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.