October 20, 2020
Written by: Rebecca Somach
What is the difference between a person and a houseplant? Although there are many possible answers, there might be fewer than you think. Just like us, plants are alive, need nutrients to grow, and have some ability to respond to the outside world. However, people have a special characteristic that makes their lives much more complicated than the typical houseplant. People can be awake and aware of their surroundings, in other words – they have consciousness. People are fascinated by consciousness, mainly because little is known about it. Some neuroscientists want to figure out whether certain cells in the brain could be the source of consciousness. While that seems like an abstract concept, trying to find what produces our consciousness has led to some interesting hypotheses.
Studying consciousness is tricky. When performing experiments, scientists need to be able to distinguish between cells that are responsible for keeping someone ‘awake’ and cells that are responsible for making someone ‘conscious’. Sometimes people can be completely awake, aware and attentive and yet still be affected by things that they aren’t conscious of. This is demonstrated by experiments that measure subliminal messaging. In one example of this type of experiment, participants were shown a face paired with an occupation 1. These faces and words were shown very quickly, for just 17 milliseconds at a time, and were mixed together with other images. Up to 25 minutes later, the participants were shown only the faces and were asked if they thought each face was someone who earned a lot of money or less money. More than half of the time, the participants matched the faces with the amount of money associated with the occupation. The researchers asked them if they recognized the faces, and the participants said that they did not. This meant that those brief images were influencing their decisions, even if they did not consciously realize they had seen them. In these experiments, the subjects are completely awake and aware, but there must be a part of their brain that is working separately from their consciousness to process the images. When designing experiments that focus on consciousness, scientists need to be wary that cells that might be active when someone is ‘awake and aware’ could be different from those that process if we are ‘conscious’.
Consciousness researchers also need to design experiments that take into account a different problem: there are systems and cells that are constantly working in the body, whether or not someone is awake to direct them. This is true for systems that we often consider to be fundamental to awareness, including our hearing and vision. In some of the fundamental research on the visual system that was done in the 1960s, the researchers Hubel and Wiesel investigated how the brain processes light from the eyes of anesthetized subjects 2. They found that neurons in the visual system were clearly active, even though the subjects were not awake. Although subjects don’t ‘see’ anything, the brain was still processing visual stimuli. Therefore, when researchers start to study cells that are responsible for consciousness, they need to take into account that cells can respond to stimuli while a subject is unconscious. These are significant challenges to some of our current methods of measuring thought and cell activity but some researchers have still proposed hypotheses of cells that they believe are involved in producing and maintaining consciousness.
One of the candidates for a ‘cell of consciousness’ is a structure called the claustrum. The claustrum is a thin sheet of cells that have very long projections. Researchers Crick and Koch proposed that the structure might be responsible for integrating information from different parts of the brain, which is thought to be a necessary component for consciousness 3. The problem with Crick and Koch’s hypothesis is that there was very little evidence to actually link consciousness with the claustrum. Researchers can inactivate or destroy the claustrum in animal models to determine if the claustrum is involved in signal integration. However, the claustrum is very thin, so it is difficult to tell if removing this region of the brain only cuts its connections or if the many regions nearby are also damaged. For ethical reasons, it is also not possible to do those types of experiments in humans. In humans, researchers can only look at people that already have damage to the claustrum or patients that have needed brain surgeries in regions near the claustrum. In one such patient, doctors implanted an electrode that happened to be near the claustrum to try to regulate the patient’s seizures. Doctors could turn on the electrode and the patient would be unresponsive and have no memory of the experience 4. This study and others like it 5, suggest that the claustrum may play a role in consciousness but there is not yet enough evidence to know conclusively.
Another hypothesis about a cell type associated with consciousness focuses on the connection between the thalamus and the cortex. The thalamus is where the information from our senses first enters the brain. The cortex is where much of this information is processed further. The cortex also makes associations with itself. There is a type of cell that participates in the connections between the thalamus and the cortex as well as the connections between the cortex and other parts of the cortex. These are cells located in the fifth layer of the cortex, and therefore called cortical layer 5 pyramidal neurons 6. These cells have multiple input areas. One input area is in the top layers of the cortex, called the apical input area 7. The other is closer to the cell body, called the basal inputs. Since the apical input area is connected to other cells that process information in the cortex, it is thought that this area can influence the activity of the layer 5 cells based on activity throughout the cortex 8. Anesthetics work because they block the integration of sensory information which is one of the roles of the pyramidal cells 9. If this is the case, since these cells are integrators of information from different cortical areas, they are a good candidate for cells associated with consciousness.
Proving that these cells are directly responsible for consciousness is difficult. In an ideal experiment, researchers would be able to directly affect those cells and measure an output that is related to consciousness. In animals, our measurements for consciousness are measuring if an animal is asleep or awake. If they are awake, then the best we can do is measure their sensory perception, which, as discussed above, is not always a good readout for ‘consciousness’. In humans, it would be unethical to alter these cells specifically or to measure them directly during periods of unconscious activity. For now, we have some hypotheses about the cells that produce consciousness but direct proof is still lacking. As it stands, the source of consciousness still remains a fascinating mystery.
To learn more about the relationship between the thalamus and consciousness, you can read this PennNeuroKnow article written by Carolyn Keating.
Want to know what happens when you lose consciousness because of anesthesia? Check out this PennNeuroKnow article written by Sarah Reitz.
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Cover Photo from Pixabay User jnittymaa0