Tag Archives: place cells

A train of discrete places

Place cells are active when an animal is moving about, when it is learning a route, when it is revisiting the path during sleep, when it is planning a route and when it is taking that route. The place cells are active in a sequence that defines the route.

ScienceDaily has an item (here) on a recent paper (B. E. Pfeiffer, D. J. Foster. Autoassociative dynamics in the generation of sequences of hippocampal place cells. Science, 2015; 349 (6244): 180). The paper describes the events in remembering a route.

Foster says, “My own introspective experience of memory tends to be one of discrete snapshots strung together, as opposed to a continuous video recording. Our data from rats suggest that our memories are actually organized that way, with one network of neurons responsible for the snapshots and another responsible for the string that connects them.

The research showed gaps between the ‘snapshot’ discrete memories of a place. “The trajectories that the rats reconstructed weren’t smooth. We were able to see that neural activity ‘hovers’ in one place for about 20 milliseconds before ‘jumping’ to another place, where it hovers again before moving on to the next point. At first, you get a ‘blurry’ representation of point A because a bunch of place cells all around point A fire, but, as time passes, the activity becomes more focused on A. Then the activity jumps to a “blurry” version of B, which then gets focused. We think that there is a whole network of cells dedicated to this process of fine-tuning and jumping. Without it, memory retrieval would be even messier than it is.

It seems to me that this discrete series of place memories may well be like consciousness – a discrete train of individual conscious moments rather than a continuous ‘movie’.

Here is the abstract:

Neuronal circuits produce self-sustaining sequences of activity patterns, but the precise mechanisms remain unknown. Here we provide evidence for autoassociative dynamics in sequence generation. During sharp-wave ripple (SWR) events, hippocampal neurons express sequenced reactivations, which we show are composed of discrete attractors. Each attractor corresponds to a single location, the representation of which sharpens over the course of several milliseconds, as the reactivation focuses at that location. Subsequently, the reactivation transitions rapidly to a spatially discontiguous location. This alternation between sharpening and transition occurs repeatedly within individual SWRs and is locked to the slow-gamma (25 to 50 hertz) rhythm. These findings support theoretical notions of neural network function and reveal a fundamental discretization in the retrieval of memory in the hippocampus, together with a function for gamma oscillations in the control of attractor dynamics.

Place tagged memories

We have another paper from scientists using epilepsy patients with electrodes implanted in their brains during treatment. This method gives very clear results because it measures very small groups of neurons with very accurate time measurements while the patients are able to behave in normal ways. In this case the researchers looked at place cells while the patients played a video game. The places were stores in a virtual city, the patients made deliveries to these stores, each delivery was unique and finally the patients tried to recall the delivered items. The pattern of place cells was recorded for each store just before the delivery was made, then during the delivery when the item was revealed and finally when each item was recalled. When patients recalled a delivery to that store by recalling the item (an event), their place cells showed the store’s signature (the place tag). So something that has been thought to be likely has been demonstrated.

The hippocampus appears to have (at least) two roles: it tracks location for a spatial memory; and it records events for episodic memory. These are interwined. “Our finding that spontaneous recall of a memory activates its neural geotag suggests that spatial and episodic memory functions of the hippocampus are intimately related and may reflect a common functional architecture,” said Kahana, head of one of the groups involved.

The connection between a place and what has happened in that place would be very useful to survival. Deciding how to act in that place would that take into account what was known of the place.

It has seemed to me that memories may also be time-tagged in the sense that episodic events have a serial order for some time. This does not have as strong an involvement in recall but has some. When we remember one event we have a tendency to remember what happened before and after that.

Here is the abstract (Miller etal. Neural Activity in Human Hippocampal Formation Reveals the Spatial Context of Retrieved Memories. Science, 2013):

In many species, spatial navigation is supported by a network of place cells that exhibit increased firing whenever an animal is in a certain region of an environment. Does this neural representation of location form part of the spatiotemporal context into which episodic memories are encoded? We recorded medial temporal lobe neuronal activity as epilepsy patients performed a hybrid spatial and episodic memory task. We identified place-responsive cells active during virtual navigation and then asked whether the same cells activated during the subsequent recall of navigation-related memories without actual navigation. Place-responsive cell activity was reinstated during episodic memory retrieval. Neuronal firing during the retrieval of each memory was similar to the activity that represented the locations in the environment where the memory was initially encoded.