Unity of consciousness

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Tim Bayne (2009), Scholarpedia, 4(2):7414. doi:10.4249/scholarpedia.7414 revision #52128 [link to/cite this article]
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Curator: Tim Bayne

At any one point in time we typically enjoy a rich phenomenal perspective. I can currently hear the music of Charles Mingus, see these words on my computer monitor, taste and smell the olives that I am eating, and sense a slight ache in my shoulders. Arguably, each of these experiences (and more) contributes to my overall conscious state—it plays a role in fixing what it is currently like to be me. These experiences are not merely had by me, they are had by me in a certain kind of way: as unified with each other in a single phenomenal perspective.

The issues raised by the unity of consciousness cluster around three headings. What forms can the unity of consciousness take? To what degree is consciousness unified? How might we explain the unity of consciousness?


The Varieties of Unity

Although it is common to speak of the unity of consciousness, there are in fact a variety of ways in which experiences can be unified. This section surveys some of the central forms of unity to be found within consciousness. I focus here on the unity of consciousness at a single time and leave to one side the unity of consciousness as it holds across time—that is, its continuity.

Subject Unity

One form of the unity of consciousness concerns the fact that conscious states are had by subjects of experience. We can describe conscious states that are had by the same subject of experience as ‘subject unified’. Your visual and auditory experiences are subject unified with each other, but neither of these experiences is subject unified with my visual or auditory experiences. What precisely subject unity amounts to depends to a large degree on what subjects of experience are, but we can agree that subject unity plays an important role in structuring consciousness without agreeing on how to conceive of subjects of experience. Closely related to subject unity is the sense that self-conscious subjects have of being a single subject of experience (Rosenthal 2003). We might think of this as the unity of subjectivity.

Representational unity

A second kind of unity to be found within consciousness concerns its representational content. It is a matter of some controversy whether all conscious states have representational content, but certainly many do. One’s visual experience might represent a black dog standing in front of a cluster of trees, one’s auditory experience might represent the barking of the dog and the approach of an ambulance, and one’s bodily experience might represent one’s hands as being raised above one’s head. The representational content of consciousness involves both what we might call object unity and spatial unity. Experiences are object unified insofar as they represent objects as unified entities. For example, my visual experience of the dog and my auditory experience of the dog are object unified in so far as they are directed at one and the same object. My visual and auditory experiences are spatially unified in that their intentional objects are presented as occurring within a common space; for example, I might experience the ambulance as moving towards the dog. More generally, one sees, hears and feels objects as bearing determinate spatial relations to each other and to oneself.

Access unity

The contents of consciousness are typically available to a range of consuming systems—systems that drive belief-formation, voluntary agency, verbal report, memory consolidation, and so on. For any set of conscious states, we can ask whether the contents of those states are available to the same consuming systems. We can say that conscious states are access unified when, and only when, their contents are available to the same consuming systems. If a pain experience is access unified with an olfactory experience, then any consuming system that has access to the content of the experience of pain will also have access to the content of the olfactory experience and vice-versa.


Each of the unity relations described thus far captures an important sense in which consciousness is unified, but there is a fundamental sense of unity within consciousness that we have not yet identified. As we noted in the introduction, a person’s simultaneous conscious states are typically contained unified within an overall phenomenal ‘perspective’ or ‘field’. My experiences of hearing the music of Mingus, seeing words on a computer monitor and tasting olives do not occur in isolation from each but occur together, as components of a phenomenal whole. States that are unified in this way are said to be either ‘co-conscious’ (or ‘phenomenally unified’).

Although co-consciousness is often treated as a primitive, a number of theorists have attempted to give accounts of it (see Tye 2003; Dainton 2006). One such analysis proceeds in terms of a subsumption relation between conscious states (Bayne and Chalmers 2003). This analysis holds that conscious states e1 and e2 are co-conscious exactly when there is a conscious state (e3) that subsumes them both. The subsumption relation can itself be understood in various ways, but at a minimum we should suppose that e3 captures the phenomenology of both e1 and e2: what it is like to be in state e3 guarantees what it is like to be in e1 and e2. On this approach, we might think of unified conscious states as parts, aspects, or components of those states that subsume them. It is an open question whether the subsumptive analysis can itself be understood in more primitive terms. Some theorists have argued that co-consciousness must be taken as a primitive unity relation (Dainton 2006). Others have suggested that co-consciousness can be analysed in representational terms—that it goes together with the closure of content under co-instantiated conjunction (Tye 2003; Hurley 1998). Exactly how best to understand co-consciousness is a matter of on-going debate.

A second issue raised by co-consciousness concerns its logical structure. Here, the literature has focused on whether co-consciousness is a transitive relation, at least where simultaneous states are concerned. (A relation R is transitive when it is such that aRb and bRc entails aRc.) Take three particular experiences: e1, e2 and e3: if e1 and e2 are each co-conscious with e3, must e1 and e2 be co-conscious with each other? Some theorists answer this question in the affirmative (Dainton 2006); others answer it in the negative (Lockwood 1989). The resolution of this debate has important implications, both for our understanding of co-consciousness and for the question of whether consciousness is necessarily unified. To that topic we now turn.

To What Extent is Consciousness Unified?

There is little consensus as to how much unity there is within consciousness. Some theorists hold that consciousness is rarely unified to any considerable degree, others hold that although consciousness is typically unified (in humans at least) this unity can break down in the context of pathologies of consciousness, and still others hold that there is some sense in which consciousness is necessarily unified. Although there are substantive disagreements here, it is difficult to disentangle them from terminological disagreements, for even a cursory glance at the literature reveals that a large number of things have been meant by the claim that consciousness is or is not unified. Some forms of unity can clearly be lost from consciousness, but other forms of unity may be deep, and perhaps even necessary, features of consciousness.

Let us begin with subject unity. There is a sense in which subject unity itself cannot be lost, for conscious states must always be the states of a particular subject of experience. However, there are various pathologies of consciousness in which the subject's sense of being a unified subject of experience is disrupted. For example, patients suffering from the schizophrenic disorder of thought insertion will describe thoughts as being put into their mind by an alien agency. Certain aspects of the sense of being a unified subject of experience may also be undermined in the depersonalization syndrome, for patients suffering from depersonalization often report that it is as if their experiences are no longer their own.

Pathologies of consciousness also provide us with examples in which aspects of the representation unity of consciousness are compromised. In apperceptive agnosia, the normal experience of objects as unified wholes is lost. Patients experience the various features of visually presented objects, but are unable to synthesize those features into representations of unitary objects. In certain types of out-of-body experience perception may lose the spatial unity that it normally has, for subjects report that they experience the world from discontinuous spatial locations.

What about access unity? Many theories of consciousness take access unity as a kind of theoretical ideal, assuming that once performance limitations of various kinds are taken into account the contents consciousness will be equally available for cognitive and behavioural consumption. It is very much an open question whether this assumption is justified. At the very least, certain experimental findings put pressure on it. We have space here to mention only Marcel’s (1993) results. Marcel presented subjects with a light for 200 ms, and required subjects to report the onset of the light in three ways at once: by blinking, by pressing a button, and by saying ‘yes’. Surprisingly, he found that subjects often gave inconsistent responses. For example, the subject’s positive button-pressing response might be at odds with his negative verbal response.

Perhaps the most intense debate regarding the question of whether consciousness is unified has concerned the relationship between subject unity and co-consciousness. According to one conception of the unity of consciousness, it is not possible for a subject to have simultaneous conscious states that are not co-conscious. Bayne and Chalmers (2003) call this ‘the unity thesis’. Let us examine some objections to the unity thesis. In order to simplify discussion I will assume that subjects of experience are organisms.

One cluster of objections to the unity thesis concern dissociative phenomena such as automatic writing, fugue states and multiple personality (since known as Dissociative Identity Disorder). These phenomena received detailed examination by Alfred Binet, William James and Morton Prince in the 19th century, and were commonly thought to show that “in certain persons, at least, the total possible consciousness may be split into parts which coexist but mutually ignore each other” (James 1890: 206). The tradition of appealing to dissociative phenomena as evidence against the claim that consciousness is necessarily unified continued into the 20th century, with Ernest Hilgard arguing that the unity of consciousness is lost under hypnosis. However, the interpretation of these syndromes is extremely challenging (see Braude 1995). It is clear that dissociation involves various kinds of representational and access disunities, but it is debatable whether it ever involves the simultaneous existence of two separate streams of consciousness in a single subject (Bayne 2007).

Since the 1970s, discussion of the unity of consciousness has been dominated by the commissurotomy (or split-brain) syndrome. The split-brain operation involves sectioning the corpus callosum in order to treat epilepsy. Although the procedure has little impact on cognitive function in everyday life, carefully controlled research seems to show that split-brain patients have a divided consciousness, at least in certain environments. In a typical split-brain experiment, the word ‘key-ring’ might be presented so that ‘key’ falls within the patient’s left visual field and ‘ring’ falls within the patient’s right visual field. The contralateral structure of the visual system ensures that stimuli projected to the left visual field are processed in the right hemisphere and vice-versa. When asked to report what she sees the patient will say that she sees only the word ‘ring’, yet, with her left hand the patient may select a picture of a key, ignoring pictures of a ring and a key-ring.

Many theorists endorse the two-streams model of the split-brain, according to which split-brain patients have two streams of consciousness, one in each hemisphere (e.g. Sperry 1984). It is this duality of consciousness which is thought to explain: (i) why the patient appears to have a conscious representation of ‘key’ and ‘ring’ but no representation of ‘key-ring’; and (ii) why the patient’s representation of ‘key’ and ‘ring’ are available to different consuming systems. Two-streams accounts of the split-brain are well-equipped to explain the behavioural disunity that patients exhibit in laboratory conditions, but they struggle to account for the unity that split-brain patients exhibit in everyday situations. Some two-stream theorists attempt to explain this unity by adopting the duplication gambit, according to which the split-brain patient has behavioural integrity because their two streams of consciousness have the same contents (Davis 1997; Moor 1982). Other two-streamers hold that the patient’s stream of consciousness is normally unified and divides into two only in laboratory conditions (Marks 1981; Tye 2003).

Another model of the split-brain holds that such patients have a fragmented stream of consciousness, in that they have triples of conscious states (e1, e2 and e3) such that e1 and e2 are both co-conscious with e3 but not with each other (Lockwood 1989). This model attempts to account for both the unity and disunity exhibited by split-brain patients. However, as we noted above, it is controversial whether consciousness can fragment in this way.

A third model of the split-brain holds that consciousness in the split-brain switches between hemispheres (Bayne 2008). A central line of evidence for this approach derives from research conducted by Levy (1977) and Trevarthen (1974) involving chimeric stimuli (that is, similar stimuli joined at the vertical mid-line). Since each hemisphere receive information about a different stimulus one would expect the subject to produce conflicting responses if both left-hemisphere and right-hemisphere representations were conscious, but no such conflict was observed. Patients gave one response on the vast majority of competitive trials, and the non-responding hemisphere gave no evidence that it had any perception at all. In light of these findings, one might suppose that consciousness in the split-brain ‘switches’ between hemispheres, and that at any one time the split-brain patient has but a single stream of consciousness. If this model is correct, then perhaps consciousness is always unified, even in those human beings in whom the corpus callosum has been severed.

Explaining the Unity of Consciousness

Of the many complex issues surrounding the unity of consciousness, explanatory issues are arguably the most obscure. Perhaps the only point that is clear is that we should not be looking for a single explanation of the unity of consciousness: not only are there different forms of the unity of consciousness, particular types of unity might demand a variety of explanations. As Hurley points out, the unity of consciousness might demand both personal and sub-personal levels of explanation (Hurley 1998).

The task of explaining object unity has been pursued under the heading of the binding problem: what is it that binds the various experiential features of an object together into a representation of a unitary object (Revonsuo 1999; Triesman 1996; 2003)? There is at present no-consensus as to how the binding problem ought to be solved; indeed, there is little consensus about the nature of the problem itself. It is also controversial whether the binding problem might be related to wider questions concerning consciousness itself. Although it has often been assumed that the mechanisms of object binding are intimately related to the mechanisms responsible for consciousness itself, convincing arguments for this claim are rather thin on the ground.

What about co-consciousness? How might we explain why it is that a person’s simultaneous experiences are usually (if not always) mutually co-conscious? Here it is useful to contrast atomistic theories of consciousness with holistic theories of consciousness. Current theories tend to take an atomistic or ‘building block’ (Searle 2000) approach to consciousness. Rather than account for the subject’s entire phenomenal field at once, they account for only particular conscious states—a pain, a visual experience, a conscious thought—on a case-by-case basis. Atomistic accounts of consciousness posit one mechanism responsible for making mental states conscious and another for making them co-conscious. There is no shortage of proposals for what the first mechanism might be, but theorists have been noticeably reluctant to tackle the question of what might bind experiential states together. Note, importantly, that this kind of binding problem is distinct from the content-based binding problem discussed in the previous paragraph. Instead of positing one mechanism responsible for consciousness and another that is responsible for the unity of consciousness, holistic theorists invoke a single mechanism responsible for both making mental states conscious and unifying them. According to such models, consciousness is created as a single global state of which the various conscious states—bodily sensations, thoughts, perceptual states, affective states, and so on—are components or abstractions. There are various ways in which the holistic approach might be developed. Accounts of consciousness that posit some form of centralized consciousness module or workspace are most naturally understood in holistic terms. One particularly appealing approach looks to the domain-general enabling mechanisms that are involved in mediating the transition from unconsciousness to consciousness. Perhaps such systems not only enable the creature to be consciousness but also play a role in ensuring that its conscious states will be unified.

The debate between atomistic and holistic approaches to consciousness is very much a live one. In large part, the plausibility of each approach turns on just how common breakdowns in co-consciousness are. Atomism would be buoyed if such breakdowns are common, for holistic approaches struggle to explain how a subject’s conscious states could be conscious without being conscious together. On the other hand, the prospects of holism would be advanced such breakdowns do not occur (or are exceedingly rare), for the absence of phenomenal disunity is precisely what one would expect were consciousness to have a fundamentally holism structure. Not surprisingly, debates about how best to explain the unity of consciousness cannot be separated from debates about the degree to which consciousness is unified.


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