| We consider the nature of the cortical slow oscillation in terms of the behaviour of neuron assemblies. Specifically, we look at the hypothesis that the low- (Ħdown˘) and high- (Ħup˘) firing states correspond to pseudo-stable states of a neuron assembly, and that the transitions between the two are equivalent to first-order phase transitions of physics. First, we analyse the temporal fluctuations in membrane potential prior to a transition from a Ħdown˘ to an Ħup˘ state, for sleeping rats. By averaging over many events, we show that changes in the power spectrum (specifically total power and frequency centroid) on the approach to the transition are broadly consistent with the physical slowing of a system close to a first-order phase transition; it is conjectured that the increased fluctuations may play a role in synaptic strengthening and weakening during the slow oscillation. Secondly, we look at the dynamics of the transition itself; this shows rapid initial movement in membrane potential away from the Ħdown˘ state, again in agreement with a mean-field first-order phase transition model. |
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