Simulation of sleep spindles and spike and wave discharges using a novel method for the calculation of field potentials in rats

We suggest a new method for calculation of extracellular field potentials generated by a large population of pyramidal cells (PCs), using a single PC compartmental model. Similar methods described earlier use the assumption that the intracellular potential or current distributions of the cells within the population are much alike as a result of simultaneous activation at about the same longitudinal location (i.e., all the PCs in the population are located on the same level and are ideally synchronized). However, the degree of synchronization of natural firing even during synchronized rhythmic discharges in the cortex is not as high. We introduce the possibility to vary the degree of synchronization of the PCs' activity in the population, thus taking into account disperse timing of cortical pyramidal cells' firing. The temporal variability in cell firing is described by a Gaussian distribution, the width of which defines the degree of synchronization/desynchronization. In addition, the suggested method allows for certain spatial spread of PCs in the population along longitudinal axis of the PCs. The method was applied to test the assumption that the transition from sleep spindles to rhythmic spike and wave discharges (SWDs) observed in absence epilepsy may occur due to an increase in pyramidal cells' firing synchronization. We show that in case of weak synchronization of PC firing in the population, the shape of field potential during rhythmic thalamic input is similar to the oscillations during a sleep spindle, while at stronger synchronization of PCs, it looks much more as a SWD, with clear expressed spikes and waves. This suggests that in large population of pyramidal cells the changes in the degree of synchronization of cell firing may explain the changes in the shape of field potential from spindle oscillations to SWDs and vice versa.