Clemens Lode
September 30, 2023
Cartesian theater with an infinite homunculus observing itself (source: Wikimedia, Jennifer Garcia).

The Search for Consciousness

If we dismiss the idea of a quantum mind and if we accept that consciousness does not reside in the immaterial world, then consciousness has to relate to one or several parts of our material brain. In this section, we will determine if there is a single brain part responsible for consciousness, if consciousness is an emergent property of the brain, or if several brain parts are involved in consciousness.

Most monist theories of consciousness imply that there is some kind of single “observer”-like element in the brain, even if the theories do not spell it out. This is what the philosopher and cognitive scientist Daniel Dennett calls the “Cartesian theater.”

"Cartesian materialism is the view that there is a crucial finish line or boundary somewhere in the brain, marking a place where the order of arrival equals the order of “presentation” in experience because what happens there is what you are conscious of. […] Many theorists would insist that they have explicitly rejected such an obviously bad idea. But […] the persuasive imagery of the Cartesian theater keeps coming back to haunt us—laypeople and scientists alike—even after its ghostly dualism has been denounced and exorcised." —Daniel Dennett, Consciousness Explained
Cartesian theater   The Cartesian theater is a term coined by Daniel Dennett to criticize most of the contemporary explanations of consciousness. At their core, such explanations all share the view that there is some sort of miniature person (“homunculus”) or entity within the brain looking at what we are looking at—an idea which ends in an infinite series of subsequently ever-smaller Cartesian theaters with ever-smaller homunculi.

Instead of admitting that they are mind-brain dualist theories, most modern theories of consciousness stay away from discussing the Cartesian theater and only describe individual aspects of consciousness, without explaining the whole picture. Dennett criticizes scientists who look only at one part without looking at the whole picture. Each study pushes the responsibility to find the final “actual observer” to someone else further down the line.

The Cartesian theater fallacy demonstrates that there is no brain part or magical objective observer that takes over the brain and starts being a “brain in a brain.” While the brain parts compete, such competition does not imply that they are conscious actors themselves. Because after all, who would control that brain or brain part? We could have another “brain” observing the previous brain and so on, ending up with the observer having in itself an observer observing the observer, and so on, ending up in an infinite series of observers. This is the so-called fallacy of the Homunculus argument (see Figure 6.7).

Figure 6.7: An illustrated example of the fallacy of the Homunculus argument: the observer himself is observed by an (even smaller) observer and so on (image source: Jennifer Garcia, wikimedia).

Homunculus argument fallacy   The homunculus argument is the fallacy of trying to explain consciousness by another (smaller) conscious person (the “homunculus”) observing and steering you. The problem with this explanation is that it remains unexplained how this smaller homunculus subsequently experiences consciousness.

This argument is reminiscent of the previous discussion about the role of the observer in quantum mechanics and consciousness. If the world exists only when we are observing it, who is observing us? This creates an infinite loop, too, with an undefined “original cause” (well, or “consciousness”) at the end of the line, but does not really explain anything that is going on. With that in mind, it seems that consciousness is both finite and infinite.

In summary, the “Cartesian theater,” in which there is some sort of miniature observer sitting in the brain and observing the incoming data, is not a satisfactory explanation of consciousness. Even if we had concrete proof of such an observer, we would simply move on and start examining that observer and question how it worked.

Emergent Property

Emergent property   An emergent property is a property of a system that emerges only when its parts are combined or interact with each other. Individual parts of that system do not have the emergent property themselves. For example, the division of labor in ant colonies allows the ants to be more efficient than if individual ants fended for themselves.

There is no central “evolution” or “creativity” authority directing nature to come up with novel solutions. We cannot point to a plant or animal and declare “that is evolution.” Evolution is a process and the way lifeforms are selected is an emergent property of this process. As such, it seems natural to ask whether consciousness is such an emergent property of our brain. Just like fish form schools that seem to behave like single entities, neurons could form consciousness.

One way to approach the question is to take a whole brain and then think about what would happen if we removed individual neurons.

As a comparison, imagine being a ruler of a country. Your cabinet advises you on policy decisions and you make the final call. Now people leave your cabinet one by one. You could still make decisions, although they would be less informed and may cause hardships among the population. In that regard, the brain is not much different: the sum of your brain’s neurons is like a committee. Even losing neurons, you are still conscious and (your committee) capable of acting but your decisions and movements will be less refined.

While losing many neurons within a short period of time is usually lethal or leads to irreparable coma, if this damage happens slowly, you might not even notice it. For example, there was a case of a man whose brain mass was, over time, compressed to less than 10% of its original mass. While he had a reduced IQ of 75, he was still fully conscious [Feuillet et al., 2007]. The loss occurred due to a huge fluid-filled chamber that took up most of the room in the center of his brain over the course of 30 years, leaving little more than a thin sheet of actual brain tissue pressed against the skull. The loss of neurons happened globally, meaning that only volume—not connectivity or brain parts—was lost. In a way, it seems that the question about consciousness is like the question about the pile of sand we discussed in Philosophy for Heroes: Knowledge (Sorites paradox): how many grains of sand do you have to pile up in order to call it a “pile” of sand? One? Two? A thousand? If 90% of the brain can be removed, what would happen if even more were removed? Can a single neuron be conscious or create consciousness?

The limit between a conscious and non-conscious brain seems to be the structure, not the number of neurons. This implies that consciousness is probably not an emergent property of individual neurons but a product of how individual brain parts are connected to each other—a system we could clearly define and describe. Now, if we can find instances of localized damage in the brain that have resulted in impairments of consciousness, we can determine that those brain parts are elements of the system that creates consciousness. This is in line with how scientists have been able to discover the functions of the neocortex by correlating localized damage to specific cognitive impairments.

If there is no single “final” observer and if consciousness is not an emergent property, the remaining alternative is to look at consciousness as a system with many different brain parts interacting with each other. To examine this option, let us look at what we most prominently connect with consciousness, namely our sense of sight. To better understand the way our eyes send signals to the visual cortex, let us look at Figure 6.8 and examine what various types of damage to our visual system would do to our visual experience:

  • Damage to 1 or 2: Damage to the eye or optical nerve can result in the loss of sight in one eye. The effect is the same as closing one eye.
  • Damage to 3: Splitting the optic chiasm in the middle means that the left visual cortex no longer receives the right side of the visual field of the right eye, and the right visual cortex no longer receives the left side of the left visual field of the left eye.
  • Damage to 4 or 5: Damage to the connection between the optic chiasm and the thalamus results in the left and right visual cortex receiving sense data only the left visual field or only the right visual field.
Figure 6.8: The basic process of perception, from initially seeing the object (in this case, an apple) to the pre-processed image in the cortex.


If the primary visual cortex itself (or the nerves leading up to it from the LGN, see 6 and 7 in the diagram) is damaged, this damage results in the same impact on the visual field as damage to 4 or 5. But the affected person can experience “blindsight” on the blind side of the visual field as some of the information is transferred from the thalamus to other parts of the brain.

Blindsight   Someone suffering from blindsight reports that he cannot see. However, experiments show that he can react to visual cues. As a result of damage in the visual cortex, information from the retina arrives in the midbrain but does not undergo conscious processing through the visual cortex.

Sufferers of blindsight have perfectly working eyes, yet they report that they are blind. Given that their visual cortex never receives visual sense data, they cannot report what their eyes are seeing. But they can, for example, navigate through a room with obstacles much more effectively than blind people whose visual system is damaged before the LGN (their eyes or the optical nerves leading to the LGN). Even under test conditions [Stoerig and Cowey, 1997] where they are presented visual stimuli, people suffering from blindsight scored significantly higher than random chance would suggest—at least when they are asked to rely on their instincts or “just guess.” It is like asking someone without sight impairments to pass an obstacle course without thinking about the obstacles and instead only trusting her muscle memory and instincts (her “blindsight”). Bruce Lee put it succinctly:

"Don’t think. Feel. It’s like a finger pointing at the moon. Do not concentrate on the finger, or you will miss all of the heavenly glory." —Bruce Lee

The fact that people suffering from blindsight have the ability to accurately grasp for objects in their blind field tells us that there is already some processing of three-dimensional structures and movements in the LGN, providing the other brain parts basic information about the environment.

Theories regarding blindsight include the assumption that some data (around 10%) from the LGN travels not just to the primary visual cortex at the very beginning of the ventral and dorsal streams but also further down the line to other brain areas (for example, the visual association area that is adjacent to the primary visual cortex).

Blindsight contradicts our intuitive understanding of blindness. If we cannot report what we see, how can our brain still see and “report” its findings to us with intuition? It certainly seems an uncomfortable thought, not being in the driver’s seat, especially while our usual inner experience suggests that we are in control. For us to be conscious of something, that information needs to be processed by particular brain parts. If it is not, it can still affect us, but we cannot translate it directly into words. The underlying principle is actually common in daily life. Try to explain to someone how to keep balance on a bicycle and you discover that you can do something (ride a bike) which you cannot explain with words. That is the reason professional trainers have to learn their movements in an abstract way, too, in order to correctly verbalize them to their students—even experts do not have direct access to their own bodies’ optimized programs of their cerebellum.

It seems that something can catch the attention of our brain, but we do not necessarily become aware of it. In this context, awareness means that the information is available for conscious processing. For example, your mind might wander when you are on the phone and taking a walk. It might take a while before you become again aware of the fact that you are walking. If there are no obstacles in your path and the conversation is deep enough, your brain might still be processing walking along the path, but it never reaches your awareness. This shows that it is possible to have attention without awareness [Webb et al., 2016].

Awareness   Awareness is a description of the process of attention. Something can grab the attention of your brain, but to talk about it, you need a model of what is happening in your brain. Awareness is such a model.

Split-Brain Syndrome

When the connective tissue between brain regions is damaged, this has implications for many regions of the brain. By far the biggest connective bridge between brain parts is the corpus callosum, connecting the left and right hemispheres of the brain (see Figure 6.9). To understand the role of the corpus callosum, we need to understand the role of each hemisphere.

Figure 6.9: The location of the corpus callosum between the left and right hemispheres (image source: Wikipedia).

Corpus callosum   The corpus callosum connects the left and right brain hemispheres, coordinating tasks requiring both sides.

In healthy people, the most obvious difference between our left and right hemisphere shows up in our handedness. Not all people are right-handed—some people are left-handed, and a few are ambidextrous. For simplicity, we will focus on right-handedness for now. As our right hand is controlled by the left hemisphere and our left hand by our right hemisphere, we can see a clear division of labor between both hemispheres (so-called lateralization); for people with right-handedness, there seems to be a preference for fine-grained control in the left hemisphere. This relates to the involvement of the left supramarginal gyrus in tactile sensory data and tool use. Still, even with this division of labor, the two hemispheres have to work together. When you look straight ahead and then try to reach for an object in your left visual field with your right hand, information from your right visual cortex has to be transferred to your left hemisphere to coordinate the hand movement with visual information. This information transfer is made possible by the corpus callosum that connects both hemispheres.

However, why is the architecture of the body so confusing, with the left hemisphere controlling the right side of the body and the right hemisphere controlling the left side of the body?

The main reason is that evolution looks for short-term advantages. This “entanglement” between the left and right side is thought to have originated very early in our evolution. The theory is that at one point in time, the body plan (a “blueprint” detailing aspects such as symmetry, segmentation, and limb disposition) of an early vertebrate was twisted by 180 degrees which left this so-called decussation in its wake [de Lussanet and Osse, 2012].

Beyond how the right hemisphere controls the left hand and the left hemisphere controls the right hand, the hemispheres are also specialized in different tasks (see Figure 6.10). As a rule of thumb regarding the architecture of both hemispheres, the left hemisphere of the brain usually deals with the concrete, while the right hemisphere deals with intentions, interpretation, and hidden meanings.

Left hemisphere

  • Right-hand control
  • Right field vision
  • Language
  • Concepts
  • Math

Right hemisphere

  • Left-hand control
  • Left field vision
  • 3D forms
  • Measurements
  • Face recognition

Figure 6.10: A (non-comprehensive) comparison of the different specializations (the lateralization) of the left and right hemispheres of a human’s neocortex.

The advantage of this architecture is that we can use it to learn more about the functioning of the brain. If the corpus callosum gets damaged, we can draw different conclusions about consciousness and its involved brain parts depending on what symptoms people experience in such a case. If people with a damaged corpus callosum retain normal consciousness, this might be an indication for mind-brain dualism. If they lose consciousness, it would mean that both hemispheres are necessary for a conscious experience. If we find that by splitting up the brain, we create two separate consciousnesses, this would contradict our previous finding that consciousness is probably not an emergent property. If we create two separate consciousnesses that have limitations, consciousness is either an emergent property, or identical structures exist in both hemispheres.

When the corpus callosum is damaged, people can experience split-brain syndrome. In the past, such damage was caused by surgery to prevent life-threatening injuries caused by severe epileptic seizures. Disconnecting the left and right hemisphere prevented the epileptic seizure from spreading to or oscillating between both hemispheres. Nowadays, this type of surgery is applied only in very rare cases, given its significant adverse effects. A severed corpus callosum means that the left hemisphere cannot directly access the right hemisphere and vice versa.

Split-brain syndrome   The split-brain syndrome can occur when the corpus callosum is damaged. This leads to problems with communication between the left and right brain hemispheres and complicates some tasks requiring both sides.

Some tasks might be more difficult for the split-brain patient to solve, as he needs to use different paths to connect both hemispheres. Because people with split-brain syndrome can compensate for the lack of connection between hemispheres, it is not easy to identify those who have the condition. Doing so requires specific experiments to determine whether or not the connection is missing. One experiment commonly used for split-brain patients involves instructing the patient to stare at a dot on a computer screen and then showing her different words and pictures on the left and right side of the dot. When asked what she is seeing, she is unable to report on what she sees on the left side of the dot (see Figure 6.11). This is because her language center is in the left hemisphere. Without a corpus callosum, it has no direct access to the right hemisphere which processes the left visual field. And without connection through the corpus callosum, this information from the right hemisphere is not available in the left hemisphere.

When the patient is asked to close her eyes and draw with her left hand what she has seen on the left side, she is able to do so (see Figure 6.12). This shows that in patients with a severed corpus callosum, the information arrives and is correctly processed in both hemispheres, but each hemisphere is unable to access the information from the other hemisphere.

Figure 6.11: The split-brain patient can tell you what is on the left side but (due to the missing connection between the left and right hemisphere) cannot draw it with her left hand.

Figure 6.12: The split-brain patient can draw with her left hand what is on the right side. But due to the missing connection between the left and right hemisphere, the patient cannot tell you what is on her right side.

While the language center is often located in the left hemisphere and the right hemisphere cannot produce speech, the right hemisphere can read and understand simple questions. Scientists have found ways to train the (otherwise silent) right hemisphere in split-brain patients to point the left arm (which is controlled by the right hemisphere) to, for example, either “Yes” or “No” written on a blackboard. That way, the researchers were able to pose questions to both the left and the right hemisphere of a split-brain patient—with surprising results. The (silent) right hemisphere could correctly answer a variety of questions (where the subject is, his or her gender, etc.) but for some questions, the answer differed from the left hemisphere’s response. For example, one patient’s left hemisphere told the scientists he believes in God, while the right said he does not. On closer inspection, this is not surprising, though, as each hemisphere bases its decisions on different data and, without the corpus callosum, the two sides cannot negotiate the final decision.

"Imagine our surprise when we noticed that in patient LB the left hemisphere said it believed in God whereas the right hemisphere signaled that it was an atheist. The inter-trial consistency of this needs to be verified but at the very least it shows that the two hemispheres can simultaneously hold contradictory views on God." —Vilaynur Ramachandran, The Emerging Mind

We see similarities between the brain of a split-brain patient and the nervous system of an octopus. Some two-thirds of its neurons are located in its limbs instead of its central brain. They can move independently of each other, even after being severed from the main body. Both the split-brain syndrome as well as examples like the octopus lead us to question the intuitive idea of a simple one-to-one relationship between “number of brains” and “consciousnesses.”

Our observation actually points to split-brain patients having two consciousnesses, each being somehow partly conscious. Both sides seem to be “conscious” but neither side has conscious access to the information or abilities (like spoken language or comprehending three-dimensional forms) of the other hemisphere. For example, as discussed above, the right hemisphere of a split-brain patient has no access to the language center in the left hemisphere and thus cannot verbally explain how it experiences consciousness. While we have uncovered more about consciousness with this examination, we need to dig deeper. From a certain perspective, the split-brain syndrome could be compared with psychophysical parallelism. There, mind and brain originate from the same substance but were split at some point in time, running in parallel from then on. Likewise, in patients with split-brain syndrome, the brain was whole at some point, but both brain hemispheres had to run in parallel after the corpus callosum was severed.

Hemispatial Neglect

Can we see something but not be aware of it?

For sufferers of hemispatial neglect, the ability to report on one side of their visual field is impaired. However, hemispatial neglect is not merely a problem of a subject’s vision. Instead, the issue is that any conscious experience of this part of the field of vision is impaired while the sense of sight (the eyes and the visual cortex) is working perfectly well [Parton et al., 2004]. Left-sided hemispatial neglect is more common than right-sided hemispatial neglect. This is because attention to the left side is processed only in the rTPJ, while attention to the right side is processed in both hemispheres.

Hemispatial neglect   Someone suffering from hemispatial neglect lacks consciousness of half of his visual field. The person is not aware that his vision is impaired in any way, making the condition different from blindness in one eye. People with this condition have to learn abstract strategies as a way of coping.

The important distinction between an impaired field of vision and the condition of hemispatial neglect is that sufferers of hemispatial neglect do not recognize that anything is missing in their visual field. The challenge for people with hemispatial neglect is not only that they are not conscious of something, but also that they do not realize that they are not conscious of it. For example, when asked to copy a clockface, a patient with hemispatial neglect ends up drawing a circle with the numbers 1 to 6, omitting 7 to 12 on the left side (or drawing all numbers 1 to 12 on the right side). If he was just visually blind on the left side (hemianopia), he would be conscious of the fact that he could not see the left side of the original clockface and turn his head.

In one study, subjects who were thought to have this condition were presented simultaneously with two line drawings of a house. In one of the drawings, the left side of the house was on fire and the right side was fine. Patients with left-sided neglect (and right-sided brain damage) reported not seeing anything wrong with the house that was, in the picture, on fire. They stated that the drawings were identical; yet when asked to select which house they would prefer to live in, they reliably chose the house that was not burning [Marshall and Halligan, 1988]

One explanation is that each hemisphere can still “communicate” with the other hemisphere indirectly through lower brain functions. For example, the left and right parts of the hippocampus are connected, and so are the left and right parts of the amygdala. Likewise, the hormonal system acts on both hemispheres. In that regard, people with hemispatial neglect can be influenced without being conscious of that influence. A patient with hemispatial neglect “just knows” and tries making up some rationalization to justify her feelings in order to appear rational. She does not have conscious access to the image of fire on the left side of a house, so in her verbal explanation of why she feels uneasy when looking at the picture, she will use some magical or far-fetched explanation of why she thinks something is wrong with it. Another example would be ducking from an approaching object. Having a ball thrown at her from the left side might cause her to duck, but she might be not conscious that there was a ball or why she ducked. She has access to the emotional evaluation of a situation but not the abstract reasoning behind it. This result can be interpreted as: in the affected patients, there is a disconnect between what their brains process and the information they have access to when trying to rationalize their decisions.

To what could we compare such an experience?

Let us say you are driving in your car and the car’s tail light is broken. You would still accelerate and break as you usually do, not conscious of possible danger. You might notice that something is wrong because other drivers flash their lights at you, but you have no idea why. Similarly, a sufferer of hemispatial neglect is not conscious that anything is missing on one side of her visual field. She would put make-up on only half of her face, comb only one side of her hair, and walk out of the door as if nothing were wrong—just like you would enter and leave your car like everything was fine. She would not be aware of what is wrong when encountering people reacting strangely to her behavior.

The difference compared to driving with a broken tail light is that even if she consciously knows that she suffers from hemispatial neglect on, for example, her left side, she is still unable to do something about it in a direct way. She is only conscious of her lack of “left side” in an abstract way because that was her diagnosis. She has no option to turn her head to be able to see what is missing from her vision as she is not conscious of her limited visual field. Without therapy, hemispatial neglect can even lead to a loss of attention to the limbs on the left side. She has to use her prefrontal cortex to develop habits that continually demonstrate to the rest of her brain that she indeed still has a left arm. To cope with their impairment, people with hemispatial neglect cannot rely on a separate “self” sitting in some sort of driver’s seat of their mind or body. To overcome cognitive defects like neglect or delusions, they have to learn strategies to cope. The major challenge with cognitive defects is that the people who have them are not necessarily conscious of the fact that there is a problem.

To imagine how it feels to explain something to which you do not have conscious access, try to explain why your favorite color is your favorite color. This might strike you as insulting, but your story will probably be an invention to justify your preferences (e.g., “I like blue because it’s the color of the sky”). As long as you do not know your neural configuration in your visual cortex but try to come up with a reason through introspection alone, you will end up confabulating instead of giving actual insight into how your brain works. This is because your left hemisphere, which provides verbal answers, believes that its interpretations and conclusions are correct. The actual cause might be some neural preference, while your rationalization for your preference is probably a non-rational explanation pointing to an unspecified “hunch” or “feeling” about why one color is preferred over another.

Some of our preferences can be changed. We can put ourselves into situations to train particular skills and form new habits. This way, the prefrontal cortex can change the rest of the brain in an indirect way. For example, by climbing up a diving tower and jumping into the water, the prefrontal cortex can demonstrate to the rest of the brain that you are able to accomplish this activity and survive. Similarly, going to the gym might initially require a lot of overcoming. After a few weeks, it becomes a habit and part of your daily routine.

What can we deduce from our examination of the split-brain syndrome, hemispatial neglect, blindsight, and AHS as these relate to consciousness?

We see that we cannot be aware of what is not fed to us by the brain. For example, as experiments with patients suffering from hemispatial neglect show, parts of the visual field we are not aware of are not “blacked out.” We cannot simply turn our head to cover the entire visual field. Instead, it seems that our options to react to our environment are limited by the attention mechanisms provided by our brain. There is no homunculus sitting in our brain with an objective overview of incoming sense data.

This leads us to the question what we mean when referring to ourselves as “us.” As experiments with people suffering from blindsight or hemispatial neglect show, sense data is still processed in the brain, but it is not directly available in our consciousness. We can still act on a subconscious hunch or feeling, but there is a qualitative difference between making a decision based on a hunch and being able to explain why you are making the decision. With obstacles in the way, it is certainly sufficient to “just know” your way around them (as people with blindsight do). Having the information available in your consciousness could help you to plan a path through the room of obstacles or think about a strategy for how to remove the obstacles altogether. In that regard, hemispatial neglect is the opposite of someone suffering from a monothematic delusion who is still fully aware of his environment, but who is unable to properly connect emotions with what he sees.

Consciousness seems to be neither an emergent property from neurons, nor the product of a single brain part. We have confirmed this by looking at damage to the visual system, the connection between the hemispheres, and syndromes that affect awareness but not attention. With this knowledge, we will examine what specific brain parts are connected with each other to form a loop of consciousness.

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September 30, 2023

About the Author

Clemens Lode

Hello! My name is Clemens and I am based in Düsseldorf, Germany. I’m an author of books on philosophy, science, and project management, and coach people to publish their books and improve their approach to leadership.

I like visiting the gym, learning to sing, observing animals, and creating videos on science and philosophy. I enjoy learning from nature and love the idea of optimizing systems.

In my youth, I was an active chess player reaching the national championship in Germany, and an active pen&paper player leading groups of adventurers on mental journeys. These activities align with my calm approach to moderating meetings, leading meetups, and focusing on details. My personality type in socionics is IEE/ENFp.

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