http://informahealthcare.com/bij ISSN: 0269-9052 (print), 1362-301X (electronic) Brain Inj, 2014; 28(9): 1156–1163 ! 2014 Informa UK Ltd. DOI: 10.3109/02699052.2014.920523

What kind of consciousness is minimal? Boris Kotchoubey1, Dominik Vogel1,2, Simone Lang3, & Friedemann Mu¨ller2 1

Institute of Medical Psychology and Behavioral Neurobiology, University of Tu¨bingen, Germany, 2Schoen Clinics for Neurological Rehabilitation, Bad Aibling, Germany, and 3University of Heidelberg, Germany

Abstract

Keywords

Primary objective: A comparison between unitary and non-unitary views on minimal consciousness. Research design: First, unitary (all-or-none) and non-unitary (gradual or continuous) models of consciousness are defined as they have been developed in both philosophy and neurophysiology. Then, the implications of these ideas to the notion the minimally conscious state (MCS) are discussed. Methods and procedures: Review and analysis of theoretical conceptions and empirical data. Main outcome and results: Both kinds of models are compatible with the actual definitions of MCS. Although unitary views may seem to contradict the description of the MCS in ‘Neurology’ 2002, the apparent contradiction can easily be solved. Most recent data, particularly those obtained using fMRI and concerning learning, emotional responsiveness and pain and suffering, speak for non-unitary models. Conclusions: Most evidence speaks for non-unitary models of minimal consciousness. If these models are correct, patients with MCS may have, in addition to temporal fluctuations, a lower level of consciousness compared with fully conscious individuals. A still lower level could characterize patients diagnosed as unresponsive wakefulness syndrome (UWS). From this point of view, therefore, the difference between UWS and MCS is gradual rather than qualitative. However, due to methodological limitations of the available studies, the evidence for nonunitary models cannot be regarded as definite.

Consciousness, emotions, fMRI, global workspace, learning, levels of consciousness, MCS, VS/UWS. History Received 21 July 2013 Revised 11 February 2014 Accepted 18 February 2014 Published online 25 July 2014

Abbreviations: ACC, anterior cingulate cortex; AI, anterior insula; GW, global workspace; MCS, minimally conscious state; PET, positron emission tomography; VS/UWS, vegetative state/ unresponsive wakefulness syndrome; WGC, weighted global connectivity

Introduction Descartes is famous by his theory that strongly separated the soul from the body. Less known, however, his dualism drew not one but three different lines of demarcation [1]. The first line was between the pure (extended) matter and the pure (non-spatial) mind. Presently this division is held by a negligible minority of neurologists, but the other two are much more viable. The second borderline separated cognition from emotion. Since emotion cannot be completely separated from bodily changes, only cognitive processes can really be designated as mental. This borderline can well survive when the other one fails; somebody may not believe that the mind is fully different from the brain, but still believe that only cognition constitutes the essence of mind. This conception was criticized by Damasio [2]. The present study is interested in the third Cartesian demarcation, which separates consciousness from other (unconscious) processes that now also Correspondence: Boris Kotchoubey, Institute of Medical Psychology and Behavioral Neurobiology, University of Tu¨bingen, Silcherstr., 5, 72076 Tu¨bingen, Germany. Tel: +49 7071 2974221. E-mail: boris. [email protected]

count to mental processes [3]. Again, this dichotomy is independent of the other two. In order to believe that conscious and unconscious mental processes are radically different from each other (third demarcation) you neither need to believe that some of these processes are independent from the brain (first demarcation), nor that conscious and unconscious processes are related to cognitions and emotions, respectively (second demarcation).

Arguments for the unitary nature of consciousness The idea that consciousness is something radically different from the rest of the world proved to be the most viable of all Cartesian dichotomies. Many classical philosophical writers, from Kant [4] to Sartre [5], believed that there cannot be anything between consciousness and the lack thereof. Also the broadly spread branch of philosophy, which regards consciousness as a function of language, implies hard-drawn borderline between verbally mediated consciousness and nonverbal unconscious mind [6]. In psychology, the main evidence for the unitary view was accumulated within the framework of the global workspace (GW) theory [7, 8]. Consciousness restricts the context and

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removes ambiguities, has a limited capacity, clear priorities, is characterized by a strongly consecutive mode of operation and builds a basis for multi-level control manifested in metacognition, self-programming and self-monitoring. Contrary to it, unconscious processes are tolerant against ambiguities, have very large capacities and diffuse priorities. These processes work in many parallel modules; they are lacking the ability to self-control [7, 9]. The GW theory regards consciousness as a ‘place’ of common access and global broadcasting for all information processing mechanisms in the brain [7, 8]. Roughly speaking, the many specialized processing mechanisms of the brain, working in relative independence of each other, can be compared with a huge working room, in different parts of which larger or smaller groups of experts are simultaneously working on a solution of many different problems. There is, however, a big blackboard in the room that can be seen by all people wherever they are sitting. The information written on this board is available for all of them. This blackboard is consciousness. The place on the blackboard is limited and if one group is able to put its own task there, the task (and the respective group) attracts everybody’s attention and, thus, gets a status of higher priority as compared with all other problems being solved during this time. Therefore, different groups continuously compete for access to the board and ‘the winner takes all’, albeit for a limited time. This blackboard metaphor is complemented by a theatre metaphor [10]. Numerous actors (sensory data of various modalities, memories, ideas) compete for the possibility to enter the scene, which, again, is observed by the members of unconscious audience of language, motivation and automatic behaviour. From the present point of view it is important to note that, while the blackboard metaphor presumes a clear ‘all-or-none’ decision of being conscious or not, the theatre metaphor appreciates the possibility of grades, because different parts of the scene are differently illuminated by a projector light. The spotlight has a central part, in which the experiences are fully aware, and a fringe containing ‘dark feelings’. The GW theory maintains, however, that the difference in grade is a function of attention, not of consciousness. All contents placed on the theatre’s scene are conscious, although they may be lightened by the beam of attention in a different degree [10, 11]. The psychological idea of global workspace has in the last years been corroborated by a continuously increasing amount of neurophysiological data. Already in the early 1980s, Grossberg [12] related conscious perception to a simultaneous and synchronous ignition of sensory brain areas by the bottom-up sensory stimulation, on the one hand, and by topdown activation from the remote high-level processing regions, on the other hand. Later on, Grossberg [13] generalized this approach into a broader theory of neurophysiological underpinnings of consciousness. In this vein, important neurophysiological findings were obtained indicating that conscious experience may be specifically related to re-entrant activations from higher-level brain regions back to primary and secondary sensory cortices [14–16] (see, however, severe methodological critique in King et al. [17]). The corresponding theoretical models were further supported by the data of calculations and simulations [18, 19].

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Furthermore, Edelman and Tononi [20] identified a system for integration of broadly distributed brain processes into a large-scale network of re-entrant loops. Being broadly integrated, conscious brain activity in such systems remains highly differentiated in the sense of a large repertoire of possible states, which is in contrast with the conditions of large-scale integrated brain activities possessing a very limited number of possible states such as epileptic petit mal or slow-wave sleep [21]. The idea of the synchronous longrange connectivity as a necessary pre-requisite of conscious experience was supported by a number of experimental data using different neurophysiological techniques [22–25]. One cannot deny the differences and controversies between the approaches briefly summarized above. However, all these approaches converge on the statement of major qualitative differences between conscious and unconscious ways of brain functioning. Their data confirm the idea of the GW theory [8–11] that consciousness is characterized by global access to information processing contents in contrast to local access typical for unconscious or pre-conscious [14] processes. A more radical version of this view maintains that local information processing of whatever ‘highest’ executive level (goal-setting, self-monitoring, complex language comprehension) remains unconscious as long as no global access on the basis of broadly distributed long-range connections is provided [26].

The state of unstable consciousness When the notion of minimally conscious state (MCS) was first introduced in the late 1990s [27], it might appear to contradict the above notion of consciousness. Thus, ‘episodes of crying that are precipitated by family voices only’ ([27], p. 109) might indicate that the respective patient’s consciousness is altered either quantitatively (e.g. awareness is so weak that only very intensive or very significant stimuli are consciously perceived) or qualitatively. On the other hand, different contents of the GW are equally conscious and neither quantitative nor qualitative differences among them can exist (except the quantitative differences determined by the spotlight of attention). In the seminal report of the Aspen group [28], this ambiguity was not resolved completely. Table 1 of Giacino et al. [28] mentions ‘partial consciousness’ in MCS, in contrast to ‘none’ in coma and VS/UWS and ‘full consciousness’ in the locked-in syndrome. The description of the critical feature of MCS remains; however, observation of ‘definite but inconsistent signs of conscious behaviour’ such as following simple commands, gestural or verbal yes/no responses, intelligible verbalization, gaze fixation and other signs of purposeful (non-reflexive) behaviour. The idea is repeated in different terms, but remains the same, e.g. ‘in MCS, cognitively mediated behaviour occurs inconsistently, but is reproducible and sustained long enough to be differentiated from reflexive behaviour’ ([28], p.351). Minimal consciousness, from this point of view, is the same consciousness, but is inconsistent in time, as shown in Figure 1(A). This ambiguity of the term MCS remained in its translations. Thus, the first translations to German were equivalent to the ‘state of minimal consciousness’ (e.g. http://www.medi

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Figure 1. (A) Minimally conscious state from a unitary point of view. The space of awareness is sub-divided into two domains: presence (1, light domain) or absence (0, dark domain) of consciousness. The xaxis presents consecutive time intervals. In each time interval, a patient with MCS (solid line) can be either conscious or not, in an ‘all-or-none’ fashion. In contrast, a patient in the vegetative state (VS/UWS) would be always in condition 0 and a wake conscious patient always in condition 1 (these simple cases are not shown in the figure). (B) Rather than two different domains (light vs. dark, i.e. consciousness vs. the lack thereof), the model implies a continuum of grey shades between the black of a loss of consciousness and the white of lucid awareness. A patient with MCS (black solid line) is characterized by strong state fluctuations. A patient diagnosed VS/UWS (white solid line) is characterized by fluctuations of a lower amplitude. A dashed line between the lighter and darker areas is not a borderline between conscious and unconscious states, but, rather, a threshold of the present methods for detection of consciousness.

cinebook.de/load.php?name¼News&file¼article&sid¼1432; http://www.dradio.de/dlf/sendungen/wib/948906/). Later on, this was corrected to an equivalent of the ‘minimally conscious state’. In the former case, the adjective ‘minimal’ is applied to the noun ‘consciousness’, while in the latter, it characterizes ‘state’ (because there is only one consciousness, which can be either present or not, but no other, ‘minimal’ form of it). To sum up, the notion of MCS is not incompatible with the GW theory and other unitary models of consciousness, as it might appear on the first glance. One need not think that consciousness in patients with MCS is really ‘minimal’ or reduced as compared with other conscious individuals. It is just unstable, that is, most of the time, patients are unconscious. However, when they are conscious, their consciousness is the same, being characterized by global broadcast of information contents.

Non-unitary views The conception of consciousness as an ‘all-or-none’ phenomenon is not, however, free from problems. At the empirical level, the data demonstrating clear-cut differences between conscious and unconscious brain information processing may simply be artifacts of the experimental methodology, in which only two conditions are compared, one of them representing conscious awareness and the other one the lack of it (e.g. strongly masked vs. unmasked stimuli; conscious patients vs. coma patients; wakefulness vs. deep anaesthesia; etc.; for a critical analysis, see Daltrozzo et al. [29]). At a conceptual level, although the unitary view accepts the existence of

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subjective experience in animals [30], integration of animal consciousness into the GW context requires an additional assumption that animal consciousness is simply human consciousness minus verbal reports. This would be in conflict with a common intuition that animal consciousness is, in some important respects, ‘less’ than or qualitatively different from human consciousness. The unitary models may also have difficulties in explanation of consciousness in small children. Do newborns have the same global access to their conscious experience as adults? If not, are they conscious at all? Some time ago, many child surgeons believed that, due to the under-development of critical brain structures, newborns could not experience pain and that their pain cries were merely physiological reactions unrelated to any subjective feeling [31]. Facing these problems, some authors developed alternatives to the unitary approach. Thus, Dennett [32] maintains that there is no such thing as ‘consciousness as such’ and, thus, no specific brain processes related to conscious experience. Instead, there are multiple patterns of processes related to those criteria, which are momentarily used as indicators of conscious experience, e.g. a motor (overt) or a bodily (covert) response according to a verbal instruction; an immediate or delayed verbal report; a record in short-term or long-term memory; the ability to select among several stimuli or responses; etc. Another model [33, 34] is less radical. It appreciates the existence of specific neural correlates of consciousness and agrees that recurrent and re-entrant neural loops are probably a necessary condition of conscious experience. The model distinguishes, however, between phenomenal experience and access consciousness, e.g. one can experience red without knowing that one is experiencing red, that is, without having access to this experience. An example is the experiments of subliminary priming, in which subjects are presented with a prime word (e.g. serious) and given the instruction to complete the following word stem (ser__), but to avoid using the prime word. When the prime is consciously perceived, subjects readily find different words for completion (e.g. service or server), but they consistently use the prime serious when they assert that they were unable to perceive this prime. This fact can be regarded as a purely unconscious priming effect, which does not have any direct implication for a theory of consciousness. However, if subjects are not only instructed to avoid using the primes but also consistently punished for such using, they do not use them even if the primes are not perceived consciously [35]! This may indicate that subjects phenomenally perceived the word serious, but that this perception was so weak that it did not get access to be broadcast for the whole cognitive system [34]. The two presumable levels of consciousness, i.e. phenomenal experience and conscious access, are related to two respective sets of different neural correlates. Phenomenal consciousness is based on recurrent loops between higherand lower-level areas within sensory cortical regions, while global conscious access is based on long-range recurrent loops including the parietal and frontal cortex. Furthermore, the global access may also have different degrees [33]. The model, therefore, can easily integrate the graduate approach. If some conscious contents are globally broadcast and some

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others are not broadcast at all, there also may be contents that are broadcast for a limited space. Similar ideas have been developed not only by philosophers but also by neurologists. Damasio [36] distinguished three hierarchical levels of consciousness, called protoconsciousness, core consciousness and extended consciousness. Core consciousness is mainly a function of medial brain structures such as the superior colliculus and the cingular cortex, while extended consciousness emerges in the complex loops of the neocortex. Similarly, Panksepp [37, 38] distinguished between affective consciousness (raw emotional feelings, largely of subcortical origin) on the one hand and neocortical cognitive awareness on the other hand. From this point of view, adult persons’ emotional awareness, including the ability to verbalize one’s emotional state, is a fact of cognitive and not affective awareness. Panksepp et al. [39] explicitly related this distinction to clinical issues. They did not mention MCS but suggested that even patients diagnosed as VS/UWS can retain certain elements of primary conscious (mainly affective) experience. They indicate that ‘the common failure not to distinguish among the evolutionary layers of consciousness can lead to many conceptual conundrums and communicative confusions’ and that if one regards consciousness as a unitary cognitive entity, one eliminates, simply by definition, ‘affective experience from the potential residual mentality of patients [with VS/UWS]’ [39]1. Of course, different non-unitary models of consciousness diverge from each other even stronger than different unitary models do. Nevertheless, all former models differ from the latter ones in that they admit different kinds or grades of awareness. Most generally, all of them distinguish between some ‘lower-level’ and ‘higher-level’ forms of consciousness even if they can radically disagree in what should be called ‘higher’ and ‘lower’, what might be the neurobiological substrates of these putative levels and whether the borders between the levels are solid or fluent. The existence of shades of grey rather than the contrast between black and white constitutes the least common denominator of all non-unitary views.

Implications for minimal consciousness If non-unitary models of consciousness are valid, pictures of minimal consciousness emerge, which are completely different from the model presented in Figure 1(A) above. One of them is illustrated by the schema in Figure 1(B). Notwithstanding strong fluctuations, an MCS patient presented in this figure remains in a zone of limited awareness. The dashed horizontal line in the middle of the grey surface is not a real borderline between the presence and the absence of conscious awareness, but, rather, the threshold of the actual methods designed to detect awareness; thus different methods would imply different position of this line. A white curve shows the fluctuating states of another patient who, as can be seen, never attained the dashed line. This patient would, therefore, be diagnosed as VS/UWS, while the patient represented by the black curve is diagnosed as MCS. Importantly, the model presented in Figure 1(B) does not ignore strong fluctuations of the state of consciousness in both VS/UWS and MCS. It only maintains that, despite the

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different diagnoses, the two patients differ rather quantitatively than qualitatively. If, as in Figure 1(A), the borderline separating patients with some instable signs of awareness (i.e. MCS) from those with no signs of awareness (i.e. VS/UWS) is real, all reliable methods of the assessment of consciousness should indicate the same borderline. If, as in Figure 1(B), the reality is gradual and the borders between the presence and the absence of consciousness are determined by the sensitivity of specific methods, different methods should result in divergent, rather than convergent, findings. During the last years, several lines of evidence have been accumulated, indicating that patients with both MCS and VS/ UWS might retain some lower functions of consciousness after the loss of its higher functions. One of these lines concerns learning. Scho¨nle and Schwall [41] were the first to demonstrate the ability of patients with VS/UWS to habituation, that is to elementary learning. Although the learning process detected by these authors happened at the level of brain stem, later on habituation of the N1 component of auditory evoked potential was also detected in many patients with the same diagnosis, indicating simple cortical learning [42]. Furthermore, several patients with the diagnosis VS/ UWS or MCS exhibit an even more complex form of learning, trace conditioning, during which subjects have to retain the conditioned stimulus in the working memory for several seconds [43]. According to the presently prevailing view, trace conditioning is a conscious process requiring participation of the hippocampus [44, 45]. Unfortunately, the hippocampus hypothesis has not been tested to date in patients with severe disorders of consciousness. The learning data can be criticized. A skeptic may say that habituation in the auditory cortex does not require consciousness, while trace conditioning, being found in only a few patients, does not indicate any low level of consciousness but simply that these patients were fully conscious (that is, they were misdiagnosed). These arguments, however, are hardly applicable to the second line of evidence regarding pain stimulation. Klein [46] was probably the first who noticed that pain-related brain structures are typically not destructed in VS/UWS and that this fact raises doubt about the presumed inability of such patients to experience pain. Two studies using radioactive 15O positron emission tomography (PET) revealed significant activations in all examined patients with VS/UWS in the thalamus and/or primary somatosensory cortex in response to pain [47, 48], but no activity was detected in the higher level regions of the pain matrix [49]. Another PET investigation did, however, obtain pain-related activation in higher-order cortical association areas in all seven examined chronic patients with VS/UWS [50]. A larger sample of 30 patients was examined in the only published to date fMRI pain study in VS/UWS [51]. Although very weak noxious stimuli (electrical shocks of 5 mA, 200 milliseconds duration) were applied in this study, activations in various parts of the pain matrix were obtained in 16 (53%) patients, including 14 patients with activations in higher-order sensorydiscriminative pain structures (the secondary somatosensory cortex, the cerebellum and the posterior insula) and nine patients with activations in the typical affective portions of the pain matrix (the anterior insula, AI; and the anterior cingulate cortex, ACC). The large number of patients

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Figure 2. An example of a brain response to electrical pain stimuli (vs. rest) in a male patient, 47 years old, anoxic brain injury following ventricular febrillation 6 years before examination. Despite a very severe brain atrophy, activations were present in the thalamus, the cerebellum, the anterior insula and the primary and secondary somatosensory cortex.

demonstrating significant pain-related responses in critical brain structures makes it implausible that all of them were misdiagnosed (Figure 2). Note that the patients of the two last studies [50, 51] were of non-traumatic, mainly anoxic origin, many of whom were more than 1.5 years post-ictum. However, in the larger study [51], patients with a shorter disease duration (3 months) and a higher motor score (i.e. flexion withdrawal) in the Coma Recovery Scale-Revised [52] were associated with higher brain reactivity to noxious stimuli. The third line of evidence is about emotional stimuli. Many patients with VS/UWS appear to respond to highly significant affectively meaningful stimuli [53, 54], but such responses are scarcely documented by experimental evidence. Although a few cases of reliable responses to social tactile interaction [55], familiar music [56, 57], familiar faces [58], affective prosody [59, 60] or the voice of a patient’s mother [61] have been reported in such patients, one should still wait before generalization of these findings. A serious problem may be the idiosyncrasy of such stimuli, which should be individually selected or, possibly, prepared for each individual patient. This peculiarity of many affective stimuli complicates their use in well-controlled studies with large patient samples. Kotchoubey et al. [62] first described a significant EEG response to her own name in a young patient with MCS, distinct from the response to another word presented with the same frequency. The patients’ own names were further used in several studies in patients with the diagnoses VS/UWS [63, 64], MCS [63, 65] or coma [66]. The results of these studies have not been convincing because the patients’ responses, even if significant, differed from the typical responses of healthy individuals. Thus, event-related brain potentials in

patients revealed an enhanced Mismatch Negativity component or other frontal negative components instead of the large parietal positivity characteristic for control subjects. Such responses do not constitute evidence for lower-level conscious perception because they can also be produced without any conscious awareness. Emotional cries indicating pain and suffering elicit in normal subjects significant fMRI-responses in all aspects of the pain matrix; the responses involve even the somatosensory cortex, although the stimulation is in the auditory rather than somatosensory modality [67]. Yu et al. [68] applied such stimuli in a sample of 44 patients with the diagnosis of VS/ UWS. Overall, brain activations similar to those in control subjects were obtained in 24 (55%) patients, including 10 patients with significant responses in the ACC, which is probably the highest level in the hierarchy of affective stimulus processing [69]. An example is shown in Figure 3. A sub-set of these patients (n ¼ 6) was then compared with six patients with MCS using the technique of Weighted Global Connectivity (WGC [70]). This method differs from other measures of brain functional connectivity, being completely theory-free and data-driven. Whereas the pattern of local responses in various emotional brain structures did not differ between VS/UWS and MCS, Figure 4 shows that extended long-range networks of functional connectivity were found in MCS and in the healthy control group, but not in the VS/UWS sample [25]. These results remind about the abovecited hypothesis [34] according to which local recurrent networks may be sufficient to bring about phenomenal experience, while long-distance global connectivity is necessary for conscious access. If this analogy is valid, at least some patients with VS/UWS might be suggested to

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Figure 3. An example of a brain response to exclamations of pain and suffering (vs. neutral sounds of human voice) in a female patient, 70 years old, 3 months after an intracerebral haemorrhage. The diagnosis during the fMRI examination was VS/UWS, changed to MCS 6 weeks later.

Figure 4. Pattern of weighted global connectivity in disorders of consciousness. The connectivity in six VS/UWS patients (blue in the color version; light grey in the grey version) is restricted by locally active brain areas such as the auditory and the insular cortex. The connectivity in six MCS patients (orange in the color version, dark-grey in the grey version) is characterized by longdistance connections involving prefrontal and parietal areas. The figure is based on the data from Ref. [25].

experience negative emotions when presented with highly unpleasant cries of pain and suffering; but patients with MCS can go beyond this and have a conscious access to their emotional states. On the other hand, the connectivity data indicate the necessity to be cautious when interpreting activation findings. Although many patients show typical affective responses in higher-order processing structures such as the AI and the ACC [51, 68], these responses do not strictly prove conscious experience (of pain and negative emotions, respectively), as far as they are not involved in the broad long-range cortical–subcortical loops [26]. The connectivity analysis, in turn, has its own serious limitations. The actual version of the WGC technique employed in Kotchoubey et al. [25] is still highly sensitive to anatomical brain abnormalities. Thus, only a limited number of patients with relatively mild dystrophy of brain tissue could be included in the analysis.

Conclusions Two principally different approaches to the notion of conscious awareness determine different concepts of minimal consciousness in patients with severe disorders of consciousness. A unitary approach conceives of consciousness as an allor-none phenomenon; brain information processes are either clearly conscious or simply unconscious, but nothing in between. This approach has several important implications. Firstly, accumulating evidence of a lack of consciousness indicates that a patient probably does not have any conscious experience. Secondly, what may appear as minimal consciousness (e.g. in patients with the diagnosis MCS) is a result of strong temporal fluctuations between the conscious and unconscious states. Thirdly, there is a true qualitative difference between the MCS, in which patients are fluctuating between awareness and the lack of it, and the VS/UWS, which implies constant and complete unawareness. Fourthly, if a

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novel method permits one to detect consciousness in a patient with the diagnosis VS/UWS [71], the diagnosis was just an error and the patient is not in VS/UWS. Non-unitary approaches imply, to the contrary, that there may be several or even many transition stages between full awareness and full unawareness. The consequences for patients with severe disorders of consciousness are opposite to those of the unitary approach. Firstly, even very strong evidence that a patient is completely lacking some kind of consciousness (e.g. cognitive awareness) does not prove or even indicate that he or she has lost other kinds of consciousness (e.g. experience of pain and pleasure). Secondly, patients with MCS and, perhaps, some patients with VS/UWS are characterized by a kind of really minimal (‘decreased’, ‘low-level’) awareness, in addition to consciousness fluctuations. Therefore (and thirdly), the difference between MCS and VS/UWS is fluid; this is a difference in degree rather than a difference between two states. Instead of viewing disorders of consciousness in terms of distinct and separate categories (VS/UWS vs. MCS) a dimensional approach might be more helpful. Fourthly, detection of low-level consciousness in a patient with the diagnosis VS/UWS does not automatically imply a diagnostic error, because it is possible that the level of consciousness found in the patient was so low that no clinical-neuropsychological method is able to reveal it. Some recent data indicate the possible existence of a lower-level conscious experience in patients completely lacking higher-level awareness, thus speaking for non-unitary models of consciousness. Each of the respective studies, however, has some limitations, thus alternative explanations for each finding cannot be completely ruled out on the basis of the extant data. The controversy between unitary and nonunitary approaches is, therefore, open for further studies. The critical issue remains that of empirical criteria for the putative ‘low levels’ of consciousness. Since these levels are, almost by definition, non-verbal, no criterion that relies on verbal behaviour or responses to verbal instructions can be used.

Acknowledgements The authors appreciate the substantial contribution of Tao Yu, Alexandra Markl, Niels Birbaumer, Christian Schwarzbauer and Susanne Merz.

Declaration of interest The study was supported by the German Research Society (Deutsche Forschungsgemeinschaft, DFG, Grant Ko-1753/10 for B.K. and S.L.), and the European Commission (Grant DECODER).

Note 1. Pages of the quotations cannot be given because the article was published online only.

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