Bistable, Irregular Firing and Population Oscillations in a Modular Attractor Memory Network
Figure 2
Spike raster showing bistability.
A: A subsample of 5880 pyramidal cells (4 out of 9 hypercolumns) is shown. Each dot represents a spike occurring at a particular time (x-axis) and in a particular cell (y-axis). In the beginning of the simulation the stable, non-specific ground state was active. When a part of a pattern (first minicolumn in 5 out of 9 hypercolumns, see Methods) was stimulated it completed and was then persistently active, even after stimulation terminated. The foreground pattern consisted of the first minicolumn in each hypercolumn, so the activity after stimulation also marks the borders between the hypercolumns. Each of the four highly active and synchronous bands is the collective spike output of 30 pyramidal cells within the first minicolumn. The three bottom hypercolumns in the raster plot received direct stimulation and activated the top hypercolumn. After stimulation, the background pyramidal cells lowered their firing rates. B: Activity histograms of 30 pyramidal cells (top) going from ground state to foreground in the active state, and 30 pyramidal cells (bottom) going from ground state to background. The vertical bar marks 10 s−1 (top) and 1 s−1 (bottom), respectively, measured as the number of spikes divided by time and number of cells. C: Zoom in of the part of the spike raster that is indicated by the dashed vertical lines in A. Only the cells in the foreground population are shown. Active minicolumns are not tightly synchronized in terms of phase. D: Synthetic LFP spectrograms. The network started out in the ground state and entered an active state after 2 seconds due to stimulation. The signal was produced from 30 pyramidal cells entering foreground (left) and background (right) respectively. The average signal from 5 runs is plotted.