Few subjects generate more argument in neuroscience and philosophy than the nature of consciousness and its function. At one extreme, some philosophers and cognitive neuroscientists regard consciousness as an epiphenomenon — a kind of story people tell themselves after the fact of behavior to make some sense of what’s going on. At the other extreme, the dualistic tradition postulates an inner, indivisible entity (traditionally called the “soul”) that is the ultimate controller of what people do and perceive.
Prior to the 21st century, most neuroscientists avoided even discussing consciousness. Consciousness was thought to be a private phenomenon not accessible to laboratory experimentation like the neural mechanisms that process sensory inputs and generate motor outputs.
The reluctance of cognitive and neuroscientists to discuss and investigate consciousness has changed, however. Much of this is due to the ability to visualize, via brain-scanning technologies such as functional magnetic resonance imaging (fMRI), the differences in brain activity associated with conscious versus unconscious processing of inputs. Consciousness is almost always associated with frontal lobe and thalamic activity triggered by the sensory inputs processed in the parietal or temporal lobe.
Today, scientists know, for example, that humans are unconscious of most of our own brain activity. If a visual stimulus is flashed for a brief period of time (usually followed by a “mask” or noise stimulus that “erases” the sensory afterimage), scientists can see about the same early brain processing activity for flash durations so short that you don’t consciously perceive the image, versus slightly longer durations for which you do. But there is a big difference in the activation of areas of prefrontal cortex, the thalamus, and portions of the parietal lobe for consciously perceived versus unconscious image processing.
Why is activity in these prefrontal cortex, thalamus, and parietal lobe areas associated with consciousness? Apparently, activation of brain areas associated with consciousness creates a global brain state in which all the aspects of the subject of perception, including memories triggered by and associated with that perception, exist in a self-reinforcing loop that dominates the state of the entire brain. Among other things, this brain state enables a type of memory called working memory, which allows complex plans to be formed, such as imagining the sequence of steps necessary to drive home from work.
Only humans can execute complex, learned, hierarchical plan sequences during which subgoals can be switched to find alternate ways of accomplishing these complex plans. Many animals have awareness, but no comparable capability for forming and conducting such complex plans. Consciousness evolved in humans with language, which allows communicating acquired knowledge across generations. It has permitted humans to become the dominant multicellular species on Earth. This view is consistent with the “functionalist” idea that some future computer operating with a similarly powerful architecture could have an equivalent sort of consciousness, because consciousness is a specific type of brain state that could be simulated.