Relationship of spikes, synaptic activity, and local changes of cerebral blood flow

Brain work and brain imaging

Functional brain imaging with positron emission tomography and magnetic resonance imaging has been used extensively to map regional changes in brain activity. The signal used by both techniques is based on changes in local circulation and metabolism (brain work). Our understanding of the cell biology of these changes has progressed greatly in the past decade. New insights have emerged on the role of astrocytes in signal transduction as has an appreciation of the unique contribution of aerobic glycolysis to brain energy metabolism. Likewise our understanding of the neurophysiologic processes responsible for imaging signals has progressed from an assumption that spiking activity (output) of neurons is most relevant to one focused on their input. Finally, neuroimaging, with its unique metabolic perspective, has alerted us to the ongoing and costly intrinsic activity within brain systems that most likely represents the largest fraction of the brain's functional activity.

The neural basis of the hemodynamic response nonlinearity in human primary visual cortex: Implications for neurovascular coupling mechanism

It has been well recognized that the nonlinear hemodynamic responses of the blood oxygenation level-dependent (BOLD) functional MRI (fMRI) are important and ubiquitous in a series of experimental paradigms, especially for the event-related fMRI. Although this phenomenon has been intensively studied and it has been found that the post-capillary venous expansion is an intrinsically nonlinear mechanical process, the existence of an additional neural basis for the nonlinearity has not been clearly shown. In this paper, we assessed the correlation between the electric and vascular indices by performing simultaneous electroencephalography (EEG) and fMRI recordings in humans during a series of visual stimulation (i.e., radial checkerboard). With changes of the visual stimulation frequencies (from 0.5 to 16 Hz) and contrasts (from 1% to 100%), both the event related potentials (ERPs) and hemodynamic responses show nonlinear behaviors. In particular, the mean power of the brain electric sources and the neuronal efficacies (as originally defined in the hemodynamics model [Friston et al. Neuroimage, 12, 466-477, 2000], here represent the vascular inputs) in primary visual cortex consistently show a linear correlation for all subjects. This indicates that the hemodynamic response nonlinearity found in this paper primarily reflects the nonlinearity of underlying neural activity. Most importantly, this finding underpins a nonlinear neurovascular coupling. Specifically, it is shown that the transferring function of the neurovascular coupling is likely a power transducer, which integrates the fast dynamics of neural activity into the vascular input of slow hemodynamics.

Modulatory role of cyclooxygenase-2 in cerebrovascular coupling

To investigate the role of cyclooxygenase-2 (COX-2) in the cerebrovascular coupling, hemodynamic and neuronal responses to forepaw stimulation were measured in alpha-chloralose-anesthetized rats (N = 18) before and after intravenous administration of Meloxicam (MEL), a preferential COX-2 inhibitor, and following a bolus of prostaglandin E(2) (PGE(2)), a prominent vasodilatatory product of COX-2 catalyzed metabolism of arachidonic acid. The cerebral blood flow (CBF) and blood-oxygenation-level-dependent (BOLD) response was quantified using continuous arterial spin labeling magnetic resonance imaging. Neuronal activity was measured by recording somatosensory-evoked potentials (SEPs) via intracranial electrodes. Both MEL and PGE(2) had a significant effect on the activation-elicited CBF (P < 10(-6)) and BOLD (P < 10(-6)) responses, without affecting the baseline perfusion. Meloxicam decreased brain COX enzymatic activity by 57 +/- 14% and decreased the stimulation-induced CBF response to 32 +/- 2% and BOLD to 46 +/- 1% of their respective pre-drug amplitudes. In turn, PGE(2) bolus resulted in a partial recovery of functional hyperemia, with the CBF response recovering to 52 +/- 3% and the BOLD response to 56 +/- 2% of their values prior to MEL administration. There was no concomitant decrease in either amplitudes or latencies of SEP components. These findings suggest a modulatory role of COX-2 products in the cerebrovascular coupling and provide evidence for existence of a functional metabolic buffer.

Functional magnetic resonance imaging and c-Fos mapping in rats following an anorectic dose of m-chlorophenylpiperazine

We have used blood-oxygenation-level-dependent (BOLD) contrast functional magnetic resonance imaging (fMRI) to characterise brain regions responsive to a regulator of appetite. An anorectic dose of the 5-HT(1B/2C) receptor agonist m-chlorophenylpiperazine (mCPP; 3 mg/kg s.c.) was used to compare BOLD contrast fMRI with expression of the c-Fos protein. mCPP was administered to rats, which were then anaesthetised and perfused with fixative 90 min later to allow immunohistochemistry. In a separate experiment, rats were imaged using a T(2)*-weighted gradient echo in a 7 T magnet for 70 min under alpha-chloralose anaesthesia. Both methods detected positive activation in areas of the limbic system: cingulate and orbitofrontal cortices, nucleus accumbens, paraventricular and dorsomedial regions of the hypothalamus. fMRI detected increased signal in the pontine nuclei, the hippocampal formation and olfactory cortex, areas that did not display c-Fos. In addition, BOLD signal was diminished in the ventral tegmental area, preoptic area and the cerebellum-presumably due to decreased neuronal signalling and, therefore, unlikely to display c-Fos. Activity in the limbic system may reflect the appetitive agonist activity of mCPP at the 5-HT(2C) receptor. We conclude that c-Fos provides excellent spatial information but is less useful for detecting inhibited regions, whereas fMRI provides greater temporal resolution. Thus, the two methodologies provide complementary details of brain activity following pharmacological challenge.

Sustained activity in topographic areas of human posterior parietal cortex during memory-guided saccades

In a previous study, we identified three cortical areas in human posterior parietal cortex that exhibited topographic responses during memory-guided saccades [visual area 7 (V7), intraparietal sulcus 1 (IPS1), and IPS2], which are candidate homologs of macaque parietal areas such as the lateral intraparietal area and parietal reach region. Here, we show that these areas exhibit sustained delay-period activity, a critical physiological signature of areas in macaque parietal cortex. By varying delay duration, we disambiguated delay-period activity from sensory and motor responses. Mean time courses in the parietal areas were well fit by a linear model comprising three components representing responses to (1) the visual target, (2) the delay period, and (3) the eye movement interval. We estimated the contributions of each component: the response amplitude during the delay period was substantially smaller (<30%) than that elicited by the transient visual target. All three parietal regions showed comparable delay-period response amplitudes, with a trend toward larger responses from V7 to IPS1 and IPS2. Responses to the cue and during the delay period showed clear lateralization with larger responses to trials in which the target was placed in the contralateral visual field, suggesting that both of these components contributed to the topography we measured.

Spatial flow-volume dissociation of the cerebral microcirculatory response to mild hypercapnia

The spatial and temporal response of the cerebral microcirculation to mild hypercapnia was investigated via two-photon laser-scanning microscopy. Cortical vessels, traversing the top 200 microm of somatosensory cortex, were visualized in alpha-chloralose-anesthetized Sprague-Dawley rats equipped with a cranial window. Intraluminal vessel diameters, transit times of fluorescent dextrans and red blood cells (RBC) velocities in individual capillaries were measured under normocapnic (PaCO2= 32.6 +/- 2.6 mm Hg) and slightly hypercapnic (PaCO2= 45 +/- 7 mm Hg) conditions. This gentle increase in PaCO2 was sufficient to produce robust and significant increases in both arterial and venous vessel diameters, concomitant to decreases in transit times of a bolus of dye from artery to venule (14%, P < 0.05) and from artery to vein (27%, P < 0.05). On the whole, capillaries exhibited a significant increase in diameter (16 +/- 33%, P < 0.001, n = 393) and a substantial increase in RBC velocities (75 +/- 114%, P < 0.001, n = 46) with hypercapnia. However, the response of the cerebral microvasculature to modest increases in PaCO2 was spatially heterogeneous. The maximal relative dilatation (range: 5-77%; mean +/- SD: 25 +/- 34%, P < 0.001, n = 271) occurred in the smallest capillaries (1.6 microm-4.0 microm resting diameter), while medium and larger capillaries (4.4 microm-6.8 microm resting diameter) showed no significant changes in diameter (P > 0.08, n = 122). In contrast, on average, RBC velocities increased less in the smaller capillaries (39 +/- 5%, P < 0.002, n = 22) than in the medium and larger capillaries (107 +/- 142%, P < 0.003, n = 24). Thus, the changes in capillary RBC velocities were spatially distinct from the observed volumetric changes and occurred to homogenize cerebral blood flow along capillaries of all diameters.

Synchronization between background activity and visually evoked potential is not mirrored by focal hyperoxygenation: implications for the interpretation of vascular brain imaging

We performed an electroencephalography and optical topography study simultaneously exploring electrophysiological and vascular response magnitude as a function of stimulus frequency. To elicit a response in the visual cortex, subjects were exposed to flicker frequencies varying from 1 to 25 Hz (1 Hz steps, eyes closed). Extending the standard view to compare magnitudes of the evoked neuronal to the evoked vascular response, we additionally investigated modulations of alpha-power, a marker of "background" EEG activity. The results show two discrepancies between the electrophysiological and vascular response: (1) VEP and alpha-power exhibit a discontinuous peak when stimulating at the individual alpha-frequency (IAF) (approximately 10-11 Hz), indicating resonance between background oscillations and evoked response; this is not mirrored by the vascular response. (2) The vascular response, in contrast, steadily increases up to a maximum at 7-8 Hz and slightly decreases with higher frequencies. This continuous frequency dependence is partly reflected by the decrease in alpha-power up to frequencies of 8-9 Hz and a slight increase in alpha-power beyond the IAF resonance. Although indicating an inverse relationship between alpha-power and vascular response, the frequency dependence of the evoked response does not show such a correlation. Thus, electrophysiological resonance between an individual's alpha-frequency and isofrequent stimulation is not mirrored by the vascular response. Also, spontaneous background EEG activity is an important modulator of the vascular response magnitude. We discuss these deviations from a simple one-to-one translation between evoked potential and vascular response amplitude in the light of questions concerning synchronization, attenuation, and induction of background oscillations such as the alpha-rhythm.

Spatial extent of CBF response during whisker stimulation using trial averaged laser Doppler imaging

The spatial pattern of activation in response to multiple whisker stimulation was studied using high-resolution laser Doppler (LD) imaging in urethane-anesthetized rats. LD flux change representing cerebral blood flow (CBF) responses were analyzed from a single trial or after averaging a number of similar trials. CBF change in a single trial was observed predominantly over pixels having low baseline flux values (microvessels), which included diffuse circular patterns of activation 400-800 microm in diameter similar to the histological dimensions of individual barrels established in the layer IV of the rat somatosensory cortex. The overall activation pattern varied considerably between each trial (only about 9-10% overlap); however, the diffuse circular pattern of activation was reproducible in every single trial within and across all rats. With trial averaging, no significant increase was observed in the outer boundary of activation, but the number of activated pixels increased within the diffuse circular patterns of activation. Emergence of further active pixels primarily within the diffuse circular regions of activity with trial averaging indicates distinct CBF responses in the septal and barrel regions, with a lesser LD signal to noise ratio in the barrel core.

Multisensory integration of dynamic faces and voices in rhesus monkey auditory cortex

In the social world, multiple sensory channels are used concurrently to facilitate communication. Among human and nonhuman primates, faces and voices are the primary means of transmitting social signals (Adolphs, 2003; Ghazanfar and Santos, 2004). Primates recognize the correspondence between species-specific facial and vocal expressions (Massaro, 1998; Ghazanfar and Logothetis, 2003; Izumi and Kojima, 2004), and these visual and auditory channels can be integrated into unified percepts to enhance detection and discrimination. Where and how such communication signals are integrated at the neural level are poorly understood. In particular, it is unclear what role "unimodal" sensory areas, such as the auditory cortex, may play. We recorded local field potential activity, the signal that best correlates with human imaging and event-related potential signals, in both the core and lateral belt regions of the auditory cortex in awake behaving rhesus monkeys while they viewed vocalizing conspecifics. We demonstrate unequivocally that the primate auditory cortex integrates facial and vocal signals through enhancement and suppression of field potentials in both the core and lateral belt regions. The majority of these multisensory responses were specific to face/voice integration, and the lateral belt region shows a greater frequency of multisensory integration than the core region. These multisensory processes in the auditory cortex likely occur via reciprocal interactions with the superior temporal sulcus.

Using functional magnetic resonance imaging to assess adaptation and size invariance of shape processing by humans and monkeys

Functional magnetic resonance imaging in awake monkeys and humans was used to compare object adaptation in shape-sensitive regions of these two species under identical and different size conditions. Object adaptation was similar in humans and monkeys under both conditions. Neither species showed complete size invariance, in agreement with single-cell studies. Both the macaque inferotemporal (IT) complex and human lateral occipital complex (LOC) displayed an anteroposterior gradient in object adaptation and size invariance, with the more anterior regions being more adaptable and size invariant. The results provide additional evidence for the homology between the macaque IT cortex and human LOC but also add to the growing list of differences between human and monkey intraparietal sulcus regions.

Brain hemodynamic changes mediated by dopamine receptors: Role of the cerebral microvasculature in dopamine-mediated neurovascular coupling

The coupling between neurotransmitter-induced changes in neuronal activity and the resultant hemodynamic response is central to the interpretation of neuroimaging techniques. In the present study, MRI experiments showed that dopamine transporter blockers such as cocaine and dopamine releasers such as amphetamine and D1 receptor agonists induced large positive increases in relative cerebral blood volume (rCBV) that were not sensitive to nitric oxide synthase inhibition. However, D1/D5 receptor antagonism with SCH-23390 prevented or blocked the hemodynamic response without any concomitant effect on dopamine release. Dopamine D2/D3 receptor agonists, in contrast, induced negative changes in rCBV in brain regions corresponding largely to those endowed with these receptors. D1 and D5 receptor mRNAs were expressed in microvessels of responsive brain areas, while D2 and D3 receptors were not consistently associated with the microvascular bed. D3 receptors had an astroglial localization. Together, these experiments show that direct effects of dopamine upon the vasculature cannot be ignored in measuring the hemodynamic coupling associated with dopaminergic drugs. These results further suggest that this coupling is partially mediated through D1/D5 receptors on the microvasculature leading to increased rCBV and through astroglial D3 receptors leading to decreased rCBV. These data provide additional support for the role of local post-synaptic events in neurovascular coupling and emphasize that the interpretation of fMRI signals exclusively in terms of neuronal activity may be incomplete.

The correlative triad among aging, dopamine, and cognition: Current status and future prospects

The brain neuronal systems defined by the neurotransmitter dopamine (DA) have since long a recognized role in the regulation of motor functions. More recently, converging evidence from patient studies, animal research, pharmacological intervention, and molecular genetics indicates that DA is critically implicated also in higher-order cognitive functioning. Many cognitive functions and multiple markers of striatal and extrastriatal DA systems decline across adulthood and aging. Research examining the correlative triad among adult age, DA, and cognition has found strong support for the view that age-related DA losses are associated with age-related cognitive deficits. Future research strategies for examining the DA-cognitive aging link include assessing (a) the generality/specificity of the effects; (b) the relationship between neuromodulation and functional brain activation; and (c) the release of DA during actual task performance.

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.

About being BOLD

The last decade has seen an unprecedented increase in the use of functional magnetic resonance imaging (fMRI) to understand the neural basis of cognition and behavior. Being non-invasive and relatively easy to use, most studies relied on changes in the blood oxygenation level dependent (BOLD) contrast as an indirect marker of variations in brain activity. However, the fact that BOLD fMRI is dependent on the blood flow response that follows neural activity and does not measure neural activity per se is seen as an inherent cause for concern while interpreting data from these studies. In order to characterize the BOLD signal correctly, it is imperative that we have a better understanding of neural events that lead to the BOLD response. A review of recent studies that addressed several aspects of BOLD fMRI including events at the level of the synapse, the nature of the neurovascular coupling, and some parameters of the BOLD signal is provided. This is intended to serve as background information for the interpretation of fMRI data in normal subjects and in patients with compromised neurovascular coupling. One of the aims is also to encourage researchers to interpret the results of functional imaging studies in light of the dynamic interactions between different brain regions, something that often is neglected.

Blood oxygenation level dependent contrast resting state networks are relevant to functional activity in the neocortical sensorimotor system

The relevance of correlations between blood oxygenation level dependent (BOLD) signal changes across the brain acquired at rest (resting state networks, or RSN) to functional networks was tested using two quantitative criteria: (1) the localisation of major RSN correlation clusters and the task-related maxima defined in BOLD fMRI signal changes from the same subjects; and (2) the relative hemispheric lateralisation (LI) of BOLD fMRI signal changes in sensorimotor cortex. RSN were defined on the basis of signal changes correlated with that of a "seed" voxel in the primary sensorimotor cortex. We found a generally close spatial correspondence between clusters of correlated BOLD signal change in RSN and activation maxima associated with hand movement. Conventional BOLD fMRI during active hand movement showed the expected wide variation in relative hemispheric lateralisation of LI for sensorimotor cortex across the subjects. There was a good correlation between LIs for the active hand movement task and the RSN (r=0.74, p<0.001). The RSN thus define anatomically relevant regions of motor cortex and change with functionally relevant variations in hemispheric lateralisation of sensorimotor cortical interactions with hand movement.

In vivo neurochemical monitoring and the study of behaviour

In vivo neurochemical monitoring techniques measure changes in the extracellular compartment of selected brain regions. These changes reflect the release of chemical messengers and intermediates of brain energy metabolism resulting from the activity of neuronal assemblies. The two principal techniques used in neurochemical monitoring are microdialysis and voltammetry. The presence of glutamate in the extracellular compartment and its pharmacological characteristics suggest that it is released from astrocytes and acts as neuromodulator rather than a neurotransmitter. The changes in extracellular noradrenaline and dopamine reflect their role in the control of behaviour. Changes in glucose and oxygen, the latter a measure of local cerebral blood flow, reflect synaptic processing in the underlying neuronal networks rather than a measure of efferent output from the brain region. In vivo neurochemical monitoring provides information about the intermediate processing that intervenes between the application of the stimulus and the resulting behaviour but does not reflect the final efferent output that leads to behaviour.

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.

Hemodynamic correlates of EEG: A heuristic

In this note we describe a heuristic, starting with a dimensional analysis, which relates hemodynamic changes to the spectral profile of ongoing EEG activity. In brief, this analysis suggests that 'activation', as indexed by increases in hemodynamic signals, should be associated with a loss of power in lower EEG frequencies, relative to higher frequencies. The fact that activation is expressed in terms of frequency (i.e., per second) is consistent with a dimensional analysis in the sense that activations reflect the rate of energy dissipation (per second). In this heuristic, activation causes an acceleration of temporal dynamics leading to (i) increased energy dissipation; (ii) decreased effective membrane time constants; (iii) increased effective coupling among neuronal ensembles; and (iv) a shift in the EEG spectral profile to higher frequencies. These predictions are consistent with empirical observations of how changes in the EEG spectrum are expressed hemodynamically. Furthermore, the heuristic provides a simple measure of neuronal activation based on spectral analyses of EEG.

Sub-area-specific Suppressive Interaction in the BOLD responses to simultaneous finger stimulation in human primary somatosensory cortex: evidence for increasing rostral-to-caudal convergence

In the primary somatosensory cortex (SI) of non-human primates, receptive field properties have been shown to differ between its sub-areas with increasing convergence in areas 1 and 2 as compared with area 3b. In this study, we searched for a similar functional organization of human SI. We performed fMRI in healthy subjects during separate or simultaneous electrical stimulation of the second and third finger of the right hand. Activation patterns in response to stimulation of single fingers reflected the somatotopical arrangement within the hand area of SI. Somatotopy was more clear-cut in area 3b as compared with areas 1 and 2. The response to simultaneous stimulation was considerably smaller than the summed responses to separate stimulation of each finger alone, pointing to a suppressive interaction effect. A region-of-interest analysis in the representational areas of the second and third finger revealed sub-area-specific differential suppressive interaction with an increase along the rostral-caudal axis (areas 3b, 1 and 2: 26, 32.7 and 42.2%, respectively). These findings on differences in the topographic as well as functional organization between sub-areas of SI support the notion of increasing convergence and integration from area 3b to areas 1 and 2 in human subjects.

fMRI Hippocampal Activity During a VirtualRadial Arm Maze

Numerous studies have shown that the hippocampus is critical for spatial memory. Within nonhuman research, a task often used to assess spatial memory is the radial arm maze. Because of the spatial nature of this task, this maze is often used to assess the function of the hippocampus. Our goal was to extrapolate this task to humans and examine whether healthy undergraduates utilize their hippocampus while performing a virtual reality version of the radial arm maze task. Thirteen undergraduates performed a virtual radial arm maze during functional magnetic resonance imaging. The brain maps of activity reveal bilateral hippocampal BOLD signal changes during the performance of this task. However, paradoxically, this BOLD signal change decreases during the spatial memory component of the task. Additionally, we note frontal cortex activity reflective of working memory circuits. These data reveal that, as predicted by the rodent literature, the hippocampus is involved in performing the virtual radial arm maze in humans. Hence, this virtual reality version may be used to assess the integrity of hippocampus so as to predict risk or severity in a variety of psychiatric disorders.

The effect of hypercapnia on the neural and hemodynamic responses to somatosensory stimulation

Modern non-invasive imaging techniques utilize the coupling between neural activity and changes in blood flow, volume and oxygenation to map the functional architecture of the human brain. An understanding of how the hemodynamic response is influenced by pre-stimulus baseline perfusion is important for the interpretation of imaging data. To address this issue, the present study measured hemodynamics with optical imaging spectroscopy and laser Doppler flowmetry, while multi-channel electrophysiology was used to record local field potentials (LFP) and multi-unit activity (MUA). The response to whisker stimulation in rodent barrel cortex was recorded during baseline (normocapnia) and elevated perfusion rates produced by two levels of hypercapnia (5 and 10%). With the exception of the 'washout' of deoxyhemoglobin, which was attenuated, all aspects of the neural and hemodynamic response to whisker stimulation were similar during 5% hypercapnia to those evoked during normocapnia. In contrast, 10% hypercapnia produced cortical arousal and a reduction in both the current sink and MUA elicited by stimulation. Blood flow and volume responses were reduced by a similar magnitude to that observed in the current sink. The deoxyhemoglobin 'washout', however, was attenuated to a greater degree than could be expected from the neural activity. These data suggest that imaging techniques based on perfusion or blood volume changes may be more robust to shifts in baseline than those based on the dilution of deoxyhemoglobin, such as conventional BOLD fMRI.

Metabolic indices shift in the hypothalamic-neurohypophysial system during lactation: implications for interpreting their relationship with neuronal activity

Metabolic indices of neuronal activity are thought to predict changes in the frequency of action potentials. There are stimuli that do not shift action potential frequency but change the temporal organization of neuronal firing following modifications of excitatory inputs by inhibitory synaptic activation. To our knowledge it is unknown whether this kind of stimulus associates with adjustments of metabolic markers of neuronal activity. Here, we used the hypothalamic-neurohypophysial system of lactating rats to address whether shifts in the temporal organization of neuronal firing relate with modifications of metabolic markers of neuronal activity. Cytochrome oxidase activity, (3)H-2-deoxyglucose uptake, and the area occupied by blood vessels increased in the paraventricular nucleus and neurohypophysis of lactating rats, as compared with their virgin counterparts. Taken together, these results suggest that metabolic demands denote shifts in the temporal organization of action potentials related with the adjustment of excitatory synaptic activation, and support that changes in metabolic markers do not necessarily reflect shifts in the frequency of action potentials.

Spatiotemporal characteristics of low-frequency functional activation measured by laser speckle imaging

Changes in neuronal activity have been shown to be accompanied by alteration in regional cerebral blood flow . In the present study, laser speckle imaging (LSI) was employed to measure stimulus-evoked neuronal activities in rat barrel cortex. The spatiotemporal characteristics of hemodynamic response to mechanical stimuli from 1 to 3 Hz were examined. Time to peak amplitude reduced from 4.5 to 3.5 s with increasing frequencies. Spatially, the response was confined to a small circular region at the beginning and then spread out asymmetrically to the surrounding regions. The maximal area of activation ranged from 2.2 to 3.5 mm2, while the time to reach maximal area occurred between 5.5 and 6 s. Moreover, there was a high correlation between LSI and laser-Doppler flowmetry in terms of peak response magnitude and the time to reach peak. These two values were linearly dependent on stimulus frequency whereas area of activation and time to maximal area appeared to be independent of this parameter. LSI's high sensitivity, low cost of the equipment, and size and complexity make this a suitable technique for fundamental neurophysiological investigations.

Investigating the neural basis for functional and effective connectivity. Application to fMRI

Viewing cognitive functions as mediated by networks has begun to play a central role in interpreting neuroscientific data, and studies evaluating interregional functional and effective connectivity have become staples of the neuroimaging literature. The neurobiological substrates of functional and effective connectivity are, however, uncertain. We have constructed neurobiologically realistic models for visual and auditory object processing with multiple interconnected brain regions that perform delayed match-to-sample (DMS) tasks. We used these models to investigate how neurobiological parameters affect the interregional functional connectivity between functional magnetic resonance imaging (fMRI) time-series. Variability is included in the models as subject-to-subject differences in the strengths of anatomical connections, scan-to-scan changes in the level of attention, and trial-to-trial interactions with non-specific neurons processing noise stimuli. We find that time-series correlations between integrated synaptic activities between the anterior temporal and the prefrontal cortex were larger during the DMS task than during a control task. These results were less clear when the integrated synaptic activity was haemodynamically convolved to generate simulated fMRI activity. As the strength of the model anatomical connectivity between temporal and frontal cortex was weakened, so too was the strength of the corresponding functional connectivity. These results provide a partial validation for using fMRI functional connectivity to assess brain interregional relations.

Integrating neuroscientific data across spatiotemporal scales

A major challenge confronting neuroscientists is associated with the multiple spatial and temporal scales of investigation of neural structure and function. I shall discuss the use of computational neural modeling as one method to bridge some of the different spatial and temporal levels. This approach will be illustrated using large-scale, neurobiologically realistic network models of auditory and visual pattern recognition that relate neuronal dynamics to fMRI data. It will be demonstrated that the models are capable of exhibiting the salient features of both electrophysiological neuronal activities and fMRI values that are in agreement with empirically observed data.

The contribution of GABA to glutamate/glutamine cycling and energy metabolism in the rat cortex in vivo

Previous studies have shown that the glutamate/glutamine (Glu/Gln) neurotransmitter cycle and neuronal glucose oxidation are proportional (1:1), with increasing neuronal activity above isoelectricity. GABA, a product of Glu metabolism, is synthesized from astroglial Gln and contributes to total Glu/Gln neurotransmitter cycling, although the fraction contributed by GABA is unknown. In the present study, we used (13)C NMR spectroscopy together with i.v. infusions of [1,6-(13)C(2)]glucose and [2-(13)C]acetate to separately determine rates of Glu/Gln and GABA/Gln cycling and their respective tricarboxylic acid cycles in the rat cortex under conditions of halothane anesthesia and pentobarbital-induced isoelectricity. Under 1% halothane anesthesia, GABA/Gln cycle flux comprised 23% of total (Glu plus GABA) neurotransmitter cycling and 18% of total neuronal tricarboxylic acid cycle flux. In isoelectric cortex, glucose oxidation was reduced >3-fold in glutamatergic and GABAergic neurons, and neurotransmitter cycling was below detection. Hence, in both cell types, the primary energetic costs are associated with neurotransmission, which increase together as cortical activity is increased. The contribution of GABAergic neurons and inhibition to cortical energy metabolism has broad implications for the interpretation of functional imaging signals.

Cortical and subcortical brain effects of transcranial magnetic stimulation (TMS)-induced movement: an interleaved TMS/functional magnetic resonance imaging study

BACKGROUND

To date, interleaved transcranial magnetic stimulation and functional magnetic resonance imaging (TMS/fMRI) studies of motor activation have not recorded whole brain patterns. We hypothesized that TMS would activate known motor circuitry with some additional regions plus some areas dropping out.

METHODS

We used interleaved TMS/fMRI (11 subjects, three scans each) to elucidate whole brain activation patterns from 1-Hz TMS over left primary motor cortex.

RESULTS

Both TMS (110% motor threshold) and volitional movement of the same muscles excited by TMS caused blood oxygen level-dependent (BOLD) patterns encompassing known motor circuitry. Additional activation was observed bilaterally in superior temporal auditory areas. Decreases in BOLD signal with unexpected post-task "rebounds" were observed for both tasks in the right motor area, right superior parietal lobe, and in occipital regions. Paired t test of parametric contrast maps failed to detect significant differences between TMS- and volition-induced effects. Differences were detectable, however, in primary data time-intensity profiles.

CONCLUSIONS

Using this interleaved TMS/fMRI technique, TMS over primary motor cortex produces a whole brain pattern of BOLD activation similar to known motor circuitry, without detectable differences from mimicked volitional movement. Some differences may exist between time courses of BOLD intensity during TMS circuit activation and volitional circuit activation.

Coupling of the cortical hemodynamic response to cortical and thalamic neuronal activity

Accurate interpretation of functional MRI (fMRI) signals requires knowledge of the relationship between the hemodynamic response and the neuronal activity that underlies it. Here we address the question of coupling between pre- and postsynaptic neuronal activity and the hemodynamic response in rodent somatosensory (Barrel) cortex in response to single-whisker deflection. Using full-field multiwavelength optical imaging of hemoglobin oxygenation and electrophysiological recordings of spiking activity and local field potentials, we demonstrate that a point hemodynamic measure is influenced by neuronal activity across multiple cortical columns. We demonstrate that the hemodynamic response is a spatiotemporal convolution of the neuronal activation. Therefore, positive hemodynamic response in one cortical column might be explained by neuronal activity not only in that column but also in the neighboring columns. Thus, attempts at characterizing the neurovascular relationship based on point measurements of electrophysiology and hemodynamics may yield inconsistent results, depending on the spatial extent of neuronal activation. The finding that the hemodynamic signal observed at a given location is a function of electrophysiological activity over a broad spatial region helps explain a previously observed increase of local vascular response beyond the saturation of local neuronal activity. We also demonstrate that the oxy- and total-hemoglobin hemodynamic responses can be well approximated by space-time separable functions with an antagonistic center-surround spatial pattern extending over several millimeters. The surround "negative" hemodynamic activity did not correspond to observable changes in neuronal activity. The complex spatial integration of the hemodynamic response should be considered when interpreting fMRI data.

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.

Cortical spreading depression produces long-term disruption of activity-related changes in cerebral blood volume and neurovascular coupling

Cortical spreading depression (CSD) is a pronounced depolarization of neurons and glia that spreads slowly across the cortex followed by a period of depressed electrophysiological activity. The vascular changes associated with CSD are a large transient increase in blood flow followed by a prolonged decrease lasting greater than 1 h. Currently, the profile of functional vascular activity during this hypovolemic period has not been well characterized. Perfusion-based imaging techniques such as functional magnetic resonance imaging (fMRI) assume a tight coupling between changes in neuronal and vascular activity. Under normal conditions, these variables are well correlated. Characterizing the effect of CSD on this relationship is an important step to understand the impact acute pathophysiological events may have on neurovascular coupling. We examine the effect of CSD on functional changes in cerebral blood volume (CBV) evoked by cortical electrophysiological activity for 1 h following CSD induction. CBV signal amplitude, duration, and time to peak show little recovery at 60 min post-induction. Analysis of spontaneous vasomotor activity suggests a decrease in vascular reactivity may play a significant role in the disruption of normal functional CBV responses. Electrophysiological activity is also attenuated but to a lesser degree. CBV and evoked potentials are not well correlated following CSD, suggesting a breakdown of the neurovascular coupling relationship.

Serotonin modulation of cerebral blood flow measured with positron emission tomography (PET) in humans

To develop a method to measure the dynamic response of the serotonin system in vivo, the effects of intravenously administered citalopram (the most selective of the serotonin reuptake inhibitors) or clomipramine on cerebral blood flow (CBF) were evaluated. CBF was measured with positron emission tomography (PET) in 27 normal subjects scanned under baseline conditions and, on the same day, after an intravenous (IV) infusion of placebo, citalopram, or clomipramine using a randomized, double-blind design. The main effects of the drugs on blood flow occurred in the thalamus, hypothalamus, and cingulate cortex. Compared to placebo, clomipramine reduced blood flow in the mediodorsal and ventral lateral nuclei of the thalamus, whereas citalopram reduced blood flow in the pulvinar nucleus and the hypothalamus. Compared to clomipramine, citalopram decreased blood flow in the cingulate cortex. The findings support previous reports showing acute central effects of citalopram and clomipramine on regional serotonergic functions measured by PET. Acute side effects may, however, require that care is taken in the selection of experimental designs for future PET studies using IV administration of these antidepressants.

The mental self

In meditation both the quality and the contents of consciousness may be voluntarily changed, making it an obvious target in the quest for the neural correlate of consciousness. Here we present the results of a positron emission tomography study of yoga nidra relaxation meditation when compared with the normal resting conscious state. Meditation is accompanied by a relatively increased perfusion in the sensory imagery system: hippocampus and sensory and higher order association regions, with decreased perfusion in the executive system: dorsolateral prefrontal cortex, anterior cingulate gyrus, striatum, thalamus, pons, and cerebellum. To identify regions active in both systems we performed a principal component analysis of the results. This separated the blood flow data into two groups of regions, explaining 25 and 18% of their variance: One group corresponded to the executive system, and the other to the systems supporting sensory imagery. A small group of regions contributed considerably to both networks: medial parietal and medial prefrontal cortices, together with the striatum. The inclusion of the striatum and our subsequent finding of increased striatal dopamine binding to D2 receptors during meditation suggested dopaminergic regulation of this circuit. We then investigated the neural networks supporting episodic retrieval of judgments of individuals with different degrees of self-relevance, in the decreasing order: self, best friend, and the Danish queen. We found that all conditions activated a medial prefrontal - precuneus/posterior cingulate cortex, thalamus, and cerebellum. This activation occurred together with the activation of the left lateral prefrontal/temporal cortex. The latter was dependent on the requirement of retrieval of semantic information, being most pronounced in the "queen" condition. Transcranial magnetic stimulation, targeting precuneus, was then applied to the medial parietal region to transiently disrupt the normal function of the circuitry. We found a decreased efficiency of retrieval of self-judgment compared to the judgment of best friend. This shows that the integrity of the function of precuneus is essential for self-reference, but not for reference to others.

Hemodynamics under Hippocampal Functional Hyperemia in Anesthetized Rat: A Greater Contribution of Red Blood Cell Velocity Compared to Its Concentration

It remains controversial which of the two regulators, red blood cell velocity (RBC-V) or concentration (RBC-C), is a main contributor to increasing flow (RBC-F) during functional hyperemia in the rat hippocampus induced by N-methyl-D-aspartate (NMDA). To address this, we monitored these parameters simultaneously under NMDA-infusion via microdialysis in the hippocampus of urethane-anesthetized rats and found a greater elevation in RBC-V than in RBC-C. This suggests that an RBC-V-dependent increase in RBC-F occurs under NMDA-induced functional hyperemia in the hippocampus as well as in the cortex.

Percept-related activity in the human somatosensory system: functional magnetic resonance imaging studies

In this paper, we review blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) studies addressing the neural correlates of touch, thermosensation, pain and the mechanisms of their cognitive modulation in healthy human subjects. There is evidence that fMRI signal changes can be elicited in the parietal cortex by stimulation of single mechanoceptive afferent fibers at suprathreshold intensities for conscious perception. Positive linear relationships between the amplitude or the spatial extents of BOLD fMRI signal changes, stimulus intensity and the perceived touch or pain intensity have been described in different brain areas. Some recent fMRI studies addressed the role of cortical areas in somatosensory perception by comparing the time course of cortical activity evoked by different kinds of stimuli with the temporal features of touch, heat or pain perception. Moreover, parametric single-trial functional MRI designs have been adopted in order to disentangle subprocesses within the nociceptive system. Available evidence suggest that studies that combine fMRI with psychophysical methods may provide a valuable approach for understanding complex perceptual mechanisms and top-down modulation of the somatosensory system by cognitive factors specifically related to selective attention and to anticipation. The brain networks underlying somatosensory perception are complex and highly distributed. A deeper understanding of perceptual-related brain mechanisms therefore requires new approaches suited to investigate the spatial and temporal dynamics of activation in different brain regions and their functional interaction.

How do memory systems interact? Evidence from human classification learning

Studies of human classification learning using functional neuroimaging have suggested that basal ganglia and medial temporal lobe memory systems may interact during learning. We review these results and outline a set of possible mechanisms for such interactions. Effective connectivity analyses suggest that interaction between basal ganglia and medial temporal lobe are mediated by prefrontal cortex rather than by direct connectivity between regions. A review of possible neurobiological mechanisms suggests that interactions may be driven by neuromodulatory systems in addition to mediation by interaction of inputs to prefrontal cortical neurons. These results suggest that memory system interactions may reflect multiple mechanisms that combine to optimize behavior based on experience.

Left inferior prefrontal cortex activity reflects inhibitory rather than facilitatory priming

Functional neuroimaging has demonstrated reduced activation correlated with behavioral priming effects, a finding generally interpreted in terms of facilitated retrieval of target items in the context of related primes. Without a neutral prime, however, one cannot separate facilitatory effects of related primes from inhibitory effects of unrelated primes. Here we report an auditory semantic priming paradigm with congruent (''The boy bounced the BALL''), neutral (''The next item is BALL''), and incongruent (''Pasta is my favorite kind of BALL'') sentence trials. As previously reported, reduced left inferior prefrontal cortex activation was observed for congruent relative to incongruent trials; however, the neutral condition allowed us to show that the effect arose from increased activation in the incongruent condition rather than reduced activation for congruent trials. Our results suggest that the left inferior prefrontal cortex inhibits interference from prepotent representations in order to select a task-appropriate target, and is consistent with its broader role in behavioral inhibition.

Hippocampal dampening during a relational memory task

The hippocampus (HPC) has an essential role in relational memory. One task used to test relational memory is the transverse patterning (TP) problem (A+ B-, B+ C-, and C+ A-), which is sensitive to HPC damage across species. Using functional magnetic resonance imaging in humans, the authors observed activation maps and time course data indicating that the HPC is involved in this task, but it is paradoxically less active during the hippocampal-dependent relational memory phase relative to both a hippocampal-independent control memory phase and to a fixation control phase. This suggests that traditional assumptions suggesting that brain regions critical for a task must produce an increased blood oxygen level-dependent response during performance of that task are probably inaccurate and alternative explanations should be entertained.

Relative changes in cerebral blood flow and neuronal activity in local microdomains during generalized seizures

There is broad agreement that generalized tonic-clonic seizures (GTCS) and normal somatosensory stimulation are associated with increases in regional CBF. However, the data regarding CBF changes during absence seizures are controversial. Electrophysiologic studies in WAG/Rij rats, an established animal model of absence seizures, have shown spike-wave discharges (SWD) that are largest in the perioral somatosensory cortex while sparing the visual cortex. Recent functional magnetic resonance imaging (fMRI) studies in the same model have also shown localized increases in fMRI signals in the perioral somatosensory cortex during SWD. Because fMRI signals are only indirectly related to neuronal activity, the authors directly measured CBF and neuronal activity from specific microdomains of the WAG/Rij cortex using a specially designed probe combining laser-Doppler flowmetry and extra-cellular microelectrode recordings under fentanyl/haloperidol anesthesia. Using this approach, parallel increases in neuronal activity and CBF were observed during SWD in the whisker somatosensory (barrel) cortex, whereas the visual cortex showed no significant changes. For comparison, these measurements were repeated during somatosensory (whisker) stimulation, and bicuculline-induced GTCS in the same animals. Interestingly, whisker stimulation increased neuronal activity and CBF in the barrel cortex more than during SWD. During GTCS, much larger increases that included both the somatosensory and visual cortex were observed. Thus, SWD in this model produce parallel localized increases in neuronal activity and CBF with similar distribution to somatosensory stimulation, whereas GTCS produce larger and more widespread changes. The normal response to somatosensory stimulation appears to be poised between two abnormal responses produced by two physiologically different types of seizures.

High trait anxiety and hyporeactivity to stress of the dorsomedial prefrontal cortex: a combined phMRI and Fos study in rats

The neural basis of trait anxiety is poorly understood. In genetically selected hyperanxious (high anxiety-related behavior; HAB) rats, diazepam induces a stronger anxiolytic response than in hypoanxious (low anxiety-related behavior; LAB) rats. A screen for neuronal response differences to diazepam between HAB and LAB rats using pharmacologic fMRI (phMRI) at 7 T revealed a blunted diazepam-induced neuronal deactivation in the dorsomedial prefrontal cortex (dmPFC) of HABs. This was not due to reduced benzodiazepine (BDZ) receptor densities in this region. Instead, dmPFC tissue oxygenation at baseline was found to be significantly lower in HABs. This suggests a tonic relative hypoactivity under the highly stressful phMRI conditions, offering an explanation for the reduced responsivity to the neural depressant effect of diazepam in the sense of a floor effect. Subsequently, Fos immunoreactivity (Fos-IR) showed that ethologically relevant stressors also cause less dmPFC activation in HABs. In the context of an anxiety-inhibiting role of the dmPFC, we propose that failure to sufficiently activate this region in stressful situations may contribute to high trait anxiety.

Relating neuronal dynamics for auditory object processing to neuroimaging activity: a computational modeling and an fMRI study

We investigated the neural basis of auditory object processing in the cerebral cortex by combining neural modeling and functional neuroimaging. We developed a large-scale, neurobiologically realistic network model of auditory pattern recognition that relates the neuronal dynamics of cortical auditory processing of frequency modulated (FM) sweeps to functional neuroimaging data of the type obtained using PET and fMRI. Areas included in the model extend from primary auditory to prefrontal cortex. The electrical activities of the neuronal units of the model were constrained to agree with data from the neurophysiological literature regarding the perception of FM sweeps. We also conducted an fMRI experiment using stimuli and tasks similar to those used in our simulations. The integrated synaptic activity of the neuronal units in each region of the model, convolved with a hemodynamic response function, was used as a correlate of the simulated fMRI activity, and generally agreed with the experimentally observed fMRI data in the brain areas corresponding to the regions of the model. Our results demonstrate that the model is capable of exhibiting the salient features of both electrophysiological neuronal activities and fMRI values that are in agreement with empirically observed data. These findings provide support for our hypotheses concerning how auditory objects are processed by primate neocortex.

Human Midbrain Sensitivity to Cognitive Feedback and Uncertainty During Classification Learning

Mesencephalic dopaminergic system (MDS) neurons may participate in learning by providing a prediction error signal to their targets, which include ventral striatal, orbital, and medial frontal regions, as well as by showing sensitivity to the degree of uncertainty associated with individual stimuli. We investigated the mechanisms of probabilistic classification learning in humans using functional magnetic resonance imaging to examine the effects of feedback and uncertainty. The design was optimized for separating neural responses to stimulus, delay, and negative and positive feedback components. Compared with fixation, stimulus and feedback activated brain regions consistent with the MDS, whereas the delay period did not. Midbrain activity was significantly different for negative versus positive feedback (consistent with coding of the “prediction error”) and was reliably correlated with the degree of uncertainty as well as with activity in MDS target regions. Purely cognitive feedback apparently engages the same regions as rewarding stimuli, consistent with a broader characterization of this network.

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.

Functional signal- and paradigm-dependent linear relationships between synaptic activity and hemodynamic responses in rat somatosensory cortex

Linear relationships between synaptic activity and hemodynamic responses are critically dependent on functional signal etiology and paradigm. To investigate these relationships, we simultaneously measured local field potentials (FPs) and optical intrinsic signals in rat somatosensory cortex while delivering a small number of electrical pulses to the hindpaw with varied stimulus intensity, number, and interstimulus interval. We used 570 and 610 nm optical signals to estimate cerebral blood volume (CBV) and oxygenation, respectively. The spatiotemporal evolution patterns and trial-by-trial correlation analyses revealed that CBV-related optical signals have higher fidelity to summed evoked FPs (SigmaFPs) than oxygenation-derived signals. CBV-related signals even correlated with minute SigmaFP fluctuations within trials of the same stimulus condition. Furthermore, hemodynamic signals (CBV and late oxygenation signals) increased linearly with SigmaFP while varying stimulus number, but they exhibited a threshold and steeper gradient while varying stimulus intensity, suggesting insufficiency of the homogeneity property of linear systems and the importance of spatiotemporal coherence of neuronal population activity in hemodynamic response formation. These stimulus paradigm-dependent linear and nonlinear relationships demonstrate that simple subtraction-based analyses of hemodynamic signals produced by complex stimulus paradigms may not reflect a difference in SigmaFPs between paradigms. Functional signal- and paradigm-dependent linearity have potentially profound implications for the interpretation of perfusion-based functional signals.

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.

Stuttering and the basal ganglia circuits: a critical review of possible relations

The possible relation between stuttering and the basal ganglia is discussed. Important clues to the pathophysiology of stuttering are given by conditions known to alleviate dysfluency, like the rhythm effect, chorus speech, and singing. Information regarding pharmacologic trials, lesion studies, brain imaging, genetics, and developmental changes of the nervous system is reviewed. The symptoms of stuttering are compared with basal ganglia motor disorders like Parkinson's disease and dystonia. It is proposed that the basal ganglia-thalamocortical motor circuits through the putamen are likely to play a key role in stuttering. The core dysfunction in stuttering is suggested to be impaired ability of the basal ganglia to produce timing cues for the initiation of the next motor segment in speech. Similarities between stuttering and dystonia are indicated, and possible relations to the dopamine system are discussed, as well as the interaction between the cerebral cortex and the basal ganglia. Behavioral and pharmacologic information suggests the existence of subtypes of stuttering.

LEARNING OUTCOMES

As a result of this activity, the reader will (1) become familiar with the research regarding the basal ganglia system relating to speech motor control; (2) become familiar with the research on stuttering with indications of basal ganglia involvement; and (3) be able to discuss basal ganglia mechanisms with relevance for theory of stuttering.

Neurovascular coupling analyzed non-invasively in the human brain

Most functional brain imaging methods detect neuronal activations indirectly through the accompanying neurovascular response. Here, we demonstrate that a novel methodological approach, the combination of DC-magnetoencephalography (DC-MEG) and near-infrared spectroscopy (NIRS), allows non-invasive assessment of the dynamics of neurovascular coupling in the human brain: detecting directly slow neuronal processes (with time constants of 30s), DC-MEG revealed, even in unaveraged recordings, sustained neuronal activations at pericentral hand cortices contralateral to repetitive finger movements; these were accompanied by the ensuing local vascular response showing similar dynamical features as quantified by simultaneously recorded NIRS. This non-invasive approach opens a new avenue for the understanding of neurovascular coupling during physiological tasks as well as in diseases involving slow neuronal depolarization shifts and alterations of blood flow, such as stroke or migraine.

Functional MRI of the immediate impact of transcranial magnetic stimulation on cortical and subcortical motor circuits

Recent studies indicate that the cortical effects of transcranial magnetic stimulation (TMS) may not be localized to the site of stimulation, but spread to other distant areas. Using echo-planar imaging with blood-oxygenation-level-dependent (BOLD) contrast at 3 Tesla, we measured MRI signal changes in cortical and subcortical motor regions during high-frequency (3.125 Hz) repetitive TMS (rTMS) of the left sensorimotor cortex (M1/S1) at intensities above and below the active motor threshold in healthy humans. The supra- and subthreshold nature of the TMS pulses was confirmed by simultaneous electromyographic monitoring of a hand muscle. Suprathreshold rTMS activated a network of primary and secondary cortical motor regions including M1/S1, supplementary motor area, dorsal premotor cortex, cingulate motor area, the putamen and thalamus. Subthreshold rTMS elicited no MRI-detectable activity in the stimulated M1/S1, but otherwise led to a similar activation pattern as obtained for suprathreshold stimulation though at reduced intensity. In addition, we observed activations within the auditory system, including the transverse and superior temporal gyrus, inferior colliculus and medial geniculate nucleus. The present findings support the notion that re-afferent feedback from evoked movements represents the dominant input to the motor system via M1 during suprathreshold stimulation. The BOLD MRI changes in motor areas distant from the site of subthreshold stimulation are likely to originate from altered synaptic transmissions due to induced excitability changes in M1/S1. They reflect the capability of rTMS to target both local and remote brain regions as tightly connected constituents of a cortical and subcortical network.

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.

“What” and “Where” in Visual Working Memory: A Computational Neurodynamical Perspective for Integrating fMRI and Single-Neuron Data

Single-neuron recordings, functional magnetic resonance imaging (fMRI) data, and the effects of lesions indicate that the prefrontal cortex (PFC) is involved in some types of working memory and related cognitive processes. Based on these data, two different models of the topographical and functional organization of the PFC have been proposed: organization-by-stimulus-domain, and organization-by-process. In this article, we utilize an integrate-and-fire network to model both single-neuron and fMRI data on short-term memory in order to understand data obtained in topologically different parts of the PFC during working memory tasks. We explicitly model the mechanisms that underlie workingmemory-related activity during the execution of delay tasks that have a “what”-then-“where” design (with both object and spatial delayed responses within the same trial). The model contains different populations of neurons (as found experimentally) in attractor networks that respond in the delay period to the stimulus object, the stimulus position, and to combinations of both object and position information. The pools are arranged hierarchically and have global inhibition through inhibitory interneurons to implement competition. It is shown that a biasing attentional input to define the current relevant information (object or location) enables the system to select the correct neuronal populations during the delay period in what is a biased competition model of attention. The processes occurring at the AMPA and NMDA synapses are dynamically modeled in the integrate-and-fire implementation to produce realistic spiking dynamics. It is shown that the fMRI data characteristic of the dorsal PFC and linked to spatial processing and manipulation of items can be reproduced in the model by a high level of inhibition, whereas the fMRI data characteristic of the ventral PFC and linked to object processing can be produced by a lower level of inhibition, even though the network is itself topographically homogeneous with no spatial topology of the neurons. This article, thus, not only presents a model for how spatial versus object short-term memory could be implemented in the PFC, but also shows that the fMRI BOLD signal measured during such tasks from different parts of the PFC could reflect a higher level of inhibition dorsally, without this dorsal region necessarily being primarily spatial and the ventral region object-related.