Thanks for this.
I agree with the general sequence of events, but I think your explanation misses 2 things:
- Parallel pathways with direct projections from the cortex to motor centers, along with the indirect ones via basal ganglia
- The multiple loops necessary to converge to an action
Here is my take on this:
At any time, numerous L5 Pyramidal Tract (PT) cells of some cortical areas are broadcasting direct messages to motor centers & basal ganglia & other structures like high-order thalamic nuclei (those messages conveyed by axon collaterals are really identical). But those messages are not sufficient by themselves to activate motor centers that are under tonic inhibition by basal ganglia, except in rare cases when they are really very strong (like when we artificially highly stimulate the cortex).
If there are still incompatible competing affordances (=potential actions) in the cortex, the BG will maintain the corresponding motor centers under tonic inhibition, and order the thalamus to move the attentional spotlight towards other sensory flows that could help the cortical convergence towards the winner affordance.
After a few cortico-BG-thalamo-cortical loops, the BG accepts the winning affordance (which can now be called “action”) suggested by the cortex by stopping its inhibition on motor centers. The thalamus receives the exact same message, and then passes the information of the coming action to the cortex. The cortex can now actively predict the outcome in advanced and then compare it to the ground truth coming from the senses (active sensing). This is what we call perception.
During the next loops, the BG will automatically accept all subsequent actions suggested by the cortex that are part of the same sequence (the sequence of actions is stored in the cortex), except of course if an unexpected events necessitate to switch behavior. Thus, the cortex is free to unfold its sequence to motor centers. The cortical code of innate actions (like basic walking, swallowing, …) is very straightforward because all the complexity of movement unfolding is hardcoded in evolutionary ancient Center Pattern Generators (CPG). But for more complex actions, we need something to replace the hardcoded CPG. This is where the cerebellum comes into the picture with its primary role in unfolding (and learning to unfold) smooth sequences of motor commands.
To make it more understandable, I skipped the difference between action & behavior.
A behavior is a sequenced list of actions. The competition between behaviors is mostly executed in the BG and some of it has been delegated to the PFC in mammals.
An action is a list of a parallel commands specified in a given cortical area. But I prefer the term “action map”.