Neurons that help us form habits discovered

MIT researchers have discovered neurons in the brain that weigh costs and benefits to drive formation of habits.

Researchers found that habit formation, at least in primates, is driven by neurons that represent the cost of a habit, as well as the reward.

"The brain seems to be wired to seek some near optimality of cost and benefit," said Ann Graybiel, an Institute Professor at Massachusetts Institute of Technology and also a member of the McGovern Institute for Brain Research.

Previous work by Graybiel and her colleagues discovered clear beginning and ending signals in the brain when habits are performed.

These signals appear in the striatum, a part of the brain that, among other things, coordinates body movements; the signals have been observed in mice, rats, and monkeys that have been trained to perform specific tasks.

A few years ago, Graybiel and Theresa Desrochers, then a doctoral student in her lab, decided to let two monkeys learn a habit on their own, without training, as a way to mimic real-life learning. They also recorded the activity of 1,600 neurons in the striatum during the learning period.

The primates learned, over several months, to visually navigate a grid of dots on a screen in search of a randomly selected one that has been "baited," meaning that the monkey will receive a squirt of juice when its eyes pass through it.

When the monkey's eyes land on the "baited" dot, the colour of the grid of dots changes, indicating a reward is coming. Over time the monkey's eyes followed the same path repeatedly, suggesting that the eye movements had became habitual.

In addition, these habitual eye-scanning patterns became more efficient. The monkeys shortened the paths they used to visit the dots. Graybiel and Desrochers published these findings in 2010.

The new paper shows the findings of the analysis of the neural recordings captured as the monkeys learned the habit.

Graybiel along with, Desrochers, now a postdoc at Brown University, and Ken-ichi Amemori, a research scientist in Graybiel's lab, observed the formation of clear beginning and ending signals at the boundaries of the habitual activity. In addition, over time, the ending signals changed dramatically.

During the early stages of learning, the signals are less precisely timed, firing throughout the time window. But as learning progresses, the neurons begin to fire at almost precisely the same time in that narrow window right after the monkey's habit ends.

To link the firing of these neurons to habit formation, the team compared the changes in neural activity with changes in behaviour, finding that the two changed in parallel.

The changes in firing of some neurons tracked with cost, measured in terms of the length of the path of the eye movements during a trial, while others correlated with reward.

Still others correlated with both cost and reward, and it was these neurons that sharpened their firing as the monkeys learned the habit and settled on a shorter, lower-cost eye movement pattern.