Reward-motivated learning: mesolimbic activation precedes memory formation

The prospect of a paycheck, good grade, or promotion wonderfully concentrates the mind, and researchers have now identified the brain circuitry responsible for such reward-motivated learning.

In an article in the May 4, 2006, Neuron, Alison Adcock and colleagues report brain-scanning studies in humans that reveal how specific reward-related brain regions "alert" the brain's learning and memory regions to promote memory formation.

In their studies, the researchers asked volunteers to participate in two types of reward-related tasks as they scanned the subjects' brains using functional magnetic resonance imaging. In this technique, harmless magnetic fields and radio waves are used to detect regions of higher blood flow in the brain, which reflects higher activity.

In the first task, the researchers aimed at identifying the region involved in anticipating rewards. This task involved presenting the subjects with such symbols as circles or squares that indicated an amount of money the subjects could gain or lose--from no money to $5--by rapidly responding to a subsequently presented target by pressing a button. The subjects were notified immediately whether they had received the reward. The researchers found that reward anticipation activated specific brain structures in the "mesolimbic" region involved in the processing of emotions.

In the second task, the researchers sought to measure how this reward center promoted memory formation. They first showed subjects a "value" symbol that signified whether the image of a scene that followed would yield $5 or ten cents if they remembered it the next day. Then they showed the subjects the scene, and the next day tested their ability to pick the scene out of a group.

The researchers found that the subjects were far more likely to remember high-value scenes than low-value scenes. Importantly, they found that the cues to the high-reward scenes that were later remembered--but not those scenes later forgotten--activated the reward areas of the mesolimbic region as well as the learning-related hippocampus in the medial temporal lobe (MTL) of the brain. Activation prior to scene visualization suggests that the brain actually prepares in advance to filter incoming information rather than simply reacting to the world. Activation of the MTL is associated with higher brain functions, including learning and memory, and subjects who showed greater activation in these regions also showed better memory performance, found Adcock and colleagues.

The researchers concluded that the learning mechanism they identified "may let an organism's expectations and motivation interact with events in the physical world to influence learning. Thus, anticipatory activation of this mesolimbic circuit may help translate motivation into memory."

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