Neuroscience

The Size-Weight Illusion is Functional, and It's About Throwing

My colleagues, Geoff Bingham and Qin Zhu, have recently published some fascinating data which has emerged from their work on the uniquely human skill, long-distance throwing. This is a novel and rich perception-action task which Bingham and Zhu (and recently, me) have been investigating for some time, with many interesting results. I'll get onto blogging about this project once I've caught up with the coordination studies and have had some time to get my head around the data I'm helping generate.

I wanted to blog about this new paper, though, because it's an exciting result which deserves all the attention it gets. The result is about the size-weight illusion, one of the most robust illusions around. As I've talked about before, illusions are a concern to ecological psychologists only in that they suggest the task has been incorrectly characterised. This paper presents data that suggests the size-weight illusion is actually functional, and that it reflects the readiness of the human perception-action system to throw objects long distances.

This paper has seen some activity in the popular press already (e.g. here and here): Geoff's hoping for the NYT Science section too!

UPDATE: Geoff being interview on NPR

The size-weight illusion
The size-weight illusion is, as I said, extraordinarily robust. It is typically described as a mis-perception of weight. Two objects of the same weight but different sizes are misperceived as being of different weight, with the larger one rated as lighter. People seem unable to perceive weight without taking size into account.

There are two basic types of theories about the cause of the illusion (references from Zhu & Bingham, 2010)

Top down: You expect the larger item to weight more, so you prepare to lift a heavier weight and the illusion is caused by it feeling lighter than you expected (e.g. Ross, 1966). This is 'top'down' in the sense it is being caused by an expectation, a cognitive state;
Bottom up: People actually perceive a complex variable composed of both size and weight and use this in their judgements (e.g. the inertia tensor, Amazeen & Turvey, 1996; density, Huang, 1945). This is 'bottom-up' in that it is about perception.

None of these types of theory have ever quite been able to fully explain all the data (indeed, the inertia tensor hypothesis was recently explicitly tested and rejected in a paper by Bingham's group that is under review, although it's still an area of active research). Both approaches entail some learning, but children as young as 18 months reliably show the illusion (Kloos & Amazeen, 2002). The expectation account fails because the illusions persists when people are fully informed about the weight of the objects, and the illusion persists even when people are actually performing lifts appropriately for the correct mass (Mon-Williams & Murray, 2000); there's no evidence in their lifting behaviour that the person was initially surprised.

Zhu & Bingham then suggest that, given the illusion seems to be about perceptual experience, rather than online action control, perhaps it is about an affordance. Gibson (1979) suggested that these are what animals perceive in order to control their behaviour; they are real properties of the environment, but not necessarily properties as described by physics. The illusion suggests that the affordance in question should require the perception of both the size and weight of an object; such an affordance is the throw-ability of an object.

The dynamics of throwing

Throwing an object to a maximum distance means optimising the parameters of projectile motion. Some of these parameters are constant or out of the control of the thrower (e.g. gravity, air resistance). Some of the parameters are about the action of throwing (release angle, release velocity) - these are what the thrower must control to produce a throw.

Critically, some of the parameters are about the object: for a given set of other parameters, maximising the distance of a projectile motion requires a specific combination of object size and weight. For throwing, this combination must fall within the range that is graspable and liftable by a human. By hypothesis, this relationship needs to be perceived when selecting an object to throw if you wish to maximise the distance of your throw, and empirically humans are very good at selecting the size/weight combination they can, actually, throw the furthest (e.g. Zhu & Bingham, 2008). Imagine the common game of throwing stones into a lake; we have very clear preferences about which stones we think we can throw the furthest, and these preferences are very accurate. Stable, accurate performance implies informational control (although the specific information variable being perceived has, as yet, failed to reveal itself; e.g. Zhu & Bingham, 2010).Information implies an affordance, and the critical object property for throwing is a combination of size and weight. Perceiving the affordance of 'throwable-to-a-maximum-distance' would require perceiving this combination. How does this variable, whatever it's precise form is, relate to the size-weight illusion?

The experiment

The study was simple. 12 participants (all capable of throwing a tennis ball 20m) were asked to make two separate sets of judgements about a set of objects. These were 48 balls which had been custom made; they came in 6 sizes (all graspable) and each size came in 8 weights (all liftable; see Table 1 in the paper for details).

Participants then made two judgements about the objects;

First, they were presented with a row of objects all the same size but different weights, and asked to choose and rank the 3 objects they thought was best suited for maximum distance throwing. Participants were allowed to hold and heft the objects (palm up, moving at the wrist).

Second, the participants were asked to heft a comparison object, and then asked to choose and rank the 3 objects from each size set which felt about the same heaviness. The comparison object was either the object selected for throwing from the smallest set, or from the largest set.

In each case, the three choices were combined into a weighted mean, with the first, second and third choices weighted as .5, .33 and .17 respectively. This weighted mean allows for the fact that the object set contains discrete weights and sizes, and the actual 'equivalent weight' may not exist in the set although be on the continuous function the set samples; the weighted mean can then be a legitimate value that just doesn't exist in the set. The results are in Figure 1.

Figure 1. Mean selected object weights for throwing judgments and heaviness

judgments as a function of object sizes (from Zhu & Bingham, 2010)

The dark line shows the object weights judged as optimal for throwing to a maximum distance as a function of size; larger objects need to be heavier to be optimal. The dotted line shows the object weights judged as equal heaviness to an object of a different size but that had been selected as optimal for throwing. At each size, an object of different weight was required before people thought it was equally heavy to the small or large comparison object; this is the size-weight illusion. People don't just pick any different weight, however; they pick specific objects. In fact, the weight chosen was the weight identified by those people as the optimal weight to throw to a maximum distance. In other words, objects at different sizes which afford throwing to a maximum distance all feel equally heavy.

Summary

The implication is clear: people do not misperceive weight as a function of size, they correctly perceive the optimal size-weight (heaviness) value for throwing to a maximum distance. The size-weight isn't an illusion, it is a side effect of asking a perceptual system to judge weight when what that system perceives is heaviness; throwability. The size-weight illusion therefore reflects a functional capacity of the human perception-action system.

This capacity seems to be unique to humans (apes and monkeys famously throw faeces, but not far and not accurately), and it is possibly an intrinsic capacity of humans. Zhu & Bingham speculate that the nervous system is 'ready' to be able to throw in the way it is 'ready' to learn language; what's innate isn't a module (a la Chomsky's universal grammar), but a perceptual bias - here, the tendency to perceive not size, nor weight, but heaviness. Throwing has long been thought a critical skill that helped us hunt animals otherwise too strong for us (e.g. mammoths) and it has been suggested the skill enabled to us to keep hunting successfully in the face of climate change during the last Ice Age. It is therefore exactly the type of skill you might expect to be favoured strongly by evolution; these data suggest we are indeed 'ready' to throw in just this way.

References

Zhu, Q., & Bingham, G. (2011). Human readiness to throw: the size–weight illusion is not an illusion when picking the best objects to throw. Evolution and Human Behavior DOI: 10.1016/j.evolhumbehav.2010.11.005

- The Size-weight Illusion Induced Through Human Echolocation
Echolocation is the ability to use sound to perceive the spatial layout of your surroundings (the size and shape and distance to objects, etc). Lots of animals use it, but humans can too, with training. Some blind people have taught themselves to echolocate...

- Learning The Affordances For Maximum Distance Throwing
Over the last couple of posts, I have reviewed data that shows people can perceive which object they can, in fact, throw the farthest ahead of time by hefting the object. Both the size and the weight of the object affect people's judgements and the...

- Is Hefting To Perceive The Affordance For Throwing A Smart Perceptual Mechanism?
In the last post, I reviewed Geoff's first paper looking at whether people can perceive the affordance for throwing an object to a maximum distance and a first swing at identifying the information specifying the affordance. People can perceive the...

- Hefting For A Maximum Distance Throw
From the task dynamic analysis of throwing for maximum distance, we've identified the fact that for a given release angle and maximum release velocity, there is an object whose size and weight optimises the distance it will travel when thrown. Can...

- Cracking The Tough Nut Of Chimp Tool Use
A paper just out in PLoSOne reports that chimpanzees, given some experience and enough of a weight difference, prefer to use heavier hammer stones when cracking hard nuts. This is apparently quite exciting: this is the first study to isolate weight as...

Neuroscience