Thesis defense Tobias Navarro Schröder (Donders series 260)

19 December 2016

Promotor: prof. dr. D. Norris, copromotor: prof. dr. C. Döller

Grids and Gradients - Structure and function for space in the brain

Without looking up from this thesis, it is most likely an easy feat for you to recall the location of a characteristic landmark in your environment relative to your own, such as the door you last passed through. This inconspicuous ability of ours actually reflects some amazing evolutionary achievements. Without having to look we can maintain a spatial representation of our environment for long periods of time and we can mentally travel back to re-visit previous locations, such as the door we last past through. Even if you were to close your eyes, and walk a few meters in an arbitrary direction, your estimate of the direction and the distance to the door would likely be reasonably accurate (Loomis et al., 1993). Similarly, in larger sparse environments people can directly return to a start location after a long, erratic outwards journey without direct vision, as Darwin already noted in 1873 (Darwin, 1873). Based on studies in rats and in humans, in the 1930s Tolman suggested, that some animals, such as mammals possess a ‘cognitive map’ (Tolman, 1948) of the spatial layout of the environment. He put forward the idea that this internal map could be used for sophisticated navigation. For example, to find the optimal path through an environment, to plan detours around obstacles and to take novel shortcuts where possible. During his time, however, behaviourism provided the dominant framework to explain learning and memory, which was seen as the result of direct associations between stimuli (such as the view of a spatial scene or a landmark) and responses (e.g. turn left at the bakery and walk ten steps). Stimulus-response matching or rule-based mechanisms can indeed explain various simple forms of navigation behaviour. However, Tolman’s suggestion went much further, since it proposed a mental process of an internalised Euclidean map that represented the spatial layout of the surrounding and could be used for planning and execution of navigation and wayfinding. Due to a lack of technological means at his time the suggested cognitive map constituted essentially unobservable processes in the ‘black box’ of the brain. His hypothesis, therefore, lay dormant for a number of decades.