New paper now out in Nature Human Behaviour!
Here is a little summary of the project:
To orient ourselves in space, and to find our way around, we form cognitive maps of our surroundings. But what happens if the coordinate system of our cognitive maps – thought to rely on the regular firing patterns of grid cells – is distorted? Could this result in distortions of our cognitive maps and our memories relying on them? Spatially tuned cells in the hippocampal-entorhinal region of the mammalian brain are thought of as the neural substrate of cognitive maps. Among them are grid cells, which provide a sort of coordinate system for our cognitive maps: They fire at many locations in an environment that form the vertices of equilateral triangles tiling the entire environment so that a symmetric grid pattern is created. The regular firing pattern is the characteristic signature of grid cells.
But in 2015 an observation published by a team of neuroscientists from the University College London challenged this notion: Here, the activity of grid cells was recorded while rats made their way through different enclosures. In a square box, the firing of the cells produced the characteristic regular grid patterns. However, when the animals navigated a trapezoid-shaped environment, the grid cells fired irregularly – so that the regularity and symmetry of their firing patterns broke down!
Our idea was to use these distortions of the grid metric to study how we remember positions in space and test our models of how grid cells contribute to our cognitive maps. Grid-like signals have been observed in the human brain, but how precisely grid cells help us to remember where important events take place is unclear. Typically, boundaries of an environment facilitate our spatial orientation. But this finding suggested that the specific arrangement of boundaries can also distort the grid metric of cognitive maps. We wanted to test the effect of these grid distortions on human spatial memory; based on theoretical work showing how regular grid patterns can be used to store positions in memory and to compute how far two positions are apart.
To make the boundaries of the environment as prominent as possible we used a highly immersive virtual reality setup: Participants wore virtual reality goggles and navigated through a square and a trapezoidal virtual environment using a motion platform that translated their physical steps and rotations into virtual movement. Each environment contained six objects and the volunteers learned which object belonged at which position in the environment. We observed that participants remembered the object positions less accurately in the trapezoid compared to the square environment. Within the trapezoid, their errors were particularly large in the narrow end of the environment – an effect that mirrors the strength of the distortions of grid-cell firing patterns observed in navigating rodents.
Next, we asked whether these distortions would persist in the memories of our participants even outside of the trapezoidal environment. Therefore, we asked them to estimate the distances separating the positions of pairs of objects. We had arranged the positions in such a way that the true underlying distances were actually identical. Yet, we observed distortions in how the participants remembered these distances: They consistently remembered distances in the trapezoid to be shorter than in the square and, within the trapezoid, distances were estimated to be longer in the narrow compared to the broad end. These are precisely the distortions we expected based on a model grid system and theoretical work describing how grid-cell computations can be used to calculate the distance and direction between positions. Together, our findings show that the shape of the environment impacted human spatial memory in a way that we could predict using a model of the grid pattern distortions. Further, by testing predictions that theoretical models make about the consequences of distorted spatial representations – here the grid metric of cognitive maps –, our study highlights how we can use distortions of these spatial representations as a tool to understand how they support human memory and other cognitive functions.
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.