PAVLOV's celebrated dogs may have needed no more than a single brain cell to learn their conditioned response to a bell that signalled the arrival of food.
Germund Hesslow and his colleagues at Lund University in Sweden studied the brains of ferrets as they were being conditioned to blink in response to hearing a tone that signalled an air puff in the face. By monitoring changes in the firing rate of individual cells called Purkinje neurons in the animals' cerebellum -- a cauliflower-shaped region behind the brainstem -- they were able to watch the formation and extinction of new memories. This revealed that the acquired associations were created and stored within single Purkinje neurons (The Journal of Neuroscience, vol 27, p 2493).
Each of these cells can respond to hundreds of thousands of different stimuli, such as different sounds, sights or feelings, but has just one output that causes a specific action or muscle movement. This means each neuron can potentially learn hundreds of thousands of triggers for the same response, Hesslow says.