Post-Comatose Disorders of Consciousnesssuch behaviors is strongly correlated with the absence of consciousness. Call this the behavio-
ral assumption.
There are two kinds of problems with the behavioral assumption. The first set of problems
is practical: it is often very difficult for clinicians to determine whether a patient possesses the
capacity to execute the relevant behaviors. Consider a patient who fails to respond to a request
to blink. Although it is possible that the patient fails to blink because she doesn’t possess the
capacity for consciousness, it is also possible that she fails to blink because she is asleep at the
time of examination. The behavioral repertoire of patients depends on their level of arousal, and
arousal in post-comatose patients can fluctuate widely throughout a 24-hour period (Candelieri
et al. 2011). Alternatively, the patient might have failed to blink because, although conscious, she:
was deaf and couldn’t hear the instruction; was aphasic and couldn’t understand the instruction;
or suffered from poor muscle control and couldn’t comply with the command. Assuming that
the patient is in a VS provides one explanation for why she failed to blink on command, yet this
will rarely be the only explanation for her failure to blink, and it may not be easy to determine
which of the various explanations on offer is the most plausible. Even if the patient does blink,
it may be unclear whether the blink was a response to a command or whether it was a purely
reflex response that was unrelated to the instruction (Majerus et al. 2005). A general challenge
here is that the VS is diagnosed on the basis of an absence of evidence of consciousness, thus mak-
ing it highly susceptible to false negatives.
Although careful and repeated clinical assessment over days or weeks can address some of these
problems, it cannot address a second challenge to the behavioral assumption, a challenge that
derives from the subjective nature of consciousness. A conscious organism is an organism that has a
subjective perspective on the world (Nagel 1974). The clinician, of course, has access only to what
a patient can—or, as the case may be, cannot—do. Bedside examinations attempt to bridge the gap
between the objective and the subjective by making assumptions about the relationship between
consciousness and behavioral capacities, but these assumptions can be challenged.
Most discussion of this issue has focused on the worry that the assumptions that are embedded
in the CRS-R are too conservative, and that many of the patients that the CRS-R regards as being
in the VS (and thus unconscious) might actually be conscious. (As we shall see, there are good
reasons to think that this worry is well-founded.) It is also possible that some of the assumptions
that are embedded in the CRS-R are too liberal, and that certain behaviors that are treated by the
CRS-R as markers of consciousness might be produced unconsciously. Consider the relationship
between consciousness and the control of eye movements. Although the CRS-R treats visual fixa-
tion and pursuit as diagnostic of the MCS, it is unclear whether these responses are robust indica-
tors of visual awareness (Spering and Carrasco 2015; Vanhaudenhuyse et al. 2008). In this regard,
it is worth noting that the clinical guidelines for the assessment of patients themselves betray a
certain amount of uncertainty about the relationship between consciousness and the control of eye
movements. Although the CRS-R takes visual fixation and pursuit to be sufficient for a diagnosis
of the MCS, the Royal College of Physicians (2003), to whom Britain and much of Europe defers
for diagnosis, regards these behaviors as consistent with, albeit atypical manifestations of, the VS.
4 From the Beside to the Scanner:
The Command-Following ParadigmIn response to these challenges, researchers have developed methods that aim to detect con-
sciousness on the basis of brain activity. The hope is that such methods could complement
behavioral examination, particularly when a patient’s clinical presentation is ambiguous.
(Indeed, some theorists have even suggested that brain-based methods might eventually displace