There’s an interesting debate going on among some neuroscientists about which parts of the brain are involved in subjective experience. On the one side are Christof Koch, Giuilio Tononi, and colleagues who argue that consciousness exists wholly in the back of the brain, that the frontal structures are not involved. On the other side are neuroscientists who, while agreeing that the back part of the brain is definitely involved, argue that the role of the front part can’t be dismissed.
To understand this debate, it’s worth doing a quick review of what is known about the functionality of the various components of the brain. (To keep things simple, I’m going to focus primarily on the neocortex, the wrinkled cover on the top of the brain. If you’re familiar with neural anatomy, this isn’t to discount the role of sub-cortical structures such as the thalamus or basal ganglia.)
The first thing to understand is that the back part of the brain seems to be dedicated to sensory perception, and the front part to planning and initiating movement. The neocortex is divided into four lobes, which are separated from each other by deep fissures.
The occipital lobe in the back is dedicated to vision. The front part of the temporal lobe on the side handles hearing. The back part of the temporal lobe handles visual recognition of objects, faces, etc. The back part of the parietal lobe handles visual perception of movement. The middle part of the parietal lobe, along with surrounding regions, appears to be involved in integration of the various senses. It’s sometimes referred to as the posterior association cortex.
A strip along the front part of the parietal lobe is the somatosensory cortex, each part of which processes touch sensations from a particular body part. It’s somewhat mirrored by a strip just across the central sulcus fissure along the back of the frontal lobe, which is the primary motor cortex involved in controlling the movement of each body part.
In addition to controlling movement, the frontal lobe also plans movement. More immediate planning happens in the regions just forward of the primary motor cortex, named appropriately enough, the premotor cortex.
As we move forward, the planning becomes progressively more forward looking and more abstract. This is the prefrontal cortex, often referred to the executive center of the brain. Its primary role is planning, including planning to plan, driving information gathering for future planning, etc. As part of its function, it acts as a conductor leading the other lobes in imagining various scenarios.
Okay, so back to the debate.
The back-only proponents cite various neurological case studies as evidence, talking about patients who had parts of their frontal lobes damaged or disconnected, but who still showed signs of being conscious. They also cite cases of patients who had a frontal lobe pathology making them unresponsive, but later recovered the use of their frontal lobes enough to relay that they were conscious the whole time, but simply lacked the will to communicate.
This kind of evidence seems problematic for a number of reasons. First, in my (admittedly inexpert) opinion, some of the cited cases in the paper seem anecdotal and based on hearsay. Second, the other cases depend on self report, which is a problem because only patients with at least somewhat functional frontal lobes can self report anything, and the accuracy of such reports hinge on them remembering their former states of mind accurately. Third, as the authors of the second paper point out, the data has something of a selection bias in it, and some of the cited evidence doesn’t check out. And finally, again as pointed out in the response paper, the exact nature of frontal lobe damage or disconnect matters, making each case unique.
But I think the actual answer to this question depends on how we define “consciousness.” If our definition only includes unfocused perception, then the back-only proponents might have a case. The problem is that we seem to perceive a lot of stuff unconsciously. And raw perception alone doesn’t quite seem to match most people’s intuition of consciousness.
That intuition also typically requires that the system have attention, emotions, imagination, and introspection.
Frontal lobe expert Elkhonon Goldberg, in his book ‘The New Executive Brain’, sees attention as a frontal lobe function. He describes the back portions of the brain as creating the stage production of subjective experience, with the audience for the resulting show being in the frontal lobes. Crucially, it’s this audience that decides what part of the show to focus on, in other words, where to direct attention.
Emotions are driven by sub-cortical structures such as the amygdala, hypothalamus, anterior cingulate cortex, and others that are sometimes referred to together as the limbic system. The signals from these structures seem to affect processing in the frontal lobe, but also the temporal lobe and the insular cortex, which exists in the fissure between the temporal and parietal lobe. In other words, emotional feeling seems to happen in both the front and back of the brain.
Imagination, simulating various action-sensory scenarios, seems to require the frontal lobes, particularly the prefrontal cortex. Not that the content of imagination takes place in the prefrontal cortex itself. It actually farms the content generation of these simulations out to the other regions, such that the vision processing centers handle the visual parts of an imagined scenario, the hearing centers handle the auditory parts, etc. The prefrontal cortex acts as the initiator, conductor, and audience, but not the content generator. Still, without the prefrontal cortex driving it, it’s hard to see imagination happening in any meaningful way.
And then there’s introspection, also known as self reflection. Without introspection, we wouldn’t even know we were conscious, so it seems vital for human level consciousness. Again, the prefrontal cortex seems heavily involved in this feedback function, although as with imagination, it depends on processing in the back portions of the brain, most likely the regions on the border between the temporal and parietal lobes.
Perhaps another way to look at this is to ask, if we somehow completely removed the brain’s frontal regions (and associated basal ganglia and thalamic nuclei), would the remaining back half still be conscious? It might have the ability to build predictive sensory models, in other words it would have perception, but the modeling wouldn’t be done with any purpose, and it wouldn’t have any mechanism to decide on what portions of those models should be focused on. Arguably, it would be a mindless modeling system.
But if we removed the rear portion and kept the frontal lobes, we’d have even less functionality since the frontal lobes are crucially dependent on the posterior ones for the content they need to do their work.
And neither of the above isolated systems would have emotions unless we retained the limbic system as part of their supporting structures.
All of which is to say, for what we intuitively think of as consciousness, we need all of the components discussed above. Subjective experience is the communication between the perception and emotion centers of the brain and the action oriented centers. Wholesale removal of any of these centers might conceivably leave us with an information processing framework, but not one most of us would recognize as conscious.
Unless of course I’m missing something?