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Object Indexes

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If infants’ lack knowledge of physical objects, how do they track them even when they cannot see them?

In adult humans, there is a system of object indexes which enables them to track potentially moving objects in ongoing actions such as visually tracking or reaching for objects, and which influences how their attention is allocated (Flombaum, Scholl, & Pylyshyn, 2008).

But what is an object index? Formally, an object index is ‘a mental token that functions as a pointer to an object’ (Leslie, Xu, Tremoulet, & Scholl, 1998, p. \ 11). If you imagine using your fingers to track moving objects, an object index is the mental counterpart of a finger (Pylyshyn, 1989, p. 68).

Leslie et al say an object index is ‘a mental token that functions as a pointer to an object’ (Leslie et al., 1998, p. 11)[1]

Object indexes have several features. They:

For our purposes, the interesting thing about object indexes is that a system of object indexes (at least one, maybe more) appears to underpin cognitive processes which are not strictly perceptual but also do not involve beliefs or knowledge states. This makes it possible to entertain a conjecture about infants’ abilities:

The CLSTX conjecture: Five-month-olds’ abilities to track briefly unperceived objects are not grounded on belief or knowledge: instead they are consequences of the operations of a system of object indexes.

This is a wonderful conjecture due to several scientists (Leslie et al., 1998; Scholl & Leslie, 1999; Carey & Xu, 2001; Scholl, 2007).

There is just one problem.[2] We saw earlier that infants’ abilities to track briefly occluded physical objects can be tested using violation-of-expectation experiments. And in these experiments infants will look at an incongruous scene for perhaps 20 seconds. This is not something we could explain by invoking object indexes. Apart from anything else, 20 seconds is much longer than an object index assigned to a vanished object could survive.

What could connect object indexes with looking times in violation-of-expectation experiments?


violation-of-expectation : Violation-of-expectation experiments test hypotheses about what infants expect by comparing their responses to two events. The responses compared are usually looking durations. Looking durations are linked to infants’ expectations by the assumption that, all things being equal, infants will typically look longer at something which violates an expectation of theirs than something which does not. Accordingly, with careful controls, it is sometimes possible to draw conclusions about infants’ expectations from evidence that they generally look longer at one event than another.


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  1. see also Scholl & Leslie (1999): ‘Pylyshyn’s FINST model: you have four or five indexes which can be attached to objects; it’s a bit like having your fingers on an object: you might not know anything about the object, but you can say where it is relative to the other objects you’re fingering.’ ↩︎

  2. This isn’t quite true: there is a second problem involving infants’ success in searching for objects hidden in milk or which have disappeared after the lights went down. There are some details about this and a proposed solution in (Butterfill, 2020). ↩︎