Written by: Rachel Wahlberg

Edited by: Olivia Pifer Alge, Austin Shannon, Andrés Rivera Ruiz, and Jennifer Baker 

Illustrated by: Hana Paz Harbman 

This blog post is part 1 of a multi-piece series on the neuroscientific study of consciousness. Stay tuned for parts 2 and 3 coming soon! 

If I were to walk up to you on the street and ask you if you were conscious, what would you say? My guess is you would answer with a “well, yes?” – and if you’re anything like me, with a nervous laugh added, wondering what sort of conversation you’ve just walked into.

Consciousness is the scaffold of every sensation, thought, or action we experience when awake. It is so embedded in our daily experience of the world that we often don’t consider its existence. And yet, despite each of our intimate experiences with what it feels like to be conscious, if we reflect on the question, “What is consciousness?” we likely have drastically different answers than the person next to us. 

Is consciousness awareness of the external world? Awareness of one’s internal world? To be conscious, do we have to be able to perform actions? Do we have to have a physical body? Is consciousness the same as the concept of a soul? Are animals conscious, and does that change how we think about our own consciousness? 

Fig 1. What is consciousness?

Yikes. There are so many potential answers to this question of how to define consciousness (figure 1).

Searching for the definition of consciousness is not a trivial feat. And yet the conversation becomes relevant in the most emotionally wrought of circumstances, such as when deciding whether to continue supporting comatose patients. It plays into conversations on animal welfare and, increasingly, on when we might consider artificially intelligent beings sentient.

Neuroscience, as the study of the brain, is uniquely positioned to probe the mysteries of consciousness. In the last thirty years, the use of neuroscientific techniques to study consciousness has rapidly increased. So how do neuroscientists currently study and think about consciousness, and how did we arrive here? 

In this first article of a three-part series, I am going to give an overview of how neuroscientists, philosophers, and psychologists have studied consciousness throughout history. In the following two articles, I’ll dive deeper into two leading neuroscientific theories of consciousness. So stick around, and I hope by the end you’ll have your own ideas of how you think we should think about consciousness.

B.C.E. – 18th century: Philosophers have been thinking about consciousness for a while

Neuroscience focuses on the physical brain and nervous system, but many questions it asks have been thought about for centuries before its birth. Similarly, the study of consciousness has, in many ways, been used as a synonymous term with “studying the mind.” Because of this, philosophy of mind – the study of what the mind is and how it relates to the physical body – is both neuroscience’s ancestor and its modern day colleague. 

Fig 2: Descartes believed the brain to be the connection between the spirit and the body.

Early examples of philosophers of mind are Aristotle (384-322 B.C.E.) and Ibn Sīnā (Avicenna; 980-1037 C.E.), who both wrote often on the mind and body relationship. However, in the seventeenth century, René Descartes (1596-1650 C.E.) introduced a linguistic shift in how we think of the word consciousness. With Descartes, consciousness came to describe the introspective psychological concept that it is today (Jorgenson, 2020). While Descartes was a proponent of dualism, or the belief that our spirit is distinct from our material body, he believed the brain to be the connection between our spirit and body (figure 2). 

Reflection on the mind/body relationship dominated the study of consciousness for several more centuries. Baruch Spinoza (1632-1677) believed that mind and body were of one substance, while Gottfried Leibniz (1646-1716), who was the first to really try and develop a theory of consciousness (Jorgenson, 2020), saw the two entities to be separate but work closely together. In the eighteenth century, however, Julien Offray de la Mettrie (1709-1751) in L’homme machine specifically argued that consciousness arises from physical processes. While this did not put to bed the debate between dualism and monism, it paved the way for the neuroscientific study of consciousness to arise later on with the advent of technologies capable of studying the brain in great physical detail (Frith and Rees, 2007).

19th century: Scientists realize they can measure brain activity – modern neuroscience begins

Fig 3: Fechner used the method of “just noticeable differences,” in which he determined the level of stimulus needed to elicit awareness of the sensation. (Note: Fechner often used comparison of weights of two balls in his experiments; no eyes were harmed).

The advent of modern neuroscience took off in the nineteenth century – at that point, scientists were experimenting with whether activity in the brain could even be measured at all. Gustav Fechner (1801-1887) showed that electrical activity in the brain is directly related to physical sensation on the body through studying what amount of stimulation resulted in electrical response (figure 3), while Hermann Helmholtz (1821-94) similarly showed that the brain responds to visual stimuli. David Ferrier (1843-1928) stimulated the parietal and temporal lobes of animals’ brains and recorded the physical sensations that resulted from these brain stimulations. John Hughlings Jackson (1835–1911), a contemporary of Ferrier, noted that epileptic seizures can be accompanied with alterations in conscious experience.

These early neuroscientific studies were fundamental in demonstrating that the brain is connected to our sensory world in a causal way. While these studies were primarily important for the general introduction of modern neuroscience, at that time, analyses of the brain and theories of consciousness were entangled enough that these studies’ advances equally contributed toward a neuroscientific theory of consciousness (Ledoux et al., 2020; Frith and Rees, 2007).

Early 20th century: Watson asks to cool it on the “conscious/unconscious” conversation

Fig 4: Watson challenged psychologists to not make claims they couldn’t objectively prove in regards to the internal minds of humans and other animals.

Sigmund Freud (1856-1939), influenced in part by Ferrier and Jackson’s studies on consciousness, became both wildly famous and infamous for his psychological theory that our conscious actions are guided by subconscious desires. This overfocus on subconscious processes, paired with other psychologists excessively claiming to know what animals were thinking, led to psychologist John Watson (1878-1958) calling for researchers to limit their studies to behaviors that can be quantitatively measured (figure 4). This introduction of behaviorism, though highly refining and standardizing psychology studies, led to a decrease of academic research into people’s internal worlds and experiences. Some neuroscientific subdisciplines that developed still focused on internal states, such as cognitive neuroscience, which is the study of how the brain processes information, forms memories, and makes decisions. However, many consider the mid-20th century to be the “dead period” of neuroscientific consciousness research – the magnitude and subjectivity of defining consciousness felt too far out of the realm of measurement to be considered a worthy scientific pursuit (Frith and Rees, 2007).

Clinicians, less bothered by the behavioral revolution in the psychology sphere, made advances toward the understanding of consciousness in the 20th century by focusing on patients experiencing clinical disorders. Wilder Penfield in the 1930s and ‘40s stimulated the brains of epileptic patients, demonstrating a direct link between electrical brain stimulation and what patients were simultaneously perceiving. Other researchers studied split brain patients, in which the two hemispheres of the brain have different experiences from one another. They also considered blindsight, in which patients can respond to visual stimuli though consciously blind (the eyes are functional, but a damaged visual cortex means patients aren’t aware they have seen anything). Through these functional experiments, these clinicians not only deepened our understanding of the brain, but gave a better understanding of the nuances of conscious experience (Ledoux et al., 2020).

Late 20th century: Modern era of consciousness research begins

Fig 5: Consciousness re-emerges as a scientific field of study using modern neuroscientific techniques.

In 1990, Francis Crick and Christof Koch proposed that consciousness can be neurobiologically defined, ushering in the modern era of consciousness research (Crick and Koch, 1990) (figure 5). In their seminal paper on the subject, they emphasized awareness and short-term memory as avenues into understanding consciousness. This proposal placed the study of consciousness as fair game into the realm of neuroscience. The neuroscientific study of consciousness still has its critics that argue consciousness is outside the realm of science, due to it being difficult to define and test. However, the field of study has continued to grow, with centers across the world devoted entirely to the science of consciousness (including the University of Michigan’s own Center for Consciousness Science). 

Since the ‘90s, many theories have emerged as to how to define consciousness. Arguably, the two most popular current theories of consciousness are the Global Neuronal Workspace Theory (GNW) and the Integrated Information Theory (IIT). The GNW, first introduced by Changeaux and Dehaene in 1998 (Dehaene et al., 1998), states that conscious experience inherently has measurable behavioral outputs. Conscious experience occurs when enough brain areas in the global network, consisting of key brain areas for consciousness, are synchronously activated (Mashour et al., 2020). The IIT, promoted by Giulio Tononi, Christof Koch, and colleagues, is rooted in the study of “what it is like” to be conscious. They strive to provide a quantitative measure of what makes an experience have a cohesive, subjective feel. IIT predicts that consciousness arises with a high enough level of integrated information between a set of specific feedback loops in the brain.

A simple summary of GNW and IIT can hardly do justice to the depth of these theories; that’s why in the next two articles I’ll be diving deeper into how to understand and conceptualize these ideas. Thanks for exploring the breadth of consciousness research with me over the centuries – now let’s dive deeper into these two specific theories!


Crick, F., & Koch, C. (1990). Towards a neurobiological theory of consciousness.

Seminars in the Neurosciences (Vol. 2). https://profiles.nlm.nih.gov/101584582X469

Dehaene, S., Kerszberg, M., & Changeux, J. P. (1998). A neuronal model of a global workspace in effortful cognitive tasks. Proceedings of the National Academy of Sciences of the United States of America, 95(24), 14529–14534. https://doi.org/10.1073/pnas.95.24.14529

Frith, C., Rees, G., Velmans, M., & Schneider, S. (2007). A brief history of the scientific approach to the study of consciousness. https://doi.org/10.1002/9781119132363.ch1

Jorgensen, Larry M., “Seventeenth-Century Theories of Consciousness”, The Stanford Encyclopedia of Philosophy (Spring 2020 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/spr2020/entries/consciousness-17th/&gt;.

Ledoux, J. E., Michel, M., & Lau, H. (2020). A little history goes a long way toward understanding why we study consciousness the way we do today. PNAS. https://doi.org/10.1073/pnas.1921623117

Mashour, G. A., Roelfsema, P., Changeux, J. P., & Dehaene, S. (2020). Conscious Processing and the Global Neuronal Workspace Hypothesis. In Neuron (Vol. 105, Issue 5, pp. 776–798). Cell Press. https://doi.org/10.1016/j.neuron.2020.01.026

About the Author

Rachel is currently a third-year Ph.D. candidate in Kamran Diba’s Neural Circuits and Memory Lab within the Neuroscience Graduate Program at the University of Michigan. Her research focuses on investigating the mechanisms underlying episodic memory formation in rat hippocampus. Additionally, she holds a keen interest in exploring the neuroscience of consciousness. Rachel completed her undergraduate studies at Bethel University in St. Paul, Minnesota, where she double-majored in neuroscience and psychology. Outside of the lab you’ll find Rachel at the piano, training for her next marathon, or experimenting with new baked creations.

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