A large and coherent body of evidence now shows that cortex and its resonant satellite, the thalamus, jointly constitute the brain basis of conscious experiences.
However, the states of waking, sleep and REM dreaming are switched on and off by basal brain nuclei that project neurochemicals to the cortex and thalamus.
This evidence comes from:
1. direct brain recordings of both reportable and unreportable input
2. Brain imaging studies including fMRI, MEG, PET, etc.
3. Various deficits in conscious perception and cognition,
4. Local brain stimulation and inhibition, either via the sensory pathways or by electrical, magnetic, cooling, chemical, and other precisely localized interventions.
5. Pathological conditions, as in “silent” (unconscious) ischemia compared to severe ischemic pain;
6. Comparisons between waking-state stimulation vs. Slow-wave (delta) sleep, coma and vegetative states, epileptiform loss of consciousness, syncope (fainting), and general anesthesia;
7. Direct recordings of the C-T oscillatory system in animals and humans. Recently voltage-sensitive study whole-cortex activity in rodents.
These methods show that the cortico-thalamic system underlies the states and contents of consciousness, in contrast to cerebellum, basal ganglia and other subcortical regions. However, to make that case clearly, we must consider a distinction between brain activities that show direct correspondence to conscious experiences, compared to those that do not correspond directly with reported experiences.
Direct Correspondence with Reported Contents
A damaged brainstem reticular formation (BRF) can abolish the state of consciousness, and therefore its contents as well. But the BRF shows no correspondence to specific contents of consciousness — such as color perception, fine visual resolution, or visual scene comprehension.
In contrast, for vision we can observe specific cortical “feature maps” whose activities correspond well to conscious contents, as reported under optimal conditions. For vision, final gestalt formation may take place in medial temporal lobe.
In sum, many lines of evidence converge to show that cortex underlies specific conscious percepts, in resonance with corresponding thalamic nuclei.
There are a few qualifications:
1. Brain regions that do not show direct correspondence can still influence the C-T system, and therefore indirectly influence conscious contents. A classical example is the cerebellum, which constant interacts with the C-T system. But cerebellum does not have neuronal feature fields whose activity patterns correspond to reported experiences. The same point applies to basal ganglia, motor tracts, amygdala, etc. The amygdala responds to fearful visual pictures, but the interoceptive feeling of fear depends on the body maps of the anterior insular cortex. Similarly, the vestibular (balance) system does not emerge in consciousness directly, though it shapes visual, sensorimotor, and interoceptive experiences.
2. Neurological impairments like visual neglect show that visual maps of the parietal cortex do not yield reportable conscious events. However, these “reaching” maps do shape direct perception of visual feature maps in the ventral stream.
3. Paleocortex enables conscious olfaction and taste. It includes hippocampus and rhinal regions, and evolved before the mammalian neocortex and thalamus. It has somewhat different visible anatomy. However, paleocortex and neocortex have strong similarities and are highly integrated in mammals.
4. “Feelings of knowing” (FOKs) may reflect conscious gestalt formation in non-sensory cortex. A famous example is the “tip of the tongue” state, which shows BOLD activity in prefrontal cortex.
Consciously Mediated Cognition
Stan Franklin has proposed the term “consciously mediated” for cognitive processes that come in and out of consciousness. In perceptual experiments we commonly present isolated stimuli, but in natural conditions, conscious moments emerge as part of a “stream of consciousness,” a series of intertwining topical threads that come to consciousness only intermittently, much like an internet chat room. Overlearned processes tend to be unconscious, using highly practiced automatisms and memory traces. Conscious cognition is needed for unpredictable, novel, significant, or effortful aspects of thinking.
A well-studied example is unconscious amygdala activation in response to fearful pictures. A conscious picture of a snake may trigger unconscious fear-related activity in amygdala, with very widespread activation of emotional, neurohormonal, and social-cognitive processing, while the conscious feeling of fears appears to require activation of the anterior insula. The conscious feeling of fear can then evoke a wide variety of other unconscious and conscious brain events, including fight and flight, but also rational planning.
Experimental evidence for the stream of consciousness comes from thought-monitoring studies, beginning with Jerome Singer and John Antrobus some fifty years ago.
These methods have been important in understanding clinical conditions like post-traumatic stress disorder.
Voluntary Control
Conscious events can evoke voluntary actions on request, notably accurate event reporting, memory storage, sensorimotor control, mental problem solving, and emotional reactions. Voluntary control governs a remarkably wide range of motor and neural activities. Recent intracranial recordings in waking patients shows that even single temporal lobe neurons can be controlled, on request, using conscious feedback from a firing cortical neuron. How the brain selects one among 86 billion neurons to control, given consistent conscious feedback, is not known.
While these indirect effects of conscious moments are obviously important, our focus here is on the direct brain basis for conscious experiences, when a correspondence can be shown between neuronal feature maps and accurate report.
[Excerpted from Part I – Cortex enables conscious experiences: The cortico-thalamic system in “On Consciousness: Science & Subjectivity.”]
Global Workspace Theory (GWT) began with this question: “How does a serial, integrated and very limited stream of consciousness emerge from a nervous system that is mostly unconscious, distributed, parallel and of enormous capacity?”
GWT is a widely used framework for the role of conscious and unconscious experiences in the functioning of the brain, as Baars first suggested in 1983.
A set of explicit assumptions that can be tested, as many of them have been. These updated works by Bernie Baars, the recipient of the 2019 Hermann von Helmholtz Life Contribution Award by International Neural Network Society form a coherent effort to organize a large and growing body of scientific evidence about conscious brains.
