Ballot Casting in Your Brain
By Annie Hunter, Behavioral Neuroscience, 2017
One of the most frustrating things about dealing with other people — what a less cynical person might call interacting — is not understanding why they do the things they do. Why does your friend disagree with you on the best way to get home? Why is a common goal of peace complicated by disagreements on how it should be reached? The small and large choices that people make shape who they are. Not understanding these choices can cause all sorts of conflict, from fighting between friends to fighting between countries.
Neuroscientists are beginning to address this issue of choice. Today, the first steps towards allowing you to understand the frustrating people in your life are being taken by exploring the neural basis of perceptual choice. Perceptual decision-making is the process of taking in information from the environment — sights, sounds, and other sensations — and using it to choose between several courses of action.
Evidence accumulation, the activity that occurs when the brain is integrating sensory information to form a decision, can be monitored to determine what exactly underlies the decision-making process. Changes in blood flow in the brain can be measured during a task with functional magnetic resonance imaging (fMRI). This tells us what areas are active, giving a broad understanding of where processes like evidence accumulation are happening.
The transmission of information in the brain during a task can be seen more specifically in groups of neurons by pairing fluorescent molecules with calcium ions, key factors in neural activity, and measuring where and when fluorescence changes. Finally, we can peek into the world of individual cells using small glass tubes called microelectrodes, which measure electrical activity. This array of techniques is used in humans and animal models to form a picture of the neural basis of perceptual decision-making.
Studies of perceptual choice conducted in animals from pigeons and mice to monkeys and humans have found remarkably similar phenomena in the brain during this simple decision-making process.It turns out that after the person or animal sees information to make a decision about, but before they have taken any action, there is a distinct change in neuronal activity in the parietal cortex.
For example, when you are asked press a button indicating the direction of moving dots, there is activity in your parietal cortex that neither seeing the dots, nor pressing the button can account for. The time between seeing and pressing is that period of evidence accumulation during which it seems that something is going on in parietal cortex neurons that are leading you reach your decision.
The change in activity in the parietal cortex, characterized by the speed and time at which each neuron is firing, seems to indicate that each neuron could encode something like a vote for either of the two decisions. For instance, consider deciding whether the group of dots from earlier is moving right or left. Each neuron may make a decision of its own and cast its ballot. Some neurons vote earlier, while others wait longer. Eventually all the ballots are added up and you reach a choice.
When exactly is a decision reached? How do cells within the parietal cortex talk to each other? How do they talk to other parts of the brain? These are all questions that are still being explored. Answers will tell us more about how decisions involved in navigation are formed and conveyed to the rest of the brain. This will chip away at the larger question of how neural activity gives rise to complex decisions. For anyone that has ever seen someone make a choice and was dying to know what was going on in his or her brain, hold on just a little longer. The answer could be right around the corner.