I think, therefore I am: modern neuroscience of consciousness uncovers Descartes' mistake


As you stare down into your screen, letters and words creep into your mind and the newly generated internal thoughts envelop your subjective experience. This constant flow of new information seems trivial, as this happens at every instant of your wakeful state. Consciousness gives rise to our existence and comprises all the emotions, memories, and perceptions that we have ever had. Arguably, it is the most fundamental aspect of being human. We are all experts of our own subjective worlds and it seems unthinkable that someone, a third party, could ever see through our eyes or try to explain objectively what we are experiencing. However, for thousands of years philosophers, and now scientists, have tried to do exactly that. They identified consciousness as one of the largest mysteries of the universe and have tried to answer both what it is and where to find it.

One of the most famous attempts at explaining consciousness comes from the 17th century philosopher Rene Descartes. He argued that humans are made out of two substances: the material and physical body and the immaterial mind. Unlike the material body, the immaterial mind inhabits a world not governed by the basic laws of physics. This type of philosophical thinking is known as mind-body dualism and it seems utmost natural from a subjective experience. It is “I,” the mind, that gives rise to the actions performed by my body. If I decide to raise my arm, I am instructing my body to do so.

Descartes’ theory of consciousness prompted many replies and engagement; one of the most famous and intricate questions was raised by Elisabeth, the princess of Bohemia. In many letters of correspondence with the philosopher, she questioned Descartes’ mind-body dualism by asking how two fundamentally different substances, the non-physical mind and the physical body, can interact. How can the mind casually give rise to the actions of the body if they do not share one substrate?

Descartes did not provide an exact answer to the question; he stated that the mind and body interact through the pineal gland, a small protruding structure found deep inside the brain. Unlike many brain regions which are found in both hemispheres, there is only one pineal gland. Descartes found this anatomical anomaly to be especially important. He stated that the immaterial mind could not be split into two parts; it had to interact with the body through one specific locale. Descartes explained where this interaction took place by identifying the “soul’s” canary cage; however, he still could not explain how the two substances interacted.

Few of today’s researchers and philosophers within the field of consciousness hold Descartes’ dualistic perspective on the mind and body, as princess Elisabeth’s interaction question still lacks a compelling answer. Rather, experts argue that all of the vast experiences that consciousness underlies arise from certain mechanisms within the material brain. Since the topic of consciousness consists of a broad range of phenomena, neuroscientists have divided it into two subfields: one investigating state consciousness and the other – content consciousness.

State consciousness is the arousal or wakefulness of your current experience. If you have become slightly more drowsy as you progressed through this article, the state of your consciousness has somewhat changed. State consciousness can be categorized into distinct types, but it is ultimately a continuous scale. Low levels of arousal or none at all result in comatose or anesthetic states where subjective experience is completely lost. The other end of the spectrum underlies high levels of arousal resulting in a normal wakeful state with its own subjective experience. Conscious states are modulated by evolutionarily old deep brain structures such as the brainstem and its communication with the thalamus. This spectrum of states can somewhat be seen as a laptop running on battery; when the battery is full, the laptop performs at its highest capacity, when there is less juice remaining and the battery-saving mode kicks-in, then the computer’s performance drops. Finally, when the battery has completely run-out, the computer does not work anymore. The computer’s battery percentage cannot tell us anything about the current applications running on screen; analogously, state consciousness does not delve into the type of experiences happening.

This fundamental question of how specific types of conscious experiences arise in the brain is tackled within the content consciousness subfield, also confusingly known as conscious awareness. The subfield’s aim is to understand the neural basis (both brain areas and their communication) that give rise to the consciousness of specific experiences, like the smell of morning coffee, a jump-scare during a horror film, or the mindless bliss experienced when viewing the panoramic scenery at the top of a mountain.

A natural question which might arise is how can science study such diverse experiences; how can it with certainty identify the type of conscious content a human has at a given moment? These questions bothered the cognitive science and neuroscience communities as well. They considered (and some still do today) consciousness research to be somewhat of a taboo topic; just like astrophysicists will not speculate the possible existence of aliens, neuroscientists in the late 20th century considered consciousness to be a confounded and convoluted term that either was generally unexplainable using the principles of science or already was explained by other psychological and cognitive constructs. In the early 2000s with the advent of more prominent functional MRI research, scientists slowly began investigating the topic. Christoph Koch provided one of the first conceptual breakthroughs in how conscious awareness should be studied. He argued that we need to identify the “neural correlates of consciousness” (NCC), specific populations of neurons that activate during a specific conscious experience and only during this experience. For example the smell of morning coffee will activate a population of neurons and these are the NCCs of the smell of coffee. However, the neural populations activating after this initial experience, such as thoughts about whether you like the type of beans or the subsequent initiation of an action to reach for the cup, no longer belong within the NCCs of the smell of coffee. NCCs for a given experience have to be distinct and distilled from other processes of the mind.

Stanislas Dehaene performed one of the initial experiments trying to identify consciousness-specific processing within the brain. Dehaene had participants scanned using functional MRI while they looked at words being presented on a screen. There was a catch – the words were presented only very briefly (for only around 30ms) and in some cases a “mask” followed immediately after the word. This mask consisted of many randomly oriented squares. In some cases, this immediate presentation of the mask made the words completely invisible; the participants reported that they did not see any word at that instant. Dehaene used this mask to manipulate when participants consciously saw the word. The researchers analyzed the results relatively simply by just comparing the activation of the brain both for consciously seen stimuli and for stimuli that did not enter consciousness. To the great surprise of Dehaene and the whole field, in both the conscious and unconscious cases the language regions of the brain activated; meaning that there was some type of language processing going on even when the word did not enter conscious awareness. Most importantly though, conscious words elicited higher activation within frontal regions of the brain, right behind the forehead . These brain regions are known for higher-order processing, such as abstract thought, working memory, and the control of behavior.

Based on further research, Dehaene and colleagues came up with currently one of the most prominent theories of content consciousness called the global neuronal workspace model (GNWM). As the results indicated in their experiment, they propose that in both conscious and non-conscious cases there is a so-called bottom-up sweep of activity, a very quick step-by-step activation of the brain starting from sensory regions (ones that initially process sensory information like in the case of vision or audition), and progress all the way to the frontal regions. The main difference between non-conscious and conscious information is that the latter then activates these frontal regions and broadcasts this information across many other areas of the brain. Only then, in what is called top-down activation (from frontal regions back) does information enter consciousness. The brain areas recruited top-down and placed into the “global workspace,” as the theory’s name suggests, are dependent on the type of conscious information. If you smell coffee again, olfactory and possibly visual imagery areas will receive top-down activation from the frontal regions. However, if you hear your favorite Backstreet Boys song, auditory areas will be recruited instead.

In any young science the main driver of progress are competing theories that aim to come up with alternative explanations for the same phenomenon. Another prominent theory in consciousness is the integrative information theory proposed by Giulio Tononi. The theory builds on the first principles of consciousness, the fact that it is subjective, and formulates mathematical models testing whether any given system can possess its own subjective world. The exact mathematical equations are beyond this article, yet they test the complexity and the interconnectedness of a given region of the brain. Unlike GNWM, the calculations of IIT predict that the aforementioned sensory areas at the back of the brain have this highest capability for subjectivity.

The two theories provide testable hypotheses: GNWM predicts the frontal areas while IIT predicts the posterior areas to be most crucial for consciousness. Non-coincidentally, this debate has been picked-up by the Templeton Foundation, a scientific charity aimed at helping to answer the deepest and most perplexing scientific questions of today. A multi-million euro grant has now enabled Koch, Tononi, Dehaene and many other colleagues involved in the field to design experiments that would pit these two theories against each other. These studies are currently being conducted across multiple universities and results favoring one or the other theory should be out in a couple of years. Even though progress seems to have been made since the times of Descartes – we think the mind is mechanistic and consciousness is an active process involving many regions of the brain rather than being housed in one location – the thousand year-old question still remains: what is consciousness and where can we find it?

The article was prepared on behalf of INA by PhD student J. Karolis Degutis


Videos on the topic:

Templeton-funded project:

An interview with Giulio Tononi:

Lecture on consciousness by Stanislas Dehaene:

Works cited:

Dehaene, S., Naccache, L., Le Clec'H, G., Koechlin, E., Mueller, M., Dehaene-Lambertz, G., ... & Le Bihan, D. (1998). Imaging unconscious semantic priming. Nature, 395(6702), 597-600.

Dehaene, S., & Naccache, L. (2001). Towards a cognitive neuroscience of consciousness: basic evidence and a workspace framework. Cognition, 79(1-2), 1-37.

Rees, G., Kreiman, G., & Koch, C. (2002). Neural correlates of consciousness in humans. Nature Reviews Neuroscience, 3(4), 261-270.

Shapiro, L. (2013). Elisabeth, Princess of Bohemia.

Tononi, G. (2015). Integrated information theory. Scholarpedia, 10(1), 4164.

Cover image:

"Brain Art" by Ars Electronica, licensed under CC BY-NC-ND 2.0