http://download.neurosky.com/docs/papers/bcq/Klimesch_1999_EEG_alpha_and_theta_oscillations_reflect_cognitive_and_memory_performance.pdf

EEG alpha and theta oscillations reflect cognitive and memory performance: a review and analysis

Clocks tick, bridges and skyscrapers vibrate, neuronal networks oscillate. Are neuronal oscillations an inevitable by-product, similar to bridge vibrations, or an essential part of the brain’s design? Mammalian cortical neurons form behavior-dependent oscillating networks of various sizes, which span five orders of magnitude in frequency. These oscillations are phylogenetically preserved, suggesting that they are functionally relevant. Recent findings indicate that network oscillations bias input selection, temporally link neurons into assemblies, and facilitate synaptic plasticity, mechanisms that cooperatively support temporal representation and long-term consolidation of information.

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Neuronal Oscillations in Cortical Networks

Damage to the hippocampal system disrupts recent memory but leaves remote memory intact. The account presented here suggests that memories are first stored via synaptic changes in the hippocampal system, that these changes support reinstatement of recent memories in the neocortex, that neocortical synapses change a little on each reinstatement, and that remote memory is based on accumulated neocortical changes. Models that learn via changes to connections help explain this organization. These models discover the structure in ensembles of items if learning of each item is gradual and interleaved with learning about other items. This suggests that the neocortex learns slowly to discover the structure in ensembles of experiences. The hippocampal system permits rapid learning of new items without disrupting this structure, and reinstatement of new memories interleaves them with others to integrate them into structured neocortical memory systems.

Why there are complementary learning systems in the hippocampus and neocortex: insights from the successes and failures of connectionist models of learning and memory.

The ionotropic glutamate receptors are ligand-gated ion channels that mediate the vast majority of excitatory neurotransmission in the brain. The cloning of cDNAs encoding glutamate receptor subunits, which occurred mainly between 1989 and 1992 ([Hollmann and Heinemann, 1994][1]), stimulated this

/pdf/the-glutamate-receptor-ion-channels-560mntczdb.pdf

The glutamate receptor ion channels

Neuronal activity in the brain gives rise to transmembrane currents that can be measured in the extracellular medium. Although the major contributor of the extracellular signal is the synaptic transmembrane current, other sources — including Na+ and Ca2+ spikes, ionic fluxes through voltage- and ligand-gated channels, and intrinsic membrane oscillations — can substantially shape the extracellular field. High-density recordings of field activity in animals and subdural grid recordings in humans, combined with recently developed data processing tools and computational modelling, can provide insight into the cooperative behaviour of neurons, their average synaptic input and their spiking output, and can increase our understanding of how these processes contribute to the extracellular signal.

/pdf/the-origin-of-extracellular-fields-and-currents-eeg-ecog-lfp-2e79zygxj0.pdf

The origin of extracellular fields and currents — EEG, ECoG, LFP and spikes

Temporal structure in spatially organized neuronal ensembles: a role for interneuronal networks.

Gamma frequency field oscillations (40-100 Hz) are nested within theta oscillations in the dentate-hilar and CA1-CA3 regions of the hippocampus during exploratory behaviors. These oscillations reflect synchronized synaptic potentials that entrain the discharge of neuronal populations within the approximately 10-25 msec range. Using multisite recordings in freely behaving rats, we examined gamma oscillations within the superficial layers (I-III) of the entorhinal cortex. These oscillations increased in amplitude and regularity in association with entorhinal theta waves. Gamma waves showed an amplitude minimum and reversed in phase near the perisomatic region of layer II, indicating that they represent synchronized synaptic potentials impinging on layer II-III neurons. Theta and gamma oscillations in the entorhinal cortex were coupled with theta and gamma oscillations in the dentate hilar region. The majority of layer II-III neurons discharged irregularly but were phase-related to the negative peak of the local (layer II-III) gamma field oscillation. These findings demonstrate that layer II-III neurons discharge in temporally defined gamma windows (approximately 10-25 msec) coupled to the theta cycle. This transient temporal framework, which emerges in both the entorhinal cortex and the hippocampus, may allow spatially distributed subpopulations to form temporally defined ensembles. We speculate that the theta-gamma pattern in the discharge of these neurons is essential for effective neuronal communication and synaptic plasticity in the perforant pathway.

https://www.jneurosci.org/content/jneuro/18/1/388.full.pdf

Gamma Oscillations in the Entorhinal Cortex of the Freely Behaving Rat

Population bursts of the CA3 network, which occur during eating, drinking, awake immobility, and slow-wave sleep, produce a large field excitatory postsynaptic potential throughout stratum radiatum of the CA1 field (sharp wave). The CA3 burst sets into motion a short-lived, dynamic interaction between CA1 pyramidal cells and interneurons, the product of which is a 200 Hz oscillatory field potential (ripple) and phase-related discharge of the CA1 network. Although many CA1 pyramidal neurons discharge during the time (50-100 msec) of each sharp wave, each wave of a ripple (approximately 5 msec) reflects the synchronization of more discrete subsets of CA1 neurons. When we used multi-site recordings in freely behaving rats, we observed ripples throughout the longitudinal extent (approximately 4-5 mm) of the dorsal CA1 region that were coherent for multiple cycles of each ripple. High-frequency ripples were also observed throughout the hippocampal-entorhinal output pathway that were concurrent but less coherent on a cycle-by-cycle basis. Single and multiunit neuronal activity was phase-related to local ripples throughout the hippocampal-entorhinal output pathway. Entorhinal ripples occurred 5-30 msec after the CA1 ripples and were related to the occurrence of an entorhinal sharp wave. Thus, during each hippocampal sharp wave, there is powerful synchronization among the neuronal networks that connect the hippocampus to the neocortex. We suggest that this population interaction (1) biologically constrains theoretical models of hippocampal function and dysfunction and (2) has the capacity to support an "off-line" memory consolidation process.

https://www.jneurosci.org/content/jneuro/16/9/3056.full.pdf

High-frequency oscillations in the output networks of the hippocampal-entorhinal axis of the freely behaving rat

Hippocampal theta activity following selective lesion of the septal cholinergic systeM

The coordinated activity of hippocampal neurons is reflected by macroscopic patterns, theta and sharp waves (SPW), evident in extracellular field recordings. The importance of these patterns is underscored by the ordered relation of specific neuronal populations to each pattern as well as the relation of each pattern to distinct behavioral states. During awake immobility, consummatory behavior, and slow wave sleep, CA3 and CA1 neurons participate in organized population bursts during SPW. In contrast, during theta-associated exploratory activity, the majority of principle cells are silent. Considerably less is known about the discharge properties of retrohippocampal neurons during theta, and particularly during SPW. These retrohippocampal neurons (entorhinal cortical, parasubicular, presubicular, and subicular) process and transmit information between the neocortex and the hippocampus. The present study examined the activity of these neurons in freely behaving rats during SPW (awake immobility) as well as theta (locomotion and REM sleep). A qualitative distinction between the activity of deep (V-VI) and superficial (II-III) layer retrohippocampal neurons was observed in relation to SPW as compared to theta. Deep layer retrohippocampal neurons exhibited a concurrent increase in activity during hippocampal SPW. In contrast, deep layer neurons were not modulated by the prominent theta oscillations observed throughout the hippocampus and entorhinal cortex. On the other hand, superficial layer retrohippocampal neurons were often phase-related to theta oscillations, but were surprisingly indifferent to the SPW-associated population bursting occurring within the deep layers. These findings indicate a concerted discharge of the hippocampal and retrohippocampal cortices during SPW that includes neurons within CA3, CA1, and subiculum as well as neurons in layers V-VI of the presubiculum, parasubiculum, and entorhinal cortex. Further, they suggest a temporal discontinuity in the input/output relations between the hippocampus and retrohippocampal structures. We suggest that SPW-associated population bursts in hippocampal and retrohippocampal cortices exert a powerful depolarizing effect on their postsynaptic neocortical targets and may represent a physiological mechanism for memory trace transfer from the hippocampus to the neocortex.

https://www.jneurosci.org/content/jneuro/14/10/6160.full.pdf

James J. Chrobak

Papers

Temporal structure in spatially organized neuronal ensembles: a role for interneuronal networks.

Gamma Oscillations in the Entorhinal Cortex of the Freely Behaving Rat

High-frequency oscillations in the output networks of the hippocampal-entorhinal axis of the freely behaving rat

Hippocampal theta activity following selective lesion of the septal cholinergic systeM

Selective Activation of Deep Layer (V-VI) Retrohippocampal Cortical Neurons during Hippocampal Sharp Waves in the Behaving Rat