Modeling Boundary Vector Cell Firing Given Optic Flow as a Cue

Boundary vector cells in entorhinal cortex fire when a rat is in locations at a specific distance from walls of an environment. This firing may originate from memory of the barrier location combined with path integration, or the firing may depend upon the apparent visual input image stream. The modeling work presented here investigates the role of optic flow, the apparent change of patterns of light on the retina, as input for boundary vector cell firing. Analytical spherical flow is used by a template model to segment walls from the ground, to estimate self-motion and the distance and allocentric direction of walls, and to detect drop-offs. Distance estimates of walls in an empty circular or rectangular box have a mean error of less than or equal to two centimeters. Integrating these estimates into a visually driven boundary vector cell model leads to the firing patterns characteristic for boundary vector cells. This suggests that optic flow can influence the firing of boundary vector cells. Author Summary: Over the past few decades a variety of cells in hippocampal structures have been analyzed and their function has been identified. Head direction cells indicate the world-centered direction of the animals head like a compass. Place cells fire in locations associated with visual, auditory, or olfactory cues. Grid cells fill open space like a carpet with their mosaic of firing. Boundary vector cells fire, if a boundary that cannot be passed by the animal appears at a certain distance and world-centered direction. All these cells are players in the navigation game; however, their interaction and linkage to sensory systems like vision and memory is not fully understood. Our model analyzes a potential link between the visual system and boundary vector cells. As part of the visual system, we model optic flow that is available to rats. Optic flow is defined as change of lightness patterns on the retina and contains information about self-motion and environment. This optic flow is used in our model to estimate the distance and direction of boundaries. Our model simulations suggest a link between optic flow and the firing of boundary vector cells.