Binding problem

The binding problem is one of a number of terms at the interface between neuroscience and philosophy which suffer from being used in several different ways, often in a context that does not explicitly indicate which way the term is being used. Of the many possible usages, two common versions may be useful anchor points. Firstly, there is the practical issue of how brains segregate elements in complex patterns of data. This can be illustrated by the question "When I see a blue square and a yellow circle, what neural mechanisms ensure that the sensing of blue is coupled to that of a square shape and that of yellow is coupled to that of a circle?" Secondly, there is the more fundamental problem of "how the unity of conscious perception is brought about by the distributed activities of the central nervous system." The first question is a difficult but conventional question within physical science that could equally be applied to a mechanical computer or any complex system with an input and output. The second question is metaphysical in the sense that the "unity of conscious perception" may be an idea outside physical science that requires a metaphysical or ontological underpinning, of the sort on which physics is generally agnostic. Thus "unity" in this sense has no physical meaning, but it does have a crucial meaning in subjective experience.

These two meanings of "binding problem" can be found in a well-defined form, chiefly in the neuroscience and philosophy literature respectively. However, there are also many instances where the two issues are conflated in ways that are difficult to be sure about. Perhaps the clearest exposition of the second meaning comes in William James's Principles of Psychology where he refers to it as the combination problem.

The practical segregation problem
In the case of visual perception, the brains of humans and other animals process different aspects of perception by separating information about those aspects and processing them in distinct regions of the brain. For example, Zeki and coworkers have shown that different areas in the visual cortex specialize in processing the different aspects of colour, motion, and shape. This type of modular coding yields a potential for ambiguity in many instances. Thus, when humans view a scene containing a blue square and a yellow circle, some neurons signal in response to blue, others signal in response to yellow, still others to a square shape and a circle shape. Here, the binding problem is the issue of how the brain represents the pairing of colour and shape, i.e. is the square blue or yellow?

Presumably somewhere further down the line of interneural signalling the output of the blue sensitive cells and the square sensitive cells are allowed to interact by converging in the inputs of further cells in a way that the output of the blue sensitive cells and the circle sensitive cells are not. Whether this is achieved by timing or some form of gating still appears to be wide open to debate. Relevant to this issue is the work of Pylyshyn, which suggests that incoming visual (and other) data get allocated "object labels" in some way, such that "blue" and "square" get tagged as "object number 1". In fact this labelling is so potent that if a blue square goes behind a screen and a yellow square appears at the other side the perception is that "object number 1 changed colour".

A popular hypothesis is that features are bound via synchronisation of the firing of different neurons in the cortex. Andreas K. Engel and his coworkers have found that two different neurons with a different receptive field produce divergent correlograms according to whether the stimuli were bound together or not. However, Thiele and Stoner found that perceptual binding of two moving patterns had no effect on synchronisation of the neurons responding to the two patterns. In the primary visual cortex, Dong et al. found that whether two neurons were responding to contours of the same shape or different shapes had no significant effect on neural synchrony.

A number of people, including Marcus have pointed out that phase of oscillation would only allow segregation of very few aspects of a visual image. At the very most ten phase states might be distinguishable, and even that would require stable oscillation rates over time. A much more plausible explanation would seem to be some sort of gating of allowed connection paths by short-term modulation of the weighting of synaptic inputs from "colour and shape cells" to downstream cells by signals from "object label cells".

In this form the binding problem is also an issue in memory. How do we remember the associations among different elements of an event? How does the brain create and maintain those associations? Both the hippocampus and prefrontal cortex seem to be important for memory binding. There is an implication that the behaviour of "object label" or perhaps "event label" cells, can be converted into some form of long-term reinforcement capacity.

In her feature integration theory, Anne Treisman suggested that binding between features is mediated by the features' links to a common location. Psychophysical demonstrations of binding failures under conditions of full attention provide support for the idea that binding is accomplished through common location tags.

The combination problem
The binding problem, as it applies to the "unity of consciousness" is related to the problem of the homunculus—a putative inner "little man" who is the true subject within the brain. The question is how coloured squares and circles can be "experienced together" as a single scene. The implication is that something is experiencing all these data. It has become popular to deny any need to give a physical account of what it is that has the experience, often with the suggestion that it is the "person as a whole" or the "system". However, to ascribe input to such vague physical domains is not without problems. Such suggestions appear to arise from the common misconception that there cannot be a limited internal physical domain that has access to, for instance, data from blue sensitive and square sensitive cells. This domain is often equated with a "paradoxical" homunculus, but it is often not appreciated that a homunculus is only paradoxical under limited conditions. Sutherland denotes the fear of the homunculus as "homuphobia": "But if you look inside the brain you can't find any little green men and this has given rise to a fear of homunculi, agents and Cartesian theatres. All this has resulted in some desperate and flawed attempts to build a bottom-up theory."

The homunculus concept is often equated with someone "watching a wonderfully integrated internal TV screen" and, as René Descartes noted, nothing is more certain than that there is an internal observer ("Cogito ergo sum"), so the only alternative option to the homunculus is infinite regress (who is watching the screen inside the homunculus?). Some materialists refuse to accept the reality of subjective consciousness and so are led to conclude that infinite regress and homunculi are equally repugnant or absurd and so adopt the third alternative: eliminativism. Daniel Dennett maintains that "homunculi are only bogey men if they repeat entirely the talents they are rung in to explain". Limited domains within brains supporting percepts based on signals that have undergone several transductive steps almost certainly have to be postulated because much of brain activity appears to be outside consciousness. How signals are finally transduced into percepts in these domains remains a major mystery but there need be no further regress (again the regress/homunculus/eliminativism alternatives). What is much less clear is whether there is one such domain per brain or many, as in Dennett's "Multiple Draft" hypothesis.

The synchronisation of oscillating cellular potentials has also been invoked as a solution to the combination problem. Thus it was never very clear whether Francis Crick was trying to solve the segregation or the combination problem in his book The Astonishing Hypothesis However, one criticism of the synchronisation idea is that experiential combination of information in separate neurons is incompatible with any standard biophysical explanation of the brain, whether or not there is any synchrony. One possible explanation for binding is that the information is integrated in each of many individual downstream neurons. This requires that percepts exist in multiple complete copies. This difficulty has led many to suggest unconventional physical explanations for percepts, often invoking quantum theory (e.g. the approach of Freeman and Vitiello). Also, multiply experiencing neurons seems to make no sense if their experience is not pooled in a "global workspace" (see Bernard Baars or Cartesian theatre). And indeed they may be another example of a "desperate and flawed attempt" to build a bottom up theory.