Figure 2.

Sequence encoding modules. (A) Illustration of a sequence encoding module architecture consisting of 3 bistable units B_i, B_i+1, B_i+2, the competitive inhibitory population I_C and the gating inhibitory input I_G. The inhibitory unit I_C prevents two bistable units B_j from having high rate at the same time, whereas the gating inhibition I_G is the ”break” that prevents activity from propagating from B_j to B_j+1. This break is externally controlled, by some mechanism that recognizes when the animal changes its location. (B) Evolution of the rates of 2 abstract rate units B_i, I_C and I_G, disposed according to the sequence encoding architecture in (A). (C) General illustration of the states the network goes through over time. (i,vi) correspond to stable states in the network while I_G is activated. Inactivating I_G induces a dynamical process that brings the network from state (ii) into state (v). This dynamical process is depicted in further detail below with (iii,iv). In (iii), due to inactivation of I_G, the activity in B_i+1 picks up, driven from the activity B_i. As shown in (iv), once both B_i and B_i+1 are active, they together activate I_C, which gives strong inhibitory input to B_i and B_i+1 and (in v) forces B_i to become inactive, while the recurrent activity permits B_i+1 to recover and stay active. (D) Spiking network of sequence cells. Above, the x-axis shows 1.2 s of simulated activity. The left y-axis shows the indices of the cells in each sequence ensemble, and a raster plot is given. The right y-axis shows the rate of each sequence ensemble. Below, the period where I_G is inactivated is emphasized. During this period, the stability of the system is broken and the dynamics described above take place, leading ultimately to B_i+1 overtaking B_i as the active sequence ensemble. (E) Analogous to (D), but we now purposedly enforce a longer time out of the global inhibitory mechanism. Observe that the system still functions much like in (D), so the period where I_G is turned off is flexible.