These choices are affected by perception, wrote the researchers, because "there are visual images and marketing messages that have insinuated themselves into the nervous systems of humans that consume the drinks."
Even though scientists have long believed that such cultural messages affect taste perception, there had been no direct neural probes to test the effect, wrote the researchers. Findings about the effects of such cultural information on the brain have important medical implications, they wrote.
"There is literally a growing crisis in obesity, type II diabetes, and all their sequelae that result directly from or are exacerbated by overconsumption of calories. It is now strongly suspected that one major culprit is sugared colas," they wrote.
Besides the health implications of studying soft drink preference, the researchers decided to use Coke and Pepsi because-- even though the two drinks are nearly identical chemically and physically--people routinely strongly favor one over the other. Thus, the two soft drinks made excellent subjects for rigorous experimental studies.
In their study, the researchers first determined the Coke versus Pepsi preference of 67 volunteer subjects, both by asking them and by subjecting them to blind taste tests. They then gave the subjects sips of one drink or the other as they scanned the subjects' brains using functional magnetic resonance imaging (fMRI). In this widely used imaging technique, harmless magnetic fields and radio signals are used to measure blood flow in regions of the brain, with such flow indicating brain activity levels. In the experiments, the sips were preceded by either "anonymous" cues of flashes of light or pictures of a Coke or Pepsi can.
The experimental design enabled the researchers to discover the specific brain regions activated when the subjects used only taste information versus when they also had brand identification. While the researchers found no influence of brand knowledge for Pepsi, they found a dramatic effect of the Coke label on behavioral preference. The brand knowledge of Coke both influenced their preference and activated brain areas including the "dorsolateral prefrontal cortex" and the hippocampus. Both of these areas are implicated in modifying behavior based on emotion and affect. In particular, wrote the researchers, their findings suggest "that the hippocampus may participate in recalling cultural information that biases preference judgments."
The researchers concluded that their findings indicate that two separate brain systems--one involving taste and one recalling cultural influence--in the prefrontal cortex interact to determine preferences.
Samuel M. McClure, Jian Li, Damon Tomlin, Kim S. Cypert, Latané M. Montague, and P. Read Montague: "Neural Correlates of Behavioral Preference for Culturally Familiar Drinks"
Published in Neuron, Volume 44, Number 2, October 14, 2004, pages 379–387.