Neural population dynamics during reaching
A very interesting paper about how the collective activity of neurons in the motor cortex is related to movement appeared in Nature recently. Mark Churchland, John Cunningham, et al. point out that rhythmicity is far more important than scientists have previously thought. This is perhaps not surprising when considering rhythmic motion like walking or chewing. However, the authors show that even non-rhythmic movements, like reaching, may be driven by oscillatory activity in the cortex. Moreover, this oscillatory activity only becomes apparent when looking at the activity of populations of cells.
There are many different reasons why this is interesting: For many years people who have tried to understand the motor cortex where guided by the insights from the visual cortex – an area of the brain which is better understood. In the visual cortex the activity of many cells directly reflects aspects of the stimuli they receive. For instance, certain neurons will respond to horizontal bars in particular locations in visual space, while others will respond to vertical bars. The present study shows that different areas of the cortex may encode information very differently. Rather than representing the direction and length of a motion to be executed, neurons in the motor cortex may encode the motion dynamically. Their collective rhythmic activity drives the movements of the muscles that translate into the particular motion.
It is interesting that this mechanism was first proposed theoretically by Rokni and Sompolinski. It is also related to the ideas put forth by Rodolfo Llinas in his book “I of the Vortex”. The central nervous system evolved from cells that drove rhythmic behavior that allowed early animals to move and digest food. Since it evolved from such pattern generators, the brain may still itself largely function to generate oscillations. Rhythmic activity is can be found everywhere in the cortex. Oscillatory activity has been implicated in coordinating the activity of distant areas in the brain. The study by Churchland, Cunningham, et al. suggests that it may have other functional roles.
It appears that in the very near we will be able to design brain-machine interfaces that may allow paralyzed people to control a robotic exoskeleton. Such interfaces need to interpret the activity of cells in the motor cortex. There are many technical obstacles that still remain. However, I believe that a good theoretical description of how neural population activity is translated into motion will also be necessary. This study shows that careful experiments together with novel mathematical and statistical techniques will lead the way.
Here is the press release for this study with some nice quotes from the authors.