Ask a Neuroscientist: How does the brain balance fear and desire?

Mike asks:

"If neurons are the driving force behind thoughts, actions and emotions is it simply the case people lean towards one decision over another when more neurons are activated in a particular brain region? For example let's say someone was deathly afraid of snakes and given an opportunity to walk through a snake enclosure. The fear response would be "no" and easy to decide. Conversely if the same person was offered a winning lottery ticket worth a million dollars their response would likely be "yes".

How much to cross this copperhead?

My question centers around what happens when someone is faced with a decision that both greatly terrifies them and something they really want? What causes the brain to lean more towards fear or reward? Are more neurons firing in one region of the brain more than others or is something else going on?”  

Mike, that’s a great question. The carrot of desire and whip of fear are powerful drivers of our actions, but what happens when they are in conflict? How does the brain balance these great motivators in a scenario like the one you propose, where you are offered a million dollars to walk through a terrifying snake enclosure?

Emotional Networks

Well, for starters, we know that the brain has separate centers specialized in processing fearful and rewarding experiences. Decades of research have shown that a brain region called the amygdala is crucial for learning to recognize and respond to frightening stimuli. Another area, the nucleus accumbens, is at the center of brain’s reward system. Together, these systems work to steer you away from danger and lure you towards pleasurable activities.

Another part of the brain, the prefrontal cortex (PFC), endows us with the ability to evaluate and control our instinctive desires and fears based on experience and context. When you are deciding whether to brave the hall of snakes, your prefrontal cortex would work to temper your amygdala’s warnings about how scary the snakes are while also evaluating your reward system’s excitement about the money you may gain. Are the snakes behind glass? Are you sure that million-dollar check is for real? Are your friends going to make fun of you if you chicken out? What’s the real risk, and what’s the real payoff? (As a side note –you may not be surprised to learn that the PFC’s ability to inhibit impulsive behavior is one of the brain functions particularly impaired by alcohol consumption.)

Weighing Options

So all of these different areas would certainly be involved in a decision that pits fear against desire. Perhaps I could predict what decision you will make simply by placing you in an fMRI scanner and looking at how much activity I can see in these different regions. But your original question was not just where do we make decisions (with what parts of the brain), but how do we make decisions. How does the firing of neurons in an are like the PFC actually take fear, desire, experience, and context into account to produce a decisive choice among different possible actions? 

This is a challenging question that neuroscientists are still struggling to answer. In part this is because the large number of variables influencing most decisions are difficult to control in scientific studies. Current theories are based on very simple decision tasks that let scientists carefully manipulate a limited number of variables. In a classic experiment, Stanford neuroscientist Bill Newsome trained monkeys to watch a screen with dots randomly moving left and right and then report which way the majority of the dots were moving. (Try it yourself!)  This task can get very tricky as the ratio gets close to 50:50 (like this), but in the end the monkey just has to make a choice and report it to the experimenters. If he is right, he gets paid in juice. This type of simple perceptual choice allowed Newsome to control key variables very precisely, including the quality of the sensory evidence and the size and probability of reward. He and his colleagues could then look for representations of these variables in the monkeys’ brains by implanting electrodes to record the activity of neurons during the decision task. 

Newsome’s studies, and others like them, suggest that distinct patterns of electrical activity involving hundreds or thousands of neurons in the PFC and other executive regions represent a monkey’s competing choices. As evidence accumulates in favor of a particular decision, one pattern of activity will eventually “win out” over the others and the monkey will make the corresponding choice. Some recent studies have even used recordings of these activity patterns for something close to mind-reading. By comparing the current activity of these neurons to previous trials of the experiment, they could predict what decision a monkey was about to make and even detect when the monkey seemed to change its mind between one option and another, as the neurons' activities shifted first towards one pattern and then towards another in a sort of neuronal tug of war.

Science of Tomorrow

The idea of mind-reading is pretty cool, but remember, we’re still just talking about a monkey reacting to dots moving on a screen. I want to be totally clear that this type of decision may be categorically different from the type of decision you asked about, which involves a decision about what to DO in the context of conflicting fear and reward. Different types of decision probably weigh the contributions of different brain regions differently, and so far neuroscience doesn’t have a clear understanding of how firing rates in different brain regions encode all types of decisions.

In particular, it’s really not clear how the PFC would balance conflicts between really extreme emotions like the ones you asked about. In fact, the mental paralysis induced by extreme fear may actually override the executive brain, making it impossible to think clearly about the fact that the snakes are safe behind glass or to give your imagination of all the things you could do with that million dollars equal weight against that overpowering dread. But exactly how powerful does fear have to be to override the promise of reward? That too waits on the science of tomorrow.

PS: One other point I didn’t have time to get into here: different people seem to have quite different set points for balancing the appeal of reward and the fear of risk. This is a major component of what how psychologists now define personality, and early research suggests that these personality differences may well be due to personal variations in functional connectivity between the brain regions we’ve been discussing. For more on this, check out our recent Brains and Bourbon podcast with neuroeconomist Daniel Hawes of the McClure lab, and this excellent article by Brenda Patoine at the Dana Foundation.

References / further reading:

•  Sugrue, LP, Corrado, GS, & Newsome, WT. Choosing the Greater of Two Goods: Neural Currencies for Value and Decision Making. Nature Reviews Neuroscience 6, 2005. (link – pdf)

• Mante et al. Context-dependent computation by recurrent dynamics in prefrontal cortex. Nature 503, 2013. (link – behind paywall)

NB: A previous version of this article appeared on the worldview.stanford.edu blog.