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 enhanced blood flow in brain regions, which reflects greater neural activity.
The researchers measured how effectively the subjects became conditioned to anticipate the juice squirts by measuring the dilation of their pupils after the stimuli and before the juice.
In analyzing the brain scans, the researchers detected significant responses reflecting learning of behavioral preferences in a region called the ventral midbrain, as well as an area of the ventral striatum. In the former region, the researchers found that the response increased with increasing preference for the juice. And in the latter area, the researchers found a "bivalent" response, with the highest responses for the most and least preferred juices.
"It has long been known that associating brand items with other rewarding or appetitive stimuli, through the process of classical conditioning, makes it possible to modulate subjects' preferences," wrote the researchers. "This process may account in large part for the efficacy and power of advertising.
"The principal implication of the present study is that it provides an account of how predictive representations, learned through classical conditioning, come to elicit activity in the human brain that relate directly to subsequent behavioral preference. We suggest that such representations play an important role in the guidance of action based upon future reward, a form of elementary behavioral decision making."
The researchers included John P. O'Doherty of the University College London in London, United Kingdom and California Institute of Technology in Pasadena, California; Tony W. Buchanan of the University of Iowa in Iowa City, Iowa; Ben Seymour and Raymond J. Dolan of the University College London in London, United Kingdom. This research was supported by a Programme Grant to R.J.D. from the Wellcome Trust. T.W.B. was supported by a short-term fellowship provided by the Human Frontiers Science Program.
O'Doherty et al.: "Predictive Neural Coding of Reward Preference Involves Dissociable Responses in Human Ventral Midbrain and Ventral Striatum." Publishing in Neuron 49, 157–166, January 5, 2006, DOI 10.1016/j.neuron.2005.11.014 www.neuron.org
Journal
Neuron