Binding problem

The binding problem is a term used at the interface between neuroscience, cognitive science and philosophy of mind that has multiple meanings.

Firstly, there is the segregation problem: a practical computational problem of how brains segregate elements in complex patterns of sensory input so that they are allocated to discrete "objects". In other words, when looking at a blue square and a yellow circle, what neural mechanisms ensure that the square is perceived as blue and the circle as yellow, and not vice versa? The segregation problem is sometimes called BP1.

Secondly, there is the combination problem: the problem of how objects, background and abstract or emotional features are combined into a single experience. The combination problem is sometimes called BP2.

However, the difference between these two problems is not always clear. Moreover, the historical literature is often ambiguous as to whether it is addressing the segregation or the combination problem.

The segregation problem is the problem of how brains segregate elements in complex patterns of sensory input so that they are allocated to discrete "objects".

Smythies defined BP1 in these terms: "How is the representation of information built up in the neural networks that there is one single object 'out there' and not a mere collection of separate shapes, colours and movements?" Revonsuo refers to this as the problem of "stimulus-related binding" – of sorting stimuli. Although usually referred to as a problem of binding, the computational problem is arguably one of discrimination. Thus, in the words of Canales et al.: "to bind together all the features of one object and segregate them from features of other objects and the background". Bartels and Zeki describe it as "determining that it is the same (or a different) stimulus which is activating different cells in a given visual area or in different visual areas".

Most experimental work is on vision, where it is known that humans and other mammals process different aspects of perception by separating information about those aspects and processing them in distinct regions of the brain. For example, Bartels and Zeki 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 has been claimed to yield a potential for ambiguity. 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 or a circle shape. Here, the binding problem is the issue of how the brain correctly pairs colour and shape, i.e. indicates that blue goes with square, rather than yellow.

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