Visual indexing theory

Visual Indexing Theory (also called FINST theory) is an account of early visual perception developed by Zenon Pylyshyn in the 1980s. It proposes a pre-attentive mechanism (a ‘FINST’) whose function is to individuate salient elements of a visual scene, and track their locations across space and time. Developed in response to what Pylyshyn viewed as limitations of prominent theories of visual perception at the time, visual indexing theory is supported by several lines of empirical evidence.

'FINST' abbreviates ‘FINgers of INSTantiation’. Pylyshyn describes visual indexing theory in terms of this analogy. Imagine, he proposes, placing your fingers on five separate objects in a scene. As those objects move about, your fingers stay in respective contact with each of them, allowing you to continually track their whereabouts and positions relative to one another. While you may not be able to discern in this way any detailed information about the items themselves, the presence of your fingers provides a reference via which you can access such information at any time, without having to relocate the objects within the scene. Furthermore, the objects' continuity over time is inherently maintained — you know the object referenced by you pinky finger at time t is the same object as that referenced by your pinky at t₋₁, regardless of any spatial transformations it has undergone, because your finger has remained in continuous contact with it.

Visual indexing theory holds that the visual perceptual system works in an analogous way. FINSTs behave like the fingers in the above scenario, pointing to and tracking the location of various objects in visual space. Like fingers, FINSTs are:

FINSTs operate pre-attentively — that is, before attention is drawn or directed to an object in the visual field. Their primary task is to individuate certain salient features in a scene, conceptually distinguishing these from other stimuli. Under visual indexing theory, FINSTing is a necessary precondition for higher level perceptual processing.

Pylyshyn suggests that what FINSTs operate upon in a direct sense is 'feature clusters' on the retina, though a precise set of criteria for FINST allocation has not been defined. "The question of how FINSTs are assigned in the first instance remains open, although it seems reasonable that they are assigned primarily in a stimulus-driven manner, perhaps by the activation of locally distinct properties of the stimulus-particularly by new features entering the visual field."

FINSTs are subject to resource constraints. Up to around five FINSTs can be allocated at any given time, and these provide the visual system information about the relative locations of FINSTed objects with respect to one another.

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