Taking yesterday's post further, the question quickly becomes: what the hell is motor preparatory activity for? Note that when I say preparatory activity I'm talking about e.g. delayed reaching tasks and other experiments where they record from the dorsal premotor cortex (PMd), in primates. What about rodents?

Maybe this question is even deeper than it sounds. Rodents are a real mystery because it turns out you can completely wipe out a rat's motor cortex and it continues moving and living normally, with one caveat: if you introduce an unexpected perturbation into the environment they are used to (i.e. some behavioral assay that they were trained on prior to the motor cortex removal), they don't seem to know what to do about it. That might be a bit of an overinterpretation of behavior from my part, but the point is the only kinds of situations where de-motor-corticated rats behave differently from rats with a motor cortex is when such unexpected perturbations are introduced. And, funnily enough, after the first time they are introduced to the perturbation they quickly adapt and deal with it just like normal rats. This result is at the crux of Adamp Kampff's work trying to figure out what the hell cortex is really for. Their working hypothesis is that it is a brain structure evolved to produce robust behaviors - behaviors that are resistant to all kinds of unexpected situations, absolutely vital for survival (e.g. https://www.youtube.com/watch?v=u73hRPH4RQs).

Zooming back in to rodents vs primates, the picture looks somewhat like this: whereas primate motor cortex directly controls muscles, rodent motor cortex seems to provide highly specialized input to subcortical structures that directly control muscles. I probably need to read up more on what exact anatomical connections exist, but clearly muscle control can be performed by rodents without their motor cortex (through spinal reflex loops + subcortical input). What is the analog of this in primates? Could it be PMd? Usually lives in the nullspace, then jumps into potent space when absolutely needed. Delayed reaching task doesn't seem like that kind of unexpected situation where this kind of processing would be needed, but then again rat motor cortex presumably is not silent during usual run-of-the-mill motor activity.

Here's an idea for nullspace computation: the motor cortex is constantly processing and updating its model of the environment (w.r.t. what motor movements are useful to make - e.g. there is a stable wall to my left and an unstable wall to my right so make sure to hold on to the left in case of an earthquake), ready to exploit this information when need be by jumping into the potent space. There's a rodent and primate experiment there. Could PMd be doing something like this? After all, cuing a reaching target is an update to the kind of information about the world needed to behave correctly in the task. As is applying a force field during reaching.