The question for this section is,

 

 

Let my try and show you stimuli that were used in an experiment (without yet telling you anything about the experiment). What do you see?

 

[If the animation doesn't work, there's a static version on the next slide.]

 

OK, so adults: (a) verbal reports. So what?

 

 

Adults will also report experiencing causal interactions including pullling, ...

 

... disintegration ...

 

... and bursting.

 

This effect, or one very like it, can also be found in infancy.

 

Of course, six-month-olds can't tell us about their expeirences. So how can we tell that they detect launching effects?

 

 

[nb: Several people have discussed this in seminars so I won't discuss it here (the reference is on your handout).]

 

In their experiment, they compared two groups of infants. The first group was habituated to the top display, which is just the sort of animation Michotte used to get reports of causal experiences in adults. After habituation, this first group was then shown the same display except that the direction was reversed. Meanwhile a second group was habituated to a display like the top display here except that there was a delay between the first object stopping and the second object starting. This delay would mean that, in adults, there are no reports of experiences of a causal interaction. After habituation, this second group was shown the same display except that the direction of movement was reversed.

 

Of interest was whether the first group showed greater dishabituation to the reversal than the second group. How could this tell us anything about infants' experiences? Suppose that infants do not have anything like what adults report as an experience of causation. They they experience merely patterns of movement. And, in this case, reversing on sequence should create no more interest than reversing the other. But now suppose that infants do have something like what adults report as an experience of causation? Then, when reversing the first sequence, there are two changes: there is a change both to the movement and to the character of the causal interaction. To put it informally, reversing direction means that the patient of the interaction becomes its agent. So the hypothesis that infants' experiences of Michotte-like stimuli resemble adults predicts that there will be greater dishabituation when the first, `direct launching` sequence is reversed.

 

And this is just what the researchers found. The table shows mean looking times in second (with standard deviations in brackets). The control group was just like the direct lanunching group except that there was no reversal

 

So can humans, adult and infant, perceive causal interactions?

 

So far I don't think we have strong reasons to accept that they do. In infants we have discrimination and in adults we have verbal reports. But we shouldn't trust verbal reports. After all, people will say all kinds of things about their experiences. This is nicely illustrated by a famous experiment on apparent behaviour by \citet{Heider:1944ts}.

 

 

But how does this help us? Importantly, tiny variations in the parameters will make big differences in the experiences reported. Let me illustrate this for the delay between the objects' movements.

 

adults: (b) they can discriminate between short gaps and long gaps.

 

That is, the can discriminate gaps of around 50ms.

 

Maybe this is clearer as a figure.

 

People can distinguish between stimuli that differ only in that the gap between two movements is approximately 50ms longer in one than the other. A 50ms difference makes the difference between reporting launching and reporting two movements.

 

We need to do more to understand the effect, ...

 

The question was how we can get beyond intuition in understanding the verbal reports.

 

Part of the answer is this. We don't worry about the content of the verbal reports. We just focus on the fact that their content changes depending on a tiny, 50 millisecond difference in the delay between two movements. Call this \emph{launching effect}.

 

This doesn't tell us \emph{what} people are detecting. But it does tell us that the effect is not merely confabulation or making it up. So we have taken a tiny step beyond intuition. But we also have to answer two questions.

 

So we have the launching effect: adults and probably infants too exhibit perceptual sensitivity to differences in timing of around 50 milliseconds, but only when such delays make the difference between a causal interaction or non-causal interaction.

 

We are still trying to understand the nature of the launching effect. To make progress we need to think about how it arises.

 

Guess how the launching effect works! A natural thought is this: first you perceive objects, then you identify causal interactions based on contiguity etc. This turns out to be completely wrong.

 

The impression of launching is judgement-independent. So it can't be a consequence of thinking about the interaction. Still, it might be a consequence of perceiving objects in certain relations to each other. However a key finding shows that this is wrong. Surprisingly, we don't first perceive objects and then get the launching effect; rather, the launching effect is tied up with perceptual process of identifying objects' surfaces.

 

[I'm about to talk about illusory causal crescents. I first show them two videos. This is a full overlap video. You can drag the slider to show them that it's full overlap, but first ask them what they see.] [Static images follow in case video doesn't work.]

 

Normally, if the two balls overlap completely, subjects report seeing a single object changing colour.

 

[This is a causal capture video with full overlap. Focus on the top sequence. Tell me what you saw! You can drag the slider to show them that it's full overlap, but first ask them what they see.] [Static images follow in case video doesn't work.]

 

Normally in this case people report the impression that the top sequence collided. That is, they didn't pass, they collided.

 

Causal capture is described by \citep{Scholl:2002eb}. As I said, normally, if the two balls overlap completely, subjects report seeing a single object changing colour. But if we show subjects a sequence like the launching effect but where the first square overlaps the second's position before it moves. When this event is shown is isolation almost all subjects see it as a single object changing colour. But when the event is shown with an unambiguous launching effect nearby, almost all subjects now see the 'overlap' event as a launching. Causal capture means that we can show subjects a sequence with complete overlap and still have the report a causal effect.

 

Why do we care about causal capture? Because it gives rise to illusory causal crescents ...

 

[Now I'll explain illusory causal cresecents.]

 

Here's a static image representing the sequence you saw first, when there was full overlap.

 

 

 

*here or later? Contrast Spelke’s view. ‘objects are conceived: Humans come to know about an object’s unity, boundaries, and persistence in ways like those by which we come to know about its material composition or its market value.’ \citep[p.\ 198]{Spelke:1988xc}.

 

(*This figure just shows when the overlap event was perceived as causal.)

 

(*This figure shows the illusory causal crescents -- the underestimation of overlap.)

 

How do we know that object indexes track some causal interactions?

 

You can sort of see that object indexes track some causal interactions by looking at Spelke’s figures. But let’s try to be more careful ...

 

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.

 

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

 

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

 

Background: object-specific preview effect

 

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

 

 

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.

 

 

Recall the table ...

 

 

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 a causal interaction because no delay between two movements is consistent with a causal interaction.

 

Michotte said this:

 

 

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 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.

 

Recall this experiment about causal interactions. Infants are more interested when the ball moves through the bench (the `inconsistent' condition). Why? Because the object index stops at the solid barrier.

 

Why do 2.5-year-olds look longer when experimenter removes the ball from behind the wrong door? Because the object is attached to an object index behind another door. But why don't they reach to the correct door? Because having an object index attached to an object is not sufficient to intitate goal-directed action.

 

Why do infants look longer at the 'launching' reversal? For the same reason you report an impression of causation: it's the weird behaviour of the object index, moving to this object, which is discrepant with what you know about the boundaries of these objects. (I.e. its explained by \citet{Krushke:1996ge})

 

The operations of object indexes (plus perceptual abilities (e.g. for texture)) can explain how infants meet all three requirements on knowledge of objects ...

 

Contrasting features with physical objects suggests three requirements on having any knowledge about particular physical objects.