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Abstract
What are the computational laws of hippocampal activity? In this paper we argue for the slowness principle as a fundamental processing paradigm behind hippocampal place cell firing. We present six different studies from the experimental literature, performed with real-life rats, that we replicated in computer simulations. Each of the chosen studies allows rodents to develop stable place fields and then examines a distinct property of the established spatial encoding: adaptation to cue relocation and removal; directional dependent firing in the linear track and open field; and morphing and scaling the environment itself. Simulations are based on a hierarchical Slow Feature Analysis (SFA) network topped by a principal component analysis (ICA) output layer. The slowness principle is shown to account for the main findings of the presented experimental studies. The SFA network generates its responses using raw visual input only, which adds to its biological plausibility but requires experiments performed in light conditions. Future iterations of the model will thus have to incorporate additional information, such as path integration and grid cell activity, in order to be able to also replicate studies that take place during darkness.
Highlights
Since their initial discovery (O’Keefe and Dostrovsky, 1971) place cells found in the hippocampus were subject to a wide range of both experimental studies as well as theoretical models and ideas
Sampling and Development To examine the (a) development and (b) sampling issues of place fields we present the results of two different simulations. (a) Figure 3 shows the development of three different place fields after 30 s and 1, 2, 4, and 8 min of exploration time
It can be seen that the hierarchical network produces distinct spatial fields of activity as early as within 2 min of exploration in an unknown environment. (b) Figure 4 shows the emergence of three fully established place fields while the animal randomly traverses the environment for 8 min
Summary
Since their initial discovery (O’Keefe and Dostrovsky, 1971) place cells found in the hippocampus were subject to a wide range of both experimental studies as well as theoretical models and ideas Their main draw is their unusual potential to directly correlate with observable behavior by developing spatially localized fields of activity, the so-called place fields. Place cells take part in planning (Robitsek et al, 2013) and goal directed behavior (Pfeifer and Foster, 2013) They react to changes in familiar environments and have been shown to adapt their firing rate (local remapping), reposition themselves (global remapping), or become silent, depending on the performed manipulation (Leutgeb et al, 2005a). Theoretical models of place cells do have to explain localized firing in the first place, and need to account for their remapping behavior in order to be able to make compelling predictions for future experiments
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