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4- to 6-month-olds can track briefly occluded objects

scenario	method	source
1 vs 2 objects	habituation	Spelke et al 1995
one unperceived object constrains another’s movement	habituation	Baillargeon 1987
where did I hide it?	violation-of-expectations	Wilcox et al 1996
wide objects can’t disappear behind a narrow occluder	violation-of-expectations	Wang et al 2004
when and where will it reappear?	anticipatory looking	Rosander et al 2004
marker of object maintenanc	EEG	Kaufman et al 2005

Pylyshyn 2001, figure 6

object indexes / files in adults and infants ...

• guide ongoing action (e.g. visual tracking, reaching)
• influence how attention is allocated
can conflict with beliefs and knowledge states
This system of object indexes does not involve belief or knowledge and may assign indexes to objects in ways that are inconsistent with a subject’s beliefs about the identities of objects (Mitroff et al., 2005; Mitroff & Alvarez, 2007)
have behavioural and neural markers
We have observed one behavioural marker of object indexes, namely the object-specific preview benefit.
There are also neural markers of object indexes. That is, in adults there is a pattern of brain activity which appears to be characteristic of processes involved in maintaining an object index for an object that is briefly hidden from view.
are subject to signature limits
The system of object indexes is also subject to signature limits. In general, a \emph{signature limit of a system} is a pattern of behaviour the system exhibits which is both defective given what the system is for and peculiar to that system.
One signature limit of a system of object indexes is that featural information sometimes fails to influence how objects are assigned in ways that seem quite dramatic. Let me illustrate ...
In this scenario, a patterned square disappears behind the barrier; later a plain black ring emerges. If you consider speed and direction only, these movements are consistent with there being just one object. But given the distinct shapes and textures of these things, it seems all but certain that there must be two objects. Yet in many cases these two objects will be assigned the same object index (Flombaum & Scholl, 2006; Mitroff & Alvarez, 2007). So one signature limit of systems of object indexes is that information about speed and distance can override information about shape and texture.
sometimes survive occlusion
As the findings I just describes imply, object indexes can survive brief occlusion. That is, an object index can remain attached to an object even if that object is briefly occluded by a screen. (Sameness of object index may be detected by the presence of an object-specific preview benefit).
To clarify terminology, I should say that whereas I’m talking about object indexes, researchers more typically interpret this research in terms of object files. I’m sticking to object indexes rather than object files for reasons of simplicity and caution. If you believe in object files then you can interpret what I’m saying as referring to object files. And if you have doubts about object files, you might still have reason to accept that a system of object indexes exists.
So far I have been talking about object indexes in adult humans.
But our interest in object indexes stems from a hypothesis about four-month-old infants’ abilities to track briefly occluded objects. According to this hypothesis, these abilities depend on a system of object indexes like that which underpins multiple object tracking or object-specific preview benefits (Leslie et al., 1998; Scholl & Leslie, 1999; Carey & Xu, 2001; Scholl, 2007). What makes this hypothesis attractive?
One reason the hypothesis seems like a good bet is that object indexes are the kind of thing which could in principle explain infants’ abilities to track unperceived objects because object indexes can, within limits, survive occlusion.
If we consider six-month-olds, we can also find behavioural markers of object indexes in infants (Richardson & Kirkham, 2004) ...
... and there are is also a report of neural markers too (Kaufman et al., 2005).
(Kaufman et al. (2005) measured brain activity in six-month-olds infants as they observed a display typical of an object disappearing behind a barrier. They found the pattern of brain activity characteristic of maintaining an object index. This suggests that in infants, as in adults, object indexes can attach to objects that are briefly unperceived.)
The evidence we have so far gets us as far as saying, in effect, that someone capable of committing a murder was in the right place at the right time. Can we go beyond such circumstantial evidence?
The key to doing this is to exploit signature limits. Carey (2009) argues that what I am calling the signature limits of object indexes in adults are related to signature limits on infants’ abilities to track briefly occluded objects.
To illustrate, a moment ago I mentioned that one signature limit of object indexes is that featural information sometimes fails to influence how objects are assigned in ways that seem quite dramatic.
There is evidence that, similarly, even 10-month-olds will sometimes ignore featural information in tracking occluded objects (Xu & Carey, 1996).% \footnote{ This argument is complicated by evidence that infants around 10 months of age do not always fail to use featural information appropriately in representing objects as persisting (Wilcox & Chapa, 2002). In fact McCurry, Wilcox, & Woods (2009) report evidence that even five-month-olds can make use of featural information in representing objects as persisting (Wilcox, 1999, p. see also). %they use a fringe and a reaching paradigm. NB the reaching is a problem for the simple interpretation of looking vs reaching! Likewise, object indexes are not always updated in ways that amount to ignoring featural information (Hollingworth & Franconeri, 2009; C. M. Moore, Stephens, & Hein, 2010). It remains to be seen whether there is really an exact match between the signature limit on object indexes and the signature limit on four-month-olds’ abilities to represent objects as persisting. The hypothesis under consideration---that infants’ abilities to track briefly occluded objects depend on a system of object indexes like that which underpins multiple object tracking or object-specific preview benefits---is a bet on the match being exact. }
Here are the results. The central column shows that infants looked longer when they saw two objects at test rather than when they saw a single object. This is not different from how they performed in a base line condition when the information about number was not present. And it is different from how they performed in the ‘spatiotemporal condition’ in which the two objects were at simultaneously visible at one point before the test phase.
While I wouldn’t want to suggest that the evidence on siganture limits is decisive, I think it does motivate considering the hypothesis and its consequences. In what follows I will assume the hypothesis is true: infants’ abilities to track briefly occluded objects depend on a system of object indexes.
The hypothesis has an advantage which I don’t think is widely recognised. This is that object indexes are independent of beliefs and knowledge states. Having an object index pointing to a location is not the same thing as believing that an object is there. And nor is having an object index pointing to a series of locations over time is the same thing as believing or knowing that these locations are points on the path of a single object. Further, the assignments of object indexes do not invariably give rise to beliefs and need not match your beliefs.
To emphasise this point, consider once more this scenario in which a patterned square disappears behind the barrier; later a plain black ring emerges. You probably don't believe that they are the same object, but they probably do get assigned the same object index. Your beliefs and assignments of object indexes are inconsistent in this sense: the world cannot be such that both are correct.
To illustrate, consider an ingenious experiment by Shinskey & Munakata (2001). There was an opaque screen that could rotate between lying flat on the ground and being raised to conceal a toy behind it. Shinskey and Munakata also used a second piece of apparatus just like the first except that the screen was transparent rather than opaque. They reasoned that infants would quite often pull the screen forwards just for fun, regardless of what is behind it. However, they also guessed that when infants know there is an interesting toy behind the screen, then they will pull it forwards more often than when they know that there is nothing behind the screen. This is just what happened when infants were presented with the apparatus involving a transparent screen: they sometimes pulled the screen forwards when there was no toy behind it, but they pulled it forwards significantly more often when the toy was behind it. What happened when infants were presented with the opaque screen? Here infants pulled the screen forwards no more often when they had observed a toy being placed behind it then when they had observed that there was nothing behind it. This is evidence that seven-month-old infants do not know that a toy they have very recently seen hidden behind a screen is behind the screen. After all, since knowledge guides action we would expect infants who know that a toy is behind an opaque screen to pull the screen forward more often than infants who know there is nothing behind the screen, just as they do when the screen is transparent.
More than two decades of research strongly supports the view that infants fail to search for objects hidden behind barriers or screens until around eight months of age (Meltzoff & Moore, 1998, p. \ 202) or maybe even later (M. K. Moore & Meltzoff, 2008). Researchers have carefully controlled for the possibility that infants’ failures to search are due to extraneous demands on memory or the control of action. We must therefore conclude, I think, that four- and five-month-old infants do not have beliefs about the locations of briefly occluded objects. It is the absence of belief that explains their failures to search.

Scholl 2007, figure 4

Carey and Xu 2001, figure 3

Xu and Carey 1996, figure 4

Shinskey & Munakata 2001, figure 1

Wynn 1992, fig 1 (part)

Wynn 1992, fig 1 (part)