A. van Rotterdam

RELATIVE CONTRIBUTIONS OF INTRACORTICAL AND THALAMO-CORTICAL PROCESSES IN THE GENERATION OF ALPHA RHYTHMS, REVEALED BY PARTIAL COHERENCE ANALYSIS

The thalamo-cortical relationships of alpha rhythms have been analysed in dogs using partial coherence function analysis. The objective was to clarify how far the large intracortical coherence commonly recorded between different cortical sites could depend on a common thalamic site. It was found that the alpha rhythms of the LGN influenced only moderately the coherence between cortical alpha rhythms whereas that of the pulvinar had much more influence, at least in relation to some cortical areas. Significant phase shifts between thalamus and cortex were also measured. The results demonstrate that there are thalamo-cortical and cortico-cortical components which interact in the generation of cortical alpha rhythms.

Models of Neuronal Populations: The Basic Mechanisms of Rhythmicity

Publisher Summary This chapter discusses the basic mechanisms of rhythmicity using the models of neuronal populations. The models presented in the chapter allows to understand the neural mechanisms underlying certain types of field potentials revealed in the EEG, particularly the generation of alpha or theta rhythms and of epileptic paroxysms. The importance of this aspect to general neurophysiology may be secondary, however, considering the fact that the field potential does not form an essential element of the activity of neuron populations. However, it is very relevant in view of the paramount importance of the EEG in clinical neurophysiology. In this respect, it should be noted that the EEG is still the neurophysiological signal most widely employed to evaluate normal and pathological human brain activity. The models of alpha and theta rhythms presented in the chapter should not be considered as the models of the EEG. They may well be relevant only for particular aspects of EEG signals; the EEG as recorded from the scalp includes a variety of phenomena about which sufficient basic information is yet not available An exception may be the case of paroxysmal activity characteristic of epileptic foci. The chapter also presents models that include excitatory and inhibitory feedback.

Dynamic characteristics of visual evoked potentials in the dog. II. Beta frequency selectivity in evoked potentials and background activity.

Abstract 1. 1. Sinusoidally modulated light (SML) evoked potentials recorded from the posterior marginal gyrus were described as generated by a beta frequency selective system. 2. 2. The beta resonance is related to the dominant mode of spontaneous activity recorded from the same brain area and characterized by means of power spectra. 3. 3. The resonance behaviour of cortical SML evoked potentials was dependent on the state of the animal and disappeared during light Nembutal anaesthesia. 4. 4. A linear model including resonance was devised to account for the cortical frequency selective system. The model was tested experimentally by obtaining the systems impulse response by means of cross-correlating the input and output of the system excited with noise modulated light. 5. 5. The implications of beta selectivity in terms of neural pathways and networks are discussed.

Dynamic characteristics of visual evoked potentials in the dog. I. Cortical and subcortical potentials evoked by sine wave modulated light

Abstract 1. 1. Potentials evoked by sinusoidally modulated light (SML) were recorded from the lateral geniculate nucleus and occipital cortex of unanaesthetized dogs. The evoked potentials were analysed by discrete Fourier analysis. 2. 2. The harmonic components of SML evoked potentials were described by frequency response functions. The range of linear and non-linear behaviour was determined. 3. 3. A linear description was only possible regarding the SML evoked potentials recorded from the posterior marginal gyrus. 4. 4. Three types of non-linearities were characterized: saturation, non-linear oscillations responsible for the generation of subharmonics and essential non-linearities. 5. 5. The essential non-linearities correspond to rectification occurring in “on” and “off” neuronal populations; they were the dominant features of SML evoked potentials in the lateral geniculate nucleus and the calcarine region of occipital cortex, but not in the posterior marginal gyrus. 6. 6. The phase functions were shown to be determined in part by a delay time. 7. 7. The relations between SML evoked potential parameters and data obtained at the unit level are discussed.

The double dipole model of theta rhythm generation: Simulation of laminar field potential profiles in dorsal hippocampus of the rat

A set of compartmental models of CA1 pyramidal, granular and polymorph cells of the dorsal hippocampus have been used to simulate membrane potentials generated by synaptic activation at various levels along these cells. From the membrane potential distributions the field potentials in dorsal CA1 and the dorsal blade of the dentate area have been simulated using a model based on volume conduction theory. Field potential profiles similar to laminar profiles, found experimentally in the dorsal hippocampus during theta rhythm, could only be simulated by assuming (almost) simultaneous synaptic excitation of the 3 cell types at given sites. The results lead to 2 alternative models for the simultaneous excitation of CA1 pyramidal cells and dentate granular cells during theta rhythm. Other electrophysiological evidence favours the model in which the two neuronal populations are activated distally near the fissure.

Electric and magnetic fields of the brain computed by way of a discrete systems analytical approach: Theory and validation

It is shown how the stationary volume conduction phenomena in the brain, namely the electric and magnetic fields can be described in discrete terms.The volume conductor is sampled in space by introducing a sampling distance corresponding to the uncertainty in the measurements. In this way, a threedimensional lattice is needed with equidistantly spaced nodes. The electric and magnetic properties of such a lattice are assumed to be equivalent to that of brain and other tissues. The electric and magnetic potential fields are calculated for each node as the output of a linear feedback system which has the impressed currents as the input. By way of the feedback loop the reflection phenomena at the boundaries between media of different conductivity can be taken into account.This discrete formalism has been implemented in a software system. To demonstrate the validity and accuracy of this system a number of analytically tractable problem in volume conduction has been evaluated.

Latency distributions in homogeneous bundles of myelinated axons.

Using a very simple hypothesis concerning the length of the depolarized area during propagation of action potentials, distributions of latencies in bundles of myelinated axons have been derived. The internodal length, the number of nodes of Ranvier, the depolarized area and the variation in internodal length are the important parameters.To demonstrate the applicability of the derivation proposed here some examples taken from neurophysiological experiments are given.