Phenomenal Unification

The binding problem is the problem of explaining how it is that our brains are able to connect the various features that we perceive of a single object together. For instance, when you see a T you are able to tell that it is both shaped like a T and red. The features redness and T-shape are bound together and associated with a single object.

The binding problem is thought to be tricky in part because our brains utilize distinct neural feature maps to represent the different features that we perceive.

Sensory processing is organized hierarchically. Different levels of the neural hierarchy represent different levels of feature specificity. For example, one part of the visual cortex represents where in the visual field there are boundaries, while another part  represents where there are lines/edges, and another part might represent where there are surfaces. Since some of the details that are represented at the earlier levels of processing are abstracted away at later levels in the hierarchy, the features that we perceive are represented in different places. The earlier levels represent very specific features without representing general features, and the later parts represent very general features without representing specific features.

What's more, at least in the visual cortex, some different kinds of properties are represented in different areas of the brain. There are two separate processing pathways that information takes after arriving in the visual cortex. One pathway is primarily involved in object tracking and contains representations of which parts of the visual field contain objects, and how fast those objects are moving, without fine-grained representations of what sorts of objects they are. The other pathway contains representations of what sorts of object's are in the visual field without precisely locating them or representing whether or not they are moving.

The binding problem is a problem of neural engineering. Somehow, our brains are able to associate the different features of an object that are represented in different feature maps together. In order to locate a red T in a field of black text, our brains must not only discern the redness and T-shape of the letter, they must also assign those features to the same object. There are different proposals for how the brain could do it, and for all we know there are many different ways that would work equally well. The binding problem is that of finding out how our brains actually do it.

The unification problem is a separate but related problem. Here is an intuitive thought: not only does our brain manage to associate different features of an object as belonging to the same object, but we sometimes experience them that way. When we see something and hear it, the sight and the sound occur to (most of) us as two separate experiences. The experiences appear before our minds simultaneously, but they are represented in distinct percepts. When we see that something is a square and watch it move, however, we don’t perceive the squareness and motion in two separate experiences. We don’t have a motion experience and a squareness experience that we intellectually connect as experiences of one object, in the way that we have connect a sound with an image of the object producing it. The perception of both properties is unified into a single percept. The unification problem is to explain why this is the case.

The binding problem and the unification problem are distinct problems. Just because some features are bound together in the brain (i.e. associated with the same object) doesn’t mean that they must be experienced in a single percept. We often associate sounds with particular objects that we also see without having merged visual and auditory experiences. 

It is also conceivable that we could perceive objects with a unified percept without our brain being able to represent the unified features as belonging to the same object. this is made plausible by the observation that it takes our brains some time to bind the features of objects together. If you flash open your eyes on a complex scene, it is thought that your brain does not have the time to associate the right properties with the right objects. Nevertheless, it is not crazy to think that we have percepts that unify the features belonging to an object together before our brain succeeds in actually binding the represented features together.

 Thus, while solving the binding problem may help us solve the unification problem, and vice versa, they aren't just the same problem.