Exploring brain connectivity: a new frontier in systems neuroscience. Functional Brain Connectivity, 4-6 April 2002, Dusseldorf, Germany.
Functional Brain Connectivity, held on 4-6 April 2002, Dusseldorf, Germany.
Brain↗
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Combining independent component analysis and correlation analysis to probe interregional connectivity in fMRI task activation datasets.
A new approach in studying interregional functional connectivity using functional magnetic resonance imaging (fMRI) is presented. Functional connectivity may be detected by means of cross correlating time course data from functionally related brain regions. These data exhibit high temporal coherence of low frequency fluctuations due to synchronized blood flow changes. In the past, this fMRI technique for studying functional connectivity has been applied to subjects that performed no prescribed task ("resting" state). This paper presents the results of applying the same method to task-related activation datasets. Functional connectivity analysis is first performed in areas not involved with the task. Then a method is devised to remove the effects of activation from the data using independent component analysis (ICA) and functional connectivity analysis is repeated. Functional connectivity, which is demonstrated in the "resting brain," is not affected by tasks which activate unrelated brain regions. In addition, ICA effectively removes activation from the data and may allow us to study functional connectivity even in the activated regions.
Adult↗
[Functional characteristics of the basic internal system of hippocampal connections].
Functional characteristics of two main intrahippocampal connections (dentate fascia-field CA3 and field CA3-field CA1) were studied in unanesthetized rabbits. Electrical stimulation of FD or CA3 was used during extracellular recording of the CA3 or CA1 neuronal activity, respectively. The system CA3-CA1 is a highly effective, with a very narrow threshold zone and wide range of active frequencies. The process of recruitment during continuous rhythmic stimulation is weak, any signs of prolonged potentiation are absent. The system FD-CA3 is characterized by low effeciency, a wide threshold zone, narrow range of active frequencies. It is able of gradual building up and prolonged preservation of potentiated state. The morphological reasons of these differences and possible role of these systems in the hippocampal activity are discussed.
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Human functional neuroimaging connectivity research in dependence.
Functional and effective connectivity are relatively new techniques in the analysis of functional neuroimaging studies in humans. They have previously been used in studies of 'normal' psychological and neurological processes such as vision before gradually transferring into use in pathological disease states such as schizophrenia. These techniques are now beginning to extend into the field of substance misuse and dependence. So far, most functional neuroimaging studies in this field have shown consistent patterns of activation in several brain regions, and theories are emerging based upon these and animal models. Studies of brain connectivity can now begin to help further unravel the tangle of disparate brain regions and their connections that underpin the psychopharmacological processes of dependence.
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Longitudinal functional network connectivity changes across the clinical stages of C9orf72 hexanucleotide repeat expansion carriers.
INTRODUCTION: Intrinsic functional connectivity network abnormalities in C9orf72 hexanucleotide repeat expansion carriers emerge during the asymptomatic phase, yet longitudinal studies remain limited. We examined cross-sectional abnormalities and longitudinal connectivity changes across clinical stages. METHODS: We analyzed task-free functional magnetic resonance imaging (fMRI) and structural MRI data in 36 asymptomatic (aSxC9), 17 prodromal (proC9), and 29 symptomatic (SxC9) carriers, and 107 healthy controls (HCs). Functional networks previously found altered in C9orf72, including salience, sensorimotor, default mode, and medial pulvinar thalamic networks, were examined. Associations between longitudinal connectivity and gray matter decline with baseline neurofilament light chain (NfL) concentrations and symptom severity were assessed. RESULTS: aSxC9 and SxC9 showed longitudinal connectivity changes within specific networks. In aSxC9, connectivity changes correlated with baseline NfL. In proC9 and SxC9, changes in connectivity and gray matter were associated with baseline NfL and symptom severity. DISCUSSION: C9orf72 expansion carriers demonstrate stage-specific network connectivity changes.
Humans↗
Morphology, electrophysiology and functional input connectivity of pyramidal neurons characterizes a genuine layer va in the primary somatosensory cortex.
Cortical layer V classically has been subdivided into sublayers Va and Vb on cytoarchitectonic grounds. In the analysis of cortical microcircuits, however, layer Va has largely been ignored. The purpose of this study was to investigate pyramidal neurons of layer Va in view of their potential role in integrating information from lemniscal and paralemniscal sources. For this we combined detailed electrophysiological and morphological characterization with mapping of intracortical functional connectivity by caged glutamate photolysis in layer Va of rat barrel cortex in vitro. Electrophysiological characterization revealed pyramidal cells of the regular spiking as well as the intrinsically burst firing type. However, all layer Va pyramidal neurons displayed uniform morphological properties and comparable functional input connectivity patterns. They received most of their excitatory and inhibitory inputs from intracolumnar sources, especially from layer Va itself, but also from layer IV. Those two layers were also the main origin for transcolumnar excitatory inputs. Layer Va pyramidal neurons thus may predominantly integrate information intralaminarly as well as from layer IV. The functional connectivity maps clearly distinguish layer Va from layer Vb pyramidal cells, and suggest that layer Va plays a unique role in intracortical processing of sensory information.
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Emergence of a powerful connection between sensory and cognitive functions across the adult life span: a new window to the study of cognitive aging?
Six hundred eighty seven individuals ages 25-103 years were studied cross-sectionally to examine the relationship between measures of sensory functioning (visual and auditory acuity) and intelligence (14 cognitive tasks representing a 5-factor space of psychometric intelligence). As predicted, the average proportion of individual differences in intellectual functioning connected to sensory functioning increased from 11% in adulthood (25-69 years) to 31% in old age (70-103 years). However, the link between fluid intellectual abilities and sensory functioning, albeit of different size, displayed a similarly high connection to age in both age groups. Several explanations are discussed, including a "common cause" hypothesis. In this vein, we argue that the increase in the age-associated link between sensory and intellectual functioning may reflect brain aging and that the search for explanations of cognitive aging phenomena would benefit from attending to factors that are shared between the 2 domains.
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Functional synaptic connections among respiratory neurons.
This presentation focuses on the application of methods to determine functional connections between neurons in the respiratory network of adult decerebrate rats. We employ a general network investigation paradigm that first examines the intracellular recordings of a respiratory neuron and then determines which neurons synapse with it to produce the observed membrane potential changes. It is used to pursue the source of respiratory excitation and inhibition from its arrival at phrenic motoneurons to respiratory neurons in the medulla, and then examine some of the interactions among these neurons that shape their patterns of activity. Findings include a demonstration that phrenic motoneuron activity is determined by excitation from medullary inspiratory premotor neurons and inhibition by Bötzinger complex expiratory neurons, and that the latter neurons inhibit both medullary inspiratory premotor neurons and themselves. We conclude that these functional interconnections explain the activity patterns of some respiratory neurons, but the connections between neurons thought to be involved in rhythm generation remain to be demonstrated in adult rats.
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Statistical mapping of functional olfactory connections of the rat brain in vivo.
The olfactory pathway is a unique route into the brain. To better characterize this system in vivo, rat olfactory functional connections were mapped using magnetic resonance (MR) imaging and manganese ion (Mn2+) as a transport-mediated tracer combined with newly developed statistical brain image analysis. Six rats underwent imaging on a 1.5-T MR scanner at pre-administration, and 6, 12, 24, 36, 48, and 72 h and 5.5, 7.5, 10.5, and 13.5 days post-administration of manganese chloride (MnCl2) into the right nasal cavity. Images were coregistered, pixel-intensity normalized, and stereotactically transformed to the Paxinos and Watson rat brain atlas, then averaged across subjects using automated image analysis software (NEUROSTAT). Images at each time point were compared to pre-administration using a one-sample t statistic on a pixel-by-pixel basis in 3-D and converted to Z statistic maps. Statistical mapping and group averaging improved signal to noise ratios and signal detection sensitivity. Significant transport of Mn2+ was observed in olfactory structures ipsilateral to site of Mn2+ administration including the bulb, lateral olfactory tract (lo) by 12 h and in the tubercle, piriform cortex, ventral pallidum, amygdala, and in smaller structures such as the anterior commissure after 24 h post-administration. MR imaging with group-wise statistical analysis clearly demonstrated bilateral transsynaptic Mn2+ transport to secondary and tertiary neurons of the olfactory system. The method permits in vivo investigations of functional neuronal connections within the brain.
Amygdala↗
A comparison of homonymous and heteronymous connectivity in the spinal monosynaptic reflex arc of the cat.
Multi-unit spike triggered averaging was used to determine functional connectivity between spindle afferent fibers from the medial gastrocnemius muscle and the motoneurons innervating the medial (homonymous connections) and the lateral gastrocnemius-soleus muscle (heteronymous connections). As many as 288 possible connections between 24 motoneurons and 12 afferent fibers were studied in single, acute experiments. The influences of morphological and topographical factors, as well as of motoneuron species on functional connectivity were analysed. The probability that a motoneuron would receive functional connections from a given population of afferent fibers was related to its size and its proximity to the spinal entry level of the afferent fibers. The faster the axonal conduction velocity of the motoneuron (i.e. the larger the motoneuron) and the closer its location to the entry zone of the afferent fibers, the higher was its probability of receiving functional connections. The greater the conduction velocity (i.e. diameter) of a stretch receptor afferent fiber, the higher was its probability of making functional connections with motoneurons. These relationships were qualitatively similar for homonymous and heteronymous connections. 58% (233/399) of the Ia and group II afferents (combined) had functional connections with homonymous motoneurons, 32% (75/234) with heteronymous motoneurons. However, homonymous and heteronymous motoneurons of similar sizes were equally likely to receive functional connections when located at the same craniocaudal level. Differences in the locations and mean sizes of homonymous and heteronymous motoneurons however, cannot account completely for the observed overall differences in homonymous and heteronymous connectivity.
Action Potentials↗
"Latent" inhibitory connections become functional during activity-dependent plasticity.
Simultaneous pre- and postsynaptic recordings from identified glycinergic inhibitory interneurons and the Mauthner cell showed that 25% of the afferents produced no or extremely small postsynaptic responses. Morphological determination of the number of contacts made by these cells on the Mauthner cell revealed a connectivity similar to that of functional neurons which always produce clear inhibitory postsynaptic potentials, suggesting that most of the endings, made by weak interneurons are silent. Intraaxonal injection of 4-aminopyridine or Ca2+ greatly enhanced transmission at functional connections but did not modify those which were ineffective. However, after eighth nerve tetanic stimuli, transmission at the weak connections was unmasked or enhanced for prolonged periods and was twice as likely to be potentiated, with a 6-fold greater mean enhancement than the potent ones. This result provides additional support for long-term potentiation of inhibitory synapses. Furthermore, weakly functional junctions represent a "reserve" pool which can be critical for the expression of plasticity within a network, and, consequently, for setting the threshold of reflex activities such as the escape reaction mediated by the Mauthner cell.
4-Aminopyridine↗
Connections among functional areas in the mustached bat auditory cortex.
Connections among functional areas in the mustached bat's auditory cortex were examined by placing anatomical tracers in physiologically defined locations. We identified at least two and probably three channels connecting the various areas. One channel is formed by interconnections among areas containing neurons sensitive to frequency-modulated components (FMs) of the pulse and echo. These neurons are tuned to echo delay, a cue for target range, and thus define a ranging channel. An additional one or two channels are formed by interconnections among areas that contain neurons sensitive to the constant frequency components (CFs) of echoes. These neurons are of two main types: either sensitive to CFs of both pulse and echo (CF/CF neurons) or sensitive to a pulse FM and echo CF (FM-CF neurons). There was only a weak connection between the largest area of each type, suggesting they lie in different channels. Connections among areas in the ranging channel and echo CF-sensitive channel(s) were weak. Thus, the interconnections among functional areas in the mustached bat's auditory cortex define parallel channels for processing different types of biosonar information. Most corticocortical connections were patchy, in a manner suggestive of a columnar organization. The average width of the patches was approximately 360 microm. Based on the sizes of the functional areas, we estimate the auditory cortex contains a total of approximately 150 columns. Individual areas contain from as many as approximately 20 to as few as 1-4 columns. Each area had abundant projections outside of the auditory cortex. Connections within the cortex included the frontal, anterior cingulate, retrosplenial and perirhinal cortices, and the claustrum. Subcortical targets included the amygdyla, auditory thalamus, pons, pretectum, superior and inferior colliculi, and central gray. Projections within the cortex were of modest strength compared with several of the subcortical projections. Thus, the auditory areas themselves are the primary source of cortically processed biosonar information to the rest of the brain.
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Functional frontoparietal connectivity during short-term memory as revealed by high-resolution EEG coherence analysis.
In this electroencephalographic study, the authors modeled the functional connectivity between frontal and parietal areas during short-term memory (STM) processes by spectral coherence analysis and the directed transfer function, that is, for the estimation of coherence "direction." A no-STM task was used as a reference. STM was characterized by an increased frontoparietal electroencephalograph coherence at high frequencies (beta and gamma, 14-45 Hz). In the control task, parietal-to-frontal flow prevailed at those frequencies. However, the STM task showed a bidirectional frontoparietal flow at the gamma band. In conclusion, frontoparietal connectivity would optimize "representational" memory during STM. In this context, the frontal areas would increase their influence on parietal areas for memory retention.
Adult↗
Determining significant connectivity by 4D spatiotemporal wavelet packet resampling of functional neuroimaging data.
An active area of neuroimaging research involves examining functional relationships between spatially remote brain regions. When determining whether two brain regions exhibit significant correlation due to true functional connectivity, one must account for the background spatial correlation inherent in neuroimaging data. We define background correlation as spatiotemporal correlation in the data caused by factors other than neurophysiologically based functional associations such as scanner induced correlations and image preprocessing. We develop a 4D spatiotemporal wavelet packet resampling method which generates surrogate data that preserves only the average background spatial correlation within an axial slice, across axial slices, and through each voxel time series, while excluding the specific correlations due to true functional relationships. We also extend an amplitude adjustment algorithm which adjusts our surrogate data to closely match the amplitude distribution of the original data. Our method improves upon existing wavelet-based methods and extends them to 4D. We apply our resampling technique to determine significant functional connectivity from resting state and motor task fMRI datasets.
Adult↗
Estimation of the effective and functional human cortical connectivity with structural equation modeling and directed transfer function applied to high-resolution EEG.
Different brain imaging devices are presently available to provide images of the human functional cortical activity, based on hemodynamic, metabolic or electromagnetic measurements. However, static images of brain regions activated during particular tasks do not convey the information of how these regions are interconnected. The concept of brain connectivity plays a central role in the neuroscience, and different definitions of connectivity, functional and effective, have been adopted in literature. While the functional connectivity is defined as the temporal coherence among the activities of different brain areas, the effective connectivity is defined as the simplest brain circuit that would produce the same temporal relationship as observed experimentally among cortical sites. The structural equation modeling (SEM) is the most used method to estimate effective connectivity in neuroscience, and its typical application is on data related to brain hemodynamic behavior tested by functional magnetic resonance imaging (fMRI), whereas the directed transfer function (DTF) method is a frequency-domain approach based on both a multivariate autoregressive (MVAR) modeling of time series and on the concept of Granger causality. This study presents advanced methods for the estimation of cortical connectivity by applying SEM and DTF on the cortical signals estimated from high-resolution electroencephalography (EEG) recordings, since these signals exhibit a higher spatial resolution than conventional cerebral electromagnetic measures. To estimate correctly the cortical signals, we used a subject's multicompartment head model (scalp, skull, dura mater, cortex) constructed from individual MRI, a distributed source model and a regularized linear inverse source estimates of cortical current density. Before the application of SEM and DTF methodology to the cortical waveforms estimated from high-resolution EEG data, we performed a simulation study, in which different main factors (signal-to-noise ratio, SNR, and simulated cortical activity duration, LENGTH) were systematically manipulated in the generation of test signals, and the errors in the estimated connectivity were evaluated by the analysis of variance (ANOVA). The statistical analysis returned that during simulations, both SEM and DTF estimators were able to correctly estimate the imposed connectivity patterns under reasonable operative conditions, that is, when data exhibit an SNR of at least 3 and a LENGTH of at least 75 s of nonconsecutive EEG recordings at 64 Hz of sampling rate. Hence, effective and functional connectivity patterns of cortical activity can be effectively estimated under general conditions met in any practical EEG recordings, by combining high-resolution EEG techniques and linear inverse estimation with SEM or DTF methods. We conclude that the estimation of cortical connectivity can be performed not only with hemodynamic measurements, but also with EEG signals treated with advanced computational techniques.
Adult↗
Relating macroscopic measures of brain activity to fast, dynamic neuronal interactions.
In this article we used biologically plausible simulations of coupled neuronal populations to address the relationship between phasic and fast coherent neuronal interactions and macroscopic measures of activity that are integrated over time, such as the BOLD response in functional magnetic resonance imaging. Event-related, dynamic correlations were assessed using joint peristimulus time histograms and, in particular, the mutual information between stimulus-induced transients in two populations. This mutual information can be considered as an index of functional connectivity. Our simulations showed that functional connectivity or dynamic integration between two populations increases with mean background activity and stimulus-related rate modulation. Furthermore, as the background activity increases, the populations become increasingly sensitive to the intensity of the stimulus in terms of a predisposition to transient phase locking. This reflects an interaction between background activity and stimulus intensity in producing dynamic correlations, in that background activity augments stimulus-induced coherence modulation. This is interesting from a computational perspective because background activity establishes a context that may have a profound effect on event-related interactions or functional connectivity between neuronal populations. Finally, total firing rates, which subsume both background activity and stimulus-related rate modulation, were almost linearly related to the expression of dynamic correlations over large ranges of activities. These observations show that under the assumptions implicit in our model, rate-specific metrics based on rate or coherence modulation may be different perspectives on the same underlying dynamics. This suggests that activity (averaged over all peristimulus times), as measured in neuroimaging, may be tightly coupled to the expression of dynamic correlations.
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Widespread functional disconnectivity in schizophrenia with resting-state functional magnetic resonance imaging.
Using resting-state functional magnetic resonance imaging, we examined the functional connectivity throughout the entire brain in schizophrenia. The abnormalities in functional connectivity were identified by comparing the correlation coefficients of each pair of 116 brain regions between 15 patients and 15 controls. Then, the global distribution of the abnormal functional connectivities was examined. Experimental results indicated, in general, a decreased functional connectivity in schizophrenia during rest, and such abnormalities were widely distributed throughout the entire brain rather than restricted to a few specific brain regions. The results provide a quantitative support for the hypothesis that schizophrenia may arise from the disrupted functional integration of widespread brain areas.
Adult↗
[Formation of functional synaptic connections between heterogeneous brain structures in an organotypic nerve tissue culture].
A directed growth of nervous fibres and formation of functional synaptic connections between heterogeneous brain structures (spinal cord and olfactory bulb of mouse embryos) were studied in organotypic nervous tissue culture by means of neuromorphological and neurophysiological methods. Within the first week in vitro fibre connections between the spinal cord and olfactory bulb explants were formed as glioneuritic bridges. In the formation of these connections glial cells play an active role and organize an oriented substrate upon which there occur growth and fasciation of axons developing later. Silver impregnation showed that connections between the explants were formed by bundles of axons or by separate nerve fibres. An electrophysiological study of the co-cultured heterogeneous brain structures revealed that functional synaptic connections were developed between the spinal cord and olfactory bulb explants by the second week of cultivation. Electrical stimuli applied to the spinal cord explants evoked short- and long-latency responses of neurons in the olfactory bulb explants. The described formation of nonspecific for the given brain structures functional synaptic connections in vitro showed a high degree of morphogenetic plasticity of growing or regenerating axons and an active role of neuroglial cells in a directed growth of nervous fibres.
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