Figure 4.

Large positive LFPs in the hilus are produced by mirrored parallel layers or a singlefolded layer of dendritically polarized GCs. A, Model estimate of LFPs using a realistic configuration of parallel layers of GCs with polarized dendritic arbors and a basket cell-type inhibitory input. Note the large amplitude and positive polarity of hilar potentials compared with dendritic sites. The lines of current (black dashed) and isopotential lines (solid colored) to the right show the collective behavior of the cell component (stacked rectangles) as a laminar dipole in which the soma layer oriented toward the hilus acts as the current source and the outer dendritic segment as the current sink. B, Perisomatic inhibitory activation of imaginary GCs with two polarized dendritic trees produced very small negative LFPs in the hilus. Only perisomatic synaptic layers exhibited sizable LFPs with matching polarity. The bulk of the inward currents were directed toward inner synaptic sites, spreading weakly through the outer volume (hence their small potential). C, Mirroring cell layers are required to develop large hilar potentials. The simulation used GC layers with similar orientation. The simultaneous homogeneous somatic inhibition in the two blades cancelled out the hilar LFPs, while distant sites beyond the cell layers developed weak negative and positive potentials. D, Stationary snapshots of the voltage distribution (amplitude and polarity) for the activation of different portions of the GC domains. Large positive potentials (red) were produced by the summation of volume-conducted currents of equal polarity in the hilus generated by two parallel laminas of GCs of different extensions (D1, D2), or by a single-folded lamina (soma-inwards) of GCs (D3). The drop-shaped dashed lines represent the convergence of the volume-conducted currents in the hilus: the wider the activated GC domains the greater the reach of the positive potentials away from cell domains. In the apex (D3), the volume-conducted currents created a headlight effect in the hilus (arrow).