Wednesday, October 24, 2012

Mechanism of unconscious internal bias in our choices

What's actually happening when we make choices that do not seem to be justifiable on purely economic or logical grounds? Wimmer and Shohamy do some interesting work showing how the hippocampus can instill an unconscious bias in our valuations, whereby an object that is not highly valued on its own, increases in value when it becomes implicitly associated with a truly high-value object. As a consequence, we then end up preferring the associated object over a neutral object of equal objective value while not really knowing why. The abstract:
Every day people make new choices between alternatives that they have never directly experienced. Yet, such decisions are often made rapidly and confidently. Here, we show that the hippocampus, traditionally known for its role in building long-term declarative memories, enables the spread of value across memories, thereby guiding decisions between new choice options. Using functional brain imaging in humans, we discovered that giving people monetary rewards led to activation of a preestablished network of memories, spreading the positive value of reward to nonrewarded items stored in memory. Later, people were biased to choose these nonrewarded items. This decision bias was predicted by activity in the hippocampus, reactivation of associated memories, and connectivity between memory and reward regions in the brain. These findings explain how choices among new alternatives emerge automatically from the associative mechanisms by which the brain builds memories. Further, our findings demonstrate a previously unknown role for the hippocampus in value-based decisions.
The details of the experiment are kind of neat. I pass on two figures:

Fig. 1 The task consists of three phases: association learning, reward learning, and decision-making. (A) In the association phase, participants were exposed to a series of pairs of pictures (S1 and S2 stimuli) while performing a cover task to detect “target” upside-down pictures. S1 stimuli were either face, scene, or body part pictures; S2 stimuli were circle images. (B) In the reward phase, participants learned through classical conditioning that half of the S2 stimuli were followed by a monetary reward (S2+), whereas the other S2 stimuli were followed by a neutral outcome (no reward, S2–). S1 stimuli never appeared in this stage. (C) In the decision phase, participants were asked to decide between two stimuli (both S1 or both S2) for a possible monetary win. No feedback was provided, and all gains were awarded at the end of the experiment. Decision bias was operationalized as the tendency to choose S1+ over S1– stimuli in this phase.

Fig. 3 Reactivation of category-specific visual areas during the first half of the reward phase is related to subsequent decision bias. (A) Example participant region of interest masks (derived from the association phase) for body, face, and scene S1 stimuli. Masks were applied to S2 presentations during the reward phase. (B) S2 presentation elicits activation in visual regions responsive to associated S1 stimuli when participants later exhibit decision bias. Error bars indicate ±SEM; a.u., arbitrary units.

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