Imagine you just woke up from a deep, dreamless sleep. Fuzzy at first, you suddenly become aware of your surrounding, your body, your reality.
We say at this point that you are conscious. But although familiar and intimate to all of us, consciousness remains one of the most puzzling phenomena in science.
How does the electric and chemical activity in your brain produce your subjective experiences; the redness of red, the taste of chocolate or the pain in your back?
So far, science hasn’t provided a plausible explanation of how these subjective qualities (called qualia) are produced by the brain.
Instead of directly tackling this “hard problem” of consciousness, neuroscience has focused on identifying the neural correlates of consciousness (NCC). These are the neural events associated with conscious experience.
Neural correlates of consciousness
Like waves, brain activity oscillates. These oscillations can be measured by neuro-imaging techniques such as EEG (where electrodes pasted on the scalp detect electrical charges of brain cell activity), MEG (a technique that maps brain activity by recording the magnetic fields of electric currents) and fMRI (that measures brain activity by detecting changes associated with blood flow), across time scales that range from milliseconds to seconds.
Early studies measuring electrical activity of the brain with EEG in sleep and awake states found that fluctuations in the awake conscious state are small and fast (alpha oscillations between 8 and 12 Hz) in comparison to the big, slow delta oscillations (between 0.25 and 4 Hz) in deep sleep, when subjects lose consciousness.
But the change of these oscillations (from fast alpha to slow delta waves) may not reflect the whole picture of what is happening in your brain when you lose or regain consciousness.
fMRI studies in the resting awake state have revealed that the low-frequency fluctuations (<0.1Hz) between distant parts of the brain are actually synchronised, forming distinct patterns of correlation across the brain. The shape of these correlation patterns actually changes when we lose consciousness.
Imagine each of these patterns as a building block making up the changing brain activity patterns, just like musical notes make up a melody. When characterised in terms of these building blocks, the dynamics of the conscious brain are composed of a richer, more flexible repertoire of correlation patterns compared to the brain during sleep or under anaesthesia.
Studying brain dynamics during loss and recovery of consciousness through these correlation patterns can give us a better understanding of its neural correlates and reveal the signature of consciousness. But why do we even need to find this “signature”?
Disorders of consciousness
Besides genuine curiosity in understanding the brain’s inner workings and the nature of consciousness, there is an urgent clinical need to understand and accurately diagnose disorders of consciousness.
After several weeks in coma – a state where patients are unconscious and unable to be aroused to consciousness by stimuli – most either die or transition into what is called a vegetative state.
Here they don’t show any behavioural signs, not even opening their eyes, and are thought to be unconscious. But recent findings show that a subgroup of patients previously diagnosed as being in vegetative state are actually minimally conscious.
This means they show inconsistent but discernible, non-reflexive behaviours, such as sustained visual fixation or responses to a verbal order, although they are still unable to communicate.
Current diagnostic methods based on observing the patient’s behaviour have led to 41% being misdiagnosed. Such a misdiagnosis could cause the patient to suffer, create legal and ethical dilemmas or even end a conscious person’s life.
A 2006 study clearly demonstrated a case of such misdiagnosis. The authors asked a 23 year old woman in a vegetative state to imagine playing tennis and walking through the rooms of her house while her brain activity was scanned using fMRI. Her activity showed similar patterns to that of healthy adults who imagine playing tennis or navigating through their houses.
Although this study pioneered the use of functional imaging to diagnose disorders of consciousness, it had one major limitation. It required a patient’s active (mental) participation and response to a command, such as “imagine playing tennis”.
But the absence of a response from the patient does not imply he is unconscious. He may simply be failing to perform the task while being conscious.
Absence of evidence
In an alternative method of diagnosis, electromagnetic pulses are sent through the scalp while the complexity of the brain’s response to these pulses is evaluated with EEG. In the awake state, these pulses lead to more complex and longer lasting changes in brain activity compared to when consciousness is lost in sleep, anaesthesia or coma.
Although this method eliminates the need for a patient’s active participation, it requires a new and not always readily available setup of transcranial magnetic stimulation (TMS) as well as a compatible EEG device.
So studying changes in resting state brain activity remains important in the search for signatures of consciousness without asking patients to perform tasks (such as imagining playing tennis) or stimulating the brain with external pulses (such as using TMS).
These tests are a major step in the developing science of consciousness and provide an important diagnostic tool. But we have to remember the absence of evidence is not evidence of absence.