Prof. dr. Edvard Moser
|Date||20 October 2010|
|Time||16:00 - 17:00|
Prof. dr. Edvard Moser
Kavli Institute for Systems Neuroscience, NTNU, Trondheim, Norway
The ability to find one's way depends on the brain's ability to integrate information about location, direction and distance. A key component of the brain network subserving such integration is the ‘grid' cell. Grid cells fire selectively at regularly spaced positions in the environment such that, for each cell, activity is observed only when the animal is at places that together define a repeating triangular pattern tiling the entire environment covered by the animal. Grid cells are known to co-localize in the medial entorhinal cortex (MEC) with head-direction cells, conjunctive grid × head direction cells, and border cells, which each contribute to a dynamically updated metric representation of current location. In the present talk I will show that grid cells are abundant also in other parahippocampal structures, including pre- and parasubiculum. The proportion of grid cells decreases from MEC to parasubiculum, and from parasubiculum to presubiculum. In all of these regions, grid cells intermingle with head-direction cells and border cells, suggesting that the parahippocampal cortex forms one extended functional network cutting across variations in network architecture and cellular response properties. Grid cells are likely to form a major component of the cortical input to place cells in the hippocampus. An important difference between grid cells and place cells is the tendency for place cells to form orthogonal representations in different environments. This orthogonalization process is thought to depend on the formation of attractor states in recurrent neuronal networks. While several experimental observations are consistent with the presence of attractors in the entorhinal cortex and hippocampus, the dynamic processes supporting attractor dynamics, at the time scale of behaviour, are not well understood. I will show that, in response to an instantaneous transition between two familiar and similar spatial contexts, hippocampal CA3 networks undergo short periods of flickering between pre-formed representations before settling in on the representation most consistent with the new cue configuration, several seconds after the cue change. During the flickering period, convergence to each representation may take place within a single theta cycle and fully expressed representations may alternate at theta time-scale frequencies. Flashbacks to previous states can occur spontaneously but are rare under unambiguous stimulus conditions. The data suggest that, in CA3, pattern completion dynamics repeats within each individual theta cycle. The repetition may facilitate error correction, thus enhancing the discriminative power of the system in the presence of conflicting input cues from spatial representations in entorhinal cortex and stored representations within the hippocampus.
The CSCA lecture is followed by informal drinks. Registration for the lecture is not necessary.