The binding problem.

Abstract

Both Ghose and Maunsell and Shadlen and Movshon point out what is perhaps the fundamental problem with the binding hypothesis: even supposing that temporal coding is the vehicle for signaling which neural populations should be bound together, the theory does not adequately address how those combinations are computed. In a sense, the binding problem is pushed back one level, for it must be solved at least partially for the necessary temporal correlations to be established. Are neurons with appropriate receptive field properties and anatomical connectivity a sufficient basis for the generation of correlated signals? What is the role of top-down connections in establishing proper synchrony? These questions are difficult to answer and difficult to model with biological realism. A similar problem rests with the output side of the temporal binding hypothesis—if correlations are the signal for binding, how are those signals read out? The readout problem is one of the most puzzling and fundamental problems for systems neuroscience in general: how is the firing of populations of neurons interpreted and transformed by other neurons to result in decision, action, perception, etc? This problem plagues most, if not all, models of brain function, for in modeling it is the modeler that attributes semantics to nodes in a network. One way to potentially avoid that problem would be to close the loop between the model and the world, so that the world impinges upon the network, which then acts upon and affects the world, thus generating semantics through action.This brings me to mention (by virtue of my editorial license) what is perhaps the most mystifying binding problem of all: the problem of consciousness. How does something as simple and mechanistic as neural firing add up to subjectivity, raw feelings, a self? Are the mechanisms that allow us to attribute the correct color and shape to an object the same ones that lead to the unity of phenomenal experience? Will the solution of the binding problem be the solution to the mystery of consciousness? I will not belabor the point, since answers will be long in coming, but although none of the scientists who authored the reviews that follow discuss binding with respect to consciousness, I will wager that the a good part of the interest, excitement, and contentiousness that surrounds the binding debate is attributable to the magnitude of the issues with which it is connected. The following pieces provide a comprehensive review of the status of the binding problem at the dawn of the new millenium. It will be extremely interesting to repeat this exercise in a decade or two, to chart the progress that we as scientists make on one of the most puzzling and fascinating issues that the brain and cognitive sciences have ever faced.*E-mail: aroskies@cell.com.