temporal relations requires the joint evaluation of responses from more than one neuron, only experiments that permit simultaneous measurements of responses 60528 Frankfurt from multiple units are considered. These include multi-Federal Republic of Germany electrode recordings from multiple individual cells, but also measurements of local field potentials (LFPs) and electroencephalographic (EEG) or magnetoencephalo-Most of our knowledge about the functional organization of neuronal systems is based on the analysis of the firing patterns of individual neurons that have been recorded one by one in succession. This approach permits as-sessment of event-related variations in discharge rate, but it precludes detection of any covariations in the amplitude or timing of distributed responses if these graphic (MEG) recordings. The signals of these latter

Synchrony among multicortical EEGs 2 Freeman, Gaál & Jörnsten Information transfer and integration among functionally distinct areas of cerebral cortex of oscillatory activity requires some degree of phase synchrony of the trains of action potentials that carry the information prior to the integration. However, propagation delays are obligatory. Delays vary with the lengths and conduction velocities of the axons carrying the information, causing phase dispersion. In order to determine how synchrony is achieved despite dispersion, we recorded EEG signals from multiple electrode arrays on five cortical areas in cats and rabbits, that had been trained to discriminate visual or auditory conditioned stimuli. Analysis by time-lagged correlation, multiple correlation and PCA, showed that maximal correlation was at zero lag and averaged.7, indicating that 50 % of the power in the gamma range among the five areas was at zero lag irrespective of phase or frequency. There were no stimulus-related episodes of transiently increased phase locking among the areas, nor EEG "bursts " of transiently increased amplitude above the sustained level of synchrony. Three operations were identified to account for the sustained correlation. Cortices broadcast their outputs over divergent-convergent axonal

: In this work we discuss Hebb's old ideas about cell assemblies in the light of recent results concerning temporal structure and correlations in neural signals. We want to give a conceptual, necessarily only rough picture, how ideas about `binding by synchronisation', `synfire chains', `local and global assemblies', `short and long term memory' and `behaviour' might be integrated into a coherent model of brain functioning based on neuronal assemblies. Keywords: cell assemblies, synchronization, gamma-oscillations, synfire chains, memory, behaviour 1 ASSEMBLIES AND ASSOCIATIVE MEMORIES 1.1 Cell Assemblies Cell assemblies have been introduced by Donald Hebb with the intention of providing a functional and at the same time structural model for cortical processes and neuronal representations of external events (Hebb, 1949). According to Hebb's ideas, stimuli, objects, things, but also more abstract entities like concepts, contextual relations, ideas, and so on are thought of being repre...

We investigate the propagation of pulses of spike activity in a neuronal network with feed-forward couplings. The neurons are of the spike-response type with a ring probability that depends linearly on the membrane potential. After ring neurons enter a phase of refractoriness. Spike packets are described in terms of the moments of the ring-time distribution so as to allow for an analytic treatment of the evolution of the spike packet as it propagates from one layer to the next. Both analytic results and simulations show that depending on the synaptic coupling strength a stable propagation of the packet with constant wave form is possible. Crucial for this observation is neither the existence of a ring threshold nor a sigmoidal gain function { both are absent in our model { but the refractory behavior of the neurons.

Introduction Recently, while it has been a unanimously accepted hypothesis that spatial or temporal spike patterns of neurons in the central nervous system ( CNS ) transact perception, knowledge, thinking and so on, very little is known as to the most fundamental question of neuroscience how these infomations are encoded and decoded in these spike patterns ([1],[8]). The `leaky integrate-and-fire' neuron model has been widely used to address this question, since this model is believed to be capturing essential properties of the firing behaviour of cortial neurons ([9], [24]). It is well known that spike trains of neurons in cerebral cortex have both spatial and temporal irregularity, which can be characterized by the fact that interspike intervals ( ISIs ) of the spike trains are quite similar to that of Poisson process spike emission ([19]). We have to face this spiking irregularity to clarify the principle of infomation encoding in CNS. There might be two different views co