Know Your Brain: The Nucleus Accumbens — Learning by Reward


Is that promotion really worth all the late nights? Is that macchiato worth the next week at the gym? Or at least the five-dollar price tag? Whether we have lofty aspirations, crave a sugary reward at the end of the day, or just feel like getting lost for a few hours chatting online or reading outside as the sun sets, our brains are hardwired to seeking out rewards. The source of this behavior is often attributed to the neurotransmitter dopamine, a vital signal controlling both motivation and basic movement. Evidence suggests that dopamine also plays a role in “incentive salience”: weighing the value of rewards and determining whether we should act, rather than simply allowing us to experience pleasure.

Dopamine is released from the brain’s prefrontal cortex into a small structure known as the “nucleus accumbens,” which differentiates rewarding and reinforcing stimuli as we experience it. Think of it as the difference between that sweet mocha flavor and those late nights that you’re putting in regularly now. However, working longer hours can give you more responsibilities and feelings of accomplishment, providing feelings of both, and the nucleus accumbens takes note of this. For these reasons, the structure plays a role in not only substance addiction but also the placebo effect and slow-wave sleep: the state of rest that allows for healing and growth as well as long-term memorization.

The nucleus accumbens is divided into two hemispheres — one for each half of the brain. The structure is located in the ventral portion of the brain’s striatum, at the center of the brain, which it shares with the olfactory tubercle — which despite its name is not used to process smells, but rather how the brain reacts to smells it recognizes. It’s one reason why the smell of a supermarket bakery always seems inviting when you first walk into the store. In addition to processing reward information, it also encodes new motor programs, releasing dopamine that will stimulate the body in order to seek out further rewards.

The nucleus accumbens is composed of a bundle of neurons, a few circuits in the brain’s reward system known as the mesolimbic pathway. The structure consists of both an outer shell — made up of medium spiny neurons that recognize pleasurable stimuli, and an inner core, in which the neurons are denser with longer branches for forming connections — that it will continue to forge with the rest of the brain’s motor systems. This core associates pleasurable feelings with the physical movements and behaviors that gave you that reward — it’s probably why you’re so particular on your current workout regimen — why you don’t feel exactly right if you don’t get in your 10,000 steps per day.

Perhaps we’re right to feel a little disappointed at a lack of accomplishment. In some ways, the nucleus accumbens is an extension of who we are — not just because of memory, but key parts of our personality, as it plays a role in impulsive behavior as well as fear conditioning — being one of the reasons we easily startle. Maternal behavior too, has been associated with increased activity in the nucleus accumbens, a trait observed in both rodents as well as humans via functional MRI studies. Even when the women who participated in the study were shown pictures of infants that were not their own, activity in the nucleus accumbens was visible, so long as they thought the pictures were cute. Life without the functions of the nucleus accumbens seems almost unfathomable.

Never Forget A Face

You could say that the popular mobile dating app Tinder uses the functioning of the nucleus accumbens to its advantage — whether it realizes it or not. A research study led by Vasily Klucharev at the Donders Centre for Cognitive Neuroimaging in the Netherlands revealed that not just mothers respond to attractive faces. As a species, we are all attuned to looking at faces we find physically attractive, due to this functioning of the nucleus accumbens.

Not only do we prefer looking at them, but we tend to believe that more attractive people are more trustworthy, competent, and even intelligent than a person with average looks. Flipping through profile pictures — or “swiping left and right” as Tinder users would call it, is actually heavy on the brain’s reward circuitry. Most of those right-hand swipes (to show interest in a profile), you won’t hear from, but every so often, you’ll hit a match — where that same person previously saw you and also swiped right. This makes Tinder a prime example of intermittent reinforcement, in which the subject’s behavior is occasionally rewarded, just frequently enough that they stay engaged.

It’s why people consistently play the slot machines in casinos — they know the reward is coming if they play long enough. Similarly, it’s why dysfunctional relation ships can continue — if things start smoothly enough, it seems logical to stay with that person long after the problems start. If your partner was romantic in the beginning, you feel that things will eventually be good again if you stay around long enough.

It typically takes a matter of seconds before you decide to swipe left or right on your phone, but regardless of how quickly we make the decision — there is an underlying mechanism determining why we swipe the way we do. Researchers have narrowed the process down to a single valuation stream. A region of the brain known as the “dorsolateral prefrontal cortex” or DLPFC helps the brain weigh options.

As your brain processes each face, the brain’s amygdala and ventral striatum both activate with emotional responses. The ventromedial prefrontal cortex also looks at subjective values — things like distance and common interests, which are readily visible on user profiles. The DLPFC mediates between all of these parts. The quickness of user responses, your own personal biases (like who’s more physically attractive), and the significance of each subsequent response are being subconsciously factored with each face. You may also want to ask yourself when you open the app if you’re looking for a relationship or just casual dating. If you hope to find your significant other right away, the DLPFC prioritizes checking Tinder often and you may find yourself swiping for hours.

You likely got a rush the first time another user swiped right on you. That’s dopamine firing through the neurons. You felt pretty reassured the second and third time, probably, but eventually the neurons may fire in response to the reward predictor — such as the notifications that you have matches before you click on each user.

Focus On The Future

While social media technology stands to benefit from understanding how the brain’s reward system works, technology can also work to improve it, and in doing so, help the rest of the body, when it comes to dealing with addictions or even modifying impulsiveness with deep-brain stimulation techniques. Low-frequency electrical impulses targeting the nucleus accumbens can even play a role in boosting the brain’s memory network.

These treatments may even be beneficial beyond the brain. A 53-year-old patient in the Netherlands sought deep brain stimulation for obsessive-compulsive disorder (OCD). A surprising side effect happened: the patient’s Type 2 diabetes also improved. Researchers suspect that a spike in dopamine activity could improve the body’s ability to break down sugar.

This doesn’t mean that deep brain stimulation is ideal for people with Type 2, but it could lead the way to less invasive therapies targeting dopamine release. To determine whether the therapy affected his improvement, endocrinologist Mireille Serlie and her colleagues at the Academic Medical Center in Amsterdam performed an experiment with 14 other men and women, all of whom were nondiabetic but were receiving the same therapy to treat their OCD. Serlie and the researchers turned off their stimulation devices for 17 hours and then measured the participants’ blood sugar levels. All of them reported increased sensitivity in insulin. Further research showed that depleting dopamine levels also decreased a patient’s sensitivity to insulin treatment.

This article was originally published in Brain World Magazine’s Fall 2018 issue.

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