In this first posting, I’d like to address what I think is the most important contribution I can make to the discussions on this forum. It concerns the distinction, repeatedly made here and in the journal club video, between the “new” brain (mammalian neocortex) and the “old” brain (subcortical structures). I know that this is a popular distinction among psychologists and many neuroscientists, and it was the prevailing view when researchers like Vernon Mountcastle were doing their pioneering work. But with all due respect to those pioneers**, that view is incorrect.
The neocortex is not a recent structure that was added on top of an old “reptilian brain”, as previously believed. In fact, comparative and developmental work in the last few decades has thoroughly rejected the notion that the neocortex is a mammalian innovation, and some are now calling it “isocortex” to avoid the misleading implications of the prefix “neo”. Topologically speaking, the part of the brain that is the neocortex in mammals is extremely old, and might even have a counterpart in lamprey. For example, at the SFN conference in Chicago a few days ago, Sten Grillner’s group presented very strong evidence that within the layered lateral pallium of lamprey there is a retinotopically organized area that receives visual input via the thalamus (like V1 receiving from LGN), right next to two somatosensory regions (one spinal, one trigeminal), of which the former partially overlaps with a motor region projecting downstream (like the M1/S1 “amalgam” proposed for early mammals). They make a strong case that this is not a case of convergent evolution, but a homologous organization that was possessed by our common ancestor ~560 million years ago. So, with all due respect to Vernon Mountcastle (who is my academic “grandfather” and a hero), the view of neocortex that was prevalent in his day has since been disproven. Topologically, the neocortex is over half a billion years old and originally evolved to support very fundamental behavioral functions such as interactive control.
The discussions here of my affordance competition hypothesis have accepted the possibility that it applies to the “old brain” (and I’m glad to hear it!), but several of you feel that this shouldn’t apply to the “new brain”. I would claim that it does, if only because the distinction between old and new brains is a false distinction. The neocortex is not only old, but it is topologically inseparable from its connections with those other brain regions. In fact, instead of thinking of it as a separate structure with isolatable functions, another way to think of it is as part of three fundamental types of circuits: 1) the “dorsal pallial” portion of a set of parallel loops through the basal ganglia (just like the hippocampus is a “medial pallial” portion of a loop through the septal nuclei); 2) the telencephalic part of a set of parallel loops through the cerebellum; 3) the telencephalic part of a set of parallel loops through the external environment. These circuits are what define neocortical functions, and many people think otherwise only because of outdated ideas about evolution, and perhaps because cortical activity is easier to study with functional imaging than subcortical activity.
If one accepts the ancient origins of the neocortex (and the data is very strong on this point), then all discussion of old brain versus new brain should be abandoned. Furthermore, even if we want to focus on the mammalian neocortex, then we are led to consider it as fundamentally a sensorimotor structure. Take a look at the following diagram of a putative mammalian ancestor as proposed by Jon Kaas (2017):
Above, the neocortex is all the stuff above the rhinal sulcus, and note how much of it is concerned with sensorimotor control. This includes the areas labeled as RS, S1, S2, CS, V1, V2, as well as all of the medial shaded regions (cingulate and retrosplenial). Together, those make up what I’ve referred to as the topographic “dorsomedial neocortex” (following Finlay & Uchiyama), though I’d also presume to include MF. That leaves only a relatively thin strip of non-topographic “ventrolateral neocortex”, including OF, g, Aud, T, and perirhinal cortex. I’d claim that those latter regions are doing a lot of the action selection, including combining key stimuli with internal state information, and in the case of temporal cortex (T) eventually specialize in object recognition.
So in summary, the neocortex is an expanded part of a very old region that served sensorimotor control in early vertebrates, and most of it is still playing that role. When people like me record neural activity in frontoparietal cortex that’s what we find – a mixture of “sensory”, “motor”, and “cognitive” variables. But it’s not because the brain strangely combines these things. It’s because the combined thing is the thing, and has been all along.
Kaas, J.H. (2017) “The evolution of mammalian brains from early mammals to present-day primates” (Chapter 3 of S. Watanabe et al, Evolution of the Brain, Cognition, and Emotion in Vertebrates: Springer Japan)
** I’d like to add that Mountcastle is my academic “grandfather” and one of the founders of my field, so when I say “respect”, I really mean it.