Boundary related enhancements in spatial firing of grid cells

Abstract

It has been established that environmental boundaries influence the firing of grid cells. However, whether they can enhance grid cell firing, and how this might occur, is not known. One possibility is that they reduce environmental uncertainty. The aim of Experiment 1 was to investigate how a wall versus a drop-edge might enhance grid cell firing at distal locations. The results showed that the 4th wall increased gridness, not just at locations proximal to the manipulated boundary, but at locations distal to the manipulated boundary. Additionally, the 4th wall, which was initially novel, increased gridness relative to the drop-edge, which was highly familiar. This is in contrast to previous research that has shown that novelty causes gridness to decrease (Barry et al., 2012). This suggests that walls provide more certainty about the environment compared to drop-edges, and this immediately counteracts the relatively subtle environmental novelty. In Experiment 2, the key hypothesis was that a barrier inserted into the environment would reduce spatial uncertainty, and that, according to Towse et al. (2014), this would reduce spatial scale. The results confirmed this hypothesis. They also demonstrated that the same manipulation can bidirectionally modify grid scale: the presence of the barrier caused grid scale to initially expand, becoming larger than grid scale recorded on the baseline day (no barrier present) occurring immediately before the first manipulation day, then contract, becoming smaller than grid scale recorded on the baseline day immediately following the last manipulation day. Considering these findings together, this thesis presents novel research showing that boundaries can have an enhancing effect upon established grid patterns. It also, at least partially, teased apart the two factors of boundariness and familiarity, and shows that enhancement effects were not simply driven by familiarity.