Side-box 1.1 Knowledge of another’s state of mind

In a behaviourally unresponsive individual, how do we distinguish whether they are in a permanent coma, persistent vegetative state, or fully conscious but with motor paralysis (locked-in syndrome)? An interesting new technique for detecting consciousness has been proposed by Owen et al. (2006). They verbally asked a ‘vegetative’ patient to imagine herself playing tennis or walking round the rooms in her home. Functional MRI imaging showed that the same areas of brain lit up as in normal individuals performing the same two tasks. The authors’ conclusion is that since she was deliberately cooperating so ‘beyond any doubt she was consciously aware of herself and her surroundings’.

The logic here is clever; a degree of intention seems intuitively to be involved, and the cortical areas activated were those expected given the nature of the imagery requested. However, it is surprising how sophisticated processing can be without consciousness: it may include symbolically-cued attention (Kentridge et al., 1999) and decision-making (Bechara et al., 1997). So how much linguistic analysis of verbal instructions and preparation for motor output might be possible too without consciousness? It also seems counter-intuitive, but nevertheless empirically demonstrated, that ‘normal’ large rises in cerebral blood flow can be evoked by sensory stimuli unaccompanied by consciousness (e.g. Goebel et al., 2001; Avidan et al., 2005). Additionally, the baseline level of brain metabolism in vegetative state can be more than 50% below normal (Laureys et al., 2004). Therefore a small rise in blood flow (<0.4% in Owen et al.’s patient) does not necessarily indicate the presence of enough extra neural activity to generate consciousness (although this argument relies on the intuitive assumption that there is a definite and absolute threshold for the amount required: see section 7.3.2). Finally, Owen et al.’s patient showed changes in blood flow that were smaller than for all but two of the 12 control subjects. The healthy volunteers’ responses ranged from 1% down to zero, so finding a rise in their patient that was marginally above those of two normals who showed negligible rises (in the spatial navigation condition), does not justify the authors’ assertion that ‘Her neural responses were indistinguishable from those observed in healthy volunteers’. So, despite this overstatement, the project to detect consciousness (as opposed to mere neural activity) by using brain scanners remains open.

References

Avidan, G., Hasson, U., Malach, R. and Behrmann, M. (2005) Detailed exploration of face-related processing in congenital prosopagnosia: 2. Functional neuroimaging findings. Journal of Cognitive Neuroscience 17, 1150-1167.

Bechara, A., Damasio, H., Tranel, D. and Damasio, A.R. (1997) Deciding advantageously before knowing the advantageous strategy. Science 275, 1293-1295.

Kentridge, R.W., Heywood, C.A. and Weiskrantz, L. (1999) Attention without awareness in blindsight. Proceedings of the Royal Society of London, Series B 266, 1801-1811.

Laureys, S., Owen, A.M. and Schiff, N.D. (2004) Brain function in coma, vegetative state, and related disorders. Lancet Neurology 3, 537-546.

Owen, A.M., Coleman, M.R., Boly, M., Davis, M.H., Laureys, S. and Pickard, J.D. (2006) Detecting awareness in the vegetative state. Science 313, 1402.