In mammals, great insights have been obtained for early stages of sensory systems where signals can be followed through hierarchical networks from receptors to primary sensory cortices. But how the mammalian brain generates its own codes, deep in the association cortices, has remained deeply mysterious. Yet this is where the understanding of subjective experience begins. A path was opened in this terra incognita in 2005 when the Mosers and their students discovered grid cells – the metric of the brain’s map for space. Grid cells are place-modulated neurons whose firing fields define a triangular array across the entire environment. These cells are thought to form an essential part of the brain’s coordinate system for metric navigation. Because their matrix-like firing is generated in the brain, far away from specific sensory inputs, grid cells provide unprecedented access to algorithms of neural coding in high-end cortices. The simplicity and the crystal-like structure of the grid cells offers opportunities for understanding, maybe for the first time, a mammalian behaviour at the level of neuronal network computation.
The first clues to a spatial mapping mechanism in the entorhinal cortex came when the group showed, in 2002, that the intrinsic circuit of the hippocampus is not necessary for basic hippocampal place signals. These studies, reflecting the beginning of a very rewarding collaboration with Menno Witter’s group in Amsterdam (now at the Kavli Institute in Trondheim), implied that the spatial signal must originate outside the hippocampus. The obvious candidate region was the entorhinal cortex, the main source of cortical input to the hippocampus. In 2004, after recording directly in the entorhinal cortex, the group reported place-specific activity in this region, and in 2005, they showed that the firing fields has a grid pattern.
After the discovery of grid cells in 2005, the Mosers and their colleagues showed how these cells interact with other cell types in entorhinal cortex to generate a continuously updated representation of self-location that can be used in any environment, irrespective of shape and landmarks. The group showed that grid cells intermingle with direction cells and they discovered border cells, cells that fire only along boundaries of the environment. They also showed how outputs of the entorhinal circuit are used by memory networks in the hippocampus, and how place memories are separated from each other during the early stages of the hippocampal memory storage. In their most recent work (2012), they provide evidence for a modular column-like organization of the grid map. Grid cells were shown to consist of functionally autonomous modules. The discrete organization of the grid map differs from the graded topography of maps for continuous variables in classic sensory systems and points to a new form of cortical circuit organization.
The discovery of grid cells and their control of population dynamics in the hippocampus has led to a revision of established views of how the brain calculates self-position. As a result of this and other work, spatial mapping is becoming one of the first non-sensory cognitive functions to be characterized at a mechanistic level in neuronal networks. Understanding the origin and the properties of the grid map is an attractive challenge for anybody wanting to know how brain circuits compute. Grid cells thus provide us with direct access to some of the most fundamental operational principles of cell assemblies and microcircuits in the brain. The Mosers wish to take advantage of the accessibility of the grid cells, as well as the recent explosion of technologies for neural circuit analysis, to unravel basic mechanisms of neural coding in the brain.