Australasian Science: Australia's authority on science since 1938

Consciousness Revealed

By Tim Hannan

Can consciousness be detected by neuroimaging?

The detection of conscious awareness in individuals who have suffered an extremely severe brain injury has implications for the provision of care, the prediction of the likelihood of recovery, and for decision-making regarding the prolongation of life.

Historically, knowing that someone is conscious has relied on the observation of a behavioural response to a verbal or nonverbal stimulus. However, recent advances in neuroimaging have enabled the identification of patterns of neural activity associated with thoughts, actions and intentions, and this promises to provide the means to detect whether a physically unresponsive brain-injured patient is aware of their environment even though they are unable to indicate this through a purposeful movement or speech.

While formal definitions of consciousness elude neuro­psychologists and philosophers alike, it is generally accepted that it involves not just wakefulness but also awareness of the self and the environment. A person who is aware is necessarily awake; however, one who has suffered an injury to the brain may be awake but not aware.

The criteria of wakefulness and awareness are employed to distinguish disorders of consciousness: coma, the vegetative state, and the minimally conscious state. Following a very severe brain injury, a patient who is neither awake nor aware is determined to be in a coma. In the vegetative state, the patient opens his eyes and demonstrates a sleep–wake cycle but does not appear to display any awareness of his surroundings. The minimally conscious patient displays wakefulness and inconsistent evidence of awareness of his environment.

Traditionally, the test of awareness employed in medical diagnosis and care has been the observation of a predictable behavioural response – such as speech, a gesture or eye blink – to a command or stimuli. The problem with this method is that it is not uncommon for severely brain-injured patients to be unable to move or speak, and thus not able to exhibit identifiably purposeful behaviour. This leaves open the possibility that an apparently vegetative patient may actually be aware but unable to produce an observable behavioural response.

A recent article by Adrian Owen in the Annual Review of Psychology examined whether brain responses to external stimuli can be reliably detected in the absence of observed behaviour. He noted that efforts to explore the neural mechanisms underlying consciousness have utilised various imaging methods, especially positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), as well as monitoring of electrical activity in the brain through electroencephalography (EEG).

Studies with healthy volunteers have demonstrated that fMRI responses can be reliably associated with the performance of specific cognitive operations in response to an examiner’s commands. In the most common paradigm, the examiner asks the participant to imagine performing different tasks, such as hitting a tennis ball or walking through their own home. Results have shown that a participant’s response to the request to wield a racquet was associated with increased activity in the supplementary motor area on fMRI , while the instruction to imagine one’s home elicited activity in the parahippocampal and posterior parietal regions, which are long associated with the performance of spatial tasks.

When this method was utilised with patients diagnosed to be in a vegetative state on the basis of the absence of observable purposeful behaviour, Owen and others found that almost 20% displayed fMRI or EEG patterns that reliably differed across the two tasks in a manner similar to what is observed in healthy volunteers. This finding was interpreted as an indication that the patients were covertly aware of the instructions and able to modulate their brain activity accordingly.

Owen argues that this necessitates a revision of the traditional diagnostic standard for the persistent vegetative state, from sole reliance on the observation of purposeful behaviour to the inclusion of neuroimaging evidence for conscious awareness.

These findings have obvious and significant implications for the diagnosis of the vegetative state, for estimating prognosis, and for end-of-life decision-making. The observation of reliable neural responses in physically unresponsive patients has also offered the hope of establishing basic yes/no communication with brain-injured patients who have some level of conscious awareness.

Tim Hannan is an Associate Professor of Clinical Psychology at Charles Sturt University, and the President of the Australian Psychological Society.