Summary of problem Smythies defines the combination problem, also known as the subjective unity of perception, as "How do the brain mechanisms actually construct the phenomenal object?". Revonsuo there are nuances of difference beyond the basic BP1:BP2 division. Smythies speaks of constructing a phenomenal object ("local unity" for Revonsuo) but philosophers such as
René Descartes,
Gottfried Wilhelm Leibniz,
Immanuel Kant, and James (see Brook and Raymont) have typically been concerned with the broader unity of a phenomenal experience ("global unity" for Revonsuo) – which, as Bayne illustrates may involve features as diverse as seeing a book, hearing a tune and feeling an emotion. Further discussion will focus on this more general problem of how sensory data that may have been segregated into, for instance, "blue square" and "yellow circle" are to be re-combined into a single phenomenal experience of a blue square next to a yellow circle, plus all other features of their context. There is a wide range of views on just how real this "unity" is, but the existence of medical conditions in which it appears to be subjectively impaired, or at least restricted, suggests that it is not entirely illusory. There are many neurobiological theories about the subjective unity of perception. Different visual features such as color, size, shape, and motion are computed by largely distinct neural circuits but we experience this as an integrated whole. The different visual features interact with each other in various ways. For example, shape discrimination of objects is strongly affected by orientation but only slightly affected by object size. Some theories suggest that global perception of the integrated whole involves higher order visual areas. There is also evidence that the
posterior parietal cortex is responsible for perceptual scene segmentation and organization. Bodies facing each other are processed as a single unit and there is increased coupling of the
extrastriate body area (EBA) and the posterior
superior temporal sulcus (pSTS) when bodies are facing each other. This suggests that the brain is biased towards grouping humans in twos or dyads. The boundary problem is another unsolved problem in neuroscience and phenomenology that is related to the binding problem. The boundary problem is essentially the inverse of the binding problem, and asks how binding stops occurring and what prevents other neurological phenomena from being included in first-person perspectives, giving first-person perspectives hard boundaries. Topological segmentation and electromagnetic field topology have been proposed by researchers at the
Qualia Research Institute as possible avenues for solving the boundary problem as well as the binding problem.
History Early philosophers René Descartes and Gottfried Wilhelm Leibniz noted that the apparent unity of our experience is an all-or-none qualitative characteristic that does not appear to have an equivalent in the known quantitative features, like proximity or cohesion, of composite matter.
William James, in the nineteenth century, considered the ways the unity of consciousness might be explained by known physics and found no satisfactory answer. He coined the term "combination problem", in the specific context of a "mind-dust theory" in which it is proposed that a full human conscious experience is built up from proto- or micro-experiences in the way that matter is built up from atoms. James claimed that such a theory was incoherent, since no causal physical account could be given of how distributed proto-experiences would "combine". He favoured instead a concept of "co-consciousness" in which there is one "experience of A, B and C" rather than combined experiences. A detailed discussion of subsequent philosophical positions is given by Brook and Raymont (see 26). However, these do not generally include physical interpretations.
Whitehead proposed a fundamental ontological basis for a relation consistent with James's idea of co-consciousness, in which many causal elements are co-available or "compresent" in a single event or "occasion" that constitutes a unified experience. Whitehead did not give physical specifics, but the idea of compresence is framed in terms of causal convergence in a local interaction consistent with physics. Where Whitehead goes beyond anything formally recognized in physics is in the "chunking" of causal relations into complex but discrete "occasions". Even if such occasions can be defined, Whitehead's approach still leaves James's difficulty with finding a site, or sites, of causal convergence that would make neurobiological sense for "co-consciousness". Sites of signal convergence do clearly exist throughout the brain but there is a concern to avoid re-inventing what
Daniel Dennett calls a
Cartesian Theater or a single central site of convergence of the form that Descartes proposed. Descartes's central "soul" is now rejected because neural activity closely correlated with conscious perception is widely distributed throughout the cortex. The remaining choices appear to be either separate involvement of multiple distributed causally convergent events or a model that does not tie a phenomenal experience to any specific local physical event but rather to some overall "functional" capacity. Whichever interpretation is taken, as Revonsuo The results showed that the visual system binds two humans in close proximity as part of an integrated whole. These results are consistent with evolutionary theories that face-to-face bodies are one of the earliest representations of social interaction.
Electron tunneling Experiments have shown that
ferritin and neuromelanin in fixed human
substantia nigra pars compacta (SNc) tissue are able to support widespread
electron tunneling. Further experiments have shown that ferritin structures similar to ones found in SNc tissue are able to conduct electrons over distances as great as 80 microns, and that they behave in accordance with Coulomb blockade theory to perform a switching or routing function. Both of these observations are consistent with earlier predictions that are part of a hypothesis that ferritin and neuromelanin can provide a binding mechanism associated with an
action selection mechanism, although the hypothesis itself has not yet been directly investigated. The hypothesis and these observations have been applied to
Integrated Information Theory.
Modern theories Daniel Dennett where color is perceived before orientation of lines and before motion by 40 and 80 ms respectively, constitutes an argument that, over these very short time periods, different attributes are consciously perceived at different times, leading to the view that at least over these brief periods of time after visual stimulation, different events are not bound to each other, leading to the view of a disunity of consciousness, at least over these brief time intervals. Dennett's view might be in keeping with evidence from recall experiments and
change blindness purporting to show that our experiences are much less rich than we sense them to be – what has been called the Grand Illusion. However, few, if any, other authors suggest the existence of multiple partial "drafts". Moreover, also on the basis of recall experiments, Lamme has challenged the idea that richness is illusory, emphasizing that phenomenal content cannot be equated with content to which there is cognitive access. Dennett does not tie drafts to biophysical events. Multiple sites of causal convergence are invoked in specific biophysical terms by Edwards and Sevush. In this view the sensory signals to be combined in phenomenal experience are available, in full, at each of multiple sites. To avoid non-causal combination, each site/event is placed within an individual neuronal dendritic tree. The advantage is that "compresence" is invoked just where convergence occurs neuro-anatomically. The disadvantage, as for Dennett, is the counter-intuitive concept of multiple "copies" of experience. The precise nature of an experiential event or "occasion", even if local, also remains uncertain. The majority of theoretical frameworks for the unified richness of phenomenal experience adhere to the intuitive idea that experience exists as a single copy, and draw on "functional" descriptions of distributed networks of cells. Baars has suggested that certain signals, encoding what we experience, enter a "Global Workspace" within which they are "broadcast" to many sites in the cortex for parallel processing. Dehaene, Changeux and colleagues have developed a detailed neuro-anatomical version of such a workspace. Tononi and colleagues have suggested that the level of richness of an experience is determined by the narrowest information interface "bottleneck" in the largest sub-network or "complex" that acts as an integrated functional unit. Lamme Cleeremans emphasizes meta-representation as the functional signature of signals contributing to consciousness. In general, such network-based theories are not explicitly theories of how consciousness is unified, or "bound", but rather theories of functional domains within which signals contribute to unified conscious experience. A concern about functional domains is what Rosenberg has called the boundary problem; it is hard to find a unique account of what is to be included and what excluded. Nevertheless, this is, if anything is, the consensus approach. Within the network context, a role for synchrony has been invoked as a solution to the phenomenal binding problem as well as the computational one. In his book,
The Astonishing Hypothesis, Crick appears to be offering a solution to BP2 as much as BP1. Even von der Malsburg, introduces detailed computational arguments about object feature binding with remarks about a "psychological moment". The Singer group also appear to be interested as much in the role of synchrony in phenomenal awareness as in computational segregation. The apparent incompatibility of using synchrony to both segregate and unify might be explained by sequential roles. However, Merker points out what appears to be a contradiction in attempts to solve the subjective unity of perception in terms of a functional (effectively meaning computational) rather than a local biophysical domain in the context of synchrony. Functional arguments for a role for synchrony are in fact underpinned by analysis of local biophysical events. However, Merker points out that the explanatory work is done by the downstream integration of synchronized signals in post-synaptic neurons: "It is, however, by no means clear what is to be understood by 'binding by synchrony' other than the threshold advantage conferred by synchrony at, and only at, sites of axonal convergence onto single dendritic trees..." In other words, although synchrony is proposed as a way of explaining binding on a distributed rather than a convergent basis, the justification rests on what happens at convergence. Signals for two features are proposed as bound by synchrony because synchrony effects downstream convergent interaction. Any theory of phenomenal binding based on this sort of computational function would seem to follow the same principle. The phenomenality would entail convergence, if the computational function does. The assumption in many of the quoted models suggest that computational and phenomenal events, at least at some point in the sequence of events, parallel each other in some way. The difficulty remains in identifying what that way might be. Merker's analysis suggests that either (1) both computational and phenomenal aspects of binding are determined by convergence of signals on neuronal dendritic trees, or (2) that our intuitive ideas about the need for "binding" in a "holding together" sense in both computational and phenomenal contexts are misconceived. We may be looking for something extra that is not needed. Merker, for instance, argues that the homotopic connectivity of sensory pathways does the necessary work. ==Shared intentionality and binding==