White Matter Matters

Tales from the science trenches

The Rhythm That (Literally) Moves Us

The strange link between turntablism and the motor cortex

For someone who has no musical talent ("tone deaf" is not infrequently used to describe my singing abilities) I am an avid consumer of music. It's not uncommon to hear bluegrass, classical, electronic, and hip-hop blaring from my computer speakers within the span of a few hours.

One of my favorite musical forms is a genre known as Turntabilism. Turntabilism has its roots in hip-hop and consists of making new music out of the parts of old music, usually old vinyl records. Turntabilists make it an art to find isolated samples of many different songs and knit them together as a sort of mosaic Gestault-whole that becomes a unique new song.

Consider this example from a french quartet called Birdy Nam Nam. 

Notice how each member plays his track at the precise moment in time relative to the other members in order to complete the melody. One plays a 1 second clip of an accordion. Another a few notes on the guitar. But it is the precise timing that binds the samples together into a song. Without the rhythm, you'd be left with a cacophany of musical fragments.

At this point, you may be wondering why I am rambling on about music on a forum for discussing psychology and neuroscience.

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I mention turntabilism because recent research suggests that this binding of random snippets to form a coherent whole may in fact be happening in our brains. Specifically, it might explain how our brains make us move... literally.

All of the actions that we perform, from opening doors to kicking cans to kissing cheeks, are controlled by an area of the brain called the motor cortex (actually, this is a bunch of little areas that work together, but I'm trying to keep this simple). This bit of tissue is the main interface between your brain and your muscles.

Now the motor cortex has a very interesting design. Unlike it's sensory counterparts, the motor cortex at first glance appears to be a rather sloppily designed piece of cortex.

Consider the visual cortex for a moment. Here the world that we see is parsed into neat and well-organized little pieces. Cells that "see" nearby areas of the world tend to sit next to each other. In addition, these cells only care about very specific things. For example, one cell might only fire when it sees an edge oriented in a particular direction in the proper area of space (called orientation tuning). Every time it sees that orientation in the right part of space, this cell will fire vigorously. Otherwise, it's fairly quiet. This rigid organization of function allows for a precise and veridical picture of the world that we see.

By comparison, the motor cortex appears to have been designed by the same principles that college freshmen use to organize their dorm rooms.

While cells in the motor cortex do seem to indicate the general direction that we move, there is tremendous variability in how movements are represented from cell to cell. This means that there is nothing akin to orientation-tuning in motor cortex. In addition, there isn't a clear spatial organization in motor cortex. One cell might control a particular muscle in your hand, but this cell's neighbor might be talking to a muscle in the forearm.

Why would it make sense for visual cortex to be so neatly organized while the motor cortex appears haphazardly designed? A recent study by Mark Churchland and colleagues has helped to shed some light on this.

In their article "Neural population dynamics during reaching", Dr. Churchland and colleagues recorded the collective activity of many cells of the motor cortex at the same time while monkey moved their arms about in different ways. Individually, one cell might not be very predictive of where a monkey was going to move. However, collectively, the population as a whole was highly informative as to where the animal was going to reach and how it was going to move. This group-level behaviour of the cells was tied together by one very interesting property: rhythm (or "oscillations" to use technical jargon).

You see, within the activity of all the cells were little bouts of synchrony that brought the collective dynamics together. This underlying rhythm appeared to contain all the information that described how the monkey's arm would move.  Each snippet of a motor command from every individual cell suddenly came in at the right time to make sense in a larger "whole" of the motor command.

Okay, so where does turntabilism come in?

Let's think of this back in terms of musical styles. The visual cortex operates like a classical symphony. Each cell has its specific sound and melody (i.e., area of space it sees and what it's looking for), tending to sit in groups of cells that play the same tune, in much the same way that all the clarinets sit together in an orchestra. The symphony of their firing provides a well-structured portraite of our visual world.

On the other hand, the motor cortex is more like a turntabilist. It pieces together snippets of seemingly random sounds and brings them together in a regular, rhythmic manner. Each little cell is relatively meaningless when taken out of context from the collective whole. What this affords is a kind of flexibility in how the motor cortex behaves. It's more like neuronal computing by free-form jazz rather than by symphony (if I'm allowed to mix my metaphors).

Of course, this isn't to say that Churchland's group has figured out exactly how our brains make us move. But they have offered a fundamentally new and interesting way to think about neural computation. One that gives me an excuse to listen to more music.

Tim Verstynen, Ph.D., is an assistant professor of psychology at Carnegie Mellon. He is interested in sensorimotor systems, plasticity, and zombie brains.

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