The cerebellum – the brain’s built-in thought editor?
/The choices we make have a massive impact on almost every aspect of our lives, and poor decision-making is a common feature across many neurologic and psychiatric diseases. So it makes sense that neuroscientists have been fascinated by decision-making for a long time, and have sought to undercover what influences our choices, which parts of the brain contribute to a decision, and how different aspects of a choice (for example, the evidence in favor of one option over another, or the value of each potential outcome) are reflected in neural activity.
Decision-making in our everyday lives is quite complicated, shaped by our senses, desires, emotions, social interactions, values, and many other factors. To study decision-making in the lab, neuroscientists can avoid these complications by using very simple choices that have clear right and wrong answers – for instance, presenting a noisy sensory stimulus, like a combination of red and green dots, and asking an experimental subject to choose which of two options (red or green in this example) is more strongly represented. Neuroscientists call this kind of choice “perceptual decision-making,” since subjects’ answers should most strongly reflect the information they perceive in the sensory stimulus presented to them.
Scientists have been studying perceptual decision-making for decades now. At first, most people thought the prefrontal cortex – the very front of the brain, thought to be important for things like planning, self-control, and logical reasoning – was responsible for these kinds of choices. This turned out to be partly true; but more recently, we have learned that many other brain regions, including more evolutionarily “primitive” structures like the brainstem, are also involved in decision-making.
One of these more primitive structures, the cerebellum, is known to be important for coordinating and correcting movements. More recently, though, it has been implicated in a much wider range of cognitive functions, including working memory, language, and self-control1-3. In a recent study in the journal eLife, Ben Deverett and colleagues at Princeton University showed that the cerebellum is important for in perceptual decision-making4.
Figure 1. Graphics show the experimental setup that the researchers used. (A) In each trial, two streams of random, temporally Poisson-distributed air puffs were delivered to the left and right whiskers. After a delay, mice licked one of two lick ports indicating the side with more cumulative puffs to receive a water reward. Gray-shaded regions from left to right: cue period, delay, intertrial interval. Decision lick: first detected lick after the delay. Source: Deverett et al., 2018.
The scientists focused on a part of the cerebellum called crus I, and they performed their experiments in mice so they could take advantage of imaging techniques that allow for visualization of neural activity patterns in specific subregions of individual neurons. Crus I has been linked to higher-order cognitive functions and has strong neural connections to and from the prefrontal cortex. Since this part of the cerebellum has been shown to respond to the sense of touch, the authors developed a new perceptual decision-making task that required mice to keep track of the number of times air was puffed at each side of their faces, wait for a “go” cue, and then lick for a water reward toward the side that had received the most puffs (Figure 1).
First, the authors established that activity in crus I was important for mice to perform the air-puff task. They applied a drug directly to crus I to temporarily inactivate it and observed that the mice who received the drug made less accurate choices. Importantly, inactivating crus I did not impair the animals’ ability to lick, suggesting that activity in this region might be important for the choice process itself.
The authors then used imaging to measure the activity of Purkinje cells –– the major output cells of the cerebellum –– in crus I. They found a group of cells that track stimulus information by increasing or decreasing their activity during the period of air puff presentation. Some cells were more active for air puffs on one side or the other, and a few cells tracked the difference in the number of rightward vs. leftward air puffs. The authors were able to use the activity in all these cells to predict the animals’ upcoming choices. Finally, the authors looked at the Purkinje cell dendrites, where the cells receive the most inputs from other neurons. They found that most of the dendrites were more active right after mice made erroneous choices. In other contexts, similar error signals have been shown to be important for learning by driving plasticity in Purkinje cells – in other words, modifying how these cells respond to the input signals they receive.
This study is exciting because it supports the possibility that the cerebellum, known to be important for motor coordination and motor learning, might also be important for coordinating and monitoring more cognitive tasks. The authors have provided evidence that neurons in crus I provide some of the signals necessary to carry out this role. However, future work will be necessary in order to uncover exactly how the activity in these cells relates to task timing, sensory signals, and choice formation in perceptual decision-making.
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