What is motor cortex for?
The neocortex is the most recent evolutionary expansion of the mammalian brain, and it is integral for complex, intelligent behaviour. It contains primary sensory areas dedicated to sight, hearing and touch, as well as complex, associative areas, involved in functions such as planning, anticipating consequences and executing motor commands. However, evolutionarily ancient sub-cortical structures such as the cerebellum, basal ganglia or superior colliculus can implement complex behaviours themselves. Frogs use sub-cortical structures to catch flies with their tongues. Octopi don’t have a neocortex, but are nonetheless able to solve complex spatial problems.
If evolutionarily ancient structures can implement such advanced behaviours, what exactly is the neocortex bringing to the table?
To answer this question, Kawai and colleagues trained rats to press a lever twice at a 700 ms interval. Rats successfully learnt to time this interval by executing a series of repetitive movements between presses. One group had motor cortex removed after learning this skill. Remarkably, lesioned rats performed as well as controls, implying this area is not necessary for executing the novel motor task and that this learnt motor behaviour is not stored in motor cortex. In a second experiment, the authors removed motor cortex before training on the interval task. After recovery, lesioned rats were able to move around as well as controls. However, rats lesioned before training were unable to learn the task, despite being trained for three times longer than control rats.
These experiments suggest motor cortex acts as a tutor, helping sub-cortical structures acquire and consolidate new sequences of motor actions. The motor cortex is embedded in a rich network of connections, having access to sensory, planning and contextual information from other neocortical areas that may not be directly available to sub-cortical motor structures. Its key role could therefore be to use this information to adapt and sequence movements stored and executed in less flexible sub-cortical structures. Kawai’s study raises interesting questions: could other neocortical areas have similar tutoring roles? And how does activity in motor cortex influence the structure and organization of sub-cortical circuits? How is the “tutoring” function implemented biologically?