On Tuesday 19 April 2016 Nobel Laureate May-Britt Moser gave a public lecture at The Royal Library, "Diamanten".
Spatial navigation and memory depend on brain structures such as the entorhinal cortex and the hippocampus. This has been known for more than a half century, since the groundbreaking studies of patient H.M. who had surgical damage to these structures. The lesion disturbed the ability to form episodic memories and to navigate successfully in space. John O’ Keefe was motivated by this information and explored the activity of hippocampal cells in the freely behaving rat. This way he discovered the place cells, cells that are active only at certain positions in the environment.
Over the past 15-20 years, we have explored the wider circuit of the mammalian positioning system. In this lecture, I will show that the medial entorhinal cortex contains grid cells – cells with firing fields that tile environments in a periodic hexagonal pattern. The tiling of an environment by a grid cell is reminiscent of an internal coordinate system. These cells – which probably serve a metric function in the navigation system - are intermingled with other cells that respond to other features of navigation, such as where the animal goes and how fast it is moving. Today we know that the entorhinal space circuit contains both speed cells, border cells and head direction cells, in addition to grid cells.
I will spend some time on the most recently discovered cell type – the speed cells – and show that running speed is represented in the firing rate of a ubiquitous but functionally dedicated population of medial entorhinal neurons. I will also show that speed is represented across a wider brain circuit that includes speed cells in the mesencephalic locomotor region, whose outputs may reach the medial entorhinal cortex via speed cells in the diagonal band of Broca. These multiple cell types of the entorhinal-hippocampal system are critical elements of a positioning system that dynamically monitors our changing location in the environment, and that may provide the spatial component of episodic memories.