We have answered in full all the 5 Aims of this ERC project.
On the technical side, we have developed new generations of 64-channel neural loggers that allow us to record spikes from dozens of neurons simultaneously in freely-flying bats (important for all Aims 1-5). We also set up a large 200-meter flight tunnel and a localization system that allows tracking the bat’s position with a 10-cm accuracy over large spatial scales (important for Aims 4-5).
On the scientific side, we achieved our goals on all the five Aims:
Aim 1: We finished this Aim, and published a paper in Science summarizing our exciting results (Sarel et al., Vectorial representation of spatial goals in the hippocampus of bats, Science 2017). In this paper, we found neurons that represent the direction and distance to navigational goals – i.e. a vectorial representation of spatial goals. Following this study, we asked whether not only navigational goals are represented in the hippocampus, but also other things “out there” are represented – and indeed we found that the location of other conspecifics (other bats) is represented by neurons in the bat hippocampus: A paper summarizing these exciting results was published in Science (Omer et al., Social place-cells in the bat hippocampus, Science 2018).
Aims 2-3: We finished these two Aims, and recently published a paper in Nature summarizing our exciting results on these two aims (Ginosar et al., Locally ordered representation of 3D space in the entorhinal cortex, Nature 2021). In this paper, we described our discovery of 3D grid cells (Aim 2) and 3D border cells (Aim 3) in the entorhinal cortex of flying bats. Grid fields were not cylindrical bur spherical, and exhibited fixed distances to nearest-fields – i.e. a fixed local distance scale – but did not exhibit a global lattice. These results revolutionize our understanding of grid cells and suggest that, at least in 3D, grid cells form not a global metric but a local metric for 3D space. In addition, we have used some of the previously acquired 3D flight- and neural-data (and also 2D data) to ask a question about the relevance for hippocampal and entorhinal processing of the theta oscillation. We found that there was no theta oscillation in bats, but we did find phase-locking (synchronization) and phase-precession (phase-coding) to the nonoscillatory field activity in the hippocampus – suggesting a novel nonoscillatory phase coding in the hippocampus. These exciting results were recently published in Cell, and they suggest that although oscillations do not generalize across mammalian species, coding principles do generalize (Eliav et al., Nonoscillatory phase coding and synchronization in the bat hippocampal formation, Cell 2018).
Aims 4-5: We finished these two Aims, and recently published a paper in Science summarizing our exciting results on these two aims (Eliav et al., Multiscale representation of very large environments in the hippocampus of flying bats, Science 2021). To this end, we first established all the technical foundations necessary for this ambitious project – including a new 64-channel neural logger (wireless electrophysiology system) and a positional-tracking system that is 100 times more accurate than GPS. We have built a long flight-tunnel at the Weizmann Institute (200 meters long) and recorded hundreds of hippocampal neurons in bats flying in this tunnel – both in wild-born bats (Aim 4) and laboratory-born bats (Aim 5). We found that the bat hippocampus exhibited a multifield multiscale spatial code in this large environment, which is very different from the single-field single-scale neural codes found in small environments, in both rodents and bats. Theoretical decoding analysis showed that this multiscale code is much more efficient for representing very large environments. Surprisingly, we found that in lab-born bats, the same multifield multiscale neural code exists – suggesting that the multifield multiscale code does not require previous experience with large environment, and indicating that the multifield multiscale code is a very robust characteristic of the hippocampus, regardless of experience.
