The Routledge Handbook of Consciousness

Post-Comatose Disorders of Consciousness

as the P300b response—that can be detected using EEG. In light of these findings, a number of
research teams have looked for evidence of the P300b response in post-comatose patients. King et
al. (2013) found a global effect in 14% of 70 VS patients and 31% of 65 MCS patients, and similar
findings have been obtained in a number of other studies (Bekinschtein et al. 2009; Faugeras et al.
2011; Chennu et al. 2013).
The fact that these methods do not require command-following has important implications.
Passive paradigms allow for the evaluation of consciousness in patients who may be conscious
but are unable to follow commands due to the high cognitive load imposed by command-
following tasks. Passive paradigms might also allow for the assessment of particular cognitive
capacities that are not recruited by command-following paradigms. The capacity to follow the
narrative structure of a film requires a number of cognitive capacities, and information about
which patients retain these capacities may be very useful for attempts to improve quality-of-life
in patients (Naci et al. 2016).

6 The Puzzle of Behavioral Inactivity

Although debate continues about the evidential force of the data surveyed in the previous two
sections, it is increasingly difficult to deny that significant numbers of patients who qualify as VS
according to current clinical guidelines are covertly conscious. This finding raises the puzzle of
(behavioral) inactivity: given that such patients are conscious, why do they not produce purposeful
motor behavior (Shea and Bayne 2010)? Locked-in syndrome patients raise no such puzzle, for
we know that they are paralyzed. By contrast, VS patients are not paralyzed. Those who ascribe
consciousness to putatively VS patients need to explain why they do not produce overt behavior
in the way that MCS patients do.
Klein (2017) has suggested that this puzzle might be solved by appealing to the idea that
such patients have motivational deficits: although they can generate motor responses, they are
simply not motivated to do so. In defense of this idea, he notes that there is a close anatomi-
cal relationship between the VS and akinetic mutism, a condition in which patients do not act
unless prompted to do so by an external stimulus. Klein’s proposal is helpful in that it might
explain why some patients fail to act. However, it cannot explain why those patients who com-
ply with commands to generate mental imagery do not also comply with commands to execute
motor responses.
A recent study by Fernández-Espejo et al. (2015)^1 may provide evidence against Klein’s
proposal. The study examined the connections between the thalamus and primary motor cor-
tex in two post-comatose patients, one of whom was VS according to behavioral criteria but
showed evidence against command-following, and one of whom was MCS. The white matter
tracts projecting from the thalamus to the motor cortex were damaged in the former patient but
not in the latter. It is plausible that these pathways are necessary for producing motor responses
but are not required for mental imagery. Damage to these pathways would explain why a
patient who was able to sustain covert command-following was overtly unresponsive. Such a
patient might have intentions to produce overt behavior, but would be unable to implement
those intentions.

7 Validation and Taxonomy

The research surveyed in this chapter promises to reshape how we think about the recovery of
consciousness following severe brain injury, for clinicians now have compelling reason to supple-
ment the behavioral assessments of patients with EEG and neuroimaging (Coleman et al. 2009).