Spatial cells in the Subiculum; Influence of boundary manipulations

External space is mapped by a widespread brain circuit located in interconnected sub regions of the hippocampal-parahippocampal cortices. To date there have been 4 functionally specialised cell types identified providing information about location (place cells), heading direction (head direction cells), self-motion (grid cells) and boundaries (boundary vector cells and border cells). In the present thesis I present novel research identifying and characterising spatial cells located in the subiculum, an under explored region within the hippocampal formation. The subiculum is a key output structure from CA1 but also provides strong projections to the entorhinal cortex and pre- and parasubiculum, placing it ideally within the brain circuitry to contribute to the representations of external space. This thesis presents evidence for a variety of functionally and morphologically diverse spatial cells located in the subiculum. Critically I present the first ever report of subicular grid cells. These were recorded alongside boundary-responsive cells and head direction cells. The thesis characterises the basic properties of subicular grid cells, as tested in a variety of environmental manipulations. This thesis explores grid cell relationships to the environment and in particular to boundary manipulations Among the key results in this thesis is the discovery that grid scale increases along the anterior-posterior axis of the subiculum, similar to MEC grid scale. This thesis also shows that subicular grid cell patterns can be disrupted with environmental manipulation. Wall removal caused grid patterns to shift orientation and increase grid scale. The grid scale expansion was related to novelty, which supports previous findings from MEC grid cells (Barry et al., 2007; 2012). This thesis also shows that grid patterns were disrupted by barrier insertion e.g. causing an inhibition to grid fields. When recorded in total darkness the grid cell patterns remained stable, suggesting that vision is not required for maintenance of the grid pattern structure. Taken together these findings provide evidence that grid cell firing patterns are at least partially determined by environmental boundaries. In addition this thesis extended upon the work of Lever et al., (2009), by presenting a detailed investigation and characterisation of subicular boundary vector cells (BVCs). I developed an empirical classification criterion, and utilised numerous manipulations to address the issue of what a BVC treats as a boundary. I also identified a new type of boundary related cell: the boundary-off cell. These cells have firing patterns which can be considered very similar to the inverse of the BVC. The inhibitory response of boundaries on these cells may provide a mechanism to explain the field inhibition seen in grid cells with barrier insertion. The data in this thesis presents novel research identifying and characterising spatial cells located in the subiculum. Of particular importance, is the discovery of grid cells in this structure that are intermingled with HD and boundary-responsive cells. The thesis focuses on characterising and investigating the importance of environment boundaries to subicular cell firing patterns. The results are discussed in relation to what is known about spatial representation in the hippocampal and parahippocampal regions, and the neural circuitry which comprises this representation.