Fig 1. The basic structure of the HD system with two-stage visual landmark processing.

(A) The systems-level network structure of the HD system which contains two pathways, processing visual landmarks (purple) and HD self-motion inputs (orange) respectively. The two pathways meet in dRSC (pink) where the integrated signal is generated and projected back to the HD attractor in order to correct for drift and keep a global real-time sense of direction. (B) Visual layers consisting of neurons tuned to specific features at specific egocentric directions. Different colors (red, blue, and green) stand for different features. Left: a circular environment; N, E, S, and W represent the allocentric directional frame. Middle: numerical degrees represent the egocentric directional frame (with 0 indicating ahead), here shown for the red and blue feature sensitive neurons. Right: Firing-rate responses across populations of feature-specific neurons. (C) The agent’s HD in simulation is taken from data-foraging rat. Top: Allocentric trajectory at the 11th minute of a 20-minute trajectory. Bottom: The corresponding angular velocity at the 11th minute. (D) The unimodal encoding of aLB cells when the agent is facing east (top) and then west (bottom). Two different aLB cells encode sceneries from different facing directions despite a large overlap in perceptual features (red cues). Darker filled circles indicate stronger activation of aLB cells, whilst darker lines indicate stronger feed-forward connections and inhibitory connections among aLB cells. The dashed line (bottom) indicates an example of a connection that has been depressed when the agent was facing East, so that the updated position of the cue in the visual field will not drive a previously active aLB neuron. Abbreviations: HD: head-direction(al); aLB: abstract landmark bearing; dRSC/gRSC: dysgranular/granular retrosplenial cortex; Vis.: visual; mOSA: modified Oja’s Subspace Algorithm; LI: lateral self-inhibition; Vel.: (angular) velocity.