Updated: Dec 31, 2022
Consciousness can be defined as wakefulness, awareness, and self-awareness. Brain imaging studies of people in different levels of consciousness (sleep, vegetative state, coma, neuropsychological disorders) can help reveal the neural correlates of consciousness. The strongest neural correlates of consciousness include attention networks, the default mode network, and the claustrum. The claustrum coordinates the transition between attention networks and the default mode network. Complex functions such as consciousness and attention require coordination amongst multiple brain areas. The location and size of these functional networks can differ between individuals.
This post is based on Christopher L. Zerr's invited Physiological Psychology lecture at Truman State University. Ten years before, he sat in the same classroom as Dr. Shaffer lectured. Chris is a Psychological & Brain Sciences Postdoc at Washington University in St. Louis. He is a gifted researcher, dedicated mentor, and amazing colleague!
This post covers a small fraction of neuroscience findings for consciousness. The authors focused on the cortex and its role in disorders of consciousness because of the extensive neuroimaging work on these topics. We did not cover auditory and visual awareness studies, a major chunk of consciousness research, to achieve a 16-minute read time.
What is Consciousness?
Consciousness is everything you experience. It is the tune stuck in your head, the sweetness of chocolate mousse, the throbbing pain of a toothache, the fierce love for your child and the bitter knowledge that eventually all feelings will end (Koch, 2018).
William James' Stream of Consciousness
Consciousness, then, does not appear to itself chopped up in bits. Such words as 'chain' or 'train' do not describe it fitly as it presents itself in the first instance. It is nothing jointed; it flows. A 'river' or a 'stream' are the metaphors by which it is most naturally described. In talking of it hereafter let us call it the stream of thought, of consciousness, or of subjective life (James, 1890, 239).
Popular Consciousness Metaphors
Three popular consciousness metaphors are the "tip of the iceberg," a "sea/ocean of consciousness," and a "theater of consciousness."
Most brain processes are not conscious. The limited capacity of the contents of consciousness at any given moment is represented by the “tip of the iceberg.” The vast store of largely unconscious knowledge and representations is not available. However, much of it is retrievable from stable knowledge stores.
Consciousness is the water in which we swim. Like fish in the ocean, we can’t jump out to see how it looks from the outside (Baars & Edelman, 2012).
Neuroscientists have also compared consciousness to a theater. Below is Truman State University's Baldwin Auditorium.
Attention selects salient information for further processing. It is a finite resource. We cannot be conscious of the entire theater at once. Our attentional spotlight "illuminates" part of the stage and gives rise to conscious experience.
A Pessimistic Definition of Consciousness
The term is impossible to define except in terms that are unintelligible without a grasp of what consciousness means. Consciousness is a fascinating but elusive phenomenon: it is impossible to specify what it is, what it does, or why it evolved. Nothing worth reading has been written on it (Sutherland, 1989).
Aspects of Consciousness
Breedlove and Watson (2020) summarized the main components of consciousness. Table © Oxford University Press.
We Can Define Consciousness in Several Ways
First, consciousness is the physiological brain state of wakefulness. This definition compares brain activity between wakefulness and sleep or in unconscious states, such as anesthesia or coma (Purves et al., 2013).
Second, consciousness is our subjective awareness of the world. This abstract definition is more nuanced than wakefulness because one can be awake yet unaware of things in their external and internal environments.
Third, consciousness involves subjective experience and self-awareness. In this definition, we have a sense of being aware of oneself as distinct from other selves in the world. We also have qualia, our singular subjective conscious experiences.
I cannot only feel pain and see red, but think to myself, ‘Hey, here I am, Steve Pinker, feeling pain and seeing red!' Pinker (1997).
Wakefulness and subjective awareness of the world illustrate the easy problem of consciousness. Subjective experience and self-awareness illustrate the hard problem of consciousness.
The Role of the Posterior Hot Zone in Conscious Experience
So it appears that the sights, sounds and other sensations of life as we experience it are generated by regions within the posterior cortex. As far as we can tell, almost all conscious experiences have their origin there. What is the crucial difference between these posterior regions and much of the prefrontal cortex, which does not directly contribute to subjective content? The truth is that we do not know (Koch, 2018).
Graphic © Scientific American.
Stimulating the posterior hot zone can trigger a diversity of distinct sensations and feelings. These could be flashes of light, geometric shapes, distortions of faces, auditory or visual hallucinations, a feeling of familiarity or unreality, the urge to move a specific limb, and so on. Stimulating the front of the cortex is a different matter: by and large, it elicits no direct experience (Koch, 2018).
The Mirror Test
Few animals recognize themselves in mirrors. Self-awareness by this criterion has been reported for:
Note. Left, chimpanzee; right, rhesus monkey.
Researchers can train some species to pass the mirror test. There are several issues with this paradigm. Some animal species may not understand the concept of a “mirror” or recognize it as an objective reflection of reality. Other animals avoid eye contact with other animals as it can be perceived as a threat. However, they touch the dot if it is located lower on the face Canines fail the mirror test, as seen below.
Studying Cognitive Function
We can also compare brain activity across different levels of awareness and consciousness. Can this reveal regions important for producing or supporting attention and consciousness?
In psychology and neuroscience, one of the best ways to investigate a cognitive function is to assess those with impairments or disorders of that cognitive function or to use animal models to produce systematic impairments to see how brain regions are linked to those cognitive functions.
For example, studying hemispatial neglect, reduced awareness of stimuli on one side of space, allows researchers to explore how consciousness disorders impact consciousness and brain activity.
The graphic below (Laureys, 2005) depicts neurophysiological states along two dimensions of consciousness: arousal (horizontal axis) and awareness (vertical axis). Graphic © John Wiley & Sons.