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Doherty and colleagues at UCL and the University of Iowa, US, ranked the preferences of human volunteers for blackcurrant, melon, grapefruit and carrot juice, and for a tasteless, odourless control drink.

The researchers scanned the volunteers’ brains using a technique called functional magnetic resonance imaging (fMRI) to detect enhanced blood flow in various brain regions – the greater the flow, the greater the neural activity in those areas.

They developed a Pavlovian-type association by flashing a geometric shape on a computer screen and giving a squirt of juice into the volunteers’ mouths. However, the volunteers did not realise that they were being conditioned in this way – they were simply told to press a button to indicate on which side of the screen the shape had appeared.

The team measured how the volunteers had become conditioned by measuring their anticipation of the juice squirts following an image by measuring the dilation of their pupils.

The fMRI scans revealed significant responses reflecting learning in the ventral midbrain and the ventral striatum. Crucially, they found that the strength of the response correlated with the volunteer’s like or dislike of the juice.

“Stronger neural responses occur in these regions to a cue that is associated with a more preferred food,” said Doherty. “This shows that when you see a cue that is predictive of a reward, you are able to access information about your subjective preferences.”

Doherty says this kind of brain programming may have an evolutionary function in helping humans and animals predict both good and bad experiences in their environment.

“For instance, if you learn that a particular fast food outlet gave you food poisoning the last time you ate there – it is going to be in your interest to know not to go there again once you see the sign for that shop in the street,” he says.