Neuroimaging Findings in Patients with DOCs

The designation DOC is used for patients with disturbed consciousness, attended by impaired verbal and motor responsiveness to diverse stimuli.^{Footnote 10} In most cases, DOC is caused by acute and severe brain injury caused by head trauma, brain hemorrhages, or hypoxic brain injury.^{Footnote 11} If the patient survives the coma and the patient’s clinical situation stabilizes, an unresponsive wakefulness syndrome (UWS) or what was previously referred to as vegetative state (VS) can follow, with spontaneous breathing, alternating states of wakefulness (with open eyes) and sleep (with eyes closed), and with the ongoing absence of any kind of responsiveness.^{Footnote 12,Footnote 13} If reproducible signs of purposeful behavior can be identified in the patient; for example, when the eyes follow a moving object, or the patient begins to follow simple commands, we use the term MCS, with differing grades.^{Footnote 14,Footnote 15} Taken together, we refer to MCS and UWS as DOCs in cases in which we do not give further differentiation. Important to note, however, is another clinical condition, often confused with DOCs: the locked-in syndrome (LIS). In contrast to DOCs, patients with LIS have an intact awareness and cognition, yet motor functions are strongly reduced or completely lost.^{Footnote 16}

The diagnosis of DOCs primarily rests on clinical examination and validated behavioral test instruments (e.g., Coma Recovery Scale-Revised), supplemented by brain imaging and electroencephalography (EEG), the latter of which detects the electrical activity of superficial cortical neurons.^{Footnote 17,Footnote 18} The issue of prognosis in such cases and finding effective treatment for patients with DOCs is extremely difficult.^{Footnote 19,Footnote 20,Footnote 21}

Nevertheless, recent results in advanced neuroscience research could indicate real progress regarding the evaluation of consciousness in patients with DOCs. An increasing number of studies in the field tend to use neuroimaging techniques for DOCs, such as functional magnetic resonance imaging (fMRI).^{Footnote 22} fMRI results are based on the principle of neurovascular coupling, according to which active brain tissue recruits more (oxygenated) blood flow than inactive tissue.^{Footnote 23} Blood oxygen level dependent response is also used in the relatively new technology of functional near infrared spectroscopy (fNIRS). Yet another functional brain imaging method applied in DOC studies is positron emission tomography (PET), which detects the metabolically active areas of the brain with the help of radioactively marked substances (usually glucose).^{Footnote 24} The specific (dis-) advantages of the mentioned neuroimaging techniques in DOC are discussed in the article by Lorina Naci et al.^{Footnote 25}

Some general difficulties arise with applying neuroimaging technologies in DOCs.^{Footnote 26,Footnote 27,Footnote 28} More relevant for our purposes are additional conceptual problems, given that there is little consensus about the nature and components of consciousness in philosophy, thus leaving it unclear as to which understanding of consciousness these empirical studies relate. It is further controversial in the philosophy of mind whether neural correlates give us answers to questions of consciousness at all.^{Footnote 29} Olivia Gosseries et al. point out that neuroimaging studies in DOCs would require a theory of consciousness that describes both the fundamental features of consciousness and sufficient conditions for consciousness.^{Footnote 30} Despite these pitfalls, we describe some relevant aspects of consciousness in the philosophical literature that could be more or less explicitly utilized with neuroimaging studies. Phenomenal consciousness, or the qualitative character of experiences, is one important aspect of consciousness in the philosophy of mind.^{Footnote 31} Another candidate is access consciousness, defined as the subject’s ability to access the representational content of his or her mental states.^{Footnote 32,Footnote 33} Still another aspect of consciousness is self-consciousness; that is, the reflective knowledge of a person experiencing him- or herself as someone who is in a certain mental state, and the ability for self-related reasoning or for conceiving of a future self.^{Footnote 34 Footnote 35,Footnote 36,Footnote 37} In neurological practice, the distinction between arousal (or wakefulness) and awareness seems more important.^{Footnote 38}

Leaving aside objections from the philosophy of mind concerning the relationship between visible neural activity and consciousness and the general difficulties of conceptualizing consciousness, we now can turn to the paradigms used by neuroimaging studies to detect consciousness in DOCs. We can distinguish two main paradigms: active and passive. In passive experiments, the patient has to do nothing in particular, there is no specific instruction; what is relevant is the spontaneous brain activity, or the reactivity to stimuli.^{Footnote 39} The active paradigm requires the subject to perform a specific task.^{Footnote 40} This active paradigm allows one to distinguish between willful brain activation after commands and reflexive brain activity, while bypassing (in usual actions necessary, but in DOCs impaired) motor output.^{Footnote 41} Therefore, results from studies that operate on the active paradigm seem to provide reasonable evidence of consciousness.^{Footnote 42,Footnote 43} In one study, patients with DOC were asked to imagine familiar activities (playing tennis and walking through the rooms of one’s house); some patients showed similar brain activation patterns to those of healthy persons.^{Footnote 44,Footnote 45} The distinct brain activation patterns in the two imagination scenarios could be used as a proxy for yes or no responses. In one case it was already noted that the patient was able to signal answers regarding personal questions using this yes-and-no code.^{Footnote 46} There are also further studies that show a higher brain activation in the auditory cortex if patients with DOCs hear their own name, compared with noises or other (common) names.^{Footnote 47,Footnote 48,Footnote 49} Although several of these studies show methodological limitations, their clinical impact on diagnosis and prognosis for patients with DOC can be immense.^{Footnote 50,Footnote 51} Active tasks or stimuli during neuroimaging could complement the diagnostic procedure and help to reduce misdiagnosis.^{Footnote 52,Footnote 53} Such results could—despite conceptual difficulties—also indicate the existence of more, or of a higher level of, consciousness in some patients with DOCs. Altered neuronal activity in cases when one’s own name is spoken may, for example, indicate evidence for the existence of phenomenal consciousness, or for access consciousness. Moreover, increased neuronal activity during active mental imagery tasks could provide evidence for at least access consciousness, or could even be interpreted as evidence for self-reference and self-consciousness.^{Footnote 54}

Such findings and their interpretation could turn out to be also ethically relevant. Guy Kahane and Julian Savulescu have, for example, argued that results of neuroimaging about the existence of phenomenal consciousness in patients with DOCs could provide a strong moral reason for not saving the lives of patients with DOC.^{Footnote 55,Footnote 56} Carl Fisher and Paul Appelbaum have discussed in detail the broad scientific, legal, and ethical issues that might be related to the latest neuroimaging findings in DOCs.^{Footnote 57} Jakob Hohwy and David Reutens called for more caution when confronted with the question of whether or not to withdraw life support in DOCs, if more consciousness in DOC is probable.^{Footnote 58} They further argued that it would become even more important, in cases of potentially conscious patients, to ensure that suffering is alleviated.^{Footnote 59} Orsolya Friedrich discusses that the knowledge of more intact consciousness in DOCs would influence end-of-life decisionmaking only from the background of certain theories of moral philosophy.^{Footnote 60} In this article we focus more in depth on the question of whether the results of such neuroimaging studies could affect the existence of personal identity in cases of DOCs and, therefore, the moral authority of ADs in patients with DOCs. To be able to address this problem in greater detail, we will briefly introduce the relevant existing debate about ADs and diachronic personal identity.

Moral Authority of ADs and Diachronic Personal Identity

Moral Authority of ADs in DOCs

Following this short introduction to the different options for necessary criteria to define diachronic personal identity, we now turn to which of these criteria of personal identity can be ascribed to patients with DOCs. Patients with DOCs seem to satisfy the physical criterion of personal identity, as they possess a brain that is identical to that which they lived with previously. The issue becomes more difficult if one suggests that the psychological continuity criterion must be satisfied in order to be able to talk about personal identity. Proponents of the non-identity thesis might use that strategy and maintain that the psychological continuity criterion needs to be applied, but is not met. They conclude that because there is no psychological continuity, the resulting lack of personal identity destroys the moral authority of ADs. Various authors have stated that with the level of neurological damage seen in the case of most DOCs, and especially in UWS, personal identity is lost, because the necessary condition of psychological continuity is not present.^{Footnote 105,Footnote 106} If this is true, it depends on the level that we set as necessary for psychological continuity. We can set the threshold of psychological continuity very low; namely, to the degree that is required for persistence, which means every state other than brain death.^{Footnote 107} If we set the limit that low, however, then we lose the difference from the physical criterion of personal identity.

If we regard psychological continuity as the relevant relation between states of personal identity, patients with DOCs present a more or less (depending on the threshold we set) existent challenge in terms of showing a sufficient continuity of states. Patients with DOCs, especially those with UWS, do not show clinical signs of psychological continuity with respect to criteria such as persisting beliefs, desires, intention, or memories. Exceptions are possible with cases in which patients meet these conditions, but in general, patients with DOCs do not possess a rich set of conscious states and do not show enough robust mental states.^{Footnote 108} The topic is far from closed, however, as recent results from neuroimaging studies change the previous picture. For patients with DOCs, the “flickering flame of consciousness”^{Footnote 109} documented in some patients in response to stimuli in new neuroimaging studies could emerge as relevant for psychological continuity, and as a condition of personal identity. Certain neuroimaging results could further be interpreted to suggest that, at least in some cases, patients with DOCs could intentionally reproduce imaginations from their past, recall names, and remember their old environment. If we could conclude with certainty from neuroscientific results that patients with DOCs show signs of intentional mental acting, which is connected to their past experiences, diachronic psychological continuity could be posited.

In light of this new evidence, we may go a step further to ask whether psychological continuity could also be claimed at a phenomenological level, even without necessarily involving higher cognitive capacities that allow for remembering or recalling past images or names. Diachronic unity of phenomenal consciousness seems to be a pivotal factor for psychological continuity. Phenomenal consciousness is characterized by its subjective experiential character, which can be described as its “phenomenal aspect” and a “nonstructural homogeneity” from a neuroscientific perspective.^{Footnote 110,Footnote 111,Footnote 112,Footnote 113,Footnote 114} Neuroimaging studies show that phenomenal unity could still be retained in patients with DOCs, even if representational unity or access unity broke down.^{Footnote 115} The question that remains for patients with DOCs is whether the “flickers of awareness”^{Footnote 116} in patients with DOCs show isolated states of consciousness or in fact offer evidence for a single stream of consciousness that is phenomenally unified.^{Footnote 117} Further empirical studies are necessary to unequivocally determine diachronic phenomenal unity of consciousness.

Depending on the conception of personal identity, psychological continuity shown in neuroimaging results could create a major shift in influencing our ascription of personal identity to patients with DOCs. Findings for patients with DOCs, which indicate that psychological continuity is a given, could justify the moral authority of ADs in patients with DOCs, if the respect for self-determination is followed as a foundation for the moral authority of ADs. Relying on such positive neuroimaging findings, which indicate psychological continuity in patients with DOCs, the non-identity thesis in DOCs could no longer be applied. Such findings could—in addition to conceptual objections—challenge the non-identity thesis, in line with clinical and social practice. Usually it is sufficient in practice if psychological continuity is created by the person at several points in time; it does not have to be present at every moment (e.g., deep sleep, fluctuating mental state). The person should, however, have the potential to always create this continuity. This opens up the option for UWS patients with a good rehabilitation prognosis to regain this continuity in a few weeks although at the moment they clearly cannot realize this continuity.

The general question of the dependence of the moral authority of ADs on diachronic personal identity cannot be solved, nor can the definitive criterion for personal identity be determined conclusively here. However, the recent empirical evidence in patients with DOCs demands a furthering of the ethical discussion, and a move beyond viewing DOCs merely as below-border cases for the non-identity thesis. These results have moral significance, as they provide new possibilities for arguing for the moral authority of ADs in patients with DOCs.