Karl Friston Publications

A Free Energy Principle for the Brain

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Science and Bayesian Probability

New Scientist Article on Bayesian Brain

 Karl Friston

Karl Friston has made major contributions to the 'Bayesian Brain' school of Neurology and Behaviour.

The Bayesian Brain model is heralded by many workers in the field as providing the most promising attempt at a unified theory of brain functions. It models numerous brain processes as performing a selection from competing internal models of the outside world developed by the brain on the basis of the sensory data available in support of each model. As more data becomes available the probabilities of certainty for each model are updated by a process analogous to Bayesian Probability. Through a Darwinian process, selecting from the competing models the one best supported by the evidence, a basis for action is chosen.

Let's say we are in a fairly stressful situation such as driving in the city and we see something out of the corner of our eye that seems to be moving towards us. Because we cannot see the object very well we may be unable to assign accurate probabilities to the various imaginable possibilities. Our brain develops numerous competing models of what the object and its significance to us might be but as it has insufficient data to favour one model over the others it must assign nearly equal probability to a number of the competing possibilities.  We are in a state of uncertainty.

In response to this situation, unconsciously, we turn our head and/or eyeballs and bring the object into focus. We gather an accurate and relevant data set concerning the object and our brain updates the probabilities assigned to its competing models on the basis of this data. Usually we are able to award one model a value approaching certainty and that model is selected to inform our actions. Our brains through a rational Darwinian process have made us better informed and optimized our response.




Many people now regard the brain as an inference machine that conforms to the same principles that govern the interrogation of scientific data.

Karl Friston