Neuronal deactivation explains decreased cerebellar blood flow in response to focal cerebral ischemia or suppressed neocortical function

Visual motion perception deficits due to cerebellar lesions are paralleled by specific changes in cerebro-cortical activity

Recent anatomical studies have revealed strong cerebellar projections into parietal and prefrontal cortex. These findings suggest that the cerebellum might not only play a functional role in motor control but also cognitive domains, an idea also supported by neuropsychological testing of patients with cerebellar lesions that has revealed specific deficits. The goal of the present study was to test whether or not cognitive impairments after cerebellar damage are resulting from changes in cerebro-cortical signal processing. The detection of global visual motion embedded in noise, a faculty compromised after cerebellar lesions, was chosen as a model system. Using magnetoencephalography, cortical responses were recorded in a group of patients with cerebellar lesions (n = 8) and controls (n = 13) who observed visual motion of varied coherence, i.e., motion strength, presented in the peripheral visual field during controlled stationary fixation. Corroborating earlier results, the patients showed a significant impairment in global motion discrimination despite normal fixation behavior. This deficit was paralleled by qualitative differences in responses recorded from parieto-temporal cortex, including a reduced responsiveness to coherent visual motion and a striking loss of bilateral representations of motion coherence. Moreover, the perceptual thresholds correlated with the cortical representation of motion strength on single subject basis. These results demonstrate that visual motion processing in cerebral cortex critically depends on an intact cerebellum and establish a correlation between cortical activity and impaired visual perception resulting from cerebellar damage.

Chronic electrical stimulation of the contralesional lateral cerebellar nucleus enhances recovery of motor function after cerebral ischemia in rats

Novel neurorehabilitative strategies are needed to improve motor outcomes following stroke. Based on the disynaptic excitatory projections of the dentatothalamocortical pathway to the motor cortex as well as to anterior and posterior cortical areas, we hypothesize that chronic electrical stimulation of the contralesional dentate (lateral cerebellar) nucleus output can enhance motor recovery after ischemia via augmentation of perilesional cortical excitability. Seventy-five Wistar rats were pre-trained in the Montoya staircase task and subsequently underwent left cerebral ischemia with the 3-vessel occlusion model. All survivors underwent stereotactic right lateral cerebellar nucleus (LCN) implantation of bipolar electrodes. Rats were then randomized to 4 groups: LCN stimulation at 10 pps, 20 pps, 50 pps or sham stimulation, which was delivered for a period of 6 weeks. Performance on the Montoya staircase task was re-assessed over the last 4 weeks of the stimulation period. On the right (contralesional) side, motor performance of the groups undergoing sham, 10 pps, 20 pps and 50 pps stimulation was, respectively, 2.5+/-2.7; 2.1+/-2.5; 6.0+/-3.9 (p<0.01) and 4.5+/-3.5 pellets. There was no difference on the left (ipsilesional) side motor performance among the sham or stimulation groups, varying from 15.9+/-6.7 to 17.2+/-2.1 pellets. We conclude that contralesional chronic electrical stimulation of the lateral cerebellar nucleus at 20 pps but not at 10 or 50 pps improves motor recovery in rats following ischemic strokes. This effect is likely to be mediated by increased perilesional cortical excitability via chronic activation of the dentatothalamocortical pathway.

Crossed cerebellar diaschisis in acute stroke detected by dynamic susceptibility contrast MR perfusion imaging

BACKGROUND AND PURPOSE

Crossed cerebellar diaschisis (CCD), the decrease in blood flow and metabolism in the cerebellar hemisphere contralateral to a supratentorial stroke, is frequently reported on positron-emission tomography (PET) and single-photon emission CT (SPECT) but is rarely described with MR perfusion techniques. This study was undertaken to determine the frequency of CCD observed in acute stroke by retrospective review of a research data base of patients with acute stroke evaluated by diffusion-weighted (DWI) and dynamic contrast susceptibility perfusion MR imaging (PWI).

MATERIALS AND METHODS

PWI scans of 301 consecutive patients with acute stroke and positive DWI abnormality from a research data base were reviewed. Contralateral cerebellar hypoperfusion was identified by inspection of time-to-peak (TTP) maps for asymmetry with an absence of cerebellar abnormalities on T2-weighted scans, DWI, or disease of the vertebrobasilar system on MR angiography. In a subset of the cases, quantitative analysis of perfusion scans was performed using an arterial input function and singular value decomposition (SVD) to generate cerebral blood flow (CBF) maps.

RESULTS

A total of 47 of 301 cases (15.61%) met the criteria of CCD by asymmetry of cerebellar perfusion on TTP maps. On quantitative analysis, there was corresponding reduction of CBF by 22.75 +/- 10.94% (range, 7.45% to 52.13%) of the unaffected cerebellar hemisphere).

CONCLUSIONS

MR perfusion techniques can be used to detect CCD, though the frequency presented in this series is lower than that commonly reported in the PET/SPECT literature. Nevertheless, with its role in acute stroke and noninvasive nature, MR perfusion may be a viable alternative to PET or SPECT to study the phenomenon and clinical consequences of supratentorial stroke with CCD.

Spatiotemporal dynamics of perfusion and oximetry during ictal discharges in the rat neocortex

Epileptic events elicit a large focal increase in cerebral blood flow (CBF) to perfuse metabolically active neurons in the focus. Conflicting data exists, however, on whether hemoglobin saturation increases or decreases in the focus and surrounding cortex, and whether CBF increases globally or is decreased in adjacent areas. How these hemodynamic events correlate with actual changes in tissue oxygenation is also not known. Using laser Doppler flowmetry, oxygen microsensors and intrinsic optical imaging spectroscopy, we demonstrate that the dip in hemoglobin in the focus correlates with a profound but temporary decrease in tissue oxygenation despite a large increase in CBF. Furthermore, CBF simultaneously decreases in the cortex immediately adjacent to the focus. These events are then replaced with a longer duration, less focal increase in CBF, cerebral blood volume, and hyperoxygenation, the duration of which correlates with the duration of the seizure. These findings raise the question of whether transient focal hypoxia and vascular steal might contribute to progressive deleterious effects of chronic epilepsy on the adult and developing brain. Possible mechanisms based on recent astrocyte-based models of neurovascular coupling are discussed.

Multimodal functional neuroimaging: integrating functional MRI and EEG/MEG

Noninvasive functional neuroimaging, as an important tool for basic neuroscience research and clinical diagnosis, continues to face the need of improving the spatial and temporal resolution. While existing neuroimaging modalities might approach their limits in imaging capability mostly due to fundamental as well as technical reasons, it becomes increasingly attractive to integrate multiple complementary modalities in an attempt to significantly enhance the spatiotemporal resolution that cannot be achieved by any modality individually. Electrophysiological and hemodynamic/metabolic signals reflect distinct but closely coupled aspects of the underlying neural activity. Combining fMRI and EEG/MEG data allows us to study brain function from different perspectives. In this review, we start with an overview of the physiological origins of EEG/MEG and fMRI, as well as their fundamental biophysics and imaging principles, we proceed with a review of the major advances in the understanding and modeling of neurovascular coupling and in the methodologies for the fMRI-EEG/MEG simultaneous recording. Finally, we summarize important remaining issues and perspectives concerning multimodal functional neuroimaging, including brain connectivity imaging.

Cerebral representations of space and time

A link between perception of time and spatial change is particularly revealed in dynamic conditions. By fMRI, we identified regional segregation as well as overlap in activations related to spatial and temporal processing. Using spatial and temporal anticipation concerning movements of a ball provided a balanced paradigm for contrasting spatial and temporal conditions. In addition, momentary judgments were assessed. Subjects watched a monitor-display with a moving ball that repeatedly disappeared. Ordered in 4 conditions, they indicated either where or when the ball would hit the screen bottom, where it actually disappeared or what its speed was. Analysis with SPM showed posterior parietal activations related to both spatial- and temporal predictions. After directly contrasting these two conditions, parietal activations remained robust in spatial prediction but virtually disappeared in temporal prediction, while additional left cerebellar-right prefrontal and pre-SMA activations in temporal prediction remained unchanged. Speed contrasted to the location of disappearance showed similar parietal decrease with maintained cerebellar-prefrontal activations, but also increased caudate activation. From these results we inferred that parietal-based spatial information was a prerequisite for temporal processing, while prefrontal-cerebellar activations subsequently reflected working memory and feedforward processing for the assessment of differences between past and future spatial states. We propose that a temporal component was extracted from speed, i.e. approximated momentary time, which demarcated minimal intervals of spatial change (defined by neuronal processing time). The caudate association with such interval demarcation provided an argument to integrate concepts of space-referenced time processing and a clock-like processing model.

Task-evoked BOLD responses are normal in areas of diaschisis after stroke

OBJECTIVE

Cerebral infarction can cause diaschisis, a reduction of blood flow and metabolism in areas of the cortex distant from the site of the lesion. Although the functional magnetic resonance imaging (fMRI) blood oxygen level dependent (BOLD) signal is increasingly used to examine the neural correlates of recovery in stroke, its reliability in areas of diaschisis is uncertain.

DESIGN

The effect of chronic diaschisis as measured by resting positron emission tomography on task-evoked BOLD responses during word-stem completion in a block design fMRI study was examined in 3 patients, 6 months after a single left hemisphere stroke involving the inferior frontal gyrus and operculum.

RESULTS

The BOLD responses were minimally affected in areas of chronic diaschisis.

CONCLUSIONS

Within the confines of this study, the mechanism underlying the BOLD signal, which includes a mismatch between neuronally driven increases in blood flow and a corresponding increase in oxygen use, appears to be intact in areas of chronic diaschisis.

Central thalamic contributions to arousal regulation and neurological disorders of consciousness

This review focuses on the contributions of the central thalamus to normal mechanisms of arousal regulation and to neurological disorders of consciousness. Forebrain arousal is regulated by ascending influences from brainstem/basal forebrain neuronal populations ("arousal systems") and control signals descending from frontal cortical systems. These subcortical and cortical systems have converging projections to the central thalamus that emphasize their role in maintaining organized behavior during wakefulness. Central thalamic neurons appear to be specialized both anatomically and physiologically to support distributed network activity that maintains neuronal firing patterns across long-range cortico-cortical pathways and within cortico-striatopallidal-thalamocortical loop connections. Recruitment of central thalamic neurons occurs in response to increasing cognitive demand, stress, fatigue, and other perturbations that reduce behavioral performance. In addition, the central thalamus receives projections from brainstem pathways evolved to rapidly generate brief shifts of arousal associated with the appearance of salient stimuli across different sensory modalities. Through activation of the central thalamus, neurons across the cerebral cortex and striatum can be depolarized and their activity patterns selectively gated by descending or ascending signals related to premotor attention and alerting stimuli. Direct injury to the central thalamus or prominent deafferentation of these neurons as a result of complex, multifocal, brain insults are both associated with severe impairment of forebrain functional integration and arousal regulation. Interventions targeting neurons within the central thalamus may lead to rational therapeutic approaches to the treatment of impaired arousal regulation following nonprogressive brain injuries. A model accounting for present therapeutic strategies is proposed.

Crossed cerebellar diaschisis in acute ischemic stroke: a study with serial SPECT and MRI

This study evaluated the relationship between crossed cerebellar diaschisis (CCD) and (1) lesion volume and location in the acute phase and 1 week after stroke onset and (2) clinical outcome. Twenty-two patients with cerebral ischemic stroke underwent single-photon emission computed tomography (SPECT) and magnetic resonance imaging (MRI) within 48 h and on day 8 from onset. Interhemispheric asymmetric indices (AI) on SPECT were calculated for medial, intermediate, and lateral zones of the cerebellum. Lesion volumes and locations were obtained from diffusion-weighted MRI. Neurological status and 3-month clinical outcome were evaluated. Within 48 h, lesion locations in the temporal association cortex and pyramidal tract of the corona radiata were independent determinants for the AI of the medial zone (R(2)=0.439). Lesion locations in the primary, premotor, and supplementary motor cortices, primary somatosensory cortex, and anterior part of the posterior limb of the internal capsule were determinants for the AI of the intermediate zone (R(2)=0.785). Lesions in the primary motor cortex, premotor, and supplementary motor cortices and in the genu of the internal capsule were determinants for the AI of the lateral zone (R(2)=0.746). On day 8, the associations were decreased. The AIs of the intermediate and lateral zones and lesion location in the parietal association cortex were independently associated with the 3-month clinical outcome (R(2)>0.555). Acute CCD is a result of functional deafference, while in the subacute phase, transneuronal degeneration might contribute to CCD. CCD in the intermediate and later zones is a better indicator than that in the medial zone.

Moderate loss of cerebellar Purkinje cells after chronic bilateral common carotid artery occlusion in rats

Pathological effects of moderate ischemia (oligemia, hypoperfusion) are relevant in relation to vascular factors in dementia. Chronic bilateral common carotid artery occlusion (BCCAO) in adult Wistar rats induces oligemia and leads to acute changes in gene expression, subacute changes in cortical astrocytes and prolonged changes in white matter tracts, while largely sparing neurons in the forebrain areas. Dilation and remodeling of the basilar artery ensures blood flow to the forebrain. The present study examined the hypoxia-sensitive Purkinje cells in the cerebellum after 6 months of BCCAO using conventional neuropathological analysis, immunohistochemistry and high-precision design-based stereologic methods. Purkinje cells in the vermis region revealed abnormally shaped nuclei. A stereologic analysis showed that the mean total number of Purkinje cells within the vermis was statistically significantly smaller in the BCCAO animals than in the control animals (d = 11.8%; P < 0.0001). BCCAO had no significant effect on the mean volumes of the molecular layer, granule cell layer and white matter in the vermis or the entire cerebellum. Remodeling of the basilar artery indicated that secondary vascular perturbations might be responsible for the effects of BCCAO on the cerebellar Purkinje cells.

Self awareness and speech processing: An fMRI study

Language production and perception imply motor system recruitment. Therefore, language should obey the theory of shared motor representation between self and other, by means of mirror-like systems. These mirror-like systems (referring to single-unit recordings in animals) show the property to be recruited both when accomplishing and when perceiving a goal-directed action, whatever the sensory modality may be. This hypothesis supposes that a neural network for self-awareness is involved to distinguish speech production from speech listening. We used fMRI to test this assumption in 12 healthy subjects, who performed two different block-design experiments. The first experiment showed involvement of a lateral mirror-like network in speech listening, including ventral premotor cortex, superior temporal sulcus and the inferior parietal lobule (IPL). The activity of this mirror-like network is associated with the perception of an intelligible speech. The second experiment looked at a self-awareness network. It showed involvement of a medial resting-state network, including the medial parietal and medial prefrontal cortices, during the 'self-generated voice' condition, as opposed to passive speech listening. Our results support the fact that deactivation of this medial network, in association with modulation of the activity of the IPL (part of the mirror-like network previously described), is linked to self-awareness in speech processing. Overall, these results support the idea that self-awareness is present when distinguishing between speech production and speech listening situations, and may depend on these two different parieto-frontal networks.

Functional neuroimaging applications for assessment and rehabilitation planning in patients with disorders of consciousness

OBJECTIVE

To describe the theoretic framework, design, and potential clinical applications of functional neuroimaging protocols in patients with disorders of consciousness.

DATA SOURCES

Recent published literature and authors' own work.

STUDY SELECTION

Studies using functional neuroimaging techniques to investigate cognitive processing in patients diagnosed with vegetative and minimally conscious state.

DATA EXTRACTION

Not applicable.

DATA SYNTHESIS

Positron-emission tomography activation studies suggest that the vegetative state represents a global disconnection syndrome in which higher order association cortices are functionally disconnected from primary cortical areas. In contrast, patterns of activation in functional magnetic resonance imaging studies of patients in the minimally conscious state show preservation of large-scale cortical networks associated with language and visual processing.

CONCLUSIONS

Novel applications of functional neuroimaging in patients with disorders of consciousness may aid in differential diagnosis, prognostic assessment and identification of pathophysiologic mechanisms. Improvements in patient characterization may, in turn, provide new opportunities for restoration of function through interventional neuromodulation.

The contribution of the left and right hemispheres to early recovery from aphasia: A SPECT prospective study

This prospective study examined the relationship between post-stroke recovery of aphasia and changes in cerebral blood flow (CBF). To address the question of right hemisphere (RH) involvement in restitution of language, we tested the hypothesis that the increase in perfusion of the RH is crucial for early recovery from aphasia. Twenty-four right-handed patients with acute aphasia following left hemisphere (LH) ischaemic stroke were examined twice with a six-month interval. At each session CBF and language scores were measured on the same stroke patients. Language was measured by selected tasks derived from the Boston Diagnostic Aphasia Examination (BDAE). The SPECT scans were obtained using (99m)Tc-ECD on a triple-head gamma camera Multispect-3. Although initial CBF measured for the whole group of aphasic patients was not a predictor for future language recovery for either hemisphere, increased perfusion of the RH during a six-month interval was found to parallel the recovery of aphasic disorders. There was a correlation between the change in the right parietal CBF (but not the left) and a change in numerous language abilities. Nevertheless, only CBF values on the left predicted performance on the language tests at initial and follow-up examinations. When the area damaged on structural imaging was excluded from perfusion analysis, only subcortical CBF change on the left showed a positive correlation with language improvement. Thus, the cerebral mechanism associated with early recovery from aphasia is a dynamic and complex process that may involve both hemispheres. Probably this mechanism involves functional reorganisation in the speech-dominant (damaged) hemisphere and regression of haemodynamic disturbances in the non-dominant (structurally intact) hemisphere.

Crossed cerebellar diaschisis: a positron emission tomography study withl-[methyl-11C]methionine and 2-deoxy-2-[18F]fluoro-d-glucose

OBJECTIVE

Crossed cerebellar diaschisis (CCD) is defined as a depression of blood flow and oxidative metabolism of glucose in the cerebellum contralateral to a supratentorial brain lesion, as detected with positron emission tomography (PET) and single photon emission computed tomography. We examined whether L-[methyl-11C]methionine (MET) uptake is affected in CCD.

METHODS

In 12 patients with a unilateral supratentorial brain tumor, we evaluated the uptake of 2-deoxy-2-[18F]fluoro-D-glucose (FDG) and MET in the cerebellar hemispheres by means of PET. Asymmetry index (AI) was defined as a difference in the average count between the ipsilateral and contralateral cerebellar hemispheres divided by the average count in both cerebellar hemispheres. Patients with AI of FDG PET more than 0.1 and those with AI equal to 0.1 or less than 0.1 were classified as CCD-positive and CCD-negative, respectively.

RESULTS

Six patients were CCD-positive and others were CCD-negative in the FDG PET study. Between CCD-positive and CCD-negative patients, mean AI of MET was not significantly different (0.017 +/- 0.023 and 0.014 +/- 0.039, respectively).

CONCLUSIONS

Different from glucose metabolism, cerebellar MET uptake was not affected in CCD. The present study may indicate that cerebellar MET uptake is independent of suppression of cerebellar neuronal activity.

Measurements and models of cerebral function in the severely injured brain

We review the emerging applications of functional and structural neuroimaging techniques for the assessment of patients with disorders of consciousness. Measurements of brain function from patients in the vegetative state (VS) and minimally conscious state (MCS) are compared, and a conceptual organization is developed that suggests models of brain mechanisms associated with different functional levels of recovery. We emphasize developing strategies to place complex brain injuries on a more equal footing using global and regional quantification of resting or activated brain activity using functional imaging techniques alongside more detailed structural assessments of neuronal integrity and axonal connectivity now available. Preliminary studies from several investigative groups suggest that some MCS patients may harbor a functional reserve in the form of recruitable cerebral networks. These findings support developing systematic characterizations of the severely injured brain and suggest that some patients may benefit from improved diagnostic assessments.

Consequential apoptosis in the cerebellum following injury to the developing rat forebrain

In focal brain lesions, alterations in blood flow and cerebral metabolism can be detected in brain areas remote from the primary injury. The cellular consequences of this phenomenon, originally termed diaschisis, are not fully understood. Here, we report that in two distinct models of forebrain injury, neuronal death in the cerebellum, a site distant to the primary injury, results as consequence of neuronal loss in the forebrain. Fourteen-day-old rats were subjected to unilateral forebrain injury, achieved by either hypoxia-ischemia (right carotid artery ligation and hypoxia) or direct needle injury to brain tissue. At defined times after injury, the presence of apoptosis was investigated by cell morphology, in situ end labeling, electron microscopy and poly-ADP-ribose polymerase (PARP) cleavage. Injury to the rat forebrain following hypoxia-ischemia increased apoptosis in the internal granular and Purkinje cell layers of the cerebellum, a site distant to that of the primary injury. The number of apoptotic cells in the cerebellum was significantly related to cell death in the hippocampus. Similarly, direct needle injury to the forebrain resulted in extensive apoptotic cell death in the cerebellum. These results emphasize the intimate relationship between defined neuronal populations in relatively distant brain areas and suggest a cellular basis for diaschisis.

Correlations of interictal FDG-PET metabolism and ictal SPECT perfusion changes in human temporal lobe epilepsy with hippocampal sclerosis

BACKGROUND

The pathophysiological role of the extensive interictal cerebral hypometabolism in complex partial seizures (CPS) in refractory mesial temporal lobe epilepsy with hippocampal sclerosis (mTLE-HS) is poorly understood. Our aim was to study ictal-interictal SPECT perfusion versus interictal fluorodeoxyglucose (FDG)-PET metabolic patterns.

METHODS

Eleven adults with refractory unilateral mTLE-HS, who were rendered seizure free after epilepsy surgery, were included. All had an interictal FDG-PET and an interictal and ictal perfusion SPECT scan. FDG-PET data were reconstructed using an anatomy-based reconstruction algorithm, which corrected for partial volume effects, and analyzed semi-quantitatively after normalization to white matter activity. Using Statistical Parametric Mapping (SPM), we compared interictal metabolism of the patient group with a control group. We correlated metabolic with ictal perfusion changes in the patient group.

RESULTS

Global cerebral grey matter glucose metabolism in patients was decreased 10-25% compared with control subjects. Interictal PET hypometabolism and ictal SPECT hypoperfusion were maximal in the ipsilateral frontal lobe. Ictal frontal lobe hypoperfusion was associated with crossed cerebellar diaschisis. The ipsilateral temporal lobe showed maximal ictal hyperperfusion and interictal hypometabolism, which was relatively mild compared with the degree of hypometabolism affecting the frontal lobes.

CONCLUSION

Interictal hypometabolism in mTLE-HS was greatest in the ipsilateral frontal lobe and represented a seizure-related dynamic process in view of further ictal decreases. Crossed cerebellar diaschisis suggested that there is a strong ipsilateral frontal lobe inhibition during CPS. We speculate that surround inhibition in the frontal lobe is a dynamic defense mechanism against seizure propagation, and may be responsible for functional deficits observed in mTLE.

Acute systemic complications in the preterm fetus after asphyxia: role of cardiovascular and blood flow responses

1. Poor perfusion of the kidneys and gut, and associated functional impairment, are major problems in the first days of life in very preterm infants. These complications can be associated with a substantial mortality and further problems such as reduced kidney growth and chronic renal problems in later childhood. 2. There is very little information, and consequently considerable debate, about how or even whether to improve perfusion of the vital organs of this most vulnerable group of babies. Current treatments simply do not consistently improve babies' perfusion generally or kidney and gut perfusion and function in particular. 3. In this review we critically examine clinical and experimental evidence that suggests that exposure to low oxygen levels before and during birth may be a significant contributor to impaired systemic perfusion, and highlight areas requiring further research. 4. This knowledge is essential to develop and refine ways of improving perfusion of the kidneys and other vital organs in premature babies.

Negative functional MRI response correlates with decreases in neuronal activity in monkey visual area V1

Most functional brain imaging studies use task-induced hemodynamic responses to infer underlying changes in neuronal activity. In addition to increases in cerebral blood flow and blood oxygenation level-dependent (BOLD) signals, sustained negative responses are pervasive in functional imaging. The origin of negative responses and their relationship to neural activity remain poorly understood. Through simultaneous functional magnetic resonance imaging and electrophysiological recording, we demonstrate a negative BOLD response (NBR) beyond the stimulated regions of visual cortex, associated with local decreases in neuronal activity below spontaneous activity, detected 7.15 +/- 3.14 mm away from the closest positively responding region in V1. Trial-by-trial amplitude fluctuations revealed tight coupling between the NBR and neuronal activity decreases. The NBR was associated with comparable decreases in local field potentials and multiunit activity. Our findings indicate that a significant component of the NBR originates in neuronal activity decreases.

Relationship between evolving epileptiform activity and delayed loss of mitochondrial activity after asphyxia measured by near-infrared spectroscopy in preterm fetal sheep

Early onset cerebral hypoperfusion after birth is highly correlated with neurological injury in premature infants, but the relationship with the evolution of injury remains unclear. We studied changes in cerebral oxygenation, and cytochrome oxidase (CytOx) using near-infrared spectroscopy in preterm fetal sheep (103-104 days of gestation, term is 147 days) during recovery from a profound asphyxial insult (n= 7) that we have shown produces severe subcortical injury, or sham asphyxia (n= 7). From 1 h after asphyxia there was a significant secondary fall in carotid blood flow (P < 0.001), and total cerebral blood volume, as reflected by total haemoglobin (P < 0.005), which only partially recovered after 72 h. Intracerebral oxygenation (difference between oxygenated and deoxygenated haemoglobin concentrations) fell transiently at 3 and 4 h after asphyxia (P < 0.01), followed by a substantial increase to well over sham control levels (P < 0.001). CytOx levels were normal in the first hour after occlusion, was greater than sham control values at 2-3 h (P < 0.05), but then progressively fell, and became significantly suppressed from 10 h onward (P < 0.01). In the early hours after reperfusion the fetal EEG was highly suppressed, with a superimposed mixture of fast and slow epileptiform transients; overt seizures developed from 8 +/- 0.5 h. These data strongly indicate that severe asphyxia leads to delayed, evolving loss of mitochondrial oxidative metabolism, accompanied by late seizures and relative luxury perfusion. In contrast, the combination of relative cerebral deoxygenation with evolving epileptiform transients in the early recovery phase raises the possibility that these early events accelerate or worsen the subsequent mitochondrial failure.

Post-hypoxic hypoperfusion is associated with suppression of cerebral metabolism and increased tissue oxygenation in near-term fetal sheep

Secondary cerebral hypoperfusion is common following perinatal hypoxia-ischaemia. However, it remains unclear whether this represents a true failure to provide sufficient oxygen and nutrients to tissues, or whether it is simply a consequence of reduced cerebral metabolic demand. We therefore examined the hypothesis that cerebral oxygenation would be reduced during hypoperfusion after severe asphyxia, and further, that the greater neural injury associated with blockade of the adenosine A(1) receptor during the insult would be associated with greater hypoperfusion and deoxygenation. Sixteen near-term fetal sheep received either vehicle or 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) for 1 h, followed by 10 min of severe asphyxia induced by complete occlusion of the umbilical cord. Infusions were discontinued at the end of the occlusion and data were analysed for the following 8 h. A transient, secondary fall in carotid artery blood flow and laser Doppler flow was seen from approximately 1-4 h after occlusion (P < 0.001), with no significant differences between vehicle and DPCPX. Changes in laser Doppler blood flow were highly correlated with carotid blood flow (r(2)= 0.81, P < 0.001). Cortical metabolism was suppressed, reaching a nadir 1 h after occlusion and then resolving. Cortical tissue P(O(2)) was significantly increased at 1, 2 and 3 h after occlusion compared to baseline, and inversely correlated with carotid blood flow (r(2)= 0.69, P < 0.001). In conclusion, contrary to our initial hypothesis, delayed posthypoxic hypoperfusion was associated with suppression of cerebral metabolism and increased tissue P(O(2)), and was not significantly affected by preceding adenosine A1 blockade. These data suggest that posthypoxic hypoperfusion is actively mediated and reflects suppressed cerebral metabolism.

Deep brain stimulation

Deep brain stimulation (DBS) has provided remarkable benefits for people with a variety of neurologic conditions. Stimulation of the ventral intermediate nucleus of the thalamus can dramatically relieve tremor associated with essential tremor or Parkinson disease (PD). Similarly, stimulation of the subthalamic nucleus or the internal segment of the globus pallidus can substantially reduce bradykinesia, rigidity, tremor, and gait difficulties in people with PD. Multiple groups are attempting to extend this mode of treatment to other conditions. Yet, the precise mechanism of action of DBS remains uncertain. Such studies have importance that extends beyond clinical therapeutics. Investigations of the mechanisms of action of DBS have the potential to clarify fundamental issues such as the functional anatomy of selected brain circuits and the relationship between activity in those circuits and behavior. Although we review relevant clinical issues, we emphasize the importance of current and future investigations on these topics.

Crossed cerebellar diaschisis in acute human stroke: a PET study of serial changes and response to supratentorial reperfusion

Crossed cerebellar diaschisis (CCD) is well described in the chronic phase of stroke, but few data describe acute CCD and its serial changes after reperfusion. Using positron emission tomography (PET), we studied acute CCD with respect to supratentorial perfusion and outcome measures. In 19 acute stroke patients receiving intravenous thrombolysis (<3 h), 15O-water PET assessed CCD and supratentorial hypoperfusion volume before thrombolysis, 3, 24 h and 14 days later. Infarct volume at day 14 and NIHSS score at 3 months were assessed. Supratentorial hypoperfusion decreased from 25 cm3 (median) before thrombolysis to 0.1 cm3 at day 14. Baseline CCD was 13.4% and decreased continuously to 6.1% after 14 days. The NIHSS score decreased from 11 to 4 pts after 3 months. Infarct volume was 1.1 cm3. Crossed cerebellar diaschisis correlated to the hypoperfusion volume within the first 24 h after stroke, but not later. Hypoperfusion correlated to outcome measures at the early stage only. In contrast, CCD correlated to outcome values at all four measurements. Reperfusion with recovery of CCD was seen in patients with small infarcts and good clinical outcome and vice versa. Our data suggest that (i) CCD occurs as early as 3 h after stroke and might be reversible; (ii) acute CCD is closely related to the volume of supratentorial hypoperfusion. At later time points, however, CCD is disconnected from supratentorial perfusion but strongly associated to outcome measures; (iii) CCD is not susceptible to non-nutritional reperfusion and adds valuable information to interpret supratentorial reperfusion patterns.

Transient NMDA receptor-mediated hypoperfusion following umbilical cord occlusion in preterm fetal sheep

Exposure to severe hypoxia leads to delayed cerebral and peripheral hypoperfusion. There is evidence in the very immature brain that transient abnormal glutaminergic receptor activity can occur during this phase of recovery. We therefore examined the role of N-methyl-D-aspartate (NMDA) receptor activity in mediating secondary hypoperfusion in preterm fetal sheep at 70% of gestation. Fetuses received either sham asphyxia or asphyxia and were studied for 12 h recovery. The specific, non-competitive NMDA receptor antagonist dizocilpine maleate (2 mg kg-1 bolus plus 0.07 mg kg h-1i.v.) or saline (vehicle) was infused from 15 min after asphyxia until 4 h. In the asphyxia-vehicle group abnormal epileptiform EEG transients were observed during the first 4 h of reperfusion, the peak of which corresponded approximately to the nadir in peripheral and cerebral hypoperfusion. Dizocilpine significantly suppressed this activity (2.7+/-1.3 versus 11.2+/-2.7 counts min-1 at peak frequency, P<0.05) and markedly delayed and attenuated the rise in vascular resistance in both peripheral and cerebral vascular beds observed after asphyxia, effectively preventing the initial deep period of hypoperfusion in carotid blood flow and femoral blood flow (P<0.01). However, while continued infusion did attenuate subsequent transient tachycardia, it did not prevent the development of a secondary phase of persistent but less profound hypoperfusion. In conclusion, the present studies suggest that in the immature brain the initial phase of delayed cerebral and peripheral hypoperfusion following exposure to severe hypoxia is mediated by NMDA receptor activity. The timing of this effect in the cerebral circulation corresponds closely to abnormal EEG activity, suggesting a pathological glutaminergic activation that we speculate is related to evolving brain injury.

Being a self: considerations from functional imaging

Having a self is associated with important advantages for an organism. These advantages have been suggested to include mechanisms supporting elaborate capacities for planning, decision-making, and behavioral control. Acknowledging such functionality offers possibilities for obtaining traction on investigation of neural correlates of self-hood. A method that has potential for investigating some of the brain-based properties of self arising in behavioral contexts varying in requirements for such behavioral guidance and control is functional brain imaging. Data obtained with this method are beginning to converge on a set of brain areas that appear to play a significant role in permitting conscious access to representational content having reference to self as an embodied and independent experiencer and agent. These areas have been identified in a variety of imaging contexts ranging from passive state conditions in which they appear to manifest ongoing activity associated with spontaneous and typically 'self-related' cognition, to tasks targeting explicitly experienced properties of self, to demanding task conditions where activity within them is attenuated in apparent redirection of cognitive resources in the service of task guidance and control. In this paper, these data will be reviewed and a hypothesis presented regarding a significant role for these areas in enabling degrees of self-awareness and participating in the management of such behavioral control.

Reading vascular changes in brain imaging: is dendritic calcium the key?

A key goal in functional neuroimaging is to use signals that are related to local changes in metabolism and blood flow to track the neuronal correlates of mental activity. Recent findings indicate that the dendritic processing of excitatory synaptic inputs correlates more closely than the generation of spikes with brain imaging signals. The correlation is often nonlinear and context-sensitive, and cannot be generalized for every condition or brain region. The vascular signals are mainly produced by increases in intracellular calcium in neurons and possibly astrocytes, which activate important enzymes that produce vasodilators to generate increments in flow and the positive blood oxygen level dependent signal. Our understanding of the cellular mechanisms of functional imaging signals places constraints on the interpretation of the data.

Modeling the minimally conscious state: measurements of brain function and therapeutic possibilities

The minimally conscious state (MCS) defines a functional level of recovery following severe brain injuries. Patients in MCS demonstrate unequivocal evidence of response to their environment yet fail to recover the ability to communicate. Drawing on recent functional brain-imaging studies, pathological data, and neurophysiological investigations, models of brain function in MCS are proposed. MCS models are compared and contrasted with models of the vegetative state (VS), a condition characterized by wakeful appearance and unconsciousness. VS reflects a total loss of cognitive function and failure to recover basic aspects of the normal physiologic brain state associated with wakefulness. MCS may represent a recovery of the minimal dynamic architecture required to organize behavioral sets and respond to sensory stimuli. Several pathophysiological mechanisms that might limit further recovery in MCS patients are considered. Implications for future research directions and possible therapeutic strategies are reviewed.

Crossed cerebellar hypoperfusion in hyperacute ischemic stroke

OBJECTIVE

In chronic stage of cerebral hemispheric infarction, contralateral cerebellar blood flow and metabolism are depressed, which is known as crossed cerebellar diaschisis (CCD). The present study was performed to elucidate (1) whether the diaschisis occurs in hyperacute stage of ischemic stroke when computed tomography (CT) scans is not able to identify infarction, and (2) which site of lesion in the cerebrum is responsible for the depression in contralateral cerebellar blood flow.

METHODS

Single photon emission computed tomography was performed in 21 patients with middle cerebral artery (MCA) embolic infarction within 6 h of the onset (3.2+/-1.1 h, mean+/-S.D.). Regions of interest (ROIs) were symmetrically located in the cerebral hemispheres including cerebral cortex and subcortex, and in the cerebellar hemispheres.

RESULTS

The side-to-side ratio of cerebellar blood flow ipsilateral to that contralateral to cerebral infarct was significantly increased compared with that in normal control (P<0.001), indicating that contralateral cerebellar blood flow was significantly depressed. In hyperacute stage, the ratio of cerebellar blood flow appeared to be associated with the ratio of cerebral blood flow in whole hemispheres (r=0.44, P<0.05), in anterior frontal lobe (r=0.44, P<0.05) and in anterior temporal lobe (r=0.58, P<0.01), but not in infarct areas (r=0.26, P=0.3). Stepwise regression analysis revealed that the ratios in cerebellar hemispheres were associated with those in anterior temporal lobe (multiple regression analysis, r=0.58, P<0.01).

CONCLUSIONS

Crossed cerebellar diaschisis occurs at hyperacute stage of stroke of the MCA infarction. It may be related to the hypoperfusion in the anterior frontal and anterior temporal lobes of the cerebrum where regional blood flow is decreased by ischemic infarction per se or by ipsilateral hemispheric depression from infarct area (diaschisis mechanism).

Ictal SPECT

The localizing value of ictal single-photon emission computed tomography (SPECT) performed with cerebral blood flow agents in patients with epilepsy is based on cerebral metabolic and perfusion coupling. Ictal hyperperfusion is used to localize the epileptogenic zone noninvasively, and is particularly useful in magnetic resonance (MR)-negative partial epilepsy and focal cortical dysplasias. Subtraction ictal SPECT coregistered with MRI (SISCOM) improves the localization of the area of hyperperfusion. Ictal SPECT should always be interpreted in the context of a full presurgical evaluation. Early ictal SPECT injections minimize the problem of seizure propagation and of nonlocalization due to an early switch from ictal hyperperfusion to postictal hypoperfusion during brief extratemporal seizures. The degree of thresholding of SISCOM images affects the sensitivity and specificity of ictal SPECT. Ictal hypoperfusion may reflect ictal inhibition or deactivation. Postictal and interictal SPECT studies are less useful to localize the ictal-onset zone. Statistical parametric mapping analysis of groups of selected ictal-interictal difference images has the potential to demonstrate the evolution of cortical, subcortical, and cerebellar perfusion changes during a particular seizure type, to study seizure-gating mechanisms, and to provide new insights into the pathophysiology of seizures.

Focal changes of oxygen consumption in cerebral cortex of patients with Parkinson's disease during subthalamic stimulation

Motor symptoms of Parkinson's disease (PD) are substantially improved by bilateral high-frequency electrical stimulation of the subthalamic nucleus (STN). Altered cerebral blood flow (CBF) in a network of frontal cortical and subcortical structures has been reported in numerous studies of patients undergoing subthalamic stimulation. However, CBF is a controversial indicator of brain activation because measures of blood flow bear a variable relation to measures of brain work and energy metabolism. We hypothesized that STN stimulation would alter the rate of oxygen consumption (CMRO(2)) in cerebral cortical areas in proportion to previously reported changes in CBF in patients undergoing stimulation at rest. We used quantitative PET to map CMRO(2) in brain of seven patients with Parkinson's disease, first in a baseline condition with pause of stimulation and medication for a period of 12 h, and again after 4 h of stimulation. Comparison of these two conditions revealed activation of CMRO(2) in the cerebellum, and in specific posterior neocortical regions, most notably in the left lingual gyrus and in the right lateral occipitotemporal gyrus, both of which latter regions are linked to higher-order visual processing. CMRO(2) was unaffected in the frontal cortex. Thus, the present findings do not support the original hypothesis, but suggest that STN stimulation increases energy metabolism in the posterior cerebral cortex, especially in regions involved in perception of movement and the direction of movement to visual cues.

Qualitative and quantitative imaging of the hippocampus in mesial temporal lobe epilepsy with hippocampal sclerosis

MR imaging allows the in vivo detection of hippocampal sclerosis (HS) and has been instrumental in the delineation of the syndrome of mesial temporal lobe epilepsy with HS (mTLE-HS). MR features of HS include hippocampal atrophy with an increased T2 signal. Quantitative MR imaging accurately reflects the degree of hippocampal damage.Ictal single photon emission computed tomography (SPECT) in mTLE-HS shows typical perfusion patterns of ipsilateral temporal lobe hyperperfusion, and ipsilateral frontoparietal and contralateral cerebellar hypoperfusion. Interictal 18fluoro-2-deoxyglucose positron emission tomography (PET) shows multiregional hypometabolism, involving predominantly the ipsilateral temporal lobe. 11C-flumazenil PET shows hippocampal decreases in central benzodiazepine receptor density. Future strategies to study the etiology and pathogenesis of HS should include longitudinal MR imaging studies,MR studies in families with epilepsy and febrile seizures, stratification for genetic background, coregistration with SPECT and PET, partial volume correction and statistical parametric mapping analysis of SPECT and PET images.

Dopaminergic modulation of response inhibition: an fMRI study

Dopamine has been hypothesized to modulate response inhibition. To test this hypothesis, we used functional magnetic resonance imaging (fMRI) to measure the effects of the dopamine prodrug levodopa on the brain responses to a well-validated response inhibition task (go/no-go, or GNG). Since abnormalities of response inhibition and dopamine have been thought to underlie tics and other symptoms of Tourette syndrome, we studied 8 neuroleptic-naive adults with tic disorders as well as 10 well-matched healthy controls. Subjects were pretreated with the peripheral decarboxylase inhibitor carbidopa, then scanned during GNG and control blocks, both before and during i.v. levodopa infusion. Both groups had similar task performance and task-related regional brain activity before and during levodopa infusion. Levodopa did not affect reaction times or accuracy, so fMRI findings can be interpreted without concern that they simply reflect a performance difference between conditions. Levodopa did affect the magnitude of GNG-related fMRI responses in the right cerebellum and right parietal cortex, significantly reducing both. Pre-levodopa activity in the right cerebellum correlated with reaction times (higher magnitudes associated with faster reaction times), and pre-levodopa activity in the right parietal cortex correlated with false alarm rate (higher magnitudes associated with higher error). In summary, right parietal and cerebellar regions important in mediating specific aspects of the GNG task were modulated by levodopa, suggesting a region-specific role for dopamine in response inhibition.

Principal neuron spiking: neither necessary nor sufficient for cerebral blood flow in rat cerebellum

Neuronal activity, cerebral blood flow, and metabolic responses are all strongly coupled, although the mechanisms behind the coupling remain unclear. One of the key questions is whether or not increases in spiking activity in the stimulated neurons are sufficient to drive the activity-dependent rises in cerebral blood flow (CBF) that form the basis of the signals used in functional neuroimaging such as the blood oxygen level-dependent (BOLD) signal. To this end the present study examined the effect of enhanced spike activity per se on CBF in rat cerebellar cortex under conditions of disinhibition, achieved by blocking GABA(A) receptors using either bicuculline or picrotoxin. Purkinje cell spiking activity and local field potentials were recorded by glass microelectrodes, and laser Doppler flowmetry was used to monitor CBF. Disinhibition increased Purkinje cell spiking rate to 200-300% of control without incurring any increase in basal CBF. This demonstrates that increased spike activity per se is not sufficient to affect basal CBF. The neurovascular coupling between excitatory synaptic activity and CBF responses evoked by inferior olive (climbing fibre) stimulation was preserved during disinhibition. Thus, the unchanged basal CBF in the presence of the dramatic rise in Purkinje cell spiking rate was not explained by impaired synaptic activity-CBF coupling. On the basis of our previous and the present studies, we conclude that increased spiking activity of principal neurons is neither sufficient nor necessary to elicit CBF responses and in turn BOLD signals, and that activation-dependent vascular signals reflect excitatory synaptic activity.

Perfusion- and BOLD-based fMRI in the study of a human pathological model for task-related flow reductions

In the present work, an arteriovenous malformation was taken as a pathological model for studying task-related flow decreases during a motor task. Combined Blood Oxygen Level Dependent (BOLD)-perfusion experiments were applied in order to evaluate the relative sensitivity of these techniques to task-related reductions in cerebral blood flow (CBF). Results shows that, by matching the sensitivity of the methods (which exhibit a different contrast-to-noise ratio) in the primary motor cortex, the spatial extent of the regions of decreased perfusion signal is larger than those of the BOLD signal reduction. The above finding suggests that perfusion imaging, that already represents a gold standard method in the detection of vascular phenomena, may estimate task-related flow decreases in a functional time-series better than BOLD.

Impaired neurovascular coupling by transhemispheric diaschisis in rat cerebral cortex

In acute brain disorders, elimination of the excitatory output from an injured brain region reduces activity in connecting brain regions remote from the lesion site (i.e., diaschisis). The authors examined the effect of functional ablation of the left cerebral cortex by cortical spreading depression (CSD) or topical application of tetrodotoxin on single cell spiking activity, baseline CBF, and neurovascular coupling in the right rat sensory cortex. CSD or tetrodotoxin in left cortex reduced the right cortical spontaneous spike rate by 36% and 45%, respectively. Baseline CBF in the right cortex was unaffected by a left-sided CSD, but decreased by 12% for left cortical application of tetrodotoxin. This suggested dissociation between spontaneous spiking activity and basal CBF. Left in-fraorbital nerve stimulation evoked local field potentials in right cerebral cortex that were reduced in amplitude by 19% for left CSD and by 23% for left tetrodotoxin application. The corresponding declines in the evoked CBF responses were 42% for CSD and 23% for tetrodotoxin. Vascular reactivity to adenosine remained unchanged in right cortex. Thus, transhemispheric diaschisis produced a pronounced decrease in the spontaneous spike rate accompanied by no reduction or a small reduction in basal CBF, and an attenuation in amplitudes of evoked synaptic responses and corresponding rises in CBF. The findings suggest that disturbed neurovascular coupling may contribute to the disturbance in brain function in acute transhemispheric diaschisis.

Spatially dissociated flow-metabolism coupling in brain activation

The relationships among cerebral blood flow (CBF), oxygen consumption (CMRO(2)) and glucose use (CMR(glc)) constitute the basis of functional brain-imaging. Here we report spatially dissociated changes of CMRO(2) and CBF during motor activity that lead us to propose a revision of conventional CBF-CMRO(2) coupling models. In the left primary and supplementary motor cortices, CBF and CMRO(2) rose significantly during finger-thumb tapping. However, in the right putamen CBF did not rise, despite a significant increase in CMRO(2). We explain these observations by invoking a central command mechanism that regulates CBF in the putamen in anticipation of movement. By this mechanism, CBF rose in the putamen before the measurements of CBF and CMRO(2) while CMRO(2) rose when actual motion commenced.

Cognitive-pharmacologic functional magnetic resonance imaging in tourette syndrome: a pilot study

BACKGROUND

Dopamine agonists and antagonists can reduce abnormal movements and vocalizations (tics) in Tourette syndrome (TS); however, dopamine-responsive abnormal function in specific brain regions has not been directly demonstrated in TS. We sought to identify dopamine-modulated brain regions that function abnormally in TS by combining functional magnetic resonance imaging (fMRI), a working memory (WM) task, and infusion of the dopamine prodrug levodopa (while blocking dopamine production outside the brain).

METHODS

We obtained complete fMRI data in 8 neuroleptic-naive adults with a chronic tic disorder and in 10 well-matched tic-free control subjects.

RESULTS

Different task-sensitive brain regions responded differently to the WM task depending on levodopa status and diagnostic group (analysis of variance [ANOVA], p <.001). Four regions showed interactions with diagnosis (ANOVA, p <.001). In TS subjects, the task induced excessive brain activity in parietal cortex, medial frontal gyrus, and thalamus. Levodopa normalized the excess activity. In left parietal cortex, the degree of normalization was greater in patients with higher levodopa plasma concentrations (n = 6; Spearman's r = -.84, p =.04) and a greater degree of diagnostic confidence of TS (r = -.71, p =.05).

CONCLUSIONS

These results are consistent with a dopamine-influenced functional abnormality of brain response in TS and suggest testable hypotheses about the mechanism by which dopamine antagonists and agonists alleviate tics.

The role of the sympathetic nervous system in postasphyxial intestinal hypoperfusion in the pre-term sheep fetus

Asphyxia in utero in pre-term fetuses is associated with evolving hypoperfusion of the gut after the insult. We examined the role of the sympathetic nervous system (SNS) in mediating this secondary hypoperfusion. Gut blood flow changes were also assessed during postasphyxial seizures. Preterm fetal sheep at 70% of gestation (103-104 days, term is 147 days) underwent sham asphyxia or asphyxia induced by 25 min of complete cord occlusion and fetuses were studied for 3 days afterwards. Phentolamine (10 mg bolus plus 10 mg h(-1)i.v.) or saline was infused for 8 h starting 15 min after the end of asphyxia or sham asphyxia. Phentolamine blocked the fall in superior mesenteric artery blood flow (SMABF) after asphyxia and there was a significant decrease in MAP for the first 3 h of infusion (33 +/- 1.6 mmHg versus vehicle 36.7 +/- 0.8 mmHg, P < 0.005). During seizures SMABF fell significantly (8.3 +/- 2.3 versus 11.4 +/- 2.7 ml min(-1), P < 0.005), and SMABF was more than 10% below baseline for 13.0 +/- 1.7 min per seizure (versus seizure duration of 78.1 +/- 7.2 s). Phentolamine was associated with earlier onset of seizures (5.0 +/- 0.4 versus 7.1 +/- 0.7 h, P < 0.05), but no change in amplitude or duration, and prevented the fall in SMABF. In conclusion, the present study confirms the hypothesis that postasphyxial hypoperfusion of the gut is strongly mediated by the SNS. The data highlight the importance of sympathetic activity in the initial elevation of blood pressure after asphyxia and are consistent with a role for the mesenteric system as a key resistance bed that helps to maintain perfusion in other, more vulnerable systems.

Contribution of somatosensory cortex to evoked cerebellar blood flow responses

Projections from the trigeminocerebellar pathway and the somatosensory cortex coincide spatially in the granule cell layer of Crus I/II of the cerebellar hemisphere. A biphasic field potential was seen: one peak at 10 ms (trigeminal input) and another at 20 ms (somatosensory input). Linear correlation analysis revealed only a weak coupling between somatosensory input and cerebellar blood flow responses to infraorbital nerve stimulation. In separate experiments, cortical spreading depression attenuated the field potential peak at 20 ms while blood flow responses remained unaltered. Thus, trigeminocerebellar activity explained the evoked blood flow responses. Our data provide further evidence that activity-dependent blood flow responses are context-sensitive and that interaction between excitatory neuronal circuits targeting the same cells may occlude vascular responses.

Evolution of Diaschisis in a Focal Stroke Model

Background and Purpose— Stroke produces diaschisis in adjacent and connected regions. The sequential changes in diaschisis over time and the relationship of regions of diaschisis to functional cortical areas and regions of poststroke neuroplasticity have not been determined.

Methods— Small cortical strokes were produced in the barrel cortex of rats. Relative glucose metabolism was determined in vivo over time with [ 18 F]fluorodeoxyglucose small-animal positron emission tomography. Cerebral blood flow was measured with [ 14 C]iodoantipyrine. Regions of hypometabolism and hypoperfusion were compared with histological damage in the same animals.

Results— Small cortical strokes produce an initial network of hypometabolism in a broad region of cortex adjacent to the stroke and in the striatum and thalamus on day 1. Cerebral blood flow is diminished only immediately around the cortical infarct on day 1. A substantial area of cortex adjacent to the stroke remains hypometabolic on day 8. This persistent cortical hypometabolism occupies the somatosensory cortex, forelimb motor cortex, and second somatosensory area.

Conclusions— Focal stroke produces ipsilateral diaschisis in connected cortical regions that is clearly distant from subtotal damage and may play a role in poststroke neuroplasticity.

Dissociation of spikes, synaptic activity, and activity-dependent increments in rat cerebellar blood flow by tonic synaptic inhibition

Functional neuroimaging relies on the robust coupling between neuronal activity, metabolism and cerebral blood flow (CBF) to map the brain, but the physiological basis of the neuroimaging signals is still not well understood. Here we applied a pharmacological approach to separate spiking activity, synaptic activity, and the accompanying changes in CBF in rat cerebellar cortex. We report that tonic synaptic inhibition achieved by topical application of gamma-aminobutyric acid type A (GABAA) (muscimol) or GABAB (baclofen) receptor agonists abolished or reduced spontaneous Purkinje cell spiking activity without affecting basal CBF. The magnitude of CBF responses evoked by climbing fiber stimulation decreased gradually over time after exposure to muscimol, being more pronounced in the superficial than in the deep cortical layers. We provide direct evidence in favor of a laminar-specific regulation of CBF in deep cortical layers, independent of dilatation of surface vessels. With prolonged exposure to muscimol, activity-dependent CBF increments disappeared, despite preserved cerebrovascular reactivity to adenosine and preserved local field potentials (LFP). This dissociation of CBF and LFPs suggests that CBF responses are independent of extracellular synaptic currents that generate LFPs. Our work implies that neuronal and vascular signals evoked by glutamatergic pathways are sensitive to synaptic inhibition, and that local mechanisms independent of transmembrane synaptic currents adjust flow to synaptic activity in distinct cortical layers. Our results provide fundamental insights into the functional regulation of blood flow, showing important interference of GABAA receptors in translating excitatory input into blood flow responses.

Quantitative analysis of attention and detection signals during visual search

Prior work has distinguished regions in the intraparietal sulcus (IPs) and frontal eye field (FEF) involved in the voluntary control of attention, from more ventral regions in the temporoparietal junction (TPJ) involved in target detection. The present results show that when subjects search for and detect a visual target stimulus among nontargets, these regions show sensory-, search-, and detection-related signals that both confirm and refine these functional distinctions. The different signals were isolated by an additive model that accounted for a large fraction of BOLD (blood oxygenation level-dependent) signal modulation over the brain. Both IPs and FEF were activated during search through nontargets, consistent with a role in maintaining attention-related signals during search. However, unlike FEF, IPs also showed stimulus-related activations, and may combine signals related to sensory and task-dependent components of salience. Although IPs-FEF showed search-related activations, the TPJ was deactivated during search. TPJ activations were confined to detection-related signals. These results provide a much stronger dissociation between the TPJ and IPs-FEF than previous work, while indicating functional differences between frontal and parietal regions that are often coactivated in studies of attention. Finally, continuous flow models of information processing predict that during search, signals from missed targets should be fed from sensory to associative regions rather than being gated by the decision criterion. Correspondingly, missed targets significantly activated parietal (e.g., right TPJ) and frontal (e.g., anterior insula, anterior cingulate) regions, although with a smaller magnitude than detected targets. Surprisingly, many cortical regions showed equivalent signals from detected targets and the completion of target-absent trials, reflecting a widespread signal unrelated to motor execution.

Long term treatment and disease severity change brain responses to levodopa in Parkinson's disease

OBJECTIVES

Degeneration of nigrostriatal neurons and subsequent striatal dopamine deficiency produce many of the symptoms of Parkinson disease (PD). Initially restoration of striatal dopamine with oral levodopa provides substantial benefit, but with long term treatment and disease progression, levodopa can elicit additional clinical symptoms, reflecting altered effects of levodopa in the brain. The authors examined whether long term treatment affects the brain's response to levodopa in the absence of these altered clinical responses to levodopa.

METHODS

Positron emission tomography (PET) measurements were used of brain-blood flow before and after an acute dose of levodopa in three groups: PD patients treated long term with levodopa without levodopa induced dyskinesias, levodopa naive PD patients, and controls.

RESULTS

It was found that the PD group treated long term responded to acute levodopa differently from controls in left sensorimotor and left ventrolateral prefrontal cortex. In both regions, the treated PD group had decreased blood flow whereas the control group had increased blood flow in response to levodopa. Levodopa naive PD patients had little or no response to levodopa in these regions. Within the treated PD group, severity of parkinsonism correlated with the degree of abnormality of the sensorimotor cortex response, but not with the prefrontal response.

CONCLUSIONS

It is concluded that long term levodopa treatment and disease severity affect the physiology of dopaminergic pathways, producing altered responses to levodopa in brain regions associated with motor function.