To orient ourselves in space, and to find our way around, we require and create cognitive maps of our surroundings. Like any other, these internal maps feature a coordinate system. But what good would a map be if its coordinate system wasn't regular, if it could be warped? It turns out our inner coordinate systems may in fact become distorted at times. What exactly that means for us is not known. Could it mean that our cognitive maps are also warped, or that we have skewed spatial memories? Quite possibly. We recently set out to answer those questions.
Cognitive maps are thought to be instantiated by various spatially-tuned cell types in the hippocampal-entorhinal region of the mammalian brain. These cells represent spatial information, for example by firing at specific positions in the environment. Among them are grid cells, thought to function as the coordinate system of our cognitive maps. A single grid cell will fire when its owner occupies particular locations in space. But not random locations. Rather, the cell will fire at many locations that tile the entire environment. Together, these firing fields form a regular, hexagonally symmetric grid pattern mapping the space around us — a striking internal coordinate system.
But in 2015 an observation published by a team of neuroscientists from the University College London challenged the notion that grid cells always maintain their strictly regular firing when representing a given space. Grid-cell activity was recorded while rats made their way through different enclosures. In a square box, the characteristic hexagonal grid pattern emerged. However, when the animals navigated through a trapezoid-shaped environment, the grid cells fired irregularly. That is, the same cell would still fire at different locations. But the regularity and symmetry of their firing patterns had broken down! If grid firing patterns really provide the coordinate system for our cognitive maps, surely this must have major repercussions?
We decided to study the influence these grid distortions might have on how we remember spatial information. Grid-like signals have been observed in the human brain and theoretical work suggests that regular grid patterns can be used to store positions in memory and compute distances between two points. Yet, how precisely grid cells help us to remember where important events take place is unclear. Typically, the boundaries of an environment guide our spatial orientation. But if the boundaries can also distort the grid, would this influence how well we remember positions?
To test this, we used highly immersive virtual reality. Participants navigated through a square and a trapezoidal virtual environment on a treadmill-like motion platform that translated their steps and rotations into virtual movement (see GIF). Each environment contained six objects, the positions of which were to be learned. We found that memory for object positions was worse in the trapezoid environment. Importantly, errors were particularly large in the narrow end of the environment, mirroring the strength of the distortions observed in the navigating rodents.
Left: the regularity of grid-cell firing patterns is distorted in a trapezoid compared to the square. Center-left & right: in our immersive virtual reality setup, participants learned object positions (circles) in a square and a trapezoidal environment. Right: participants made larger errors for objects in the trapezoid environment. Paralleling the distortions of grid patterns, the accuracy of participants’ spatial memory was particularly low in the narrow end of the trapezoid.
Having observed these distortions in human spatial memory in the trapezoid, we wondered if participants' memory would be distorted even outside of the virtual world. Unbeknownst to the participants, the actual distance between sets of objects was identical. Yet, when asked to estimate how far apart pairs of objects were, distances in the trapezoid were consistently remembered as shorter. And again, there was also a difference between the narrow and broad end of the trapezoid, so that distances in the narrow end were remembered to be longer. These behavioural effects were completely in line with predictions based on a model grid system.
Our findings show that the shape of an environment impacts human spatial memory, and importantly, in a way that we could predict using a model of grid pattern distortion. By testing predictions that theoretical models make about the consequences of distorted spatial representations – for example, biased distance estimations – our study highlights how we can use such distortions as a tool to understand how grid cells support human memory and cognitive function more generally.
If you are interested, you can find the original paper here:
JLS Bellmund, W de Cothi, TA Ruiter, M Nau, C. Barry, & CF Doeller (2019). Deforming the metric of cognitive maps distorts memory. Nature Human Behaviour, https://www.nature.com/articles/s41562-019-0767-3.
Want to find out more about using virtual reality in research?
Check out this Nature Video documentary including a demonstration of the VR setup used in this study.