Human behavior has historically been examined under one of two perspectives. The first asserts that the choices people make are rational, consistent, and relatively unbiased by emotions. In contrast, another opinion holds that humans are inconsistent and often irrational due to limited attention spans and skewed viewpoints. Recent technological advances in the field of neuroscience, such as fMRI and DTI brain imaging, have given researchers a way to examine these different points of view quantitatively. By measuring brain activity in areas known to be responsible for the processing of specific and sometimes conflicting stimuli, researchers can examine how patterns in brain activity predict certain choices. Professor Brian Knutson from Stanford University has shown that activation in a handful of brain regions can reliably predict an individual’s preference on a variety of choice – oriented tasks (1).

An active nucleus accumbens under fMRI. Copyright Wiki Commons

Before the experiment began, two brain areas in particular were chosen as possible predictors of choice. The first is the anterior insula, a region of the cerebral cortex, the “folded” portion of the brain, that lies just beneath the lateral sulcus, near the temples. Previous studies have shown that this area is heavily involved in processing stimuli that elicit disgust or displeasure (2). Another area chosen for its importance was the mesolimbic dopamine system, a collection neurons in the ventral tegmental area (VTA) near the brain stem that is thought to be responsible for feelings of pleasure and action motivation (3). Neurons in the VTA project to both the medial prefrontal cortex and the nucleus accumbens, both of which later showed increasing activity relative to the desirability of the stimulus.

The first task used by Knutson et al. was the monetary incentive delay task (MID). After receiving a set amount of starting money, participants were shown a symbol that indicated that they were about to either lose or gain a certain amount in US dollars. Following a delay, the participant had to press a button within a certain span of time to gain the displayed amount or avoid losing the displayed amount. fMRI scans during the period of delay between the presentation of the trial and the chance to gain/intervene showed that anticipation of gaining money selectively activated the nucleus accumbens. There was also a positive correlation between the activation of the nucleus accumbens and the amount of money to be gained. Conversely, and as predicted, anticipation of the loss of money activated the anterior insula.

Similar results were seen for the following task, which involved choosing between “good” and “bad” stocks, which were arbitrarily represented as squares and triangles. The participant learned to distinguish between the two through trial and error. In addition to the two stocks, there was a safe “bond” option that always gave the participant $1. Researchers found that nucleus accumbens activity preceded risky behavior, which involved the participant choosing what he or she perceived to be the good stock. They also found that anterior insula not only preceded risk averse behavior, choosing the safe bond, but that it strongly predicted an irrational decision. When the participant was late enough in the trials to have an intuition as to which stock was the good option, choosing the bad alternative elicited strong insula activity.

The last experiment involved a more familiar paradigm for the subjects. In the shopping task, participants were shown a series of pictures of items, as well as their price. After a delay, during which the fMRI took its measurements, they had the opportunity to decide whether or not to buy the selected item. Along with the predicted nucleus accumbens activity for trials in which the item was bought, subjects also showed increasing activity in the medial prefrontal cortex with regards to what the researchers termed the “bargain vs. ripoff” margin. Over the course of the experiment, items were presented more than once and at various prices to determine at what point and in which trial the buyer would refuse to buy the item. Activity in the trials in which the price was lowered was compared to that of this baseline trial. The aforementioned increase in MPFC activity was matched by a decrease in insula activity, both of which became more pronounced with the lowering of the price. It is worth noting that the MPFC has been shown in numerous other studies to be involved in executive decision-making (4)(5).

While the neural underpinnings of choice do not fully reconcile the differing viewpoints of human behavior, they go a long way in trying to understand the mechanisms behind the decision making process. Specifically, Knutson et al. have shown activity in the VTA and anterior insula to be excellent predictors of how people perceive the relative value of a stimulus.

References

1. B. Knutson and S. Greer, Anticipatory Affect: Neural Correlates and Consequences for Choice(Stanford University – Philosophical Transactions of the Royal Society), 2008

2. K. Phan, Tor Wager, Stephan Taylor, Israel Liberzon, Functional Neuroanatomy of Emotion: A Meta-Analysis of Emotion Activated Studies in PET and fMRI. NeuroImage. 16, 331-348(2008)

3. K. Davism D. Charney, J. Coyle, C. Nemeroff, in Neuropsychopharmacology – 5th Generation of Progress (Lippincott, Williams, & Wilkins, Philadelphia, 2002)

4. A.  Bechara, D. Tranel, H. Damasio, Characterization of the Decision-Making Deficit of Patients with Ventromedial Prefrontal Cortex Lesions. Brain. 123, 2189-2202(2000)

5. Y. Yang and A. Raine, Prefrontal Structural and Functional Brain Imaging findings in Antisocial, Violent, and Psychopathic Individuals: A Meta-Analysis. Psychiatry. 174, 81-88(2009)