousness is in the genes, some of which are already known.

Molecular concentration of deoxyHb in human prefrontal cortex predicts the emergence and suppression of consciousness

This is the first study to use fNIRS to explore anaesthetic depth and awakening during surgery with general anaesthesia. A 16 channel continuous wave (CW) functional near-infrared system (fNIRS) was used to monitor PFC activity. These outcomes were compared to BIS measures. The results indicate that deoxyHb concentration in the PFC varies during the suppression and emergence of consciousness. During suppression, deoxyHb levels increase, signalling the deactivation of the PFC, while during emergence, deoxyHb concentration drops, initiating PFC activation and the recovery of consciousness. Furthermore, BIS and deoxyHb concentrations in the PFC display a high negative correlation throughout the different anaesthetic phases. These findings suggest that deoxyHb could be a reliable marker for monitoring anaesthetic depth, and that the PFC intervenes in the suppression and emergence of consciousness.

Multiphasic modification of intrinsic functional connectivity of the rat brain during increasing levels of propofol

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.

General anesthesia and human brain connectivity

General anesthesia consists of amnesia, hypnosis, analgesia, and areflexia. Of these, the mechanism of hypnosis, or loss of consciousness, has been the most elusive, yet a fascinating problem. How anesthetic agents suppress human consciousness has been investigated with neuroimaging for two decades. Anesthetics substantially reduce the global cerebral metabolic rate and blood flow with a degree of regional heterogeneity characteristic to the anesthetic agent. The thalamus appears to be a common site of modulation by several anesthetics, but this may be secondary to cortical effects. Stimulus-dependent brain activation is preserved in primary sensory areas, suggesting that unconsciousness cannot be explained by cortical deafferentation or a diminution of cortical sensory reactivity. The effect of general anesthetics in functional and effective connectivity is varied depending on the agent, dose, and network studied. At an anesthetic depth characterized by the subjects' unresponsiveness, a partial, but not complete, reduction in connectivity is generally observed. Functional connectivity of the frontoparietal association cortex is often reduced, but a causal role of this change for the loss of consciousness remains uncertain. Functional connectivity of the nonspecific (intralaminar) thalamic nuclei is preferentially reduced by propofol. Higher-order thalamocortical connectivity is also reduced with certain anesthetics. The changes in functional connectivity during anesthesia induction and emergence do not mirror each other; the recovery from anesthesia may involve increases in functional connectivity above the normal wakeful baseline. Anesthetic loss of consciousness is not a block of corticofugal information transfer, but a disruption of higher-order cortical information integration. The prime candidates for functional networks of the forebrain that play a critical role in maintaining the state of consciousness are those based on the posterior parietal-cingulate-precuneus region and the nonspecific thalamus.

Towards an integrative theory of consciousness: part 1 (neurobiological and cognitive models)

The study of consciousness is poised today at interesting crossroads. There has been a surge of research into various neurobiological underpinnings of consciousness in the past decade. The present article looks at the theories regarding this complex phenomenon, especially the ones that neurobiology, cognitive neuroscience and cognitive psychology have to offer. We will first discuss the origin and etymology of word consciousness and its usage. Neurobiological correlates of consciousness are discussed with structures like the ascending reticular activating system, the amygdala, the cerebellum, the thalamus, the frontoparietal circuits, the prefrontal cortex and the precuneus. The cellular and microlevel theories of consciousness and cerebral activity at the neuronal level contributing to consciousness are highlighted, along with the various theories posited in this area. The role of neuronal assemblies and circuits along with firing patterns and their ramifications for the understanding of consciousness are discussed. A section on the role of anaesthesia and its links to consciousness is presented, along with details of split-brain studies in consciousness and altered states of awareness, including the vegetative states. The article finally discusses the progress cognitive psychology has made in identifying and theorising various perspectives of consciousness, perceptual awareness and conscious processing. Both recent and past researches are highlighted. The importance and salient features of each theory are discussed along with the pitfalls, if present. A need for integration of various theories to understand consciousness from a holistic perspective is stressed, to enable one to reach a theory that explains the ultimate neurobiology of consciousness.

Brain and effort: brain activation and effort-related working memory in healthy participants and patients with working memory deficits

Despite the interest in the neuroimaging of working memory, little is still known about the neurobiology of complex working memory in tasks that require simultaneous manipulation and storage of information. In addition to the central executive network, we assumed that the recently described salience network [involving the anterior insular cortex (AIC) and the anterior cingulate cortex (ACC)] might be of particular importance to working memory tasks that require complex, effortful processing.

METHOD

Healthy participants (n = 26) and participants suffering from working memory problems related to the Kleine-Levin syndrome (KLS) (a specific form of periodic idiopathic hypersomnia; n = 18) participated in the study. Participants were further divided into a high- and low-capacity group, according to performance on a working memory task (listening span). In a functional magnetic resonance imaging (fMRI) study, participants were administered the reading span complex working memory task tapping cognitive effort.

PRINCIPAL FINDINGS

The fMRI-derived blood oxygen level dependent (BOLD) signal was modulated by (1) effort in both the central executive and the salience network and (2) capacity in the salience network in that high performers evidenced a weaker BOLD signal than low performers. In the salience network there was a dichotomy between the left and the right hemisphere; the right hemisphere elicited a steeper increase of the BOLD signal as a function of increasing effort. There was also a stronger functional connectivity within the central executive network because of increased task difficulty.

CONCLUSION

The ability to allocate cognitive effort in complex working memory is contingent upon focused resources in the executive and in particular the salience network. Individual capacity during the complex working memory task is related to activity in the salience (but not the executive) network so that high-capacity participants evidence a lower signal and possibly hence a larger dynamic response.

Thalamic control of layer 1 circuits in prefrontal cortex

Knowledge of thalamocortical (TC) processing comes mainly from studying core thalamic systems that project to middle layers of primary sensory cortices. However, most thalamic relay neurons comprise a matrix of cells that are densest in the "nonspecific" thalamic nuclei and usually target layer 1 (L1) of multiple cortical areas. A longstanding hypothesis is that matrix TC systems are crucial for regulating neocortical excitability during changing behavioral states, yet we know almost nothing about the mechanisms of such regulation. It is also unclear whether synaptic and circuit mechanisms that are well established for core sensory TC systems apply to matrix TC systems. Here we describe studies of thalamic matrix influences on mouse prefrontal cortex using optogenetic and in vitro electrophysiology techniques. Channelrhodopsin-2 was expressed in midline and paralaminar (matrix) thalamic neurons, and their L1-projecting TC axons were activated optically. Contrary to conventional views, we found that matrix TC projections to L1 could transmit relatively strong, fast, high-fidelity synaptic signals. L1 TC projections preferentially drove inhibitory interneurons of L1, especially those of the late-spiking subtype, and often triggered feedforward inhibition in both L1 interneurons and pyramidal cells of L2/L3. Responses during repetitive stimulation were far more sustained for matrix than for core sensory TC pathways. Thus, matrix TC circuits appear to be specialized for robust transmission over relatively extended periods, consistent with the sort of persistent activation observed during working memory and potentially applicable to state-dependent regulation of excitability.

Vagus nerve stimulation to augment recovery from severe traumatic brain injury impeding consciousness: a prospective pilot clinical trial

OBJECTIVES

Traumatic brain injury (TBI) has high morbidity and mortality in both civilian and military populations. Blast and other mechanisms of TBI damage the brain by causing neurons to disconnect and atrophy. Such traumatic axonal injury can lead to persistent vegetative and minimally conscious states (VS and MCS), for which limited treatment options exist, including physical, occupational, speech, and cognitive therapies. More than 60 000 patients have received vagus nerve stimulation (VNS) for epilepsy and depression. In addition to decreased seizure frequency and severity, patients report enhanced mood, reduced daytime sleepiness independent of seizure control, increased slow wave sleep, and improved cognition, memory, and quality of life. Early stimulation of the vagus nerve accelerates the rate and extent of behavioral and cognitive recovery after fluid percussion brain injury in rats.

METHODS

We recently obtained Food and Drug Administration (FDA) approval for a pilot prospective randomized crossover trial to demonstrate objective improvement in clinical outcome by placement of a vagus nerve stimulator in patients who are recovering from severe TBI. Our hypothesis is that stimulation of the vagus nerve results in increased cerebral blood flow and metabolism in the forebrain, thalamus, and reticular formation, which promotes arousal and improved consciousness, thereby improving outcome after TBI resulting in MCS or VS.

DISCUSSION

If this study demonstrates that VNS can safely and positively impact outcome, then a larger randomized prospective crossover trial will be proposed.

Actigraphy assessments of circadian sleep-wake cycles in the Vegetative and Minimally Conscious States

BACKGROUND

The Vegetative and Minimally Conscious States (VS; MCS) are characterized by absent or highly disordered signs of awareness alongside preserved sleep-wake cycles. According to international diagnostic guidelines, sleep-wake cycles are assessed by means of observations of variable periods of eye-opening and eye-closure. However, there is little empirical evidence for true circadian sleep-wake cycling in these patients, and there have been no large-scale investigations of the validity of this diagnostic criterion.

METHODS

We measured the circadian sleep-wake rhythms of 55 VS and MCS patients by means of wrist actigraphy, an indirect method that is highly correlated with polysomnographic estimates of sleeping/waking.

RESULTS

Contrary to the diagnostic guidelines, a significant proportion of patients did not exhibit statistically reliable sleep-wake cycles. The circadian rhythms of VS patients were significantly more impaired than those of MCS patients, as were the circadian rhythms of patients with non-traumatic injuries relative to those with traumatic injuries. The reliability of the circadian rhythms were significantly predicted by the patients' levels of visual and motor functioning, consistent with the putative biological generators of these rhythms.

CONCLUSIONS

The high variability across diagnoses and etiologies highlights the need for improved guidelines for the assessment of sleep-wake cycles in VS and MCS, and advocates the use of actigraphy as an inexpensive and non-invasive alternative.

Sleeping well

In a study by Cruse et al. published in BMC Medicine, patients with severe brain damage who were in the Vegetative or Minimally Conscious States (VS or MCS, respectively) from traumatic and nontraumatic etiologies had assessments of circadian rhythms using an actigraph, a device worn on a limb to evaluate circadian rhythmicity, in this population. This is a novel approach and is being used as a surrogate for polysomnography and other reference standards. Cruse et al. showed more disruption in circadian rhythms in the VS when compared to the MCS. This suggests that more brain injury occurs in the areas that control circadian rhythmicity in VS than in MCS patients. The study provides opportunities for improved prognostication and rehabilitation strategies in this patient population.

Central thalamic deep brain stimulation for support of forebrain arousal regulation in the minimally conscious state

This chapter considers the use of central thalamic deep brain stimulation (CT/DBS) to support arousal regulation mechanisms in the minimally conscious state (MCS). CT/DBS for selected patients in a MCS is first placed in the historical context of prior efforts to use thalamic electrical brain stimulation to treat the unconscious clinical conditions of coma and vegetative state. These previous studies and a proof of concept result from a single-subject study of a patient in a MCS are reviewed against the background of new population data providing benchmarks of the natural history of vegetative and MCSs. The conceptual foundations for CT/DBS in selected patients in a MCS are then presented with consideration of both circuit and cellular mechanisms underlying recovery of consciousness identified from empirical studies. Directions for developing future generalizable criteria for CT/DBS that focus on the integrity of necessary brain systems and behavioral profiles in patients in a MCS that may optimally response to support of arousal regulation mechanisms are proposed.

Gating of attentional effort through the central thalamus

The central thalamus plays an important role in the regulation of arousal and allocation of attentional resources in the performance of even simple tasks. To assess the contribution of central thalamic neurons to short-term adjustments of attentional effort, we analyzed 166 microelectrode recordings obtained from two rhesus monkeys performing a visuomotor simple reaction time task with a variable foreperiod. Multiunit responses showed maintained firing rate elevations during the variable delay period of the task in ∼24% of recording sites. Simultaneously recorded local field potentials demonstrated significant decreases in power at ∼10-20 Hz and increases in power at 30-100 Hz during the delay period when compared against precue baselines. Comparison of the spectral power of local field potentials during the delay period of correct and incorrect trials showed that, during incorrect trials, similar, but reduced, shifts of spectral power occurred within the same frequency bands. Sustained performance of even simple tasks requires regulation of arousal and attention that combine in the concept of "attentional effort". Our findings suggest that central thalamic neurons regulate task performance through brief changes in firing rates and spectral power changes during task-relevant short-term shifts of attentional effort. Increases in attentional effort may be reflected in changes within the central thalamic local populations, where correct task performance associates with more robust maintenance of firing rates during the delay period. Such ongoing fluctuations of central thalamic activity likely reflect a mix of influences, including variations in moment-to-moment levels of motivation, arousal, and availability of cognitive resources.

Moving toward a generalizable application of central thalamic deep brain stimulation for support of forebrain arousal regulation in the severely injured brain

This review considers the challenges ahead for developing a generalizable strategy for the use of central thalamic deep brain stimulation (CT/DBS) to support arousal regulation mechanisms in the severely injured brain. Historical efforts to apply CT/DBS to patients with severe brain injuries and a proof-of-concept result from a single-subject study are discussed. Circuit and cellular mechanisms underlying the recovery of consciousness are considered for their relevance to the application of CT/DBS, to improve consciousness and cognition in nonprogressive brain injuries. Finally, directions for development, and testing of generalizable criteria for CT/DBS are suggested, which aim to identify neuronal substrates and behavioral profiles that may optimally benefit from support of arousal regulation mechanisms.

The effect of caffeine on working memory load-related brain activation in middle-aged males

Caffeine is commonly consumed in an effort to enhance cognitive performance. However, little is known about the usefulness of caffeine with regard to memory enhancement, with previous studies showing inconsistent effects on memory performance. We aimed to determine the effect of caffeine on working memory (WM) load-related activation during encoding, maintenance and retrieval phases of a WM maintenance task using functional magnetic resonance imaging (fMRI). 20 healthy, male, habitual caffeine consumers aged 40-61 years were administered 100 mg of caffeine in a double-blind placebo-controlled crossover design. Participants were scanned in a non-withdrawn state following a workday during which caffeinated products were consumed according to individual normal use (range = 145-595 mg). Acute caffeine administration was associated with increased load-related activation compared to placebo in the left and right dorsolateral prefrontal cortex during WM encoding, but decreased load-related activation in the left thalamus during WM maintenance. These findings are indicative of an effect of caffeine on the fronto-parietal network involved in the top-down cognitive control of WM processes during encoding and an effect on the prefrontal cortico-thalamic loop involved in the interaction between arousal and the top-down control of attention during maintenance. Therefore, the effects of caffeine on WM may be attributed to both a direct effect of caffeine on WM processes, as well as an indirect effect on WM via arousal modulation. Behavioural and fMRI results were more consistent with a detrimental effect of caffeine on WM at higher levels of WM load, than caffeine-related WM enhancement. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Central thalamic deep brain stimulation to promote recovery from chronic posttraumatic minimally conscious state: challenges and opportunities

BACKGROUND

Central thalamic deep brain stimulation (CT-DBS) may have therapeutic potential to improve behavioral functioning in patients with severe traumatic brain injury (TBI), but its use remains experimental. Current research suggests that the central thalamus plays a critical role in modulating arousal during tasks requiring sustained attention, working memory, and motor function. The aim of the current article is to review the methodology used in the CT-DBS protocol developed by our group, outline the challenges we encountered and offer suggestions for future DBS trials in this population. RATIONAL FOR CT-DBS IN TBI:  CT-DBS may therefore be able to stimulate these functions by eliciting action potentials that excite thalamocortical and thalamostriatal pathways. Because patients in chronic minimally conscious state (MCS) have a very low probability of regaining functional independence, yet often have significant sparing of cortical connectivity, they may represent a particularly appropriate target group for CT-DBS. PIlOT STUDY RESULTS:  We have conducted a series of single-subject studies of CT-DBS in patients with chronic posttraumatic MCS, with 24-month follow-up. Outcomes were measured using the Coma Recovery Scale-Revised as well as a battery of secondary outcome measures to capture more granular changes. Findings from our index case suggest that CT-DBS can significantly increase functional communication, motor performance, feeding, and object naming in the DBS on state, with performance in some domains remaining above baseline even after DBS was turned off.

CONCLUSIONS

The use of CT-DBS in patients in MCS, however, presents challenges at almost every step, including during surgical planning, outcome measurement, and postoperative care. Additionally, given the difficulties of obtaining informed consent from patients in MCS and the experimental nature of the treatment, a robust, scientifically rooted ethical framework is resented for pursuing this line of work.

"Connectomic surgery": diffusion tensor imaging (DTI) tractography as a targeting modality for surgical modulation of neural networks

Deep brain stimulation (DBS) is being used to treat a growing number of neurological disorders. Until recently, DBS has been thought to act mainly by suppressing local neuronal activity, essentially producing a functional lesion. Numerous studies are now demonstrating that DBS has widespread network effects mediated by white matter pathways. The new science of connectomics aims to map the connectivity between brain regions in health and disease. Targeting DBS specifically to pathways which exhibit pathological connectivity could greatly expand the possibilities for treating brain diseases. This brief review examines the current state of brain imaging for visualization of these networks and describes how DBS might be used to restore normal connectivity in pathological states.

What is the role of brain mechanisms underlying arousal in recovery of motor function after structural brain injuries?

PURPOSE OF REVIEW

Standard neurorehabilitation approaches have limited impact on motor recovery in patients with severe brain injuries. Consideration of the contributions of impaired arousal offers a novel approach to understand and enhance recovery.

RECENT FINDINGS

Animal and human neuroimaging studies are elucidating the neuroanatomical bases of arousal and of arousal regulation, the process by which the cerebrum mobilizes resources. Studies of patients with disorders of consciousness have revealed that recovery of these processes is associated with marked improvements in motor performance. Recent studies have also demonstrated that patients with less severe brain injuries also have impaired arousal, manifesting as diminished sustained attention, fatigue, and apathy. In these less severely injured patients, it is difficult to connect disorders of arousal with motor recovery because of a lack of measures of arousal that are independent of motor function.

SUMMARY

Arousal impairment is common after brain injury and likely plays a significant role in recovery of motor function. A more detailed understanding of this connection will help to develop new therapeutic strategies applicable for a wide range of patients. This requires new tools that continuously and objectively measure arousal in patients with brain injury, to correlate with detailed measures of motor performance and recovery.

Brain and behavioral pathology in an animal model of Wernicke's encephalopathy and Wernicke-Korsakoff Syndrome

Animal models provide the opportunity for in-depth and experimental investigation into the anatomical and physiological underpinnings of human neurological disorders. Rodent models of thiamine deficiency have yielded significant insight into the structural, neurochemical and cognitive deficits associated with thiamine deficiency as well as proven useful toward greater understanding of memory function in the intact brain. In this review, we discuss the anatomical, neurochemical and behavioral changes that occur during the acute and chronic phases of thiamine deficiency and describe how rodent models of Wernicke-Korsakoff Syndrome aid in developing a more detailed picture of brain structures involved in learning and memory.