Trends in Neurosciences
ReviewThe anatomical and computational basis of the rat head-direction cell signal
Section snippets
HD cells are not sensitive to the geomagnetic field
One possibility for how these direction-specific firing patterns are generated could be that the cells are somehow sensitive to the local geomagnetic field, so that they can detect heading, in earth-centered coordinates, directly. Numerous observations, however, have suggested that this is not the case 3, 4, 5, 6. For example, Taube et al.3 recorded from HD cells in the postsubiculum as rats foraged in a cylindrical enclosure equipped with a single, white card located on one portion of the wall
HD cells are linked together to form an attractor network
Interestingly, almost all the models developed to explain the HD cells are similar in their basic features 11, 12, 13, 14, 15, 16. Most begin with the idea that the HD cells are somehow linked together, via a set of excitatory and inhibitory connections, to form an attractor network. One example of such a network is provided in Fig. 3a. In this network, cells that are meant to represent adjacent directions are located next to one another. The excitatory HD cells provide excitatory input to
HD cells are located in a set of interconnected brain regions
HD cells were initially discovered in the postsubicular cortex of the hippocampal formation 1, 2. Subsequently, they have been discovered in several additional, closely-related brain regions, including the retrosplenial cortex 17, 18, the anterior thalamic nucleus (ATN) (Refs 19,20), the lateral dorsal thalamic nucleus (LDN) (Ref. 21), the lateral mammillary nucleus (LMN) (Refs 22,23), the dorsal tegmental nucleus (DTN) of Gudden 24 and the striatum 25, 26.
Fig. 4 depicts these areas and their
LMN and DTN might constitute the attractor-path integration network
The fact that HD cells exist in so many different brain regions raises the question of whether the postulated attractor-path integration functions are distributed across all (or many) of these areas, or whether, by contrast, the crucial network is located in just a subset of these regions. As noted elsewhere 42, recent data have suggested that the LMN and DTN might constitute crucial components of the HD cell system.
Concluding remarks
The HD cell system provides a remarkably salient, robust, and tractable neural signal. Because of this, it has been possible to make quite rapid progress in the characterization of the basic behavioral features of these cells, in addition to obtaining clues about where and how the signal might be generated. Although recent evidence suggests that the signal might be generated in the LMN–DTN complex, further work is necessary to substantiate this claim.
If these two nuclei do, in fact, constitute
Acknowledgements
Research support was from NIH (MH11102 and NS35191), Whitehall Foundation (A94-06), and NSF (9120131). We thank three anonymous reviewers who provided helpful comments on this manuscript.
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