J. J. Wright

Unified neurophysical model of EEG spectra and evoked potentials

Abstract. Evoked potentials – the brains transient electrical responses to discrete stimuli – are modeled as impulse responses using a continuum model of brain electrical activity. Previous models of ongoing brain activity are refined by adding an improved model of thalamic connectivity and modulation, and by allowing for two populations of excitatory cortical neurons distinguished by their axonal ranges. Evoked potentials are shown to be modelable as an impulse response that is a sum of component responses. The component occurring about 100 ms poststimulus is attributed to sensory activation, and this, together with positive and negative feedback pathways between the cortex and thalamus, results in subsequent peaks and troughs that semiquantitatively reproduce those of observed evoked potentials. Modulation of the strengths of positive and negative feedback, in ways consistent with psychological theories of attentional focus, results in d istinct responses resembling those seen in experiments involving attentional changes. The modeled impulse responses reproduce key features of typical experimental evoked response potentials: timing, relative amplitude, and number of peaks. The same model, with further modulation of feedback, also reproduces experimental spectra. Together, these results mean that a broad range of ongoing and transient electrocortical activity can be understood within a common framework, which is parameterized by values that are directly related to physiological and anatomical quantities.

Intracortical connectivity of pyramidal and stellate cells: estimates of synaptic densities and coupling symmetry

A method is outlined for estimating the the average number of synapses forming between cortical neurons as a function of their intercellular separation and the geometry of their dendritic and axonal arborization. Consideration is confined to the formation of local intracortical connections and to the case where the distribution of axonal and dendritic fibres has spherical symmetry. Parameters are deduced from quantitative anatomical studies in murine cortex. It is demonstrated that the majority of local connections forming within a given volume of isotropic cortex can be accounted for on the assumption that local connections between neurons form randomly.From these computations the symmetry of connection between neurons, the likely position for synapse formation on the dendritic tree and the relative synaptic densities attributable to long-and short-range interaction between excitatory and inhibitory neural subsets is determined. Local intracortical couplings appear to be highly asymmetric, and account fo...

Neurophysical Modeling of Brain Dynamics

A recent neurophysical model of brain electrical activity is outlined and applied to EEG phenomena. It incorporates single-neuron physiology and the large-scale anatomy of corticocortical and corticothalamic pathways, including synaptic strengths, dendritic propagation, nonlinear firing responses, and axonal conduction. Small perturbations from steady states account for observed EEGs as functions of arousal. Evoked response potentials (ERPs), correlation, and coherence functions are also reproduced. Feedback via thalamic nuclei is critical in determining the forms of these quantities, the transition between sleep and waking, and stability against seizures. Many disorders correspond to significant changes in EEGs, which can potentially be quantified in terms of the underlying physiology using this theory. In the nonlinear regime, limit cycles are often seen, including a regime in which they have the characteristic petit mal 3 Hz spike-and-wave form.

Synchronous cortical gamma-band activity in task-relevant cognition.

Widespread synchronous oscillatory activity, particularly in the gamma (‘40 Hz’) band, has been postulated to exist in the brain as a mechanism underlying binding. A new method of examining phase synchronicity across multiple electrode sites in specific EEG frequency bands as a function of time was employed, in a conventional cognitive ERP paradigm in 40 normal subjects. A significant late post-stimulus gamma synchronicity response occurred for task-relevant stimuli, whereas for task-irrelevant stimuli no such response was evident. However, an early response was seen for both task-relevant and irrelevant stimuli. This is the first empirical demonstration that widespread synchronous high frequency oscillations occur in humans in relation to cognition.

Maternity Blues: Phenomena and Relationship to Later Post Partum Depression

As part of a prospective investigation of postnatal psychiatric disorder, 66 women, randomly selected antenatally, completed visual analogue scales postpartum and rated their mood state daily for two weeks following childbirth. They were then interviewed using the Goldberg Standardised Psychiatric Interview on four occasions up to 14 months postpartum. Dysphoric mood was temporally related to childbirth. Emotional lability was the important affective component of the puerperium. A significant correlation was found between the blues and subsequent postnatal depression. Lability of mood in the puerperium was related to psychiatric symptoms up to 14 months postpartum and was the strongest predictor of later psychopathology. Maternity blues ratings were not accounted for by labour variables.

Autoregression models of EEG

This paper considers the properties of parameters (natural frequencies and damping coefficients) obtained from segment-by-segment autoregression analysis of ECoG of rat. The use of a reference signal as control for parameter estimate errors, and multiple regression analyses indicate that the dependencies among parameters calculated from ECoG in the alert (desynchronised) state are of a form consistent with imposition of time-invariance assumptions (implicit in autoregression) on an inherently non-stationary, multimodal, linear and near-equilibrium “thermal” process.

A millimetric-scale simulation of electrocortical wave dynamics based on anatomical estimates of cortical synaptic density

Estimates of the connectivity and synaptic densities of pyramidal and stellate cells of the cerebral cortex obtained earlier permit simulation of a lumped cellular network in which synaptic densities are translated into network coupling coefficients.The simulation reproduces spectral, autoregression and frequency/wavenumber properties of real electrocortical waves, including all major cerebral rhythms.The system control parameter is the mean strength of non-specific afferent inputs to the cortex, by analogy with non-specific cortical activation by the reticular formation and associated pathways. This control regulates total power, spectral density, spatial damping and wave velocity. The range of velocities also corresponds to physiological measurement.

Reticular activation and the dynamics of neuronal networks

It is postulated that during arousal the cortical system is driven by a spatially and temporally noisy signal arising from non-specific reticulo-cortical pathways. An elementary unit of cortical neuroanatomy is assumed, which permits non-linear dynamics to be represented by stochastic linear equations. Under these assumptions the resonant modes of the system of cortical dendrites approach thermodynamic equilibrium. Specific sensory signals perturb the dendritic system about equilibrium, generate low frequency, linear, non-dispersive waves corresponding to the EEG, which in turn regulate action potential sequences, and instantiate internal inputs to the dendritic field. A large and distributed memory capacity in axo-synaptic couplings, resistance to interference between functionally separate logical operations, and a very large next-state function set emerge as properties of the network. The model is able to explain the close association of the EEG with cognition, the channel of low capacity corresponding to the field of immediate attention, the low overall correlation of action potentials with EEG, and specificity of action potentials in some neurons during particular cognitive activity. Predictions made from hypothesis include features of thermal equilibrium in EEG (determinable by autoregression) and expectation that the cortical evoked response can be accounted for as the response to a sensory impulse of specific time characteristics.

Late Component ERPs are Associated with Three Syndromes in Schizophrenia

Previous studies have revealed various abnormalities in late-component ERP amplitude and latency in schizophrenia, considered as a diagnostic category. The aim of this study was to investigate the within-sample associations between late-component ERPs and three primary syndromes of schizophrenia Reality Distortion, Psychomotor Poverty and Disorganisation. Subjects included 40 schizophrenics and 40 age and sex matched nonpsychiatric controls. Auditory ERPs (N100, N200, P200, P300) were elicited using an auditory oddball paradigm. Between-group analyses of target data showed reduced N100, N200 and P300 amplitude, increased P200 amplitude and delayed N200 latency in schizophrenics compared to controls. For non-target data, schizophrenics showed similarly reduced N100 amplitude and delayed N200 latency. Within-group analyses of target data showed that the three syndromes (determined by principal component analysis of PANS ratings) were differentiated by ERP latency, but not amplitude (Disorganisation-delayed left hemisphere P200 and P300 latency; Reality Distortion earlier global, midline and left hemisphere N200 latency; Psychomotor Poverty delayed posterior N100 latency). Notably, only Disorganisation showed a divergent pattern of associations with non-target ERP data: reduced P200 amplitude and delayed N100 latency.

Dynamics of SCR, EEG, and ERP activity in an oddball paradigm with short interstimulus intervals

Studies of concurrent central, and autonomic activity using a conventional event-related potential (ERP) oddball paradigms, are considered useful in elucidating the relationship between central and autonomic responses, but the autonomic response tends to overlap. A new method was used to decompose and score overlapping skin conductance responses (SCR). This method enabled examination of dynamic relationships of phasic SCR, prestimulus electroencephalogram (EEG), and ERP to auditory target stimuli in 50 normal adults. SCR amplitude was negatively correlated to EEG and N200 amplitude. The SCR amplitude changes over time exhibited an exponential decline opposite to those of N200, alpha, and beta. All the fitted exponential functions had a time constant of 1-2 min. The findings suggest that a N200 component, active in the auditory sensory discrimination, is concomitant with the SCR. The narrow range of the time constant may provide a clue to the conjoint processes underlying central and autonomic adaptive functions.