Disclaimer: I’m not a neuroscientist either
I have also been thinking quite a lot about this very question during the last months. Many structures project to motor centers, so what is the added value of cortical projections to motor centers? I don’t have a definitive answer, but my personal intuition is that those cortical projections have a teaching role, not a controlling role.
It may seem counter-intuitive because we like to think about the cortex as the smartest part of the brain which can directly send complex and specialized motor commands to execute complex actions. But in nominal operating mode, those projections are not enough to activate motor centers by themselves. I speculate that they only act as learning signals for subcortical structures.
To elaborate a bit, there are two main types of corticofugal projection cells from L5:
- Pyramidal Tract cells (PT cells) which project ipsilaterally to cortex, striatum, thalamus & motor centers
- Intra Telencephalic cells (IT cells) which project bilaterally to cortex & striatum
PT cells are remarkable in that they send axon collaterals to many subcortical structures (including motor centers). They are in a good position to coordinate a complex process distributed over several different structures. I posit that this process is dedicated to learning and I was glad to read a recent paper which stated that a mouse could execute an already learned behavior (but not learn a new behavior) while corticostriatal projections were desactivated, in line with my intuition: https://twitter.com/mthiboust/status/1189972405395107842
IT cells help the coupling between cortical areas (to converge towards compatible action maps) and send relevant information to the striatum in regards to action selection. Those messages could be of two kinds: valence of actions (the corticostriatal connections which terminate in striosomes) & specification of actions (terminate in matrix).
My view was first inspired by the work of Paul Cisek and Sten Grillner (action valences & specifications), and by the “3 visual streams” paper by Randall O’Reilly (learning signal via corticothalamic loops)
Here is a speculative visual summary:
For clarity reasons, not all connections are represented in this schema (thalamocortical, thalamostriatal, …)
I described my view on the story of an action here (where I also talk briefly about basal ganglia, thalamus & cerebellum):
Would be glad to hear your critics on this! 