3.2 The Neural Correlates of Consciousness

summary

For every conscious experience, there will be some biophysical mechanisms in the brain that generate it.

Neural correlates of consciousness (NCC) are the minimal set of neuronal events and mechanisms jointly sufficient to generate a specific conscious percept. With modern biomedical imaging techniques neuroscientists are beginning to physically identify these components in the brain.
Stimulating an NCC should result in the generation of a corresponding conscious percept. This can be tested by stimulating (or suppressing) neuronal activity with electricity, magnetic fields, and light.
The only way to locate NCCs and map them to specific conscious experiences is to systematically probe the nervous system and search for any structures or functions that correspond to consciousness.

We see with our minds, not our eyes.

Visual stimulation from the environment is detected with our eyes, but they are not solely responsible for the images we consciously experience. The brain constructs a visual perception of the world which is not necessarily a one-to-one representation of the input to our eyes. This is why we are able to have visual experiences when our eyes are closed or when we are sleeping.
A large part of our visual experience is due to processing in the brain. For instance, our eyes are constantly moving about 3-4 times a second but our visual frame always seems very stable. That is because the visual processing regions of our brain edit out the brief moments of motion in order to create a more stable perceptive experience.
Each of our eyes also has a blind spot, a small portion of the visual field that corresponds to the position of the optic disk (which has no photoreceptors) within the retina. Yet we don’t perceive this “hole” in our visual field, because there are active mechanisms in our cortex that compensate for this and “fill in” the blind spot.
These examples illustrate the fact that our perceptive powers are not as good as we assume and are based on a significant amount of internal interpretation and processing. All of our perception, not just vision, is an active construction of a description of the environment.

Consciousness is likely generated in the cortex.

The cerebellum is a region of the brain that is predominantly involved with fine motor control. Of the 86 billion neurons in the human brain, 69 billion are in the cerebellum, but it is unlikely to be the seat of consciousness.
There are rare cases of patients who lack cerebellums but continue to live predominantly normal lives with full consciousness. One famous case was of a 24-year-old woman whose missing cerebellum only resulted in moderate motor deficiency, mild speech problems, and slurred pronunciation, indicating that consciousness is not associated with the cerebellum.
The cortex, the thin convoluted sheet of neurons on the outer surface of the brain, has been experimentally determined to be the main brain structure that generates consciousness.
It is still not well understood why some regions of the brain, such as the cortex, give rise to consciousness and others, such as the cerebellum, do not.

Not all cortical activity is associated with consciousness.

Despite being the most likely source of consciousness, the cortex does not exclusively generate consciousness. Numerous neuropsychology experiments have shown that other mental activities, like attention, are correlated to cortical activity.
Many studies have successfully dissociated cortical activity related to attention and consciousness, by manipulating both variables separately. Watanabe et al (2011) discovered that in the primary visual cortex, cortical activity was stimulated only by paying attention to a possible target. Whether or not the subjects actually saw the target did not influence activity in that cortical area.
The primary sensory cortices can even activate in patients in persistent vegetative states without giving rise to consciousness, suggesting that these areas do not generate consciousness.
Higher-order parts of the cortex do give rise to consciousness. For instance, the right fusiform face area is a region of cortex that is selectively active when we see faces. Stimulating this region of the cortex with electrical signals will cause people to perceive faces as distorted. This suggests that the right fusiform face area is the NCC that gives rise to the conscious percept of the face.
There is still significant debate about which regions of cortex actually correspond to conscious perceptions. It is clear though that only certain parts of the cortical sheet give rise to experience.