Object Indexes and Causal Interactions

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Attention to Objects

(from figure caption): ' A number (here eight) of identical objects are shown (at t = 1), and a subset (the `targets') is selected by, say, ̄ashing them (at t 􏰈 2), after which the objects move in unpredictable ways (with or without self-occlusion) for about 10 s. At the end of the trial the observer has to either pick out all the targets using a pointing device or judge whether one that is selected by the experimenter (e.g. by ̄ashing it, as shown at t 􏰈 4) is a target.' \citep[p.\ 142]{Pylyshyn:2001hl}

Highlight the case where subject is asked whether this is one of the objects identified.

(If a target disappears, subjects can also say where it was and which direction it was moving in.)

Limit of 3, maybe 4, objects will be important later.

Pylyshyn 2001, figure 6

What does this tell us?

If attention is organised around objects, the perceptual system must be capable of identifying and tracking objects.

object index (/ FINST)

Leslie et al say an object index is 'a mental token that functions as a pointer to an object' \citep[p.\ 11]{Leslie:1998zk}

'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. (ms. 19-20)' \citep{Scholl:1999mi}

object indexes require segmentation

object indexes can survive occlusion

object indexes require tracking some causal interactions ...

Object indexes are linked to causation.

In order to track objects, a perceptual system has to be sensitive to be causal interactions

Why is this true?

Because when you have a causal interaction, there's a conflict between principles of object perception e.g. distinct surfaces=>two objects, vs good continuity of motion=>one object

The perceptual system needs to know when conflicts should be reconciled and when they should be written off.

We get perceptual effects of causal interactions when there are conflicts among cues of object identity.

This is a point Michotte made. He found that launching occurs when there is a conflict between cues to object identity: good continuity of movement suggests a single object whereas the existence of two distinct surfaces indicates two objects.

It is plausible that other types of causal interaction also involve conflicts between cues to object identity.

Recall the question I asked earlier about 70ms.

How is launching detected? For example, does it involve perceptual processes?
Why is a delay of up to around 70ms consistent with the launching effect occuring?

This is an important question insofar as we are concerned with detecting causal interactions. Is what people detect when the launching effect occurs a causal interaction? You might say, it can't be because no delay between two movements is consistent with a causal interaction.

Michotte said this:

‘anyone not very familiar with the procedure involved in framing the physical concepts of inertia, energy, conservation of energy, etc., might think that these concepts are simply derived from the data of immediate experience.’

\citep[p.\ 223]{Michotte:1946nz}

Michotte (1946, p. 223)

How is this consistent with the laws of mechanics—surely no pause can be tolerated? Ingeniously, Michotte compares launching with the movement of a single object. The single object moves half way across a screen then pauses before continuing to move. Michotte found that the longest pause between the two movements consistent with subjects experiencing them as a single movement is around 80ms, exactly the longest pause consistent with experiences characteristic of launching \citep[pp.\ 91--8, 124]{Michotte:1946nz}. Accordingly, the experience characteristic of launching appears to require that the two movements be experienced as uninterrupted—this is why they can be separated by a pause of up to but no longer than 80ms.

The question for this section was:

Can humans perceive causal interactions?

Now I think we have achieved an answer.

Perceptual systems identify certain kinds of causal interaction in the course of tracking objects.

The perceptual system responsible for identifying objects must also concern itself with certain kinds of causal interaction in order to reconcile conflicting cues to object identity.

In slightly more detail: one function of our perceptual systems is to identify and track objects; this is done by means of various cues; sometimes the visual system is faced with conflicting cues to object identity which need to be resolved in order to arrive at a satisfactory interpretation; when certain types of causal interaction occur there is a conflict among cues to object identity; these conflicts must be treated differently from other conflicts because they do not indicate failures of object identification and so do not require resolution or further perceptual processing. So object perception depends on sensitivity to certain types of causal interaction and this is why the launching effect occurs.

Before concluding I want to mention some further evidence for this view.

further evidence

This further evidence exploits something called the object-specific preview effect.

So before I can go on, I need to explain what this is.

object-specific preview effect

Background: object-specific preview effect

We can measure object indexes using the object-specific preview effect.

The \emph{object-specific preview effect}: ‘observers can identify target letters that matched the preview letter from the same object faster than they can identify target letters that matched the preview letter from the other object.’

\citep[p.\ 2]{Krushke:1996ge}

Kahneman et al 1992, figure 3

Krushke and Fragassi (1996) have shown that the object-specific preview effect vanishes in launching but not in various spatio-temporally similar sequences. Since the object-specific preview effect is regarded as diagnostic of feature binding, this is evidence that in launching sequences, features of the second object (such as motion) remain bound to the first object for a short time after the second object starts to move.

Causal interactions are detected by the perceptual processes involved in segmenting and tracking objects.

This is unexpected insofar as perception is often supposed to be limited to features of the world less abstract that causal interactions. Indeed, the notion that perceptual processes represent three-dimensional objects rather than mere surfaces was at one time controversial. The research we have reviewed shows that perceptual processes represent not only three-dimensional but properly physical objects, that is, objects capable of causally interacting with each other.

As we'll see in a moment, this is relevant to understanding a problem we encountered in the lecture on objects.