Níall McLaughlin and Yeoryia Manolopoulou speak to Dr Hugo Spiers, a cognitive and behavioural neuroscientist at UCL. He is interested in brain function and spatial cognition: how our brain constructs representations of the world and then uses these to navigate space and – through memory and imagination – time. Hugo tells Níall and Yeoryia that we begin to mentally represent spaces before we have even entered them: we predict the geometry of a room based on our learned experiences of other, similar rooms. For example, most museums are composed of rectilinear rooms, so that is what our brains have come to expect: ‘you’d be surprised if you entered a museum and all the rooms (were) circular’. He goes on to explain that in humans, these preconceptions are principally based on visual information, as this is our primary method of understanding space: ‘our brain…builds up visual information and massively transforms it’.
Hugo says that he is particularly interested in the way in which our brain models the geometry within a space, and the relationships it creates between space and the self. He describes how we orientate ourselves, explaining that there is a dedicated circuit of cells in our brain that act as ‘a constantly-humming compass’. These head-direction cells are not related to magnetic north, but to a specific and consistent axis that they set the first time a space is entered. The same cells are activated again and again in that space, even over a number of years and under varying conditions, as long as the geometry remains unchanged. However, he explains later that rats with genetic impacts similar to those caused by Alzheimer’s disease are incapable of holding these stable models.
The orientation information described above is overlaid with localisation information, or the sense of ‘where I am’. This seems to come from place cells in the hippocampus, which tell the location cells when to become excited. Place cells are very geometrically driven, but are also sensitive to smell and sound: they are sophisticated, multi-modal cells, embedded at the deepest part of the brain. Hugo describes them as constituents of our ‘internal matrix of how we think the world is’.
Hugo goes on to explain that these place cells also seem to encode memory: hence the links between memory and place. He tells Níall and Yeoryia that the connections between brain cells – the synapses – are the ‘raw stuff of memory’. A memory is stored by strengthening a particular set of synaptic connections. Hugo explains that ‘like a stream, a river running’, visual data passes through the retina and journeys through the brain, penetrating certain cells which have become preconditioned to receive this information. Then another set of cells interprets the data. Níall asks if a spatial model is created in the mind, to which Hugo replies that the ‘sets of neurons that are “coding” this information…are carrying out a model-like function’. In explaining the link between space and memory, Hugo says that it seems ‘we have evolved to map space, and embed new experiences or memories into this map’.
Interestingly, our place cells don’t appear to tell us where we are when we are sleeping. However, it has been shown in sleeping rats that sometimes the mind quickly replays the sequence of cell patterns it has just been through. Níall asks if this is evidence of a further link between memory and place: is this process evidence of the mind spatially laying down memories as we sleep? Hugo clarifies that memory and place are inextricably linked in terms of our ability to ‘record what happens as the world around us unfolds: they’re spatio-temporally linked’. However, over time, the brain abstracts this information, overlaying and linking multiple recordings and experiences to store fact-like perceptions of the world.
Given the emphasis on geometry evident from Hugo’s descriptions of spatial perception, Yeoryia asks how our brains perform in continuous, seemingly featureless environments, like deserts. He explains that, even in places like these, the brain assigns a reference point, and is still able to encode how far the feet have travelled from that point. There are inner semi-circular canals in our heads (otoliths) that sense gravity and movement, and it seems that there are cells within the hippocampus which use this information to generate tessellating grid patterns; creating a sort of latitude/longitude signal in the brain that the place cell then relates to.
Discussing the genesis of an individual’s spatial cognition – when the place cell/grid cell/head-direction cell network starts to develop in the brain – Hugo says that studies of rat pups suggest that the systems seem to be present but ‘offline’ until the pup starts moving. Then, the first system to activate seems to be the basic head-direction cells.
Hugo and his colleagues have studied the head-direction signal, and shown that it comes from the entorhinal cortex. They discovered that they could predict how good an individual’s navigational ability was by testing the reliability of the signal from their entorhinal cortex. Compellingly, they also found evidence of a degraded head-direction signal in young, healthy people with a gene that predisposes them to Alzheimer’s disease.
Hugo, Níall and Yeoryia discuss the nature of the brain’s spatial ‘model’, and Hugo emphasises the importance of geometry to our understanding and navigation of our environment. Yeoryia points out that this is a very architectural logic; that most people say they ‘navigate, or remember, or know where they are because of landmarks; because of objects. They are not going to say: “because I remember the proportions of the room.”’ Hugo acknowledges this, but argues that it is partly due to differences in language between architects and non-architects, and partly because so much of our navigation is habitual; it is not subject to a continuous process of perception: ‘there’s an entirely separate habit system in the brain…an autopilot. When I get into this building in the morning…I’ve been here since 2007…I don’t think about how to get to my office: it’s a habit.' He explains that part of the brain, the striatum, slowly learns through progressive repetition of the body’s movements. This habitual model is overlaid with our other sources of spatial cognition to create our understanding of the world.