The question of whether the prefrontal cortex (PFC) is crucially involved in conscious experience is one that I have been interested in for quite a while. The issue has flared up again recently, especially with the defenders of the Integrated Information Theory of Consciousness defending an anti-PFC account of consciousness (as in Christof Koch’s piece in Nature). I have talked about IIT before (here, here, and here) and I won’t revisit it but I did want to address one issue in Koch’s recent piece. He says,
A second source of insights are neurological patients from the first half of the 20th century. Surgeons sometimes had to excise a large belt of prefrontal cortex to remove tumors or to ameliorate epileptic seizures. What is remarkable is how unremarkable these patients appeared. The loss of a portion of the frontal lobe did have certain deleterious effects: the patients developed a lack of inhibition of inappropriate emotions or actions, motor deficits, or uncontrollable repetition of specific action or words. Following the operation, however, their personality and IQ improved, and they went on to live for many more years, with no evidence that the drastic removal of frontal tissue significantly affected their conscious experience. Conversely, removal of even small regions of the posterior cortex, where the hot zone resides, can lead to a loss of entire classes of conscious content: patients are unable to recognize faces or to see motion, color or space.
So it appears that the sights, sounds and other sensations of life as we experience it are generated by regions within the posterior cortex. As far as we can tell, almost all conscious experiences have their origin there. What is the crucial difference between these posterior regions and much of the prefrontal cortex, which does not directly contribute to subjective content?
The assertion that loss of the prefrontal cortex does not affect conscious experience is one that is often leveled at theories that invoke activity in the prefrontal cortex as a crucial element of conscious experience (like the Global Workspace Theory and the higher-order theory of consciousness in its neuronal interpretation by Hakwan Lau and Joe LeDoux (which I am happy to have helped out a bit in developing)). But this is a misnomer or at least is subject to important empirical objections. Koch does not say which cases he has in mind (and he does not include any references in the Nature paper) but we can get some ideas from a recent exchange in the Journal of Neuroscience.
One case in particular is often cited as evidence that consciousness survives extensive damage to the frontal lobe. In their recent paper Odegaard, Knight, and Lau have argued that this is incorrect. Below is figure 1 from their paper.
Figure 1a from Odegaard, Knight, and Lau
This is brain of Patient A, who was reportedly the first patient to undergo bi-lateral frontal lobectomy. In it the central sulcus is labeled in red along with Brodman’s areas 4, 6, 9, and 46. Labled in this way it is clear that there is an extensive amount of (the right) prefrontal cortex that is intact (basically everything anterior to area 6 would be preserved PFC). If that were the case then this would hardly be a complete bi-lateral lobectomy! There is more than enough preserved PFC to account for the preserved conscious experience of Patient A.
Boly et al have a companion piece in the journal of neuroscience and a response to the Odegaard paper (Odegaard et al responded to Boly as well and made these same points). Below is figure R1C from the response by Boly et al.
Figure R1C from response by Melanie Boly, Marcello Massimini, Naotsugu Tsuchiya, Bradley R. Postle, Christof Koch, and Giulio Tononi
Close attention to figure R1C shows that Boly et al have placed the central sulcus in a different location than Odegaard et al did. In the Odegaard et al paper they mark the central sulcus behind where the 3,1,2 white numbers occur in the Boly et al image. If Boly et al were correct then, as they assert, pretty much the entire prefrontal cortex is removed in the case of patient A, and if that is the case then of course there is strong evidence that there can be conscious experience in the absence of prefrontal activity.
So here we have some experts in neuroscience, among them Robert T. Knight and Christof Koch, disagreeing about the location of the central sulcus in the Journal of Neuroscience –As someone who cares about neuroscience and consciousness (and has to teach it to undergraduates) this is distressing! And as someone who is not an expert on neurophysiology I tend to go with Knight (surprised? he is on my side, after all!) but even if you are not convinced you should at least be convinced of one thing: it is not clear that there is evidence from “neurological patients in the first half of the 20th century” which suggests that the prefrontal cortex is not crucially involved in conscious experience. What is clear is that is seems a bit odd to keep insisting that there is while ignoring the empirical arguments of experts in the field.
On a different note, I thought it was interesting that Koch made this point.
IIT also predicts that a sophisticated simulation of a human brain running on a digital computer cannot be conscious—even if it can speak in a manner indistinguishable from a human being. Just as simulating the massive gravitational attraction of a black hole does not actually deform spacetime around the computer implementing the astrophysical code, programming for consciousness will never create a conscious computer. Consciousness cannot be computed: it must be built into the structure of the system.
This is a topic for another day but I would have thought you could have integrated information in a simulated system.
