Characteristics of interhemispheric impulse conduction between prelunate gyri of the rhesus monkey

Conduction velocity, G-ratio, and extracellular water as microstructural characteristics of autism spectrum disorder

The neuronal differences contributing to the etiology of autism spectrum disorder (ASD) are still not well defined. Previous studies have suggested that myelin and axons are disrupted during development in ASD. By combining structural and diffusion MRI techniques, myelin and axons can be assessed using extracellular water, aggregate g-ratio, and a new approach to calculating axonal conduction velocity termed aggregate conduction velocity, which is related to the capacity of the axon to carry information. In this study, several innovative cellular microstructural methods, as measured from magnetic resonance imaging (MRI), are combined to characterize differences between ASD and typically developing adolescent participants in a large cohort. We first examine the relationship between each metric, including microstructural measurements of axonal and intracellular diffusion and the T1w/T2w ratio. We then demonstrate the sensitivity of these metrics by characterizing differences between ASD and neurotypical participants, finding widespread increases in extracellular water in the cortex and decreases in aggregate g-ratio and aggregate conduction velocity throughout the cortex, subcortex, and white matter skeleton. We finally provide evidence that these microstructural differences are associated with higher scores on the Social Communication Questionnaire (SCQ) a commonly used diagnostic tool to assess ASD. This study is the first to reveal that ASD involves MRI-measurable in vivo differences of myelin and axonal development with implications for neuronal and behavioral function. We also introduce a novel formulation for calculating aggregate conduction velocity, that is highly sensitive to these changes. We conclude that ASD may be characterized by otherwise intact structural connectivity but that functional connectivity may be attenuated by network properties affecting neural transmission speed. This effect may explain the putative reliance on local connectivity in contrast to more distal connectivity observed in ASD.

Cortical mapping of callosal connections in healthy young adults

The corpus callosum (CC) is the principal white matter bundle supporting communication between the two brain hemispheres. Despite its importance, a comprehensive mapping of callosal connections is still lacking. Here, we constructed the first bidirectional population-based callosal connectional atlas between the midsagittal section of the CC and the cerebral cortex of the human brain by means of diffusion-weighted imaging tractography. The estimated connectional topographic maps within this atlas have the most fine-grained spatial resolution, demonstrate histological validity, and were reproducible in two independent samples. This new resource, a complete and comprehensive atlas, will facilitate the investigation of interhemispheric communication and come with a user-friendly companion online tool (CCmapping) for easy access and visualization of the atlas.

The influence of inter-regional delays in generating large-scale brain networks of phase synchronization

Large-scale networks of phase synchronization are considered to regulate the communication between brain regions fundamental to cognitive function, but the mapping to their structural substrates, i.e., the structure-function relationship, remains poorly understood. Biophysical Network Models (BNMs) have demonstrated the influences of local oscillatory activity and inter-regional anatomical connections in generating alpha-band (8-12 Hz) networks of phase synchronization observed with Electroencephalography (EEG) and Magnetoencephalography (MEG). Yet, the influence of inter-regional conduction delays remains unknown. In this study, we compared a BNM with standard "distance-dependent delays", which assumes constant conduction velocity, to BNMs with delays specified by two alternative methods accounting for spatially varying conduction velocities, "isochronous delays" and "mixed delays". We followed the Approximate Bayesian Computation (ABC) workflow, i) specifying neurophysiologically informed prior distributions of BNM parameters, ii) verifying the suitability of the prior distributions with Prior Predictive Checks, iii) fitting each of the three BNMs to alpha-band MEG resting-state data (N = 75) with Bayesian optimization for Likelihood-Free Inference (BOLFI), and iv) choosing between the fitted BNMs with ABC model comparison on a separate MEG dataset (N = 30). Prior Predictive Checks revealed the range of dynamics generated by each of the BNMs to encompass those seen in the MEG data, suggesting the suitability of the prior distributions. Fitting the models to MEG data yielded reliable posterior distributions of the parameters of each of the BNMs. Finally, model comparison revealed the BNM with "distance-dependent delays", as the most probable to describe the generation of alpha-band networks of phase synchronization seen in MEG. These findings suggest that distance-dependent delays might contribute to the neocortical architecture of human alpha-band networks of phase synchronization. Hence, our study illuminates the role of inter-regional delays in generating the large-scale networks of phase synchronization that might subserve the communication between regions vital to cognition.

Receptive field sizes and neuronal encoding bandwidth are constrained by axonal conduction delays

Studies on population coding implicitly assume that spikes from the presynaptic cells arrive simultaneously at the integrating neuron. In natural neuronal populations, this is usually not the case-neuronal signaling takes time and populations cover a certain space. The spread of spike arrival times depends on population size, cell density and axonal conduction velocity. Here we analyze the consequences of population size and axonal conduction delays on the stimulus encoding performance in the electrosensory system of the electric fish Apteronotus leptorhynchus. We experimentally locate p-type electroreceptor afferents along the rostro-caudal body axis and relate locations to neurophysiological response properties. In an information-theoretical approach we analyze the coding performance in homogeneous and heterogeneous populations. As expected, the amount of information increases with population size and, on average, heterogeneous populations encode better than the average same-size homogeneous population, if conduction delays are compensated for. The spread of neuronal conduction delays within a receptive field strongly degrades encoding of high-frequency stimulus components. Receptive field sizes typically found in the electrosensory lateral line lobe of A. leptorhynchus appear to be a good compromise between the spread of conduction delays and encoding performance. The limitations imposed by finite axonal conduction velocity are relevant for any converging network as is shown by model populations of LIF neurons. The bandwidth of natural stimuli and the maximum meaningful population sizes are constrained by conduction delays and may thus impact the optimal design of nervous systems.

Coherence fails to reliably capture inter-areal interactions in bidirectional neural systems with transmission delays

Accurately measuring and quantifying the underlying interactions between brain areas is crucial for understanding the flow of information in the brain. Of particular interest in the field of electrophysiology is the analysis and characterization of the spectral properties of these interactions. Coherence and Granger-Geweke causality are well-established, commonly used methods for quantifying inter-areal interactions, and are thought to reflect the strength of inter-areal interactions. Here we show that the application of both methods to bidirectional systems with transmission delays is problematic, especially for coherence. Under certain circumstances, coherence can be completely abolished despite there being a true underlying interaction. This problem occurs due to interference caused in the computation of coherence, and is an artifact of the method. We motivate an understanding of the problem through computational modelling and numerical simulations. In addition, we have developed two methods that can recover the true bidirectional interactions in the presence of transmission delays.

The functional characterization of callosal connections

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The functional characterization of callosal connections is informed by anatomical data.

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Callosal connections play a conditional driving role depending on the brain state and behavioral demands.

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Callosal connections play a modulatory function, in addition to a driving role.

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The corpus callosum participates in learning and interhemispheric transfer of sensorimotor habits.

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The corpus callosum contributes to language processing and cognitive functions.

The brain operates through the synaptic interaction of distant neurons within flexible, often heterogeneous, distributed systems. Histological studies have detailed the connections between distant neurons, but their functional characterization deserves further exploration. Studies performed on the corpus callosum in animals and humans are unique in that they capitalize on results obtained from several neuroscience disciplines. Such data inspire a new interpretation of the function of callosal connections and delineate a novel road map, thus paving the way toward a general theory of cortico-cortical connectivity. Here we suggest that callosal axons can drive their post-synaptic targets preferentially when coupled to other inputs endowing the cortical network with a high degree of conditionality. This might depend on several factors, such as their pattern of convergence-divergence, the excitatory and inhibitory operation mode, the range of conduction velocities, the variety of homotopic and heterotopic projections and, finally, the state-dependency of their firing. We propose that, in addition to direct stimulation of post-synaptic targets, callosal axons often play a conditional driving or modulatory role, which depends on task contingencies, as documented by several recent studies.

Measurement of ultra-fast signal progression related to face processing by 7T fMRI

Given that the brain is a dynamic system, the temporal characteristics of brain function are important. Previous functional magnetic resonance imaging (fMRI) studies have attempted to overcome the limitations of temporal resolution to investigate dynamic states of brain activity. However, finding an fMRI method with sufficient temporal resolution to keep up with the progress of neuronal signals in the brain is challenging. This study aimed to detect between‐hemisphere signal progression, occurring on a timescale of tens of milliseconds, in the ventral brain regions involved in face processing. To this end, we devised an inter‐stimulus interval (ISI) stimulation scheme and used a 7T MRI system to obtain fMRI signals with a high signal‐to‐noise ratio. We conducted two experiments: one to measure signal suppression depending on the ISI and another to measure the relationship between the amount of suppression and the ISI. These two experiments enabled us to measure the signal transfer time from a brain region in the ventral visual stream to its counterpart in the opposite hemisphere through the corpus callosum. These findings demonstrate the feasibility of using fMRI to measure ultra‐fast signals (tens of milliseconds) and could facilitate the elucidation of further aspects of dynamic brain function.

Estimating axon conduction velocity in vivo from microstructural MRI

The conduction velocity (CV) of action potentials along axons is a key neurophysiological property central to neural communication. The ability to estimate CV in humans in vivo from non-invasive MRI methods would therefore represent a significant advance in neuroscience. However, there are two major challenges that this paper aims to address: (1) Much of the complexity of the neurophysiology of action potentials cannot be captured with currently available MRI techniques. Therefore, we seek to establish the variability in CV that can be captured when predicting CV purely from parameters that have been reported to be estimatable from MRI: inner axon diameter (AD) and g-ratio. (2) errors inherent in existing MRI-based biophysical models of tissue will propagate through to estimates of CV, the extent to which is currently unknown. Issue (1) is investigated by performing a sensitivity analysis on a comprehensive model of axon electrophysiology and determining the relative sensitivity to various morphological and electrical parameters. The investigations suggest that 85% of the variance in CV is accounted for by variation in AD and g-ratio. The observed dependency of CV on AD and g-ratio is well characterised by the previously reported model by Rushton. Issue (2) is investigated through simulation of diffusion and relaxometry MRI data for a range of axon morphologies, applying models of restricted diffusion and relaxation processes to derive estimates of axon volume fraction (AVF), AD and g-ratio and estimating CV from the derived parameters. The results show that errors in the AVF have the biggest detrimental impact on estimates of CV, particularly for sparse fibre populations (AVF<0.3). For our equipment set-up and acquisition protocol, CV estimates are most accurate (below 5% error) where AVF is above 0.3, g-ratio is between 0.6 and 0.85 and AD is high (above 4μm). CV estimates are robust to errors in g-ratio estimation but are highly sensitive to errors in AD estimation, particularly where ADs are small. We additionally show CV estimates in human corpus callosum in a small number of subjects. In conclusion, we demonstrate accurate CV estimates are possible in regions of the brain where AD is sufficiently large. Problems with estimating ADs for smaller axons presents a problem for estimating CV across the whole CNS and should be the focus of further study.

An enhanced role for right hV5/MT+ in the analysis of motion in the contra- and ipsi-lateral visual hemi-fields

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TMS applied to MT/TO-1 and MST/TO-2 disrupts translational motion.

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In the right hemisphere, disruption affects contra-and ipsi-lateral hemi-fields.

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In the left hemisphere, disruption is restricted to the contra-lateral hemi-field.

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Suggests enhanced role for right hemisphere in full-field motion perception.

Previous experiments have demonstrated that transcranial magnetic stimulation (TMS) of human V5/MT+, in either the left or right cerebral hemisphere, can induce deficits in visual motion perception in their respective contra- and ipsi-lateral visual hemi-fields. However, motion deficits in the ipsi-lateral hemi-field are greater when TMS is applied to V5/MT + in the right hemisphere relative to the left hemisphere. One possible explanation for this asymmetry might lie in differential stimulation of sub-divisions within V5/MT + across the two hemispheres. V5/MT + has two major sub-divisions; MT/TO-1 and MST/TO-2, the latter area contains neurons with large receptive fields (RFs) that extend up to 15° further into the ipsi-lateral hemi-field than the former. We wanted to examine whether applying TMS to MT/TO-1 and MST/TO-2 separately could explain the previously reported functional asymmetries for ipsi-lateral motion processing in V5/MT + across right and left cerebral hemispheres. MT/TO-1 and MST/TO-2 were identified in seven subjects using fMRI localisers. In psychophysical experiments subjects identified the translational direction (up/down) of coherently moving dots presented in either the left or right visual field whilst repetitive TMS (25 Hz; 70%) was applied synchronously with stimulus presentation. Application of TMS to MT/TO-1 and MST/TO-2 in the right hemisphere affected translational direction discrimination in both contra-lateral and ipsi-lateral visual fields. In contrast, deficits of motion perception following application of TMS to MT/TO-1 and MST/TO-2 in the left hemisphere were restricted to the contra-lateral visual field. This result suggests an enhanced role for the right hemisphere in processing translational motion across the full visual field.

Corticocortical Systems Underlying High-Order Motor Control

Cortical networks are characterized by the origin, destination, and reciprocity of their connections, as well as by the diameter, conduction velocity, and synaptic efficacy of their axons. The network formed by parietal and frontal areas lies at the core of cognitive-motor control because the outflow of parietofrontal signaling is conveyed to the subcortical centers and spinal cord through different parallel pathways, whose orchestration determines, not only when and how movements will be generated, but also the nature of forthcoming actions. Despite intensive studies over the last 50 years, the role of corticocortical connections in motor control and the principles whereby selected cortical networks are recruited by different task demands remain elusive. Furthermore, the synaptic integration of different cortical signals, their modulation by transthalamic loops, and the effects of conduction delays remain challenging questions that must be tackled to understand the dynamical aspects of parietofrontal operations. In this article, we evaluate results from nonhuman primate and selected rodent experiments to offer a viewpoint on how corticocortical systems contribute to learning and producing skilled actions. Addressing this subject is not only of scientific interest but also essential for interpreting the devastating consequences for motor control of lesions at different nodes of this integrated circuit. In humans, the study of corticocortical motor networks is currently based on MRI-related methods, such as resting-state connectivity and diffusion tract-tracing, which both need to be contrasted with histological studies in nonhuman primates.

Hyperactive Response of Direct Pathway Striatal Projection Neurons to L-dopa and D1 Agonism in Freely Moving Parkinsonian Mice

Dopamine (DA) profoundly stimulates motor function as demonstrated by the hypokinetic motor symptoms in Parkinson's disease (PD) and by the hyperkinetic motor side effects during dopaminergic treatment of PD. Dopamine (DA) receptor-bypassing, optogenetics- and chemogenetics-induced spike firing of striatal DA D1 receptor (D1R)-expressing, direct pathway medium spiny neurons (dSPNs or dMSNs) promotes movements. However, the endogenous D1R-mediated effects, let alone those of DA replacement, on dSPN spike activity in freely-moving animals is not established. Here we show that using transcription factor Pitx3 null mutant (Pitx3Null) mice as a model for severe and consistent DA denervation in the dorsal striatum in Parkinson's disease, antidromically identified striatonigral neurons (D1R-expressing dSPNs) had a lower baseline spike firing rate than that in DA-intact normal mice, and these neurons increased their spike firing more strongly in Pitx3Null mice than in WT mice in response to injection of L-dopa or the D1R agonist, SKF81297; the increase in spike firing temporally coincided with the motor-stimulating effects of L-dopa and SKF81297. Taken together, these results provide the first evidence from freely moving animals that in parkinsonian striatum, identified behavior-promoting dSPNs become hyperactive upon the administration of L-dopa or a D1 agonist, likely contributing to the profound dopaminergic motor stimulation in parkinsonian animals and PD patients.

The influence of filtering and downsampling on the estimation of transfer entropy

Transfer entropy (TE) provides a generalized and model-free framework to study Wiener-Granger causality between brain regions. Because of its nonparametric character, TE can infer directed information flow also from nonlinear systems. Despite its increasing number of applications in neuroscience, not much is known regarding the influence of common electrophysiological preprocessing on its estimation. We test the influence of filtering and downsampling on a recently proposed nearest neighborhood based TE estimator. Different filter settings and downsampling factors were tested in a simulation framework using a model with a linear coupling function and two nonlinear models with sigmoid and logistic coupling functions. For nonlinear coupling and progressively lower low-pass filter cut-off frequencies up to 72% false negative direct connections and up to 26% false positive connections were identified. In contrast, for the linear model, a monotonic increase was only observed for missed indirect connections (up to 86%). High-pass filtering (1 Hz, 2 Hz) had no impact on TE estimation. After low-pass filtering interaction delays were significantly underestimated. Downsampling the data by a factor greater than the assumed interaction delay erased most of the transmitted information and thus led to a very high percentage (67–100%) of false negative direct connections. Low-pass filtering increases the number of missed connections depending on the filters cut-off frequency. Downsampling should only be done if the sampling factor is smaller than the smallest assumed interaction delay of the analyzed network.

A Novel Approach for Studying the Physiology and Pathophysiology of Myelinated and Non-Myelinated Axons in the CNS White Matter

Advances in brain connectomics set the need for detailed knowledge of functional properties of myelinated and non-myelinated (if present) axons in specific white matter pathways. The corpus callosum (CC), a major white matter structure interconnecting brain hemispheres, is extensively used for studying CNS axonal function. Unlike another widely used CNS white matter preparation, the optic nerve where all axons are myelinated, the CC contains also a large population of non-myelinated axons, making it particularly useful for studying both types of axons. Electrophysiological studies of optic nerve use suction electrodes on nerve ends to stimulate and record compound action potentials (CAPs) that adequately represent its axonal population, whereas CC studies use microelectrodes (MEs), recording from a limited area within the CC. Here we introduce a novel robust isolated "whole" CC preparation comparable to optic nerve. Unlike ME recordings where the CC CAP peaks representing myelinated and non-myelinated axons vary broadly in size, "whole" CC CAPs show stable reproducible ratios of these two main peaks, and also reveal a third peak, suggesting a distinct group of smaller caliber non-myelinated axons. We provide detailed characterization of "whole" CC CAPs and conduction velocities of myelinated and non-myelinated axons along the rostro-caudal axis of CC body and show advantages of this preparation for comparing axonal function in wild type and dysmyelinated shiverer mice, studying the effects of temperature dependence, bath-applied drugs and ischemia modeled by oxygen-glucose deprivation. Due to the isolation from gray matter, our approach allows for studying CC axonal function without possible "contamination" by reverberating signals from gray matter. Our analysis of "whole" CC CAPs revealed higher complexity of myelinated and non-myelinated axonal populations, not noticed earlier. This preparation may have a broad range of applications as a robust model for studying myelinated and non-myelinated axons of the CNS in various experimental models.

Glial Regulation of the Neuronal Connectome through Local and Long-Distant Communication

If "the connectome" represents a complete map of anatomical and functional connectivity in the brain, it should also include glia. Glia define and regulate both the brain's anatomical and functional connectivity over a broad range of length scales, spanning the whole brain to subcellular domains of synaptic interactions. This Perspective article examines glial interactions with the neuronal connectome (including long-range networks, local circuits, and individual synaptic connections) and highlights opportunities for future research. Our understanding of the structure and function of the neuronal connectome would be incomplete without an understanding of how all types of glia contribute to neuronal connectivity and function, from single synapses to circuits.

Psychophysical inference of frequency-following fidelity in the neural substrate for brain stimulation reward

The rewarding effect of electrical brain stimulation has been studied extensively for 60 years, yet the identity of the underlying neural circuitry remains unknown. Previous experiments have characterized the directly stimulated ("first-stage") neurons implicated in self-stimulation of the medial forebrain bundle. Their properties are consistent with those of fine, myelinated axons, at least some of which project rostro-caudally. These properties do not match those of dopaminergic neurons. The present psychophysical experiment estimates an additional first-stage characteristic: maximum firing frequency. We test a frequency-following model that maps the experimenter-set pulse frequency into the frequency of firing induced in the directly stimulated neurons. As pulse frequency is increased, firing frequency initially increases at the same rate, then becomes probabilistic, and finally levels off. The frequency-following function is based on the counter model which holds that the rewarding effect of a pulse train is determined by the aggregate spike rate triggered in first-stage neurons during a given interval. In 7 self-stimulating rats, we measured current- vs. pulse-frequency trade-off functions. The trade-off data were well described by the frequency-following model, and its upper asymptote was approached at a median value of 360 Hz (IQR = 46 Hz). This value implies a highly excitable, non-dopaminergic population of first-stage neurons. Incorporating the frequency-following function and parameters in Shizgal's 3-dimensional reward-mountain model improves its accuracy and predictive power.

Distribution of axon diameters in cortical white matter: an electron-microscopic study on three human brains and a macaque

The aim of this study was to obtain information on the axonal diameters of cortico-cortical fibres in the human brain, connecting distant regions of the same hemisphere via the white matter. Samples for electron microscopy were taken from the region of the superior longitudinal fascicle and from the transitional white matter between temporal and frontal lobe where the uncinate and inferior occipitofrontal fascicle merge. We measured the inner diameter of cross sections of myelinated axons. For comparison with data from the literature on the human corpus callosum, we also took samples from that region. For comparison with well-fixed material, we also included samples from corresponding regions of a monkey brain (Macaca mulatta). Fibre diameters in human brains ranged from 0.16 to 9 μm. Distributions of diameters were similar in the three systems of cortico-cortical fibres investigated, both in humans and the monkey, with most of the average values below 1 μm diameter and a small population of much thicker fibres. Within individual human brains, the averages were larger in the superior longitudinal fascicle than in the transitional zone between temporal and frontal lobe. An asymmetry between left and right could be found in one of the human brains, as well as in the monkey brain. A correlation was also found between the thickness of the myelin sheath and the inner axon diameter for axons whose calibre was greater than about 0.6 μm. The results are compared to white matter data in other mammals and are discussed with respect to conduction velocity, brain size, cognition, as well as diffusion weighted imaging studies.

Neocortical dynamics due to axon propagation delays in cortico-cortical fibers: EEG traveling and standing waves with implications for top-down influences on local networks and white matter disease

The brain is treated as a nested hierarchical complex system with substantial interactions across spatial scales. Local networks are pictured as embedded within global fields of synaptic action and action potentials. Global fields may act top-down on multiple networks, acting to bind remote networks. Because of scale-dependent properties, experimental electrophysiology requires both local and global models that match observational scales. Multiple local alpha rhythms are embedded in a global alpha rhythm. Global models are outlined in which cm-scale dynamic behaviors result largely from propagation delays in cortico-cortical axons and cortical background excitation level, controlled by neuromodulators on long time scales. The idealized global models ignore the bottom-up influences of local networks on global fields so as to employ relatively simple mathematics. The resulting models are transparently related to several EEG and steady state visually evoked potentials correlated with cognitive states, including estimates of neocortical coherence structure, traveling waves, and standing waves. The global models suggest that global oscillatory behavior of self-sustained (limit-cycle) modes lower than about 20 Hz may easily occur in neocortical/white matter systems provided: Background cortical excitability is sufficiently high; the strength of long cortico-cortical axon systems is sufficiently high; and the bottom-up influence of local networks on the global dynamic field is sufficiently weak. The global models provide “entry points” to more detailed studies of global top-down influences, including binding of weakly connected networks, modulation of gamma oscillations by theta or alpha rhythms, and the effects of white matter deficits.

EEG functional connectivity, axon delays and white matter disease

OBJECTIVE

Both structural and functional brain connectivities are closely linked to white matter disease. We discuss several such links of potential interest to neurologists, neurosurgeons, radiologists, and non-clinical neuroscientists.

METHODS

Treatment of brains as genuine complex systems suggests major emphasis on the multi-scale nature of brain connectivity and dynamic behavior. Cross-scale interactions of local, regional, and global networks are apparently responsible for much of EEG's oscillatory behaviors. Finite axon propagation speed, often assumed to be infinite in local network models, is central to our conceptual framework.

RESULTS

Myelin controls axon speed, and the synchrony of impulse traffic between distant cortical regions appears to be critical for optimal mental performance and learning. Several experiments suggest that axon conduction speed is plastic, thereby altering the regional and global white matter connections that facilitate binding of remote local networks.

CONCLUSIONS

Combined EEG and high resolution EEG can provide distinct multi-scale estimates of functional connectivity in both healthy and diseased brains with measures like frequency and phase spectra, covariance, and coherence.

SIGNIFICANCE

White matter disease may profoundly disrupt normal EEG coherence patterns, but currently these kinds of studies are rare in scientific labs and essentially missing from clinical environments.

The primate amygdala combines information about space and value

A stimulus predicting reinforcement can trigger emotional responses, such as arousal, as well as cognitive ones, such as increasing attention towards that stimulus. Neuroscientists have long appreciated that the amygdala mediates spatially non-specific emotional responses, but it remains unclear whether the amygdala links motivational and spatial representations. To test whether amygdala neurons encode spatial and motivational information, we presented reward-predictive cues in different spatial configurations while assessing whether these cues influenced spatial attention. Cue configuration and predicted reward magnitude modulated amygdala neural activity in a coordinated fashion. Moreover, fluctuations in activity were correlated with trial-to-trial variability in spatial attention. Thus the amygdala integrates spatial and motivational information, which may influence the spatial allocation of cognitive resources. These results suggest that amygdala dysfunction may contribute to deficits in cognitive processes normally coordinated with emotional responses, such as directing attention towards the location of emotionally-relevant stimuli.

Areal differences in diameter and length of corticofugal projections

Cortical areas differ in the size and distribution of neuronal cell bodies, density, and distribution of myelinated axons, connections, and functional properties. We find that they also differ in the diameter of long corticofugal axons, with the thickest axons originating from primary motor, somatosensory, and visual areas and the thinnest ones from prefrontal and temporal areas. Since diameter is proportional to axonal conduction velocity, it can be inferred that action potentials issued from the different areas will be relayed to their targets at different speed. Conduction delays also depend on conduction distance. By computing conduction velocity and conduction distances, we found the longest conduction delays for the primary visual and temporal areas and the shortest for the premotor, primary motor, and somatosensory areas, compatible with the available electrophysiological data. These findings seem to establish a new principle in cortical organization relevant to the pathophysiology of neurological or psychiatric illnesses as well as to the speed of information processing in cortical circuits.

Development and evolution: two determinants of cortical connectivity

The production of genotypic and phenotypic diversity (differentiation) is the final outcome of both development and evolution, of nervous systems and of their components. Cortical axons, in particular, differentiate into a variety of phenotypes which are responsible for computational transformations of messages exchanged among neurons. One aspect of this differentiation concerns axon diameters whose diversity in development, but also within and across species, is enhanced by the addition of a relatively small proportion of thicker axons to some axonal pathways. This, combined with differences in the length of pathways and in brain volumes, has rescaled and expanded the temporal range of interneuronal communication. In both development and evolution, this and other aspects of axonal phenotypes, namely the structure and distribution of axonal arbors, differentiate by the combined action of cell intrinsic (genetic) variation, overproduction, and selection by the environment.

Characterization of the causality between spike trains with permutation conditional mutual information

Uncovering the causal relationship between spike train recordings from different neurons is a key issue for understanding the neural coding. This paper presents a method, called permutation conditional mutual information (PCMI), for characterizing the causality between a pair of neurons. The performance of this method is demonstrated with the spike trains generated by the Poisson point process model and the Izhikevich neuronal model, including estimation of the directionality index and detection of the temporal dynamics of the causal link. Simulations show that the PCMI method is superior to the transfer entropy and causal entropy methods at identifying the coupling direction between the spike trains. The advantages of PCMI are twofold: It is able to estimate the directionality index under the weak coupling and against the missing and extra spikes.

Transfer entropy--a model-free measure of effective connectivity for the neurosciences

Understanding causal relationships, or effective connectivity, between parts of the brain is of utmost importance because a large part of the brain's activity is thought to be internally generated and, hence, quantifying stimulus response relationships alone does not fully describe brain dynamics. Past efforts to determine effective connectivity mostly relied on model based approaches such as Granger causality or dynamic causal modeling. Transfer entropy (TE) is an alternative measure of effective connectivity based on information theory. TE does not require a model of the interaction and is inherently non-linear. We investigated the applicability of TE as a metric in a test for effective connectivity to electrophysiological data based on simulations and magnetoencephalography (MEG) recordings in a simple motor task. In particular, we demonstrate that TE improved the detectability of effective connectivity for non-linear interactions, and for sensor level MEG signals where linear methods are hampered by signal-cross-talk due to volume conduction.

Transfer entropy--a model-free measure of effective connectivity for the neurosciences

Understanding causal relationships, or effective connectivity, between parts of the brain is of utmost importance because a large part of the brain’s activity is thought to be internally generated and, hence, quantifying stimulus response relationships alone does not fully describe brain dynamics. Past efforts to determine effective connectivity mostly relied on model based approaches such as Granger causality or dynamic causal modeling. Transfer entropy (TE) is an alternative measure of effective connectivity based on information theory. TE does not require a model of the interaction and is inherently non-linear. We investigated the applicability of TE as a metric in a test for effective connectivity to electrophysiological data based on simulations and magnetoencephalography (MEG) recordings in a simple motor task. In particular, we demonstrate that TE improved the detectability of effective connectivity for non-linear interactions, and for sensor level MEG signals where linear methods are hampered by signal-cross-talk due to volume conduction.

Axonal velocity distributions in neural field equations

By modelling the average activity of large neuronal populations, continuum mean field models (MFMs) have become an increasingly important theoretical tool for understanding the emergent activity of cortical tissue. In order to be computationally tractable, long-range propagation of activity in MFMs is often approximated with partial differential equations (PDEs). However, PDE approximations in current use correspond to underlying axonal velocity distributions incompatible with experimental measurements. In order to rectify this deficiency, we here introduce novel propagation PDEs that give rise to smooth unimodal distributions of axonal conduction velocities. We also argue that velocities estimated from fibre diameters in slice and from latency measurements, respectively, relate quite differently to such distributions, a significant point for any phenomenological description. Our PDEs are then successfully fit to fibre diameter data from human corpus callosum and rat subcortical white matter. This allows for the first time to simulate long-range conduction in the mammalian brain with realistic, convenient PDEs. Furthermore, the obtained results suggest that the propagation of activity in rat and human differs significantly beyond mere scaling. The dynamical consequences of our new formulation are investigated in the context of a well known neural field model. On the basis of Turing instability analyses, we conclude that pattern formation is more easily initiated using our more realistic propagator. By increasing characteristic conduction velocities, a smooth transition can occur from self-sustaining bulk oscillations to travelling waves of various wavelengths, which may influence axonal growth during development. Our analytic results are also corroborated numerically using simulations on a large spatial grid. Thus we provide here a comprehensive analysis of empirically constrained activity propagation in the context of MFMs, which will allow more realistic studies of mammalian brain activity in the future.

Evolution amplified processing with temporally dispersed slow neuronal connectivity in primates

The corpus callosum (CC) provides the main route of communication between the 2 hemispheres of the brain. In monkeys, chimpanzees, and humans, callosal axons of distinct size interconnect functionally different cortical areas. Thinner axons in the genu and in the posterior body of the CC interconnect the prefrontal and parietal areas, respectively, and thicker axons in the midbody and in the splenium interconnect primary motor, somatosensory, and visual areas. At all locations, axon diameter, and hence its conduction velocity, increases slightly in the chimpanzee compared with the macaque because of an increased number of large axons but not between the chimpanzee and man. This, together with the longer connections in larger brains, doubles the expected conduction delays between the hemispheres, from macaque to man, and amplifies their range about 3-fold. These changes can have several consequences for cortical dynamics, particularly on the cycle of interhemispheric oscillators.

Frontal eye field neurons with spatial representations predicted by their subcortical input

The frontal eye field (FEF) is a cortical structure involved in cognitive aspects of eye movement control. Neurons in the FEF, as in most of cerebral cortex, primarily represent contralateral space. They fire for visual stimuli in the contralateral field and for saccadic eye movements made to those stimuli. Yet many FEF neurons engage in sophisticated functions that require flexible spatial representations such as shifting receptive fields and vector subtraction. Such functions require knowledge about all of space, including the ipsilateral hemifield. How does the FEF gain access to ipsilateral information? Here, we provide evidence that one source of ipsilateral information may be the opposite superior colliculus (SC) in the midbrain. We physiologically identified neurons in the FEF that receive input from the opposite SC, same-side SC, or both. We found a striking structure-function relationship: the laterality of the response field of an FEF neuron was predicted by the laterality of its SC inputs. FEF neurons with input from the opposite SC had ipsilateral fields, whereas neurons with input from the same-side SC had contralateral fields. FEF neurons with input from both SCs had lateralized fields that could point in any direction. The results suggest that signals from the two SCs provide each FEF with information about all of visual space, a prerequisite for higher level sensorimotor computations.

Different contributions of the corpus callosum and cerebellum to motor coordination in monkey

We investigated the different contribution of the corpus callosum (CC) and cerebellum to motor control in two macaque monkeys trained to perform a precision grip task with one or both hands. Recordings were made from antidromically identified CC cells and nearby unidentified neurons (UIDs) in the hand representation of the supplementary motor area (SMA) and compared with cells from the deep cerebellar nuclei (DCN). All cells showed their greatest modulation in activity (rate change locked to particular task event) during the movement epochs of the task (CC, 21.3 +/- 22.2; UIDs, 36.2 +/- 30.1 spike/s for contralateral trials; DCN, 63 +/- 56.4 for ipsilateral trials; mean +/- SD). Surprisingly, CC cells fired at very low basal rates compared with UIDs (3.9 +/- 4.9 vs. 10 +/- 9.1 spike/s) or DCN neurons (50.8 +/- 23.8 spike/s). However, SMA cells had the greatest rate modulation to baseline ratio (CC: 12.1 +/- 13.7; UID: 5.3 +/- 5.4; DCN: 1.7 +/- 2.0). This would allow them to code the timing of a behavioral event with better fidelity than DCN cells. A multivariate regression analysis between cell firing and EMG measured cells' representation of moment-by-moment modulations in muscle activity. CC neurons coded these real-time behavioral parameters significantly less well than the other cells types, using both linear and nonlinear models. Basal firing rate substantially constrains cell function. CC cells with low basal rates have restricted dynamic range for coding continuous parameters, but efficiently code the time of discrete behavioral events. DCN neurons with higher basal rates are better suited to control continuously variable parameters of movement.

Functional connections within the human inferior frontal gyrus

The highly convoluted and cytoarchitectonically diverse inferior frontal gyrus (IFG) of humans is known to be critically involved in a wide range of complex operations including speech and language processing. The neural circuitry that underlies these operations is not fully understood. We hypothesized that this neural circuitry includes functional connections within and between the three major IFG subgyri: the pars orbitalis, pars triangularis, and pars opercularis. To test this hypothesis we employed electrical stimulation tract-tracing techniques in 10 human patients undergoing surgical treatment for intractable epilepsy. The approach involved delivering repeated bipolar electrical stimuli to one site on the IFG while recording the electrical response evoked by that stimulus from a 64-contact grid overlying more distant IFG sites. In all subjects, stimulation of a site on one subgyrus evoked polyphasic potentials at distant sites, either on the same subgyrus or on an adjacent subgyrus. This provided prima facie evidence for a functional connection between the site of stimulation and the sites of the evoked response. The averaged evoked potentials tended to aggregate as response fields. The spatial spread of a response field indicated a divergent projection from the site of stimulation. When two or more sites were stimulated, the resulting evoked potentials exhibited different waveforms while the respective response fields could overlap substantially, suggesting that input from multiple sites converged but by engaging different neural circuits. The earliest deflection in the evoked potential ranged from 2 to 10 msec. No differences were noted between language-dominant and language-nondominant hemispheres.

Cortico-cerebellar coherence during a precision grip task in the monkey

We studied the synchronization of single units in macaque deep cerebellar nuclei (DCN) with local field potentials (LFPs) in primary motor cortex (M1) bilaterally during performance of a precision grip task. Analysis was restricted to periods of steady holding, during which M1 oscillations are known to be strongest. Significant coherence between DCN units and M1 LFP oscillations bilaterally was seen at approximately 10-40 Hz (contralateral M1: 25/87 units; ipsilateral: 9/87 units). Averaged coherence between DCN units and contralateral M1 LFP showed a prominent approximately 17-Hz coherence peak and an average phase of approximately -pi/2 radians, implying that the DCN units fired around the time of maximal depolarization of M1 cells. The lack of a time delay between DCN and M1 activity suggests that the cerebellum and cortex may form a pair of phase coupled oscillators. Although coherence values were low (mean peak coherence, 0.018), we used a computational model to show that this probably resulted from the nonlinearity of spike generating mechanisms within the DCN. DCN unit discharge and DCN LFPs also showed significant coherence at approximately 10-40 Hz, with similarly low magnitude (mean peak coherence, 0.012). The average coherence phase was -2.5 radians for the 6- to 14-Hz range and -1.1 radians for the 17- to 41-Hz range, suggesting different frequency-specific underlying mechanisms. Finally, 4/40 pairs of simultaneously recorded DCN units showed a significant cross-correlation peak, and 16/40 pairs showed significant unit-unit coherence. The extensive oscillatory synchronization observed between cerebellum and motor cortex may have functional importance in sensorimotor processing.

Myelination: an overlooked mechanism of synaptic plasticity?

Myelination of the brain continues through childhood into adolescence and early adulthood--the question is, Why? Two new articles provide intriguing evidence that myelination may be an underappreciated mechanism of activity-dependent nervous system plasticity: one study reported increased myelination associated with extensive piano playing, another indicated that rats have increased myelination of the corpus callosum when raised in environments providing increased social interaction and cognitive stimulation. These articles make it clear that activity-dependent effects on myelination cannot be considered strictly a developmental event. They raise the question of whether myelination is an overlooked mechanism of activity-dependent plasticity, extending in humans until at least age 30. It has been argued that regulating the speed of conduction across long fiber tracts would have a major influence on synaptic response, by coordinating the timing of afferent input to maximize temporal summation. The increase in synaptic amplitude could be as large as neurotransmitter-based mechanisms of plasticity, such as LTP. These new findings raise a larger question: How did the oligodendrocytes know they were practicing the piano or that their environment was socially complex?

A Functional Connection Between Inferior Frontal Gyrus and Orofacial Motor Cortex in Human

The inferior frontal gyrus (IFG) of humans is known to play a critical role in speech production. The IFG is a highly convoluted and cytoarchitectonically diverse structure, classically forming 3 subgyri. It is reasonable to speculate that during speaking the IFG, or some portion of it, influences by corticocortical connections the orofacial representational area of primary motor cortex. To test the hypothesis that such corticocortical connections exist, electrical-stimulation tract tracing experiments were performed intraoperatively on 14 human subjects undergoing surgical treatment of medically intractable epilepsy. Bipolar electrical stimulation was applied to sites on the IFG, while the resulting evoked potentials were recorded from orofacial motor cortex, using a multichannel recording array. Stimulation of the IFG evoked polyphasic waveforms on motor cortex of both language-dominant and -nondominant hemispheres. The evoked waveforms had consistent features across subjects. The responses were seen in discrete regions on precentral cortex. Stimulation of motor cortex also evoked responses on portions of IFG. The data provide evidence for a functional connection between the human IFG and orofacial motor cortex.

Source activity in the human secondary somatosensory cortex depends on the size of corpus callosum

If corpus callosum (CC) mediates the activation of the secondary somatosensory area (SII) ipsilateral to the side of stimulation, then the peak latencies of the contra- and ipsilateral SII activity as well as the amplitude of the ipsilateral SII activity should correlate with the size of CC. Innocuous electrical stimuli of five different intensities were applied to the ventral surface of the right index finger in 15 right-handed men. EEG was recorded using 82 closely spaced electrodes. The size of CC and of seven callosal regions was measured from the mid-sagittal slice of a high-resolution anatomical MRI. The activation in the contralateral and ipsilateral SII was evaluated using spatio-temporal source analysis. At the strongest stimulus intensity, the size of the intermediate part of the callosal truncus correlated negatively with the interpeak latency of the sources in ipsi- and contralateral SII (r = -0.83, P < 0.01). Stepwise regression analysis showed that the large size of the intermediate truncus of CC was paralleled by a latency reduction of peak activity of the ipsilateral SII, whereas both contra- and ipsilateral peak latencies were positively correlated. The peak amplitude of the ipsilateral SII source correlated positively with the size of the intermediate truncus of CC, and with the peak amplitudes of sources in the primary somatosensory cortex (SI) and in the mesial frontal cortex. The results suggest that in right-handed neurologically normal men, the size of the intermediate callosal truncus contributes to the timing and amplitude of ipsilateral SII source activity.

Comparison of cortico-cortical and cortico-collicular signals for the generation of saccadic eye movements

Many neurons in the frontal eye field (FEF) and lateral intraparietal (LIP) areas of cerebral cortex are active during the visual-motor events preceding the initiation of saccadic eye movements: they respond to visual targets, increase their activity before saccades, and maintain their activity during intervening delay periods. Previous experiments have shown that the output neurons from both LIP and FEF convey the full range of these activities to the superior colliculus (SC) in the brain stem. These areas of cerebral cortex also have strong interconnections, but what signals they convey remains unknown. To determine what these cortico-cortical signals are, we identified the LIP neurons that project to FEF by antidromic activation, and we studied their activity during a delayed-saccade task. We then compared these cortico-cortical signals to those sent subcortically by also identifying the LIP neurons that project to the intermediate layers of the SC. Of 329 FEF projection neurons and 120 SC projection neurons, none were co-activated by both FEF and SC stimulation. FEF projection neurons were encountered more superficially in LIP than SC projection neurons, which is consistent with the anatomical projection of many cortical layer III neurons to other cortical areas and of layer V neurons to subcortical structures. The estimated conduction velocities of FEF projection neurons (16.7 m/s) were significantly slower that those of SC projection neurons (21.7 m/s), indicating that FEF projection neurons have smaller axons. We identified three main differences in the discharge properties of FEF and SC projection neurons: only 44% of the FEF projection neurons changed their activity during the delayed-saccade task compared with 69% of the SC projection neurons; only 17% of the task-related FEF projection neurons showed saccadic activity, whereas 42% of the SC projection neurons showed such increases; 78% of the FEF projection neurons had a visual response but no saccadic activity, whereas only 55% of the SC projection neurons had similar activity. The FEF and SC projection neurons had three similarities: both had visual, delay, and saccadic activity, both had stronger delay and saccadic activity with visually guided than with memory-guided saccades, and both had broadly tuned responses for disparity stimuli, suggesting that their visual receptive fields have a three-dimensional configuration. These observations indicate that the activity carried between parietal and frontal cortical areas conveys a spectrum of signals but that the preponderance of activity conveyed might be more closely related to earlier visual processing than to the later saccadic stages that are directed to the SC.

Human area V5 and motion in the ipsilateral visual field

We have studied area V5 of the human brain with visually-evoked potential (VEP) and functional magnetic resonance imaging (fMRI) methods, using hemifield motion stimuli. Our results confirmed the presence of an ipsilateral field representation in V5 and found: (i) a delay in the ipsilateral response in V5, irrespective of the hemifield stimulated; (ii) a longer ipsilateral delay for left hemifield than for right hemifield stimulation; and (iii) in a patient with a section of the splenium, an absent ipsilateral response for right but not left hemifield stimulation. Together with neurophysiological and anatomical evidence in the monkey, our non-invasive spatial and temporal imaging studies in man reveal that ipsilateral V5 is activated by motion signals transferred from contralateral V5. The asymmetry of ipsilateral delay in normal subjects and the asymmetrical loss of ipsilateral response following splenial section imply that signals related to visual motion are transferred from one V5 to the other through two segregated pathways.

Steady-state visual evoked potentials and travelling waves

OBJECTIVE

The amplitude and phase of the steady-state visual evoked potential (SSVEP) is sensitive to cognition and attention but the underlying mechanism is not well understood. This study examines stimulus evoked changes in the SSVEP phase topography and the putative role of travelling waves.

METHODS

Eighteen subjects viewed a central-field checkerboard and full-field flicker stimulus temporally modulated at the peak alpha rhythm frequency. EEG was recorded from 10 midline scalp sites and the bipolar SSVEP obtained from differences between adjacent electrodes.

RESULTS

The SSVEP phase comprised either progressive variations consistent with travelling waves or a phase reversal consistent with standing waves. The checkerboard pattern elicited travelling wave patterns in 14 subjects with estimated phase velocities ranging from 7 to 11 m/s after correcting for folded cortex. The flicker stimulus elicited phase reversals in 9 subjects, suggesting standing waves. Six subjects demonstrated a phase topography specific to the stimulus with travelling wave patterns associated with the checkerboard and standing wave patterns associated with the flicker.

CONCLUSIONS

These differences suggest the emergence of travelling and standing waves under different spatial configurations of visual input to the cortex and that wave phenomena contribute to the spatiotemporal dynamics of the SSVEP.

Brain electrical source analysis of laser evoked potentials in response to painful trigeminal nerve stimulation

Cerebral generators of long latency brain potentials in response to painful heat stimuli were identified from potential distributions in 31 EEG leads, using the brain electrical source analysis (BESA) programme in the multiple spatio-temporal dipole mode. Data were taken from a study with 10 young healthy male subjects who participated in 3 identical sessions, 1 week apart, with 4 blocks of 40 stimuli (randomized intensities above mean pain threshold). Brief infrared laser heat pulses were applied to the right temple; laser evoked brain potentials (LEPs) were averaged over 40 stimuli per block. BESA was applied to the grand mean maps averaged over the 10 subjects, 3 sessions and 4 stimulus blocks per session, as well as to the individual maps. In all cases 4 generators could consistently be identified by BESA, which were able to explain up to 98.8% of the total variance in scalp distributions at certain time intervals: dipole I with a maximum activity at 106.3 msec in the contralateral somatosensory trigeminal cortex, 19.0 mm beneath the surface; dipole II with a maximum activity at 112.1 msec at the corresponding ipsilateral area at a depth of 13.6 mm; dipole III with a maximum activity at 130.4 msec in the frontal cortex; dipole IV with 2 relative maximum activities at 150.6 and 220.5 msec, localized centrally under the vertex at a depth of 33.1 mm, which described both the late vertex negativity and the consecutive positivity. BESA applied to the individual LEP maps of each individual and session yielded again 4 major generators with sites, strengths and orientations comparable to those of the grand mean evaluations. The standard deviation (S.D.) of site coordinates within subjects was less than 3 mm for dipoles I, II and IV (5 mm for dipole III). The between-subject standard deviation was considerably larger (15 mm), which was attributed to individual differences in head geometry, size and anatomy. Dipoles I and II are assumed to be generators in secondary somatosensory areas of the trigeminal nerve system with bilateral representation, though significantly stronger in the contralateral site. Dipole III in the frontal cortex may be related to attention and arousal processes, as well as to motor cortical initiation for eye movements and muscle effects. The central dipole IV describing all late activity between 150 and 220 msec is probably a representative of perceptual activation and cognitive information processing; it was located in deep midline brain structure, e.g., the cingular gyrus.

Propagation patterns of temporal spikes

In standard EEG recordings, spikes appear as single events characterized mainly by the scalp location of the their peak voltage. The signal-to-noise ratio of raw EEG is usually too high to permit more detailed analysis. We used spike averaging to improve the resolution of interictal spikes in 40 patients with temporal lobe epilepsy. Spikes were identified visually in raw, digitally stored EEG. When multiple spike types were present in a patient, they were grouped separately. Spikes were synchronized for averaging by aligning their negative peaks in a designated channel. Sixteen patients demonstrated spike propagation from anterior temporal to posterior temporal electrode locations. Thirty-six patients demonstrated spread of spikes from anterior temporal to fronto-polar electrode sites. While anterior temporal and fronto-polar spikes were often synchronous, fronto-polar spikes followed anterior temporal discharges in 25% of cases and preceded them in 13%. Spike averaging revealed propagation patterns not apparent on visual inspection of raw EEG. We speculate that these patterns may reflect inherent physiological properties of temporal and frontal neuronal circuits, possibly utilized by the epileptogenic process.

Propagation of interictal epileptic activity in temporal lobe epilepsy

We recorded interictal spikes with closely spaced scalp electrodes and sphenoidal electrodes in four patients with temporal lobe epilepsy. We used multiple dipole modeling to study the number, three-dimensional intracerebral location, time activity, and functional relationship of the neuronal sources underlying the epileptic spike complexes. In all patients, we found two significant sources generating the interictal spikes which showed considerable overlap in both space and time. Source 1 was located in the mesiobasal temporal lobe and generated a restricted negativity at the ipsilateral sphenoidal electrode and a widespread positivity over the vertex. Source 2 could be attributed to the lateral temporal neocortex and was associated with a relatively restricted negativity at the ipsilateral temporal electrodes and a more widespread positivity over the contralateral hemisphere. The sources were well separated in space, with an average distance of 45 mm between them. The time activities of both sources showed similar biphasic patterns, with the mesial source leading the lateral source by approximately 40 msec, suggesting propagation of interictal epileptic activity from the mesiobasal to the lateral temporal lobe.

Long lasting excitability changes in human peripheral nerve

When pairs of equal but submaximal electrical stimuli are delivered to a peripheral nerve, the second stimulus does not always excite the same number of fibers as the first. The number of fibers responding to the second stimulus depends on the interstimulus interval; the refractory period, a well-defined period of hypoexcitability, is followed by longer lasting and less well-characterized periods of hyper- and hypoexcitability. These cycles last at least 200 ms after the initial stimulus. We have carefully studied these cycles of excitability in human peripheral nerve in 12 normal subjects. The magnitude of excitability changes were found to be much greater in motor fibers than in mixed nerve; under some conditions, the motor response was reduced by more than 80% at interstimulus intervals of 40 ms, while the mixed nerve response never varied by more than 20%. In addition, the amplitude of the excitability changes varied as a function of the stimulus strength, so that stimuli that were near threshold or evoked near maximal responses were associated with smaller excitability changes than stimuli evoking midrange responses. Given that the excitability fluctuations are of large magnitude and occur at interresponse intervals easily achieved during physiological firing, it is suggested that they may be important modifiers of firing rate under experimental or physiological conditions.

Cytological and quantitative characteristics of four cerebral commissures in the rhesus monkey

The number, types, and distribution of distinct classes of axons and glia in four cerebral commissures of the adult rhesus monkey (Macaca mulatta) were determined using electron microscopic and immunocytochemical methods. The two neocortical commissures, the corpus callosum, and the anterior commissure contain small but cytologically distinct archicortical components: the hippocampal commissure, which lies ventral to the splenium of the corpus callosum, and the basal telencephalic commissure, which forms a small crescent at the anterior margin of the anterior commissure. Each archicortical pathway is delineated from the adjacent neocortical commissure by a glial capsule. The glia cells that form this border are immunoreactive with antisera directed against glial fibrillary acidic protein (GFAP) and issue long processes that form numerous desmosomal junctions with one another. Braids of these glial processes envelop axonal fascicles within the archicortical commissures. In contrast, the GFAP-positive cells of the corpus callosum and anterior commissure are randomly distributed cells with relatively short stellate processes that do not form boundaries around axon fascicles. Quantitative electron microscopic analysis reveals that approximately 60 million axons connect the two cerebral hemispheres: the corpus callosum contains 56.0 million +/- 3.8 million axons (n = 8), the anterior commissure contains 3.15 million +/- 0.24 million axons (n = 8), the hippocampal commissure has 237,000 axons +/- 31,000 (n = 6), and the basal telencephalic commissure has 193,000 axons +/- 28,000 (n = 5). The number of axons is not directly proportional to the cross-sectional area in any of the commissures because of variation in axonal composition. On the basis of an estimate of approximately 3 billion neurons in the monkey cortex (Shariff, '53), we estimate that between 2 and 3% of all cortical neurons project to the opposite cerebral hemisphere. Subregions of the corpus callosum as well as each of the other commissures consist of characteristic subsets of five classes of axons and contain different proportions of myelinated to unmyelinated fibers. The largest myelinated axons and the smallest proportion of unmyelinated axons (approximately 6%) are found in regions of the corpus callosum that carry projections from primary sensory cortices, whereas the smallest myelinated axons and largest proportion of unmyelinated axons (approximately 30%) are found in regions of the corpus callosum that carry projections from association cortices. Axon composition in the anterior commissure is uniform and resembles that of callosal sectors that contain association projections.(ABSTRACT TRUNCATED AT 400 WORDS)

Neocortical propagation in temporal lobe spike foci on magnetoencephalography and electroencephalography

Propagation of the neuronal population of the interictal epileptic spike was quantified in 5 patients with complex partial epilepsy arising from temporal lobe using electroencephalography and magnetoencephalography. During the spike complex in each patient there was a spike at the deep sphenoidal electrode and a spike at the superficial scalp electrode on spontaneous electroencephalography. In each patient the sphenoidal spike had a different peak latency than the scalp spike, consistent with spike propagation. Electroencephalography was used to trigger two magnetoencephalographic averages of stereotyped spikes during the sphenoidal peak and the scalp peak. Magnetoencephalography discriminated the centers of two cortical spike populations at different latencies, showing deeper localization with sphenoidal trigger and more superficial localization with scalp trigger in each patient (p less than 0.05). Latency differences and propagation distances of spikes were consistent with the conduction velocity of corticocortical fibers. Noninvasive estimates of the cortical surface area of the spikes agreed with estimates obtained by electrocorticography over temporal neocortex. These findings indicate propagation of neuronal populations active during human interictal spikes between deep and superficial cortex of temporal lobe, likely by monosynaptic or oligosynaptic pathways. This interictal system appears to be partly independent of the hippocampal interictal system in complex partial epilepsy.

Computer simulation of neuronal circuit models of rhythmic behavior in the electroencephalogram

A computer program for modeling some features of the electroencephalogram (EEG) recorded by scalp electrodes in terms of the time- and space-dependent interactions of populations of neurons in different hypothetical brain configurations was developed. The input of the model consists of: (1) geometric and anatomic data characterizing the brain configuration; (2) physiologic features characterizing neurons; and (3) functions describing the time-dependent afferent impulses to the brain configuration under study. The output of the model consists of plots of selected intracellular and extracellular potentials as a function of time. In application of the model to various brain configurations, some configurations were sufficiently compact spatially that propagation times of action potentials between elements were either taken to be constant or were nearly independent of distance within the accuracy of the calculations. Other configurations represented cerebral cortex alone or a combined thalamocortical system in which many elements interacted via global interconnections. The basic requirement for rhythmic behavior is the existence of circuits containing inhibitory neurons. The characteristic frequencies of rhythmic oscillations are primarily determined by the local circuit parameters and are relatively independent of global circuit parameters.

Supernormal excitability of fibers of the dorsal hippocampal commissure

The excitability modifications induced by a conditioning stimulus were studied in the three groups of fibers coursing in the dorsal hippocampal commissure of the guinea pig. The response of all fibers was potentiated after a conditioning stimulus greater than and equal to the test stimulus. The potentiation lasted 120 to 600 ms, 180 to 1000 ms, and 120 to 700 ms, respectively, in the three fiber groups. The data indicate that the fibers of the dorsal hippocampal commissure exhibit a prolonged supernormal period during which threshold is reduced for both previously discharged and previously inactive fibers.

Activity-related changes in electrical thresholds of pyramidal tract axons in the behaving monkey

In monkeys generating torques about the wrist we investigated changes in the excitability of pyramidal tract (PT) axons, measured as the probability of evoked antidromic responses in motor cortex with constant juxtathreshold stimuli delivered in the brain stem. When PT stimuli were delivered 2-20 ms after an orthodromic action potential in the PT neuron, the excitability of axons was elevated, with a characteristic post-spike time course. Excitability peaked at a post-pike delay of 7.0 +/- 2.7 ms (n = 33). Axonal thresholds typically dropped to 80-90% of the unconditioned values (obtained for stimuli with no preceding spike). Controlling for such post-spike threshold changes by delivering stimuli at fixed post-spike delays, we found that excitability of many PT axons also fluctuated with the wrist responses, being slightly higher during flexion or extension. The place of movement in which excitability increased had no consistent relation to the phase of movement in which the PTN fired. Task-related threshold changes were also seen in PTNs whose discharge was not modulated with the wrist response. Delivering a subthreshold conditioning stimulus also increased the excitability of most PT axons to a subsequent test stimulus. Such post-stimulus changes may be mediated by the effects of adjacent fibers activated by the conditioning stimuli. The post-spike and post-stimulus changed added in a nonlinear way. All three types of threshold change may be mediated by a common mechanism: changes in the ionic environment of the axon produced by activity of the axon itself or its neighbors.(ABSTRACT TRUNCATED AT 250 WORDS)

Axonal projections and conduction properties of olfactory peduncle neurons in the armadillo (Chaetophractus vellerosus)

Extracellular unit recording was employed to study the axonal properties and efferent projections of antidromically identified neurons in the olfactory peduncle (OP) region of a primitive eutherian macrosmatic mammal, the south american armadillo (Chaetophractus vellerosus). Of 72 cells which satisfied the criteria for antidromic invasion, 55 (76%) and 17 (24%) responded to ipsi- and contralateral olfactory bulb (IOB; COB) stimulation, respectively. The absolute refractory period (3.25 +/- 0.3 ms; mean +/- SE) and the conduction velocity (CV; 1.94 +/- 0.2 m/s; mean +/- SE) of IOB and COB driven neurons were negatively correlated (r = -0.52; p less than 0.001). In paired-shock tests (8-1950 ms interval), and early supernormal period (SPN) of increased CV and excitability was found following the relative refractory period in 82% of tested cells (N = 50); this period was followed by a late subnormal phase (SBN) of decreased CV and increased threshold in 58% of neurons (N = 50). Significant correlations were found to exist between: CV and absolute magnitude of latency variation (r = -0.55; p less than 0.001; n = 43), CV and duration of SPN and SBN periods (r = -0.60; p less than 0.002; n = 24 and r = 0.58; p less than 0.02; n = 19, respectively) and between duration of SPN and SBN phases (r = 0.79; p less than 0.001; n = 30). Maximum latency variation during the SPN and SBN periods was attained in a gradual, additive manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Decrement of somatosensory evoked potentials during repetitive stimulation

In normal subjects, cerebral potentials were evoked by brief, passive extension of the wrist joint at various interstimulus intervals (ISIs). The resulting somatosensory evoked potentials (SEPs) were found to decrease during repetitive stimulation. The greatest decrement occurred between the first and second responses of each series. After cessation of stimuli, the SEP amplitude returned to control values over a prolonged, exponential time course. The authors postulate that the observed response decrement may be a form of habituation, which provides a model for studying the neuronal substrates of behavior.

Prior activity influences the velocity of impulses in frog and cat optic nerve fibers

Under normal visual stimulation, simultaneous recording from ganglion cells in the retina and from the axons of these cells in the brain revealed activity-dependent differences in the velocity of impulse propagation. In frog ganglion cells, spikes initiated 5-500 ms after a previous impulse showed supernormal increases in conduction velocity of up to 17%; spikes initiated 500-2000 ms after a previous one traveled more slowly than a spike initiated after a long period of rest. Cat ganglion cell impulses showed much smaller supernormality (maximum 3%), but exhibited pronounced slowing due to refractoriness and long-term fatigue associated with their high levels of spontaneous activity.