Fig. 6.

Mechanisms of phase-locked networks of cortical ripples and their selection. (A) Potential mechanisms for ripple phase locking across broad cortical regions: (A, i) multiple cortico-cortical reentrant connections between the cortical oscillators; (A, ii) distributed driving oscillations from a subcortical location (e.g., hippocampal ripples). (B) Multiple synergistic mechanisms enabled by cortico-cortical corippling could select a spatiotemporal neural network. (B, i) Spikes arriving at the hyperpolarized phase (e.g., cell #1→3) will be ineffective relative to those arriving at the depolarized phase (2→3) in eliciting spikes in the rippling target area. (B, ii) Coincident spikes arriving from two source areas that are phase-locked with the target area are more likely to trigger an action potential, especially if they arrive in the depolarized phase. Note that conduction times between areas do not need to match the phase lag but could equal the phase lag plus multiples of the cycle time (e.g., 1→3 vs. 2→3). (B, iii) Reentrant activation across cycles could also result from reciprocal connections. Initial (1→2) and return (2→1) distances are equal and so their conduction delays should be similar. (B, iv) The synapses effectively evoking spikes (1→3) would be strengthened with spike-timing-dependent plasticity, whereas those arriving from a non-phase-locked area (2→3) would arrive after the target cell fires and thus be weakened. Whereas the mechanisms in B, i–iii would act synergistically to reinforce in-phase firing between corippling sites within a given ripple, the mechanism in B, iv would act across ripples to reinforce a particular network of corippling sites.