A code for space

Memory involves different brain areas. In humans and animals, damage to the hippocampus leads to problems in the ability to remember new information (anterograde amnesia) and in recall of old memories (retrograde amnesia).

In rodents, the hippocampus is often studied in the context of spatial memory and navigation. Recordings of the activity of single hippocampal neurons in behaving animals have revealed how strongly they correlate to space. Each of these ‘place cells‘ has a preferred location (‘place field’) in which it fires at a high rate. Together, the place fields of all hippocampal neurons cover the entire environment and hence it can be said that the hippocampus contains a distributed, probabilistic code for space.

This video shows the activity of several hippocampal neurons. Whenever a spike occurs, the location of the rat is marked with a dot.

Spatially modulated neurons are also found outside the hippocampus. One example is provided by neurons that are tuned to the direction the animal is facing. These head direction cells are found in several brain areas, including thalamus, presubiculum and entorhinal cortex. The entorhinal cortex also harbors cells with other types of behavioral correlates, such as grid cells and border cells. These cells, together with place cells, may be part of system that supports spatial navigation.

Decoding memories

The ensemble activity of hippocampal neurons carries information about the location of an animal in its environment. For example, one may find that cell 1 is only active when a rat is in location A; and cell 2 is only active when the rat is in location B. Conversely, if at some time in the future we observe that cell 2 is active, then we would predict that the rat is most likely in location B. This technique is called neural decoding. More generally, once the spatial correlates of population of hippocampal neurons has been established, then at any time in the future the animal’s location can be decoded based solely on the observed activity of these neurons.

Hippocampal place cells do not signal an animal’s current physical location at all times. The activity pattern switches when the animal is sleeping or when it sits quietly. During these ‘offline’ states, hippocampal neurons are mainly silent, except for short bursts that occur about one or two times per second. We applied neural decoding to these population bursts to analyze their content. We showed that during such bursts spatial sequences (i.e. trajectories) are replayed at high speed. We study the role of hippocampal memory replay in planning behavior, evaluating past performance and memory consolidation.

In this video, the (virtual) location of a rat is estimated solely based on the spiking pattern of hippocampal neurons. It is as if the animal is running along the track in his mind.