On the mechanisms of effort encoding in the rodent hippocampus and anterior cingulate cortex

Abstract

Spatial cognition plays a vital role in helping an organism respond to its environment, and thus survive. An often overlooked element of spatial cognition is the energetic costs of different routes. Most studies to date have investigated spatial cognition by using small, flat mazes which are not necessarily reflective of natural environments. Natural environments can have difficult, uneven terrain where an organism must consider the energy demands of different routes. Currently, how the brain represents difficult terrain is not well understood.

The principal aim of this thesis was to build upon the field of spatial cognition and investigate how effort affects the neural underpinnings of spatial cognition in energetically demanding environments. To this end, neural activity was recorded from rat brain regions previously found to be important to spatial cognition (hippocampus) and the encoding of physical effort (anterior cingulate). As rats travelled in energetically demanding scenarios, single cell and population activity was recorded. Communication between regions was investigated through their interactions in the theta rhythm, a neural oscillation known to facilitate long distance communication in the brain. The hippocampus has been shown to play a vital role in spatial cognition and navigation. It is theorised that the hippocampus contributes to the formation of cognitive maps of environments which aid in navigation. However, little is known about how the hippocampus encodes energetically demanding environments. By having rats run on different terrain slopes, here it was found that effort can have an organising effect on hippocampal cell ensembles. Situations which are similar in their energetic demands are represented by similar ensemble activity. In contrast, experiences which differ in effort are represented by disparate ensembles. These findings help elucidate how neural systems store and organise memories of effortful experiences. The anterior cingulate cortex (ACC) is capable of encoding the effort associated with behaviours. Specifically, the ACC evaluates the benefits and energetic costs of a route and biases an organism’s behaviour in favour of routes which have the highest value. However, it is not well understood how the ACC encodes effort and value when only one route is available. Here it was found that ACC neurons will encode the net value of rewards when a rat is foraging in a non-choice paradigm. These results demonstrate that the ACC may be continually assessing behaviours in order to learn their values, which would help an organism maximise the value of its environment. The hippocampus and ACC have been previously found to work together in spatial cognition tasks. Here it was hypothesised that the hippocampus and ACC would alter their communication when an environment became energetically demanding. However, it was found that effort did not alter the communication patterns between the hippocampus and ACC. Thus, there may be other means of communication that these two regions use to represent and evaluate energetically demanding scenarios. Lastly, the roles of the hippocampus and ACC are considered here in the context of ecological psychology. The importance of the environment when investigating neural systems is discussed.