For centuries, it was philosophers who shaped our early understanding of what it meant to have a consciousness — to consider yourself as separate from your environment. By the end of the 18th century, this question took on a completely new form when Immanuel Kant argued that some ideas, like the concept of space, are already pre-conceived within the mind. We don’t have to learn how to actively locate and remember our position in a new room or on our walk back home from work, we do it intuitively. For the next two hundred years, the details of this preconfigured system for space remained vague and hypothetical because there was no way to test it, until recently.
Current research has identified the medial entorhinal cortex (mEC) as a hub for spatial information processing. It’s a region in the brain that is home to specialized neurons like head-directional cells, which fire based on which direction your head is facing, and grid cells, which fire in a symmetrical grid of equilateral triangles for a given environment. These neurons respond selectively to spatial and directional changes in the environment in order to form maps that are unique to different locations. However, the question of just how sensitive each category of cells are to changes in the environment is still not entirely answered. A study by Fyhn et al. (2007) showed that grid cells will alter their firing pattern with large contextual changes to the environment, but were unphased by minor manipulations like changing the color of interior walls. In contrast, very little is known about how sensitive nongrid cells — including place cells, head-directional cells, boundary cells — are to changes in the environment. The Leutgib lab at University of California San Diego looked to address that very question and develop a better understanding of how these two specialized cells come together to help us navigate through our surroundings.
The first step was to identify where grid and nongrid cells were in the mEC of seven rats. To look at neuronal activity, two tetrodes were implanted directly above the mEC and just outside the mEC. These are specialized electrodes that are able to detect electrical changes in the neuron that indicate when it is firing in real time. With these in place, the researchers underfed the rats and had them forage in two different contexts meant to manipulate three aspects of the environment: shape, color, and location. One space was an open field enclosure with flexible walls that could be bent into either a square or a 16-sided polygon that resembled a circle. A separate room had a square open field enclosure with reversible walls, with one side of the wall completely white and the other completely black. Lastly, these two contexts were compared with each other since they were conducted in separate rooms to determine neuronal response to location change. By looking at the firing patterns detected by the tetrode, the researchers found that roughly 18% of the mEC was composed of grid cells while 68% were nongrid cells. With these two populations distinguished from one another, they could start to analyze the differences and similarities in their activity for each context.
They found that when the shape or color of the enclosure changed, the firing patterns of the grid cells were relatively stable, showing only minor changes. Not only were these cells firing at the same rate, but also their alignment was left unchanged in either context. Grid cells dispersed throughout the mEC will naturally align to fire in a symmetrical grid. For either context, the grid formed by the rats were visually identical, confirming earlier studies that showed that grid cells were not receptive to changes in color or shape. On the contrary, nongrid cells showed a drastic redistribution of their firing. When the rats were moved from one context to another, the activity of the nongrid cells was displaced by nearly centimeters. Essentially, different contexts promoted the activity of different nongrid cells, which stands in direct contrast to the activity of grid cells in response to the same manipulation. The results demonstrate that nongrid cells have a much higher sensitivity to more subtle changes in the environment than grid cells, ultimately indicating that nongrid cells help compliment for their more broadly perceptive counterparts. This provides major insight into how all different types of specialized spatial processing neurons work together to form a complete and distinct map of our constantly changing surroundings.
- Burgess, Neil. “The 2014 Nobel Prize in Physiology or Medicine: A Spatial Model for Cognitive Neuroscience.” Neuron, vol. 84, no. 6, 2014, pp. 1120–1125
- Diehl, Geoffrey W., et al. “Grid and Nongrid Cells in Medial Entorhinal Cortex Represent Spatial Location and Environmental Features with Complementary Coding Schemes.” Neuron, vol. 94, no. 1, 2017, pp. 83–92
- Fyhn, M., Hafting, T., Treves, A., Moser, M.B., and Moser, E.I. (2007). Hippocampal remapping and grid realignment in entorhinal cortex. Nature 446, 190–194