How does the brain represent rhythm?

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Humans are drawn to rhythms as demonstrated by our love for all different types of music. Despite this and despite the fact that we are able to follow and generate many different patterns of rhythmic behavior, the brain-related mechanisms underlying the representation of such rhythm remains unknown. On the other hand, the production of movement, especially sequences of movement, are much better understood. For example, the  medial premotor cortex (MPC) is known to aid in controlling sequences of movements and increased activity in this area has been associated with movement sequences during specific tasks. For this reason, researchers at Augsburg College (Minnesota) and the Instituto de Neurobiologia (Mexico) aimed at studying the MPC and its cells and determining how the activity of these cells is related to rhythmic movement.

In the most recent issue of Journal of Neuroscience, Crowe et al. recorded activity in neurons of the MPC to better understand the activity related to sequences of movement in tested monkeys. In this study, the authors first taught the animals how to perform the necessary task. In this task, monkeys were required to push a button in response to a presented stimulus . Importantly, each stimulus was separated by a constant interval, producing consecutive, synchronized movements by the monkey. While the monkeys performed this task, the authors were able to record the activity of 1083 cells located in the MPC via the use of extracellular recording from an electrode (an electrode placed in the brain adjacent to neurons of interest). Thus, the researchers were able to monitor how the activity of these cells related to the production of movement by the monkey subjects.

Upon analyzing the activity of these 1,000+ cells, it was observed that 352 of them had activity that was related to the interval between taps. Additionally, these same cells showed even more elevated activity with intervals that were repeated cyclically. These results indicated that certain cells are important for coding serial-order. The authors also observed that a separate set of 298 cells displayed activity related to the actual instructed interval duration, with cells showing a preference for either long or short duration intervals. An addition 118 cells showed sensitivity in activity to both duration of interval AND serial order. Due to this variable coding, this created what that authors call “dynamic patterns of activity” throughout the task that the monkeys performed, indicating that specific neuron groups were responsible for encoding the sequence and timing for the button-pushing task.

Next, the authors used a decoding algorithm to further analyze the dynamic activity observed above. This analysis showed that neurons code for both duration and serial order in a dynamic manner, displaying time-varying changes in activity. Furthermore, the authors showed that the representation of both serial order and duration depends on different neural sets, activated consecutively during the interval between taps. These data suggest that during this type of behavior, groups of neurons in the MPC are activated in a consecutive manner, where activation of one group provides enough drive to activate the next and so on, creating a “chain of events” type of phenomena.

Though this study was done in monkeys, these results suggest that the consecutive activation of neighboring groups of neurons is responsible for coding both temporal and serial features of rhythmic behaviors. This type of coding exhibited in monkeys in this task could therefore also be present in rhythmic behavior in humans such as dancing or strumming a guitar, or tapping to your favorite song. This type of coding seems to make a lot of sense….rhythmic behavior is just that….performing a sequence of similar movements in a specific pattern, repeatedly. Thus, the fact that the authors here show that groups of neurons in an area of the brain related to movement sequences are consecutively activated in relation to the order and duration of the task used in this study, is an appealing model for how neurons behave during rhythm in general. The authors note however, that there are differences in the ability to recognize rhythms between humans and monkeys, and therefore there are probably differences in the exact coding of rhythm. Either way, it is very likely that a similar mechanism probably exists in the human brain, and it is an interesting thought to wonder whether or not a similar coding mechanism is responsible for any related behavior where there are repeated sequences of rhythmic behaviors such as tremors (associated with Parkinson’s disease) and stutters.

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