Non‐Canonical Subiculum Circuit Organization and Function

The subiculum is highly interconnected with the hippocampus, sub-regions of the thalamus, and the entorhinal and retrosplenial cortices. Together, these regions form a distributed network that plays critical roles in spatial cognition and learning and memory. Despite recent discoveries detailing subiculum's circuitry and neural dynamics, a unique role for subiculum in this system has yet to be determined. Traditionally, the subiculum has been considered the "fourth leg" and output region of the trisynaptic pathway. However, recent evidence highlights the subiculum as a site of integration, receiving and redistributing outputs from the hippocampus, anterior thalamus, retrosplenial cortex, and entorhinal cortex. We review how these afferents may explain the diverse forms of spatial and directional tuning observed in the subiculum, including location coding, boundary-related signals, axis of travel, and head orientation. We also discuss more recently identified "non-canonical" connections that suggest additional roles for the subiculum in refining hippocampal processing. Together, these findings call for a reconceptualization of the subiculum's role in spatial cognition, memory, and integration across thalamic, cortical, and hippocampal networks.