167 In contrast to these states, the effectiveness of a represen

167 In contrast to these states, the effectiveness of a representation system is characterized by high variability with minimal interactions among the constituents, furthermore improvement of behavioral performance in behavioral tasks is often characterized by increased interspike variability and reduced spike correlation among neurons.109,165,168-172 This view appears to be at striking odds with suggestions that neuronal oscillations enhance the coding ability of neurons.4,5,17,58,122 There seems to be a compromise between the information processing capacity of neuronal networks and the need for some level of synchronization

of their neurons for Akt phosphorylation efficient Inhibitors,research,lifescience,medical communication. Fast communication among pyramidal neurons in the cortex is established by AMPA-type glutamate receptors, which are weak and have high probability of transmission failures.173 Therefore, in order to convey information to peer neurons, pyramidal cells should come together in transient coalitions, or cell assemblies.46,53 As discussed Inhibitors,research,lifescience,medical above, the goal of the temporal assembly formation is

to discharge the downstream neurons. Thus, for effective communication there is a requirement for a minimum level of synchrony and it may be that γ cycle-defined cell assemblies represent a useable packet of information, which contain varying Inhibitors,research,lifescience,medical specific content.53 Cross-frequency coupling between local γ oscillations and the more global slower rhythms is established by phase synchrony.

However, this coupling mechanism does not imply that the slower oscillation magnifies the synchrony in the γ period. Quite the contrary, γ cycle-embedded cell assemblies are distributed across the phase space of the modulating slow oscillation, as demonstrated Inhibitors,research,lifescience,medical above for θ oscillations. In inhibition-based oscillations, the recruitment of assemblies is deliberately protracted over time. Instead of narrowing the Inhibitors,research,lifescience,medical time window of synchronization, multiple assemblies are separated in nested γ cycles within the slower, modulating θ cycle. In contrast to the intuitive “oscillations-enhance-synchrony” picture, temporal synchrony among principal neurons is lowest during θ oscillations and highest during slow-wave sleep. Another counterintuitive observation is that the coefficient of variation (CV) of the interspike intervals of hippocampal principal cells is also largest during θ-associated exploration almost behaviors and smallest during slow-wave sleep.174 The potential mechanism underlying such “paradoxical” asynchrony and spike variability-promoting functions of θ oscillation is its ability to temporally coordinate excitatory and inhibitory synaptic inputs to neurons.12,31,175 Modeling studies have demonstrated that temporally precise correlated fluctuations of excitatory and inhibitory currents can reduce or cancel each others’ action and decorrelate the synaptic current of postsynaptic neurons.

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