Figure 5.

Site-specificity of recurrent EPSP effects (model). (A) In network simulations, recurrent EPSPs, present in only one area, produce asynchrony between gamma oscillations in that area and gamma oscillations in an adjacent area. e-cell (e) denotes pyramidal cell. When recurrent excitation is absent (Left), oscillations are tightly synchronized between cell groups 1 and 5 (at opposite ends of the network), owing to the presence of interneuron doublets (not shown). Similarly, when recurrent excitation is present in spatially uniform fashion (Center), correlation across the network still exists. EPSPs tend to appear after action potentials (eg. arrowheads). When EPSPs are present only in groups 1 and 2 (Right), however, then groups 1 and 5 lose their correlation. (B) In simulations, recurrent EPSPs can tighten synchronization between distant oscillating sites when a spatial gradient in pyramidal cell stimuli exists. The tonic excitatory synaptic conductance to pyramidal cell apical dendrites (four compartments) was spatially nonuniform, ranging from 60 nS in group 1 to 120 nS in group 5. Recurrent EPSPs were absent in the left simulation. In the right simulation (eg., arrowhead) they were spatially uniform but the conductance increased with time (as in many of the experiments), unitary peak conductance increasing from 1.97 nS to 2.34 nS over the illustrated interval. Introducing recurrent EPSPs reduces the group 1/group 5 phase lag from 6 ms to 2.5 ms and increases the cross-correlation amplitude by 35%.