Now, John O'Doherty and his colleagues have traced where in the reward-processing regions of the brain such associations are developed. They described their findings in an article in the January 5, 2006, issue of Neuron. More broadly than offering insights into food preference, they said, their findings aid understanding of the fundamental neural machinery by which the brain establishes all preference behavior.
In their experiments with human volunteers, they first determined the subjects' rank-order preference of four juices--blackcurrant, melon, grapefruit, and carrot--and a tasteless, odorless control solution.
They then scanned the subjects' brains using functional magnetic resonance imaging (fMRI) as they established a Pavlovian conditioning association in the subjects. Such conditioning is the same type that Pavlov used to condition dogs to associate an otherwise irrelevant stimulus such as a bell with food. However, in this case, the researchers conditioned the subjects to associate each juice with an arbitrary visual stimulus--a geometric shape flashed on a screen.
In these experiments, the subjects were not told that the appearance of a specific shape would be associated with a subsequent squirt of the corresponding juice into their mouths. Rather, their instruction was to indicate with a button-press on which side of the screen the shape appeared.
As the subjects performed the task--becoming unconsciously conditioned to associate the shapes with the juices--the researchers used fMRI to search for tell-tale activity in brain regions known to be associated with reward and reward-related learning. The widely used fMRI technique uses harmless magnetic fields and radio waves to detect
Contact: Heidi Hardman