Among all types of brain cells, neurons are a minority, and it is unclear that they should be sufficient for causing consciousness. Moreover, the brain also consists of 20% extracellular space—a molecular sea that is anything but empty and whose biology is, likewise, poorly understood.
There are historical reasons why cognitive brain science has had limited cell scope. Neuroscience was effectively created by Ramon y Cajal as a study of minority cells in the brain. In proving that neurons were distinct entities that communicated with each other through action potentials, he cleared a path for future cognitive brain science. Cajal may be dead, but the neural agenda of cognitive brain science is not.
Brain science is about more than neurons, and they may or may not have causal powers of consciousness. A biologically liberal perspective, inclusive of all causal biological possibilities, would include neurons, other brain cells, and whatever floats in extracellular space. A biological liberalist looks beyond the brain as a signaling machine—beyond neurofunctionalism. Any brain cells can be viewed as having signaling properties, whether they are cashed out as neural firings with neurotransmitters, purely electrical synapses, hormones, messenger molecules, peptides, calcium waves, or other means. The liberalist acknowledges the use of signaling models but does not look at the brain and say that it is a signaling machine or that brain cells are nothing but signal processors.
From the perspective of biological liberalism, nonsignaling properties of neurons, as well as of glia and of extracellular content, may be involved in consciousness. Cognitive and computational neuroscience understands neurons as signaling entities. As an example, computational neuroscience ascertains a stream of processing going from the retinas to the primary visual cortex. On the basis of retinal receptor cell firings, sequences of neural maps are topographically constructed in isomorphic relations to the visual field through signal transformations. Neural network modelers simulate and analyze such transformations. However, when we discuss the brain in abstract terms of how signals propagate, we play the game of neurofunctionalism, and it is unclear how such signaling patterns could yield experiences. Neurons may cause consciousness, but if so, it is at a level of abstraction other than signal processing that we must understand them. If neurons cause consciousness, we need to explain how they do it in terms of biological properties that are genuinely causal with respect to consciousness, not simply properties picked because they lend themselves to signaling scheme interpretations, however useful such schemes may be for other purposes.
Cajal was fascinated by neurons. The way they interacted suggested a neural universe that sustained our minds. The glial cells, lacking action potentials and firing capability, were not neglected. He was interested in them and tried to fit them into his neurocircuitry. Cajal proved the neuron doctrine as a result of being able to stain neurons so he could view their structure. He had no similarly refined technique that worked for the variety of glia. According to brain scientist Douglas Fields, who specializes in neuron-glia (NG) interaction research, it was only in 2001 that we learned what astrocytes look like. These common glia within the central nervous system look star shaped when stained with traditional methods. However, in 2001, their true appearance was discovered:
Up until then, anatomists had used various stains to reveal astrocytes in brain tissue. Because the fibrous protein in these cells took up stains particularly well, astrocytes were immediately recognizable by their star-like structure, which had inspired their name. Rather than using a stain, Ellisman and his colleagues pierced an astrocyte in the hippocampus of a rat with a fine glass capillary and filled the cell with a fluorescent dye. They saw that all the images of astrocytes heretofore had been mere ghosts, skeletons actually, for the stains anatomists had relied upon to identify astrocytes exposed only the fibrous skeleton within these cells. Astrocytes were not star-like at all; they were as bushy as the hair on Ellisman’s head, and they were roughly two times bigger than they appeared with stains that revealed only their internal skeleton. Once again, Nature had fooled scientists into giving a class of brain cells a name that described not reality, but rather a relic of incomplete staining. (Fields 2010, p. 252)
One can easily conceive of what computational neuroscience might be about. It has to do with simulating information processing in neural networks. However, it is more difficult to conceive of what computational glial science could be like. Glia often disperse molecules, which aggregate in molecular clouds and spread slowly across large numbers of brain cells over minutes or hours. While glia operating on this time scale could be interesting to explore in contrast to quick-fire neurons, it is unclear how glia-produced, slow moving, widely dispersed chemical clouds would fit with neural network modeling. Glia biologists are received with skepticism when their work seems alien to neurofunctionalism:
Research by glial biologists is easily dismissed as unimportant by the establishment view that neurons are the only cells important for information processing in the brain. As a result, research on the other brain [glial brain] is one hundred years behind research on the neuronal brain. (Fields 2010, p. 251)
It is easy to understand why neural network modeling developed the way it did and why neurofunctionalism became so popular in cognitive science and philosophy. It is also plain why glia biologists are viewed with skepticism by neurofunctionalists—it is doubtful whether their work could be integrated with computational neuroscience and neural network modeling. However, where the neurofunctionalist sees information-processing hindrances, the biological liberalist finds opportunities. Could it be that consciousness is caused, at least partly, by the largely unexplored glial brain? Could the failings of neurofunctionalism guide us in understanding how the brain causes consciousness from a nonfunctionalist perspective?
Glia are involved in repair, toxic clean up, immune defense, scaffolding, and brain cell nourishment, but recent research indicates that they do more than the classical view suggests. Glia regulate neural activity and can absorb and release neu- retransmitters. This makes the term “neurotransmitter” misleading—the chemical flux involved with them is not monopolized by neurons. The current use of the term “neurotransmitter” makes sense within a view saying that glia are merely assisting neurons with nourishment, cleaning services, and other noncognitive tasks. It would be less confusing if we reserved the term “neurotransmitter” for those chemicals that are used exclusively by neurons. Transmitters deployed by both neurons and glia could then be called simply “transmitters.” One striking example of how glia influence neural activity is their regulation of brain waves during sleep. Another example comes from epilepsy. Evidence suggests that unbalanced glial systems create neural brainstorms that we know as epileptic attacks. Glia can send calcium waves throughout intercellular space to create systemic effects. These waves have been clearly discerned using modern calcium imaging techniques and can happen in response to neural events. The waves can go on for minutes or hours, and there can be feedback from glia to neurons. Such calcium waves are one aspect of an NG system in action.
Glial cells cannot be removed from an adequate analysis of the mind and consciousness. We might see research during the next decades that destabilizes neurofunctionalism as the accepted foothold of cognitive brain research. We may or may not speak of neuroscientists and neuroscience in a decade or so; we may instead speak of brain scientists and brain science. A shift in vocabulary might not have any greater impact. What will have impact is the way that research on glia will play out. We ought to consider the alternatives. One possibility is NG functionalism. This view results from a commitment to brain cell functionalism. An NG functionalist adopts the position that the problem with neurofunctionalism is not with functionalism but with cell scope. The NG functionalist thinks that glia must be incorporated into functionalism. Here is how an NG functionalist might reason:
NG functionalist: “We know that neurons process information. We have known this since Cajal discovered how information flows from axons to dendrites by means of action potentials. The nervous system is for information processing, and we have come a long way with our models of how this works. However, other cells process information as well. Glia are in the same line of business as neurons—they, too, are information workers. We must figure out how to extend our neurocomputational models and make them work not only for neurons but also for glia. Yes, there is a glial brain, and yes, we have neglected it—partly through ignorance—but now we see that it also processes information, we have amazing opportunities to make new discoveries. It is not going to be easy, but nothing is easy in this field. We are going to need increased funding in computational neurobiology in order to develop new models that are inclusive of both neurons and glia.”
The NG functionalist is convinced the research on neurocomputational biology has not been a waste of time. The NG functionalist argues that much has been learned over the past few decades and that it will be useful for understanding how the glial brain computes and processes information. However, we might also consider an alternative view. Suppose what is wrong with neurofunctionalism is not cell scope but inadequate consideration of biology. NG systems may have features that go beyond the scope of functionalism, and it may be that such features are necessary for a full account of the mind and consciousness. Someone who believes this takes on a liberal biological view of the NG brain. The liberal biologist is open to the possibility that the NG system can have properties that are radically different from what falls within the explanatory scope of functionalism. Here is what an NG biologist might say:
NG biologist: “We know the brain causes consciousness. We don’t know how, but it does.
Up until now, the standard approach has been to look for the neural correlates of consciousness (NCC). The idea is that we would find the minimal neural structures that cause consciousness. This approach is wide of the mark because it has focused excessively on neurons. A few of us who have worked on the NCC have thought that there may be more to consciousness than neural structures. The situation is reminiscent of how psychologists used to pay lip service to behaviorism during its glory days. Some of us have been more interested in glia than neurons. It may be that glia are involved in causing consciousness. It makes better sense to talk about the neuroglia correlates (NGCC) rather than the NCC. Perhaps we need an even more expanded view of the correlates. Who knows, maybe it will turn out that some flux of chemicals in intercellular space is the causal structure for consciousness. I am not suggesting this as a particularly interesting or plausible hypothesis, but just to indicate my sentiment that we need to keep a biologically open mind. The mistake of many of my colleagues—whom I respect—is that they have such a restrictive focus. They focus on neurons, as do most people in cognitive brain science. But that is not the only problem. Some of them are coming around and want to include glia in their research, but they have such a limiting view of what brain cells are capable of. I am talking about my friends in computational neurobiology. They are starting to incorporate glia into their models, but I don’t see where this research is going. If they are trying to explain consciousness, it won’t help to include glia in their information-processing models. However, they have been doing this sort of computational modeling for a long time, and there is a community centered around it with prestigious conferences, and so on. To be honest, I don’t care if there is information processing going on or not in the structures I study. All I care about is finding whatever brings the conscious field about. The way I see it, there is nothing essential about this field that has to do with information processing. Where did they get this idea? Suppose NGCC is the right approach, and we succeed one day in finding the causal NG structures. Nothing says that those structures would have to have anything to do with processing information. It would not be a valid refutation to say that they could not be the right structures, because no one knows how they process information—no one knows how to fit them into computational neurobiology. Searle has also pointed out that information processing is an observer-relative notion—not inherent to biology. I am sympathetic to that. We could view anything in the brain as processing information, but doing so is imposing a layer of abstraction with zippo causal efficacy.”
The view I have illustrated as that of the NG biologist depends neither on information processing nor on functionalism. It depends only on biology and finding the minimal causal structures for consciousness. It is tempting to think, in a mysterian fashion, that even if we did find the NGCC, it would still be mysterious how it would work. But mysteries are observer relative. For example, from the point of view of Einstein’s theory of relativity, gravity is no mystery. It is simply part of the space-time continuum. However, from the point of view of quantum mechanics, it is still enigmatic. At the scale of quantum mechanics, Einstein’s theory of relativity does not hold, and we are, at present, unable to explain how gravity works. Do the explanatory issues related to gravity at the quantum scale prove that gravity is mysterious? Yes and no. It is a mystery at the quantum scale but not at higher scales. It is, for example, by taking into account Einstein’s view of gravitational effects on time that we have been able to build a global positioning satellite system with remarkable accuracy. Without this understanding of gravitational effects on time, it would not work. Our understanding of gravity also helped us get to the moon. Scientifically speaking, we do understand gravity. We have a scientific grip on the phenomenon that allows us to predict and control gravitational phenomena in remarkable ways. At the quantum scale, we don’t have this grip. One hypothesis is that, at that scale, there is a particle called a graviton that, while massless, nevertheless explains gravity. Physicists are seriously considering the possibility of gravitons jumping in and out of some other dimension. They are now building the largest particle accelerators we have ever seen in order to detect a loss of energy that would reveal graviton interdimensional jumps. Whether the graviton exists or not, and whether it can move between different dimensions, and what that would mean, I leave to future physicists to elaborate on.
Perhaps we can view consciousness in a similar light to gravity. It is not difficult to see how it may turn out that we do find the NGCC in the future and that consciousness at the NG level becomes a nonmystery. If we find such an NGCC, then it will not be one whose explanatory basis derives from NG functionalism, because of its abstract, nonphysical nature. What we will be left with, then, is simply a biological structure that causes the conscious field and, as with gravitational fields, we may want to seek lower-level explanations. We may think it is still a mystery how the NGCC works at lower levels of physical reality, beyond brain science, perhaps. It depends on how we choose to view the phenomenon. It hardly belongs to the essence of science to arrive only at ultimate explanations. If it did, then it is doubtful we would have any scientific explanations at all, for in what case of scientific explanation could we not conceive of the possibility of more fundamental ways of explaining reality? Hume and Kant were both right that, ultimately, reality is a mystery to us. We must admit that, from a skeptical point of view, it is. But skepticism is a point of view that we had better abandon to live our lives and get on with science. I don’t think Hume would have disagreed with this.
Leaving radical skepticism to the side, it is crucial to pay attention to the philosophical questions that matter for the problem of consciousness. These questions will naturally arise from new discoveries in biological brain science. There is no mysterious or difficult conceptual problem of consciousness in brain science. The problem of consciousness is conceptually simple but empirically difficult—what causes consciousness?
If science manages to answer this question, we can still enjoy the larger mystery of how we are here to observe the universe. How does our universe come to observe itself through consciousness? We can ponder our true nature as conscious beings and how consciousness shines through portals of the physical structures we call brains. As we start to ponder those questions, we are inevitably drawn to metaphysics. But metaphysics has largely been banned from Western philosophy ever since the beginning of the twentieth century. What can we possibly hope to gain from opening a discussion of metaphysics? It seems to me that we must pursue such a discussion to do full justice to the question of consciousness. Philosophy began with the quest of trying to understand reality and, after thousands of years of attempting this, we now live in a world in which that task has been relegated to science—yet the very fact that science is at a loss when it comes to explaining ultimate reality prompts us to take on what can be seen as a Kantian dialectic. I will try to do that in the next chapter, which will inevitably be speculative in nature.