Neural representation of two- and three-dimensional space in the hippocampal formation of behaving bats
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1
Weizmann Institute of Science, Israel
The hippocampus and medial entorhinal cortex (MEC), both regions in the mammalian hippocampal formation, are considered pivotal elements in the neural circuitry underlying spatial memory and navigation. 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 any 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, I used a novel animal model - the Egyptian fruit bat. Here, I will describe two of my PhD research projects aimed at elucidating two central questions regarding the neural mechanisms governing spatial representation in these two brain regions.
Project 1: What are the underlying neural mechanisms giving rise to the grid formation?
Two competing classes of theoretical models govern the field: 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 MEC of freely crawling bats. We found that, in the bat, grid cells existed in the absence of continuous theta oscillation and theta modulation of spiking activity which causally argues against the oscillatory interference class of models (Yartsev et al., Nature 2011).
Project 2: How is three dimensional (3-D) space represented in the neural activity of hippocampal neurons?
Most animals, on air, water or land, navigate in three-dimensional (3D) environments. Despite previous experiments in outer space and on Earth, it remains unclear how brain circuits encode the animal's 3D position. To address this, we used a wireless neural-telemetry system, which enabled recording the activity of single neurons in freely-flying bats. We focused on the hippocampus and recorded the neural activity in flying bats performing a natural foraging task. We found that single hippocampal units were active in confined 3D volumes, and represented all axes with similar resolution. 3D place-fields from different neurons spanned different locations, and collectively represented uniformly the entire available space in the room. Surprisingly, theta rhythmicity was not present in the firing patterns of 3D place-cells, arguing against a temporal phase-code for 3D positions. These results suggest the mammalian hippocampus represents 3D space by a uniform and isotropic rate-code.
Keywords:
bats,
flying,
Hippocampus,
Place Cells,
Place fields,
theta
Conference:
Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012.
Presentation Type:
Invited Symposium (only for people who have been invited to a particular symposium)
Topic:
Learning, Memory and Behavioral Plasticity
Citation:
Yartsev
MM and
Ulanovsky
N
(2012). Neural representation of two- and three-dimensional space in the hippocampal formation of behaving bats.
Conference Abstract:
Tenth International Congress of Neuroethology.
doi: 10.3389/conf.fnbeh.2012.27.00018
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Received:
15 Apr 2012;
Published Online:
07 Jul 2012.
*
Correspondence:
Mr. Michael M Yartsev, Weizmann Institute of Science, Rehovot, Israel, myartsev@berkeley.edu