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. There is evidence that consciousness is generated by cortical top-down processing. Three cortical gradients correspond with dimensions of consciousness, including arousability, awareness, and sensory organization. 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.
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Credit
This post is based on Christopher L. Zerr's invited Physiological Psychology lecture at Truman State University. He sat in the same classroom as Dr. Shaffer lectured 11 years before. Chris is a Psychological & Brain Sciences Postdoc at Washington University in St. Louis. He is a gifted researcher, dedicated mentor, and amazing colleague!
Disclaimer
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." Graphic © Alones/Shutterstock.com.
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.
Ocean graphic © somavarapu madhavi/Shutterstock.com.
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).
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).
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:
(1) land mammals: apes (chimpanzees, bonobos, orangutans, and gorillas), elephants, and pigs
(2) cetaceans: bottlenose dolphins, killer whales, and possibly false killer whales
(3) birds: magpies, pigeons (can pass the mirror test after training in the prerequisite behaviors; Breedlove & Watson, 2023; Chang et al., 2017)
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.
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).
Neuroanatomy Refresher
Anatomical terms can be long and confusing, but they try to help you by giving you location information. Pay attention to complementary pairs. An area specified as dorsal typically has a counterpart of ventral. Dorsal areas closer to the top are often used in more top-down cognition, whereas ventral areas are often in bottom-up situations.
Superior and inferior are another complementary pair. The superior colliculus is located above the inferior colliculus.
Neuroscientists assign different names to the same areas, especially for function and structure. For example, the Dorsal Attention Network (DAN) is also called the Dorsal Frontoparietal Network (D-FPN), and the Default Mode Network (DMN) = Medial Frontoparietal Network (M-FPN).
An analogy for thinking about functional brain networks. We have superimposed a map of the Truman State University on the brain.
What is a possible function of a “network,” including Centennial Hall, Missouri Hall, and Ryle Hall? Eating food!
However, like brain regions and networks, these dorms are functionally important for more than just eating food at the dining hall. They also support sleeping, studying, socially interacting, and pregaming.
There is greater network activation for functions like eating, sleeping, resting, and studying but greater deactivation for going to class, the library, and leaving campus for breaks.
Functional brain networks are brain regions that exhibit correlated activity when performing functions like attention.
There Are No Task Areas
A functional brain area is not a task area. There is no "tennis forehand area" to be discovered. Likewise, no brain area is devoted to a very complex function. "Attention" or "language" is not localized in a particular Brodmann area or lobe. Tasks or "functions" utilize a complex and distributed set of brain areas (Petersen & Fiez, 1993).
Cortical Regions Implicated in Attention
Attention is selecting stimuli for enhanced processing and analysis. The dorsal attention network mediates voluntary attention. The intraparietal sulcus (IPS) encodes a salience (priority) map that controls voluntary attention shifts. The frontal eye field (FEF) directs gaze following cognitive goals (top-down processes) rather than stimulus characteristics (bottom-up processes).
The ventral attention network mediates reflexive attention. The temporoparietal junction (TPJ) is involved in reflexive (bottom-up) shifts to a new location after a target appears, especially if the stimulus is unexpected (right-hemisphere lateralized). The ventral frontal cortex (VFC) closely communicates with the amygdala and often detects salient and behaviorally relevant stimuli, especially when unattended.
Voluntary attention depends on a dorsal stream from the frontal cortex to the IPS.
Reflexive attention depends on a ventral stream from the visual cortex to the TPJ.
Connections between TPJ and IPS allow novel stimuli to interrupt and reorganize attentional priorities.
In order to smoothly integrate overall control of attention, the networks underlying voluntary attention and reflexive attention need to interact extensively and operate as a single coordinated system.
Subcortical Regions Implicated in Attention
Based on functional connectivity analysis, subcortical structures are critical to DAN and VAN functioning. These structures include brainstem nuclei, the superior colliculi, the pulvinar, and the large anterior head of the caudate nucleus (Alves et al., 2022). Thalamus graphic © Songkram Chotik-anuchit/Shutterstock.com.
Neural Correlates of Consciousness (NCC)
Neural correlates of consciousness (NCC) consist of the activations and deactivations associated with conscious states. For example, the feeling of the pressure of the chair you’re currently sitting in is pressing against your body. The sensory information coming from your legs was no different 10 seconds ago than it is now, yet without attending to it, you weren’t aware of it.
Vegetative State
When awakened, vegetative state patients exhibit reflex behavior with eyes open.
Despite no evidence of consciousness, the brain of someone in a vegetative state generated appropriate activity patterns to simple verbal commands.
A 23-year-old woman was uncommunicative for five months following a car crash. Staff assumed that she was not consciously aware of anything because she showed no signs have consciousness. They asked this unresponsive woman to imagine playing tennis or walking around the house. . Her brain's spatial navigation regions activated to about the same amount as healthy controls performing the same task.
This was very surprising and has fueled debate surrounding life-support patients who appear unconscious for years. It showed that behaviorally, even if someone does not seem conscious, the brain can still attend to stimuli (e.g., speech) in the external environment, and perform certain tasks. This was an earthshattering finding for consciousness researchers that has also influenced unresponsive patients' quality of care. Physicians originally thought that they had no sense of the world around them. However, some unresponsive patients are consciously aware but can't convey that overtly.
Coma
The fMRI scans for those in vegetative and minimally conscious states reflect more activity than predicted based on their behavioral diagnoses.
Now, hospitals typically scan these individuals and administer fMRI tests like this. They may assign an additional diagnosis based on the fMRI evidence that may be different than their behavioral diagnosis. These patients eventually recovered consciousness, communication, and functional independence, highlighting that prognostic estimates of consciousness can be poor at predicting future outcomes.
Locked-In Syndrome (LIS)
LIS patients are fully paralyzed. They can typically only move their eyelids, sometimes their eyes. However, they are fully conscious and aware. LIS typically occurs when the pons in the brainstem is damaged, often from a stroke. Many LIS cases have occurred during chiropractic manipulations that damage arteries in the spine and neck and lead to stroke.
Surprisingly, LIS patients have self-reported mental health, personal and general health, and bodily pain levels close to healthy controls (Luié et al., 2009).
A LIS patient, Jean-Dominique Bauby, had suffered a stroke at age 43 but wrote a book about his experiences using only his left eyelid and 200,000 blinks.
fMRI scans of different consciousness disorders reveal surprising amounts of brain activity during TASKS, but what about during REST?
The Default Mode Network (DMN)
A network supporting a “default mode” of brain function that is engaged in the absence of any particular cognitive task. The DMN spans the Ventral Medial Prefrontal Cortex (VMPC), Dorsal Medial Prefrontal Cortex (DMPC), and Posterior Cingulate Cortex (PCC).
The DMN is highly activated when people are awake and aware but are not pursuing any particular goal, thought, or task. The DMN is typically active when we direct attention inwards, as when we daydream, think about our day, and allow our minds to wander.
The DMN is highly deactivated when we engage in tasks or pursue goals and direct attention externally (e.g., learning, attending to stimuli, and reading).
The DMN is sometimes referred to as a “task-negative network” (TNN) because of decreased activity (deactivations) during tasks.
Despite patients with consciousness disorders demonstrating sufficient brain activity in response to tasks and stimuli, they typically display little-to-no DMN activity during rest (Guldenmund et al., 2012).
During attention-demanding tasks, the DMN deactivates, so it is task-negative. In contrast, attentional networks activate, so they are task-positive. The activity of these networks is anti-correlated (Fox et al., 2005).
Author note. The task-positive networks include the following cortical regions bilaterally: anterior insula/frontal operculum, supplementary motor/dorsal medial frontal lobe, lateral premotor cortex (includes frontal eye fields), anterior middle frontal gyrus, superior parietal lobule/anterior inferior parietal lobule, lateral inferior posterior temporal gyrus (lateral area 37). There is evidence that the more lateral components of the task-positive network, particularly the dorsolateral frontal and parietal cortices are more important for externally directed attention, while the medial components of the default-mode network participate in internally directed attention
Abnormalities in DMN Functioning
Portions of the DMN are overactive in those diagnosed with anxiety, presenting with increased rumination, and depression.
The DMN is underactive in patients with consciousness disorders (vegetative state, coma), Alzheimer's disease and dementia, and epileptic seizures.
In addition to cognitive and memory impairment in Alzheimer’s disease and dementia,
impairments in consciousness and one’s own subjective sense of self may be just as prevalent.
For those who have loved ones with Alzheimer’s, you may notice they sometimes no longer seem like the same person you once knew.
The DMN is overactive and underactive in schizophrenia and is associated with hallucinations and delusions (Menon, 2011).
The Claustrum
The claustrum may be implicated in conscious experience. Electrical stimulation of the claustrum can interrupt a person’s conscious awareness. In one case study, a woman’s conscious awareness was instantaneously switched off when a current was passed through an electrode planted in the region of the claustrum and instantly restored when the electrode was turned off.
The claustrum is bidirectionally connected with many cortical areas and is one of the most highly connected brain structures by volume.
More broadly, the claustrum may help facilitate functional network transitions between and modulate task-positive and task-negative states. The claustrum may help ramp up attention as tasks become more difficult.
Dysfunctions in DMN activity are also related to decreased claustrum volume (Nikolenko et al., 2021).
There are several reasons for limited fMRI studies involving the claustrum. It is often hard to locate. Because it is subcortical, researchers need scanners with stronger magnets (i.e., higher Tesla ratings), which are becoming increasingly available. Third, the subtractive approach typically used in fMRI experiments creates the illusion that the claustrum is inactive when it is active during both tasks.
Combining Neural Correlates of Consciousness
Attention networks allow us to perceive and attend to the world around us. The DMN allows us to perceive and attend to our internal world. potentially a subjective sense of self-awareness.
The claustrum (task-rest interactions) may support conscious experience by moderating our perception, attention, and cognition of the internal and external worlds.
Top-Down Control of Consciousness
Research by Li and colleagues (2023), which incorporated behavioral, neuroimaging, electrophysiological, and transcriptomic data, indicated that the overall state of consciousness involves top-down hierarchical processing. Histaminergic neurons in the ascending reticular activating system appear to play an integral role in top-down cortical information processing.
We Can Map Consciousness Along Three Dimensions
Graphic © Gordodenkoff/Shutterstock.com.
“Consciousness is complex and studying it is like solving a scrambled Rubik’s cube,” said Zirui Huang, Ph.D., research assistant professor in the University of Michigan Medical School Department of Anesthesiology. “If you look at just a single surface, you may be confused by the way it is organized. You need to work on the puzzle looking at all dimensions.” (University of Michigan Medicine)
Huang and colleagues (2023) mapped consciousness along three dimensions. The team created a map of cortical gradients of consciousness by utilizing fMRI data from individuals who were conscious, under anesthesia, in a coma-like state, or diagnosed with psychiatric conditions such as schizophrenia.
They organized recordings from 400 distinct brain regions into gradients and examined how these gradients altered in relation to different states or diagnoses. The team identified three cortical gradients that appeared to correspond with dimensions of consciousness, specifically arousability, awareness, and sensory organization.
Limitations in Studying the Neural Correlates of Consciousness
Consciousness disorders tend to be unique between individuals – cases and prognoses can be very different. Many fMRI studies use small numbers of patients or case studies. Functional brain networks also tend to be quite variable between people, so tying one person’s network to their particular consciousness disorder can be difficult (Laumann et al., 2015).
Summary
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. Complex functions such as consciousness and attention require coordination amongst multiple brain areas and involve top-down cortical processing. The location and size of these areas can differ between individuals.
Glossary
attention: selecting stimuli for enhanced processing and analysis.
bottom-up: reflexive attention in response to stimulus properties that depends on a dorsal stream from the frontal cortex to the IPS.
consciousness: an awareness that we are conscious and perceiving activity in our minds and environment.
default mode network (DMN): task-negative circuit, active when we are reflective and deactivated when we attend to the environment.
dorsal: toward the top.
dorsal attention network: a circuit including the intraparietal cortex (IPI) and frontal eye fields (FEF) that mediates voluntary attention.
easy problem of consciousness: explaining how neural activity patterns create specific conscious experiences.
frontal eye field (FEF): a structure that directs gaze following cognitive goals (top-down processes) rather than stimulus characteristics (bottom-up processes).
functional brain networks: brain regions that exhibit correlated activity when performing functions like attention.
functional connectivity analysis: assessing the statistical dependencies between specific physiological signals detected from anatomically distinct regions over time.
hard problem of consciousness: explaining the neural basis of our subjective experience of consciousness called qualia.
hemispatial neglect: a syndrome caused by right inferior parietal cortex damage in which attention is paid to the left side of the body or to things presented to that side.
inferior: below.
intraparietal sulcus (IPS): a structure involved in top-down attention control. The IPS encodes a salience map to guide voluntary attentional shifts.
locked-in syndrome (LIS): a disorder caused by damage to the pons in which patients are both fully paralyzed and conscious.
minimally-conscious state: when awoken from a coma with eyes open, patients show reflex behavior, pain localization, and non-reflex movement.
mirror test of consciousness: the ability to recognize one's reflection in a mirror.
neural correlates of consciousness: the activations and deactivations associated with conscious states.
pulvinar: posterior thalamic nucleus that helps to process visual stimuli, orient and redirect attention, and filter stimuli.
qualia: subjective conscious experiences we cannot fully communicate to others.
reflexive attention: bottom-up attention guided by the ventral attention network.
stream of consciousness: a metaphor for the continuously changing elements of consciousness (e.g., ideas, sensations, and emotions). James argued that we cannot understand a coursing river by studying a pail of its water.
superior: above.
superior colliculus: the midbrain structure that directs attentional eye movements.
task-negative: deactivates during an attentional task.
task-positive: activates during an attentional task.
temporoparietal junction (TPJ): a region that is involved in reflexive (bottom-up) shifts to a new location after a target appears, especially if the stimulus is unexpected (right-hemisphere lateralized).
top-down: voluntary attention that depends on a dorsal stream from the frontal cortex to the IPS.
unresponsive wakefulness syndrome/vegetative state (UWS/VS): Patients only show reflex behavior when awoken from a coma with eyes open.
ventral: toward the bottom.
ventral attention network: a circuit including the temporoparietal junction (TPJ) and ventral frontal cortex (VFC) that mediates reflexive attention.
ventral frontal cortex (VFC): a region that closely communicates with the amygdala and often detects salient and behaviorally relevant stimuli, especially when unattended.
voluntary attention: top-down attention guided by the dorsal attention network.
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