Characterising the neural compass across the human lifespan : a multimethod investigation

Abstract

Head direction (HD) cells are important for navigating throughout our environment. They have been found in several brain regions of both rodents and, recently, humans. Their firing rate increases when the animal faces a specific direction and decreases when they move their head away from their preferred orientation. Thus, these cells have been considered to act as an internal compass. However, these HD cells must remain stable in relation to their environment to function properly. This stabilisation can be achieved via landmarks, which are supposed to give concrete directional information. In Chapter 1, we investigated the impact of landmark stability on the HD coding in the human brain. We found that the primary visual cortex (V1) and retrosplenial cortex (RSC) coded the landmark stability similar to previous studies. We also decoded a HD signal associated with stable landmarks in the RSC, unstable landmarks in the thalamus, and both landmarks types in the presubiculum. Moreover, another gap in the HD system literature was the influence of ageing on the system. Indeed, several studies reported a decline in spatial navigation skills due to ageing, but there is a lack of studies on how the HD system is affected. Therefore, Chapter 2 investigated the effect of ageing on the HD system using immersive virtual reality. In addition, to test for the accumulation of noise during a stationary orientation, a delay period was introduced in half of the trials before the response phase. We found that older participants produced higher angular errors than young participants. Additionally, both age groups were negatively impacted by the delay period suggesting an accumulation of noise while stationary. To help explain the differences between age groups, in Chapter 3, we created a model using ring attractor networks, a typical architecture when modelling the HD system. Previous HD models could not correctly assess ageing and the influence of noise sources related to physiological changes. Hence, we assessed ageing by implementing three distinct sources of noise: synaptic changes, loss of cerebral volume and vestibular input. We found that synaptic noises and increased neuron death led to a tendency to get stuck in local minima, which could lead to the increased range of errors observed in older participants. Altogether, the work from this thesis helped to understand in more depth the effect of ageing on the HD system and is a first step in characterising how the system degrades in ageing.