International Business Department           Liu Bojia           August 19, 2024

  In 2014, Mr and Mrs May-Britt Moser (May-Britt Moser and her husband Edvard Moser) and John O'Keefe from Norway rose to the top of the scientific ladder and shared that year's Nobel Prize in Physiology or Medicine.

  Together, the three scientists revealed the ‘locus coeruleus’ in the brain. Professor O'Keefe discovered in 1971 that there is a ‘place cell’ in the hippocampus that is activated when an individual is in a particular spatial location, and more than 30 years later, the Moser's found another important piece of the brain's localisation system. In the area of the olfactory cortex between the hippocampus and the cortex, they found the brain's ‘spatial coordinate system’ - the grid cell. When an individual moves, these neurons provide ‘coordinates’ in the form of a grid, similar to latitude and longitude, allowing the individual to precisely orientate and find their way around.

  Position cells and grid cells together form the brain's ‘GPS’, determining where we are. If you think about it further, is there some system in our brain that can predict an individual's future location in addition to their current location?

  In a new study published in Science, two scientists at the RIKEN Brain Science Centre have discovered a new type of predictive grid cell, in addition to the known grid cells, to support forward planning in spatial navigation.

  In this study, the scientists further analysed the medial entorhinal cortex (MEC) brain region to explore how this area creates maps of future locations.

  To do this, the team devised a goal-directed behavioural task in which rats were asked to walk back and forth between two water rewards on a square, open flat area. The locations of the two water sources changed every once in a while so that the rats' movement trajectories could cover the entire area. At the same time, they used probes to record the activity of individual neurons in the rat's MEC.

  To assess whether the MEC neurons could encode future spatial information, the researchers constructed neuron firing frequency maps using the actual positions of the rats, before and after moving along the trajectory, respectively.

  As a result, the authors observed that a portion of the MEC neurons did not show grid features in the discharge frequency maps at the original location, but exhibited a clear grid structure in the discharge frequency maps at the future location. For example, some MEC neurons would be activated only 30 to 40 cm before the rat arrived at a specific location, encoding spatial information about this future location.

  Thus, these neurons appear to predictively code for future locations, and these neurons have come to be known as predictive grid cells, a spatial system quite different from the Moser's findings.

  Next, the team analysed what these cells further encoded. The authors found that these predictive grid cells encoded both distance and time when predicting future locations, but that the cells were more ‘gridded’ when considering distance. In addition, the rats encoded future locations both when travelling to a specific target and when foraging randomly, implying that the function of the predictive grid cells is not limited to goal-directed behaviours.

  Taken together, this new study suggests that MEC provides a predictive map that supports forward planning in spatial navigation and plays an important role in the integration and prediction of temporal information. Professor Shigeyoshi Fujisawa, corresponding author of the paper, notes that this study provides important insights into the mechanisms of spatial navigation and situational memory formation in the hippocampal and MEC circuits, and that the team next hopes to elucidate the organisational mechanisms of such predictive grid cells.