Role of the dual entorhinal inputs to hippocampus: a hypothesis based on cue/action (non-self/self) couplets

The hippocampus sits at the highest level of memory processing circuits and receives two major inputs, one coming from the lateral entorhinal cortex and one coming from the medial entorhinal cortex. This duality must be of fundamental importance, but its functional meaning remains unclear. A computational model used for robot navigation (Verschure, P.F., et al. (2003). Nature, 425: 620-624) has a dual information structure that may provide insight. In this model, information is stored as couplets consisting of information about the current sensory cues and information about the current action of the robot. Sequences of such couplets are stored in a short-term memory buffer and transferred to a long-term memory store whenever a goal is found. The overall system enhances the ability of the robot to find reward sites because stored sequences enable the robot to retrace the path to a goal site whenever any of the cues along the path to a goal is subsequently encountered. A review of the literature suggests that the idea of cue/action couplets can be usefully mapped onto the function of the entorhinal cortex. Cue information may be supplied by the lateral entorhinal cortex whereas action (motor) information may be supplied by the medial entorhinal cortex. However, given that self-position information is prominent in the medial pathway and that this is not directly related to action, a modified formulation of the duality is proposed in which the fundamental distinction is between information about non-self vs. information about self. According to this view, the lateral entorhinal pathway carries information about external (non-self) cues and their positions (in egocentric coordinates) whereas the medial entorhinal pathway carries information about the organism itself, including its position (in allocentric coordinates), motor actions and goals.