Conscious and unconscious states

Consciousness is usually defined as the subjective feeling of what it is like to have an experience. That is somewhat circular, but we can define consciousness with reference to its absence. There are many mental processes that we think of as being entirely unconscious. I am not conscious of the brain processes that are enabling me to see things in three dimensions. I am not conscious of the processes that contract certain muscles as I move my hands. There is a subset of our mental lives that we are conscious of, and we can communicate that to others. We can experience it in a way that allows us to remember it, to do things with it, to manipulate it and then ultimately tell each other about it.

The word consciousness is often used in a variety of different ways. One useful distinction to make in this field is between the notion of global states of consciousness; when you vary from being in a deep sleep to being anesthetized to being fully awake, you are changing in your global state of consciousness. Even when you are fully awake and alert, there might be some brain processes, some mental processes, that you are unconscious of and others that you are conscious of. Consciousness scientists are interested in both of those notions of the word consciousness, but unfortunately, we have this single word that applies to both. It is really important to keep them conceptually distinct.

Reality monitoring is the capacity of the brain to distinguish between activity that is internally generated — for instance, activity generated by expectations, working memory or imagination — and activity generated by a feature of the outside world. We think one explanation for conscious experience is that consciousness is tracking those brain states that are reliable reflections of the outside world, and it is discarding the ones that are simply internally simulated, generated or due to internal noise. One hypothesis about the functions of conscious experience is that consciousness is tagging certain neural representations as being reliable reflections of the outside world, and this is why consciousness has what philosophers call assertoric force. It forces us to believe in the reality of the outside world — that is its job.

The trick we have in the lab is that on some trials we gradually fade in a real stimulus. That stimulus can either be the same as the one they are trying to imagine, or it can be something different. We find that people readily experience confusions between reality and imagination in this setup. When we ask people to judge whether a stimulus has really been presented, if they happen to be vividly imagining that same stimulus on a particular trial, then they are more likely to say something real was out there. It is as if the signals from imagination and perception are becoming intermixed in the mind. Using functional MRI, we find that this intermixing is taking place in mid-level visual cortex, towards the back of the brain, in an area known as the fusiform gyrus. We then see other regions of the brain in the insula and the medial prefrontal cortex, towards the front of the brain, tracking how strong those signals are, how reliable they are, and using that information to judge whether something is real or imagined. We think that these areas of the brain working together can accomplish reality monitoring.

The brain then faces a difficult problem: it needs to figure out whether activation of those neural populations in the visual cortex is being caused by the real world out there or is somehow internally generated. We think that the operation the brain is trying to engage in is key to understanding distortions in how people confuse imagination for reality, for instance in cases like hallucinations.

If we turn up the dial on these internal processes, we might start activating perceptual areas of the brain in a way that is completely divorced from reality. If, in those situations, we also have distortions in reality monitoring — if people are unable to realize that those internal processes are in their imaginations — then they might think that they are as real as the outside world. This perspective can explain why someone may not only experience hallucinations but also lack insight into the fact that those experiences are hallucinatory. A distortion in metacognition or reality monitoring can lead people to not have the self-awareness that their perceptual experiences might not actually reflect the outside world.

These patients are unable to speak or produce any behavioral response, but if they are placed inside a functional MRI scanner and asked to imagine doing things like playing tennis or walking around the house, some — a small subset — can voluntarily activate circuits in their brains that are normally activated when a healthy person imagines similar experiences. This suggests a residual capacity for imagination, and therefore for conscious experience, in patients who are otherwise entirely behaviorally unresponsive. It shows that, in a small number of cases, obviously with huge clinical importance, there are patients in these states of unresponsiveness who are nevertheless still conscious at some level.

Editor’s note: This article has been faithfully transcribed from the original interview filmed with the author, and carefully edited and proofread. Edit date: 2025