Patterns created by the brain's grid cells, which are believed
to guide navigation, are modified by the shape of the environment,
according to UCL researchers. This means grid patterns aren't a
universal metric for the brain's GPS system to measure distance, as
previously thought.
Grid cells in the brain appear to form an internal map of the
local environment by signalling periodically to create a
'grid-pattern' that helps animals to navigate, even in the dark.
Until now, it was believed that all grid patterns were hexagonal,
providing the brain with uniformly spaced regions across which
distances could be measured. The new research dispels this theory
as it shows grid patterns distort to align with the local
environment's geometry, changing the distances between
grid-regions.
Dr Julija Krupic of UCL said: "If you imagine the pattern made
by grid cells is a ruler for our brains to measure distance, we're
seeing the ruler bending and stretching depending on the geometry
of our external environment. This causes grid patterns to change
markedly between enclosures of different shapes and within the same
enclosure."
The study in rats shows that the structure of the grid pattern
is responsive to environmental shape to such an extent that highly
polarised trapezoid-shaped environments cause the usual hexagonal
grid pattern to break as the grid aligns to the enclosure
walls.
The scientists studied the grid patterns created in the
entorhinal cortex part of the brain of 41 rats as they foraged in
circular, square or trapezoid shaped environments. They found grid
patterns aligned at an angle of 8.8° to the walls in polarized
enclosures and discovered that this influence was strong enough to
cause distortions in trapezoid shaped spaces.
The team investigated whether the changes in patterns were
caused by external cues such as visual landmarks, textures and
smells, or altered behaviour in the speed or directional movement
of the rats, and found the primary influence was the impact of
environmental geometry.
Dr Marius Bauza of UCL said: "We were surprised to see how
important environmental boundaries are in permanently changing grid
patterns and just how local the activity of grid cells is. We found
those anatomically close to each other in the entorhinal cortex
responded coherently. We're planning to use this new information to
refine the mathematical models we've developed to help us
understand the behaviour of grid cells and how grid patterns
form."
Professor John O'Keefe of UCL added: "We know that other cells
involved in the brain's GPS system such as place cells and boundary
cells are affected by environmental geometry and now we see the
same is true for grid cells. It might be that grid patterns aren't
used as a ruler by the brain and are doing something different. Of
course, they may still be trying to measure distance in trapezoids
but this is now distorted, leading to the interesting idea that our
brains perceive distance differently in environments with polarised
geometry. Our next step will be to find out why grid patterns
change based on our environment and what this means for the role of
grid cells in helping us navigate and form memories."
The paper, Grid cell symmetry is shaped by environmental geometry, is
published in Nature.