Solutions for the binding problem

Abstract

Visual cortical neurons are broadly tuned to one or a few feature dimensions, like color and motion. This is advantageous because broadly tuned neurons can contribute to the representation of many visual scenes. However, if there are multiple objects in a visual scene, the cortex is at risk to combine features of different objects as if they belong to a single object. The term 'binding problem' was introduced to refer to the difficulties that may occur in sorting out those responses that are evoked by a single perceptual object. The present article reviews proposals suggesting that the binding problem is solved by labelling an assembly of neurons that is responsive to a single perceptual object. Evidence is reviewed in favor of two possible assembly-labels: Rate enhancement due to visual attention and neuronal synchrony. Assembly-labels should be spread through the cortical network to all neurons that have to participate in an assembly. The present article tries to shed light on the mechanisms that subserve such a selective spread of assembly labels. Moreover, it is suggested that assembly labels may fulfill an equivalent role in the motor system, since binding problems can also occur during the generation of useful patterns of motor activity.