How do egocentric boundary cells depend upon the coordinate system of environmental features?

Neurons in the retrosplenial (RSC) (Alexander et al 2020a, LaChance & Hasselmo 2024) and postrhinal cortex (POR) respond to environmental boundaries and configurations using egocentric coordinates relative to an animals current position. Neurons in these structures and adjacent structures also respond to spatial dimensions of self-motion such as running velocity (Carstensen et al 2021, Robinson et al 2023). Experimental and modeling data suggest that these responses could be essential for guiding behaviors such as obstacle avoidance and goal-directed navigation (Erdem & Hasselmo 2012, Erdem & Hasselmo 2014). However, these findings still leave the unanswered question: What specific features, and in what coordinate frames, drive these egocentric neural responses? Here we present models of the potential circuit mechanisms generating egocentric responses in RSC. One model posits that neurons encode internal representations of barriers in head-centered coordinates, defined by distance and angle, which are modulated by running velocity to enable trajectory planning and obstacle avoidance. We contrast this with a complementary hypothesis in which neurons respond to retinotopic features--such as the top, bottom, or edges of walls, which may serve as precursors to head-centered representations. Additional hypotheses include trajectory-based forward scanning (e.g., ray tracing) for barrier detection or comparing optic flow across the visual field. These hypotheses generate complementary modeling predictions about how changes in environmental parameters could alter the neural responses of egocentric boundary cells that are presented here.