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Perception of Causation

The question for this section is,

Can humans perceive causal interactions?

adults: (a) verbal reports. So what?

Thines et al (1991)

‘There are some cases … in which a causal impression arises, clear, genuine, and unmistakable, and the idea of cause can be derived from it by simple abstraction in just the same way as the idea of shape or movement can be derived from the perception of shape or movement’
\citep[p.\ 270--1]{Michotte:1946nz}
Adults will also report experiencing causal interactions including pullling, ...

Scholl & Tremoulet 2001, figure 2

... disintegration ...
... and bursting.
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}.

Heider & Simmel 1946, figure 1

How to get beyond intuition?

Michotte: the experience of launching depends on interactions among various factors including

  • the relative speeds of the two objects
  • the delay between the first and second objects’ movements
  • the spatial gap between the two objects
  • the trajectories of the two objects.
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.

Michotte 1946 [1963], p. 115 table IX (part)

Maybe this is clearer as a figure.

Michotte 1946 [1963], p. 115 table IX (part)

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, ...

detecting launching effects at 6 months

... but first note that 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?
Infants at around six months of age seem also to distinguish launching from other sequences, much as adults do \citep{Leslie:1987nr}.
[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.

Leslie & Keeble 1987, figure 4

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

Leslie & Keeble 1987, table 4

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

How to get beyond intuition?

The launching effect: detecting a 50ms difference in the delay between two movements.

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 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.
  1. How is launching detected? For example, does it involve perceptual processes?
  2. Why is a delay of up to around 70ms consistent with the launching effect occuring?
We'll focus on the first question and come back to the second one later.
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.

Guess how the launching effect works!

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.


Thines et al (1991)

illusory causal cresecents.
This depends on causal capture \citep{Scholl:2002eb}.
Normally, if the two balls overlap completely, subjects report seeing a single object changing colour.

Scholl and Nakayama 2004, figure 2 (part)

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.
‘when there is a launching event beneath the overlap (or underlap event) timed such that the launch occurs at the point of maximum overlap, observers inaccurately report that the overlap is incomplete, suggesting that they see an illusory crescent.’
\citep[p.\ 461]{Scholl:2004dx}
Why does the illusory causal crescent appear? Scholl and Nakayama suggest a
‘a simple categorical explanation for the Causal Crescents illusion: the visual system, when led by other means to perceive an event as a causal collision, effectively ‘refuses’ to see the two objects as fully overlapped, because of an internalized constraint to the effect that such a spatial arrangement is not physically possible. As a result, a thin crescent of one object remains uncovered by the other one-as would in fact be the case in a straight-on billiard-ball collision where the motion occurs at an angle close to the line of sight.’
\citep[p.\ 466]{Scholl:2004dx}
*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 just shows when the overlap event was perceived as causal; not essential.)
‘just as the visual system works to recover the physical structure of the world by inferring properties such as 3-D shape, so too does it work to recover the causal … structure of the world by inferring properties such as causality’
\citep[p.\ 299]{Scholl:2000eq}

Scholl and Nakayama 2004, figure 5

The question was,
  1. How is launching detected? For example, does it involve perceptual processes?
  2. Why is a delay of up to around 70ms consistent with the launching effect occuring?[This question is blurred]
We've just taken a step towards answering this first question, but more is needed.
The second question remains open.

Summary so far

The question for this section was:
Can humans perceive causal interactions?


  1. Adults report experiencing a launching effect.
  2. These reports show that they can make minute distinctions.They can detect tiny, 50 millisecond differences in the delay between two movements.
  3. Infants detect apparent causal interactions from 6 months of age or earlier.
  4. Detecting apparent causal interactions is bound up with perceiving objects.It's not that we first perceive objects and surfaces, and then identify causal interactions. Rather, identifying causal interactions is part of perceiving surfaces and objects. So the launching effect does seem to involve perceptual processes.
This fourth point suggests we should step back and consider how objects are perceived. That is what we'll do next.