Figure 2. The phases of endotaxis during exploration, goal-tagging, and navigation.

A portion of the circuit in Figure 1 is shown, including a single goal cell that responds to the water resource. Bottom shows a graph of the environment, with nodes linked by edges, and the agent’s current location shaded in orange. Each node has a point cell that reports the presence of the agent to a corresponding map cell. Map cells are recurrently connected (green) and feed convergent signals onto the goal cell. (A) Initially the recurrent synapses are weak (empty circles). (B) During exploration, the agent moves between two adjacent nodes on the graph, and that strengthens (arrowhead) the connection between their corresponding map cells (filled circles). (C) After exploration, the map synapses reflect the connectivity of the graph. Now the map cells have an extended profile of activity (darker = more active), centered on the agent’s current location ${\displaystyle x}$ and decreasing from there with distance on the graph. (D) When the agent reaches the water source ${\displaystyle y}$, the goal cell gets activated by the sensation of water, and this triggers plasticity (arrowhead) at its input synapses. Thus, the state of the map at the water location gets stored in the goal synapses. This event represents tagging of the water location. (E) During navigation, as the agent visits different nodes, the map state gets filtered through the goal synapses to excite the goal cell. This produces a signal in the goal cell that declines with the agent’s distance from the water location.