Part of weighing up any decision involves considering the consequences that are going to come of it, and scientists have now identified neurons in the brain that seem to encode the outcomes of actions so that they can be properly evaluated.
The study was carried out in mice, but the researchers think it could broadly apply to human brains too. The findings build on many previous studies that have found a link between a part of the brain called the striatum and decisions that involve assessing outcomes.
Here, the team looked at the neural activity associated with cost-benefit decisions, where an action can have a mix of both positive and negative outcomes. These options we face often have a balance of risk and reward that need to be thought about.
During their experiments with the mice, the researchers were able to identify a particular group of neurons that became active during these decisions – and particularly so when a behavior led to an outcome that wasn't expected: a sign of adaptation.
"A lot of this brain activity deals with surprising outcomes, because if an outcome is expected, there's really nothing to be learned," says neuroscientist Bernard Bloem, formerly of the Massachusetts Institute of Technology (MIT).
"What we see is that there's a strong encoding of both unexpected rewards and unexpected negative outcomes."
Through the course of the study, mice were trained to spin a wheel to the left or right. Each turn would result in a mix of both a positive outcome (a drop of sugary water) and a negative outcome (a small puff of air).
As time went on, the mice learned to maximize the level of reward and to minimize the level of air puffs. However, the probabilities of each outcome were continually being shifted by the researchers, forcing the mice to keep on adjusting their behavior.
The subsequent brain activity recorded in the striosomes – clusters of neurons in the striatum – varied depending on whether actions were 'good' or 'bad', as expected. However, the researchers also noticed many of the neurons encoding the relationship between actions and both types of outcome.
Stronger reactions were observed when the unexpected happened – when a wheel turn produced a different result than it had done the previous time. The scientists think these 'error signals' help the brain decide when it's time to change its approach to a task.
"The striosomes seem to mostly keep track of what the actual outcomes are," says Bloem. "The decision whether to do an action or not, which essentially requires integrating multiple outcomes, probably happens somewhere downstream in the brain."
Previous research has linked striosomes with messages sent to other parts of the brain – messages about how to plan movement, for example, or when to produce dopamine, the brain's reward chemical. They're intrinsic to decisions on whether or not to act and any subsequent rewards.
But as some of the 'decision' neurons fire with both good and bad outcomes, it's easy to see how the process could go awry.
"Our ability to make our movements or our thoughts in what we call a normal way depends on those distinctions, and if they get blurred, it's real trouble," explains MIT neuroscientist Ann Graybiel.
This could be behind neuropsychiatric disorders including anxiety and depression, where slight disruptions in the neural networks identified by the researchers could result in impulsive decision-making at one end of the scale, and being paralyzed by indecision at the other.
If our brains are confused about what's 'good' and what's 'bad', that might also lead to making decisions that are bad for us. A greater understanding of how this neural activity works could eventually improve treatments for such conditions.
The research could also be useful in understanding the decisions we all make every day, like whether or not to eat ice cream instead of a healthier choice: There's a short-term reward in terms of its sweetness, but potentially a longer-term negative when it comes to weight and fitness.
"From a value perspective, [these two outcomes] can be considered equally good," says Bloem.
"What we find is that the striatum also knows why these are good, and it knows what are the benefits and the cost of each. In a way, the activity there reflects much more about the potential outcome than just how likely you are to choose it."
The research has been published in Nature Communications.