File:Hippocampus assemblies.png

Interleaved cell assemblies. A. Three example model neurons (color-coded) with identical oscillation frequency but different phase onset, according to their maximal discharge location. Temporal distance T is the time needed for the rat to run the distance between the peaks of the two place fields (real time). Tau, time offset between the two neurons within the theta cycle (theta time). Bottom, the summed activity of the entire population of model neurons (black dashed line) oscillates slower than each transiently active individual neuron (color-coded). B. The phase of the three example neurons with respect to the oscillation of the population is plotted against time. Note that the neuronal spikes phase-precess approximately 360o due to the interference between the oscillatory spiking frequency of the most active neurons and the oscillation frequency of the entire population. Right: spike density for the example neurons. C. Interleaved neuron sequences represent position and distance relationships. The width of the bars indicates firing intensity of the hypothesized assemblies while the theta-time scale temporal (phase) differences between assemblies reflect their respective distance representations. In successive theta cycles, assemblies representing overlapping place fields (P1 to P8) shift together in time and sustain a temporal order relationship with each other so that the assembly that fires on the earliest phase represents a place field whose center the animal traverses first. This "temporal compression" mechanism (Skaggs et al., 1996) allows distances to be translated into time. Approximately, 7±2 assemblies/gamma cycles are present in a given theta period (Bragin et al., 1995; Lisman and Idiart, 1995). A and B, modified after Geisler et al. (2010). C, modified after Dragoi and Buzsáki (2006).