Here I want to stir some ideas about the substrate of consciousness. Let us start with a few intuitive ideas: a human brain is conscious; an animal brain is probably conscious; a stone is not conscious; my stomach is not conscious; a single neuron or cell is not conscious; the brainstem or the visual cortex is not a separate conscious entity; two people do not form a single conscious entity.

Many of these ideas are in fact difficult to justify. Let us start with single cells. To see the problem, think first of organisms that consist of a single cell. For example, bacteria, or ciliates. In this video, an amoeba’s engulfs and then digests two paramecia. At some point, you can see the paramecia jumping all around as if they were panicking. Are these paramecia conscious, do they feel anything? If I did not know anything about their physiology or size, my first intuition would be that they do feel something close to fear. However, knowing that these are unicellular organisms and therefore do not have a nervous system, my intuition is rather that they are not actually conscious. But why?

Why do we think a nervous system is necessary for consciousness? One reason is that organisms to which we ascribe consciousness (humans and animals, or at least some animals) all have a nervous system. But it’s a circular argument, which has no logical validity. A more convincing reason is that in humans, the brain is necessary and sufficient for consciousness. A locked-in patient is still conscious. On the other hand, any large brain lesion has an impact on conscious experience, and specific experiences can be induced by electrical stimulation of the brain.

However, this tends to prove that the brain is the substrate of my experience, but it says nothing about, say, the stomach. The stomach also has a nervous system, it receives sensory signals and controls muscles. If it were conscious, I could not experience it, by definition, since you can only experience your own consciousness. So it could also be, just as for the brain, that the stomach is sufficient and necessary for consciousness of the gut mind: perhaps if you stimulate it electrically, it triggers some specific experience. As ridiculous as it might sound, I cannot discard the idea that the stomach is conscious just because I don’t feel that it’s conscious; I will need arguments of a different kind.

I know I am conscious, but I do not know whether there are other conscious entities in my body. Unfortunately, this applies not just to the stomach, but more generally to any other component of my body, whether it has a nervous system or not. What tells me that the liver is not conscious? Imagine I am a conscious liver. From my perspective, removing one lung, or a foot, or a large part of the visual cortex, has no effect on my conscious experience. So the fact that the brain is necessary and sufficient for your conscious experience doesn’t rule out the fact that some other substrate is necessary and sufficient for the conscious experience of another entity in your body. Now I am not saying that the question of liver consciousness is undecidable, only that we will need more subtle arguments than those exposed so far (discussed later).

Let us come back to the single cell. Although I feel that a unicellular organism is not conscious because it doesn’t have a nervous system, so far I have no valid argument for this intuition. In addition, it turns out that Paramecium, as many other unicellular organism including (at least some) bacteria, is an excitable cell with voltage-gated channels, structurally very similar to a neuron. So perhaps it has some limited form of consciousness after all. If this is true, then I would be inclined to say that all unicellular organisms are also conscious, for example bacteria. But then what about a single cell (eg a neuron) in your body, is it conscious? One might object that a single cell in a multicellular organism is not an autonomous organism. To address this objection, I will go one level below the cell.

Eukaryotic cells (eg your cells) have little energy factories called mitochondria. It turns out that mitochondria are in fact bacteria which have been engulfed in cells a very long (evolutionary) time ago. They have their own DNA, but they now live and reproduce inside cells. This is a case of endosymbiosis. If mitochondria were conscious before they lived in cells, why would they have lost consciousness when they started living in cells? So if we think bacteria are conscious, then we must admit that we have trillions of conscious entities in the cells of our body – not counting the bacteria in our digestive system. The concept of an autonomous organism is an illusion: any living organism depends on interactions with an ecosystem, and that ecosystem might well be a cell or a multicellular organism.

By the same argument, if we think unicellular organisms are conscious, then single neurons should be conscious, as well as all single cells in our body. This is not exclusive of the brain being conscious as a distinct entity.

A plausible alternative, of course, is that single cells are not conscious, although I have not yet proposed a good argument for this alternative. Before we turn to a new question, I will let you contemplate the fact that bacteria can form populations that are tightly coupled by electrical communication. Does this make a bacteria colony conscious?

Let us now turn to another question. We can imagine that a cell is somehow minimally conscious, and that at the same time a brain forms a conscious entity of a different nature. Of course it might not be true, but there is a case for that argument. So now let us consider two people living their own life on opposite sides of the planet. Can this pair form a new conscious entity? Here, there are arguments to answer negatively. This is related to a concept called the unity of consciousness.

Suppose I see a red book. In the brain, some areas might respond to the color and some other areas might respond to the shape. It could be then that the color area experiences redness, and the shape area experience bookness. But I, as a single conscious unit, experiences a red book as a whole. Now if we consider two entities that do not interact, then there cannot be united experiences: somehow the redness and the bookness must be put together. So the substrate of a conscious entity cannot be made of parts that do not interact with the rest. Two separated people cannot form a conscious entity. But this does not rule out the possibility that two closely interacting people may not form a conscious superentity. Again, I do not believe this is the case, but we need to find new arguments to rule this out.

Now we finally have something a little substantial: a conscious entity must be made of components in interaction. From this idea follow a few remarks. First, consciousness is not a property of a substrate, but of the activity of a substrate (see a previous blog post on this idea). For example, if we freeze the brain in a particular state, it is not conscious. This rules out a number of inanimate objects (rocks) as conscious. Second, interactions take place in time. For example, it takes some time, up to a few tens of ms, for an action potential to travel from one neuron to another. This implies that a 1 ms time window cannot enclose a conscious experience. The “grain” of consciousness for a human brain should thus be no less than a few tens of milliseconds. In the same way, if a plant is conscious, then that consciousness cannot exist on a short timescale. This puts a constraint on the kind of experiences that can be ascribed to a particular substrate. Does consciousness require a nervous system? Maybe it doesn’t, but at least for large organisms, a nervous system is required to produce experiences on a short timescale.

I want to end with a final question. We are asking what kind of substrate gives rise to consciousness. But does consciousness require a fixed substrate? After all, the brain is dynamic. Synapses appear and disappear all the time, all the proteins get renewed regularly. The brain is literally a different set of molecules and a different structure from one day to the next. But the conscious entity remains. Or at least it seems so. This is what Buddhists call the illusion of self: contrary to your intuition, you are not the same person today and ten years ago; the self has no objective permanent existence. However, we can say that there is a continuity in conscious experience. That continuity, however, does not rely on a fixed material basis but more likely on some continuity of the underlying activity. Imagine for example a fictional worm that is conscious, but the substrate of consciousness is local. At some point it is produced by the interaction of neurons at some particular place of the nervous system, then that activity travels along the worm’s spine. The conscious entity remains and doesn’t feel like it’s travelling, it is simply grounded on a dynamic substrate.

Now I don’t think that this is true of the brain (or of the worm), but rather that long-range synchronization has something to do with the generation of a global conscious entity. However, it is conceivable that different subsets of neurons, even though they might span the same global brain areas, are involved in conscious experience at different times. In fact, this is even plausible. Most neurons don’t fire much, perhaps a few Hz on average. But one can definitely have a definite conscious experience over a fraction of second, and that experience thus can only involve the interaction of a subset of all neurons. We must conclude that the substrate of consciousness is actually not fixed but involve dynamic sets of neurons.

A summary of these remarks. I certainly have raised more questions than I have answered. In particular, it is not clear whether a single cell or a component of the nervous system (stomach, brainstem) is conscious. However, I have argued that: 1) any conscious experience requires the interaction of the components that produce it, and this interaction takes place in time; 2) the set of components that are involved in any particular experience is dynamic, despite the continuity in conscious experience.