The existence of brain waves (rhythmic fluctuations of electrical activity believed to reflect the brain’s state) is not a new discovery and neuroscientists know that the brain’s activity during rest slows down to an alpha rhythm of approximately 8 to 10 cycles or hertz per second.
Neuroscientists at the Massachusetts Institute of Technology (MIT) conducted a study to evaluate if these waves have a cognitive significance, if any, in terms of functions, such as learning and memory. Findings revealed that a switch between two of these rhythms is critical for learning habitual behavior.
The study, published this week in the Proceedings of the National Academy of Sciences shows that in rats that learned to run a maze, activity in a brain region that controls habit formation shifts from a fast and chaotic rhythm to a slower, more synchronized pace. According to Professor Ann Graybiel, senior author of the PNAS paper and leading researcher at the McGovern Institute for Brain Research at MIT, the switch that happens the moment the rats start to master the maze is probably the signal that a habit has been formed. This process represents a key role in understanding how the brain reorganizes itself during learning.
Rhythms in the brain
Research has observed several brain waves of different frequencies in humans and other animals. Lead author Graybiel and graduate student Mark Howe decided to examine if they could link these rhythms to changes in brain state that accompany learning. Their study focused on beta waves (Band width 15 to 28 hertz), which are linked to lack of movement and high gamma waves (Band width 70 to 90 hertz) with highly attentive states.
Graybiel’s lab proved earlier that patterns of electrical activity in the basal ganglia part of the brain are critical for habit formation. Habits start when taking a particular action reaps some kind of benefit, however, with time this action turns into second nature and is carried out even if it no longer gains a reward. In extreme cases, it could mean for example continuing to scratch part of the body even after the itching has stopped.
Howe investigated brain rhythms in a region at the very bottom of the basal ganglia, known as the ventral striatum, which is necessary for responding to pain or pleasure but also highly involved in addiction. To measure brain activity, Howe used rats running along a T-shaped maze. The animals had to learn to turn left or right in response to a sound; if turning correctly reaching the end of the maze they received a reward: chocolate milk.
During the first few runs, e.g. whilst the rats were still learning the maze, the researchers observed bursts of ventral striatum activity in the gamma frequency range shortly before the rats finished the maze. This activity spread throughout the ventral striatum: Cells synchronized with the rhythm at different times, in a fairly uncoordinated fashion.
Once the rats started to learn how the reward was earned, the gamma activity started to fade away being replaced with short bursts of activity in the lower frequency beta band, just after they finished the maze. Compared with the first runs, the activity became much more coordinated throughout the entire ventral striatum.
Reinforcing habits
The researchers also measured activity from single neurons in the ventral striatum to achieve a deeper insight of what was happening during this frequency shift and discovered that activity in two groups of neurons coordinated with the oscillations. They observed that output neurons, controlling the ventral striatum’s communication with the rest of the brain, spiked during the peaks of both gamma and beta oscillations, with another type inhibiting the output neurons, spiking at the troughs of the oscillations.
Howe stated: “Whenever you have a strong rhythm, these two populations of neurons oscillate in opposite directions.”
This finding indicates that during the rats’ learning process of a new behavior, the high-frequency activity in the output neurons of the ventral striatum transmits messages to the rest of the brain directing it to learn a new behavior, which was reinforced by the chocolate reward. Afterwards, once the behavior is learned and formed a habit, those messages are no longer needed. They are shut off by inhibitory neurons during the beta oscillations.
Graybiel explained:
“As the rats were learning, that reinforcement signal goes away, because you really don’t need it. This helps the brain because once that habit is formed what you want to do is free up that bit of brain so you can do something else – form a new habit or think a great thought.”
The researchers, including Howe, Graybiel, and other lab members Hisham Attalah, Dan Gibson and Andrew McCool plan further investigations as to whether habit formation is interrupted if they alter the brain rhythms in the ventral striatum and to identify the neurons that are involved more specifically. The identification and control of such neurons might produce new approaches in helping to combat addiction, an extreme form of habitual behavior.
Written by Petra Rattue