Spatial navigation is a complex mental task which is strongly dependent upon memory. As we make our way around a new environment, we look for easily recognisable landmarks and try to remember how their locations are related in space, so that when we return to it we can negotiate our path.
We know that spatial representations are encoded in the medial temporal lobe, and numerous studies implicate the hippocampus in particular as being crucial for the formation of spatial memories. Information about the environment is believed to be encoded by large populations of neurons distributed throughout this part of the brain, but little is known about this information is encoded.
Now researchers from UCL have made a significant advance in our understanding of how spatial memories are encoded. In a paper published online today in the journal Current Biology, they report that they can decode the activity of the hippocampus to accurately predict an individual's exact location within a simple virtual reality environment.
In rodents, spatial memory and navigation involves at least four different types of neuron, located in and around the hippocampus, whose properties are well characterised. Place cells, for example, fire selectively when the animal is in a specific location; grid cells fire periodically as it traverses a space; head direction cells increase their activity when it faces a particular way; and border cells, whose existence was reported just a few months ago, fire when it is close to anything that resembles a border. Yet, despite the relative wealth of cellular data, the neural mechanisms underlying spatial navigation are still unclear.
We do not yet know what specific features of the environment the place cells are sensitive to, or how ensembles of neurons in the hippocampus encode spatial memories. In the visual system, the topographical features of an image are mapped directly, so that adjacent parts of the image are encoded by neighbouring cells in the retina and early visual processing areas of the brain. In the hippocampus, however, there is no obvious relationship between place cell activity and an organism's precise location within its environment. Location is therefore thought to be encoded randomly in large populations of cells distributed sparsely and widely throughout the hippocampus.
For the new study, Demis Hassabis of the Wellcome Trust Centre for Neuroimaging in London and his colleagues constructed a virtual reality environment, consisting of two separate rooms with distinct layouts and landmarks (above), using a modified version of the graphics engine used for the role-playing video game Fable. Four participants were asked to navigate this virtual space, and to find specified locations within it, while their brains were scanned with high spatial resolution functional magnetic resonance imaging (fMRI).
The researchers analysed their data with an algorithm similar to the one which was recently used to reconstruct visual images from the activity of the visual cortex. They focused on the activity of several dozen clusters of neurons in the posterior part of the hippocampus, which is known to be involved in spatial memory. Each of these clusters comprises some 10,000 cells, so that the activity of several million of the approximately 40 million hippocampal neurons was recorded.
Thus the researchers identifed the activation patterns associated with a number locations in the virtual environment. They found that they could distnguish between these patterns, and were therefore able to predict the precise location of their participants in the virtual space. The researchers were effectively reading spatial memories as they were being encoded and consolidated in the hippocampus.
This study confirms that spatial memories are encoded by the co-ordinated activity of large numbers of neurons distributed widely throughout the posterior portion of the hippocampus. But by decoding the activity associated with the memories of specific locations, the researchers have demonstrated that the brain's representations of space are encoded in an orderly, structured manner, and not randomly, as previously thought. The study therefore marks a big step toward understanding how spatial memories are encoded, and how this process breaks down in conditions such as Alzheimer's Disease.
- Developmental topographagnosia
- Rats know their limits with border cells
- Reading the contents of working memory
- Visual images reconstructed from brain activity
Hassabis, D. et al (2009). Decoding Neuronal Ensembles in the Human Hippocampus Curr. Biol. DOI: 10.1016/j.cub.2009.02.033.
I'd be interested in a clarification of the differences between these two studies, and what each contributes to our knowledge that the other does not. To me, untrained in the field, they look pretty much identical.
This was a more balanced assessment of the paper than the one produced by the New Scientist yesterday. One would think that we could read the past actions of people based on fMRI.
The only dispute that I have with your write-up is the mention of scientists watching the memories "as they were being encoded and consolidated". Encoded, yes. Consolidated, no. Consolidation and reconsolidation are very hot topics in the literature at the moment, but in rodents, consolidation typically refers to the replay of place sequences "offline" -- during resting periods after "performing" or during sleep. These events are represented by physiological phenomena now called sharp-wave ripples, representing previous spatial experience manifested in a condensed (temporally) sequence of activity detected with field potentials. There are lots of fantastic papers on this from the labs of Matt Wilson (MIT) and Gyuri BuzsÃ¡ki (Rutgers).
Although this study does provide evidence for encoding and suggests some structure to spatial representation in the human hippocampus, it does not attempt to examine reconsolidation and replay.
Adrian and Noah: Your comments were filtered as spam because they contain multiple links.
@Adrian: You're right to think that they look identical, because both press releases are in fact about the study discussed here.
@Noah: You raise a fair point about encoding vs. consolidation. I suppose I had always assumed that consolidation begins at the moment a memory is encoded, provided, of course, that "rehearsal" is not interrupted.
I think it would have been very cool if the study included another set of trials in which the participants were asked to recall each different location, to see the activation patterns associated with retrieval of the spatial memories.
Spatial Disorientation (SD) is an incipient symptom in Alzheimer's Disease(SD). In fact it has been posited that SD along with anterograde memory loss (AML) may be the first behavioral symptoms of AD. Both of these processes are dependent on hippocampal integrity. Thus, neuroimaging studies such as the this may provide a method for early diagnosis of incipient AD prior to the onset of behavioral symptoms of SD and AML.
@Roland: I thought it was well known that spatial disorientation is one of the earliest symptoms of Alzheimer's.