Michael Yartsev, Department of Neurobiology, Weizmann Institute of Science

Room 1061, Meyer building, Electrical Engineering, Technion at 16:00

21 June, 2011
The Neural Basis of Spatial Representation of 2-D and 3-D Environments in the Hippocampus and Entorhinal Cortex of Bats.

Abstract:
Hippocampus and medial entorhinal cortex (MEC) both play a major role in spatial representation of the environment in the brain.  The hippocampus contains 'place cells', neurons which are active when the animal passes through a particular location in the environment (the 'place field'), and the MEC contains 'grid cells', which are activated when the animal's position in space coincides with a vertex of a spatial hexagonal grid spanning the entire environment.  To date, the vast majority of available electrophysiological data on how space is represented in these two interconnected brain regions was obtained from recordings in rats navigating in one- or two-dimensional environments.  To extend current knowledge regarding the neural basis of spatial representation in the mammalian brain, a novel animal model - the Egyptian fruit bat, was used. In this talk, I will describe two project aimed at elucidating two of the biggest mysteries regarding the neural mechanisms governing spatial representation in these two brain regions.

Question 1: What are the underlying neural mechanisms giving rise to the grid formation?

Two competing classes of theoretical models were proposed in the past: network models, based on attractor dynamics versus oscillatory interference models, based on continuous theta-band oscillations (4–10 Hz) in single neurons. To date these models could not be dissociated experimentally, because rodent grid cells always co-exist with continuous theta oscillations. To address this, we conducted the first electrophysiological recording from the entorhinal cortex of freely crawling bats. We find that, in the bat, grid cells existed in the absence of continuous theta oscillation and theta modulation of their spiking activity which causally argues against the oscillatory interference class of models.

Question 2: How is three dimensional (3-D) space represented in the neural activity of hippocampal neurons?

Much research has focused on studying the detailed properties of place cells in rodents moving in either one- or two-dimensional environments (linear tracks or open field arenas, respectively), but only few attempts have been made to study the representation of 3-D space in the mammalian hippocampus. This question is important, because both animals and humans move daily through 3-D environments; and in order to understand the role of place-cells in real-life navigation, we must record from place-cells as animals move through all three dimensions. Using a tetrode microdrive and a custom, lightweight multi-channel neural telemetry system, we conducted the first electrophysiological recordings from the hippocampus of freely flying mammals. Individual well-isolated single units were recorded from the bat hippocampus during flight. These neurons had spatially-restricted place fields that represented all three dimensions, including the height (z-dimension), suggesting that all 3 dimensions are represented by hippocampal neurons.
 
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