Research published in Nature this week suggests that the computational power of the brain is even greater than we previously thought. The new findings show how single cells in the brain can represent more than one experience at the same time - such as where you are and what you are doing. These results could lead to a greater understanding of how the brain processes memories.

John Huxter of Bristol University said: 'Our findings suggest that individual brain cells can represent different types of information at the same time. This is very important for understanding how the brain codes information in parts of the brain crucial to event memory and memory disorders such as Alzheimer's disease.'

Mammals such as the rat carry around internal maps of familiar places in their brains. These maps are built out of 'place' cells, each of which fire when in specific locations within a familiar environment.  By observing which particular place cells are firing at any given moment it is actually possible to predict where an animal is. However, the timing of firing also signals position, and until now it was unclear whether the timing was directly determined by the rate. If rate determines timing, these two 'codes' cannot represent different information.

Contrary to recent predictions, scientists at Bristol University and University College London have demonstrated that in rats the rate and timing of firing are independent of each other.

They trained rats to run back and forth on a runway, while recording the activity of their place cells and the timing of firing. They found that the firing rates increased as  the rat's speed increased, but the relationship between the timing of firing and position remained constant, even during 10-fold increases in the firing rate.

Even when they manipulated the rat's environment and shortened the runway which resulted in a lowering of the firing rate, the induced changes in firing rate were not reflected in changes to the timing.

These results show conclusively that in rats the rate and timing of firing are independent of each other, contradicting proposed models in which timing is directly determined by firing rate.  They show how both may be used to simultaneously encode multiple aspects of daily experiences, such as where you were and what you were doing - critical components of event memories.