The Temporal Correlation Hypothesis Review of Visual Feature Integration: Still Alive and Well

alike. This surge of interest has been driven by the realBinding Is a Diverse Set of Functions ization that there are few, if any, places in the nervous Binding isn’t a problem for nervous systems, as evolusystem where all the information necessary to carry out tion has sculpted their organization to solve the problem a particular task is localized. The inescapable concluefficiently and effectively. It is only a problem for those sion is that sensory, cognitive, and motor processes of us trying to understand how the nervous system result from parallel interactions among large populations achieves the task. Part of our difficulty stems from the of neurons distributed among multiple cortical and subfact that binding is a broadly defined term. It is a function cortical structures. Binding, in a general sense, is the that occurs over multiple spatial and temporal scales of process responsible for functionally linking this distriborganization in the nervous system. It can be rapid and uted activity. In the visual system, this problem correpreattentive or may require focused attention. It can sponds to identifying the relationships among features occur within a single sensory modality or may require in an image so that objects can be recognized. Much cross-modal or sensory–motor integration. Consequently, of this process relies on focused attention. It has also confusion and disagreement can easily arise among been recognized, however, that an early preattentive competing theories of the binding process and experimechanism operates rapidly, and in parallel, to segment mental tests of those theories. visual scenes into their candidate objects. This mechaTo illustrate the richness of the binding process, connism, often referred to as perceptual grouping, enables sider a simple everyday example of a sensory–motor features to be grouped together according to Gestalt, task. You want to find a certain object that’s located on and other, criteria. These processes are also faced with one of several adjacent shelves, but you’re not sure a binding problem. Some form of representation must which shelf, and there are many other objects on the be available to establish relationships among features shelves besides the one you’re searching for. To find it, and separate one set of grouped features from another. you have to visually scan the scene until you recognize Milner (1974) and von der Malsburg (1981, 1985) prothe object. At each location of gaze, your visual system posed that these problems could be overcome if conmust parse the scene into its component objects. This stellations of grouped features were represented dyinvolves identifying which features in the scene belong namically by the formation of cell assemblies, defined to which objects. Once this is accomplished, or perhaps by the selective synchronization of distributed neuronal at the same time, the identity of a particular object can activities. Such a mechanism, they argued, would permit be evaluated. If it doesn’t match what you’re searching the segregation of one representation from another and for, you must direct your gaze to a new location and provide highly salient neuronal signals to an objectrepeat the process. This redirection of gaze may itself based attentional selection mechanism. be guided by the prior parsing of the scene. Once you’ve In the ensuing years, evidence has accumulated to identified the correct object, you can reach out, grasp support this hypothesis, but critics have argued that it it, and carry on with the task at hand. is insufficient to warrant the theory’s acceptance. They It is not entirely clear which components of this task have also argued that the theory is flawed on physiologiwe should refer to as binding and hence which neuronal cal grounds, that cortical networks do not possess the mechanisms solve the “binding problem. ” Perhaps the requisite mechanisms to generate and make use of semost common view is that binding is the process of lective patterns of neuronal synchronization. In this recombining stimulus features to form an object represenview, I will argue to the contrary that the theory, particutation. This is intuitively appealing, because it is well larly as it applies to preattentive vision, is well grounded established that the locations and properties of local in the psychophysical and neurophysiological domains. stimulus features are represented within multiple retinoI will focus on the problem of visual perceptual grouping topic cortical maps. The features comprising objects and define the process within the context of a set of must somehow be “bound” together by combining the principles articulated by Gestalt psychology. I will lay appropriate activities within the relevant cortical maps. out the framework of the theory for how perceptual However, a number of other processes in the task degrouping is implemented in the visual system. And I scribed above can be thought of equally well as binding. will present arguments, couched in neurophysiological Associating the location of an object in space with its