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, ...

[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

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.

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.

[Now I'll explain illusory causal cresecents.]

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

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

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

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.

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 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})