Affordances are not probabilistic functions

The journal Ecological Psychology is hosting a special issue with papers from a Festschrift for Herb Pick. Karen Adolph and John Franchak have a paper that caught my eye about treating affordances as probabilistic functions, effectively applying standard psychophysical techniques to the study of affordance perception. 

The idea is this: affordance research typically treats affordances as all-or-none, categorical properties. You can either reach that object or you can't; you can either pass through that aperture without turning or you can't. You then measure a bunch of people doing the task as you alter some key parameter (e.g. the distance to the target, or the width of the aperture) and find the critical point, the value of some body-scaled measurement of the parameter where behaviour switches from success to failure. For instance, you might express the aperture width in terms of the shoulder width and look for the common value of this ratio where people switch their behaviour from turning to not turning.

The affordances of objects and pictures of those objects

People interested in how perception and action affect cognition have begun talking about affordances. This should be great news; the ecological approach suggests that affordances are the properties of the world that we perceive that enable us to control our actions, so if you are interested in how action can ground, say, memory or language, then discussing affordances should enable real progress.

The term 'affordance', however, is a technical term, and it refers to very particular properties of an organism's environment. There are methods for experimentally identifying exactly how these properties are composed, and there are methods for testing our perception of them. If you aren't using these methods, and if you aren't using the term correctly, then you aren't studying affordances.

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 distance thrown; however, only weight affects the dynamics of throwing (release angle and velocity are unaffected by changes in size). This rules out the smart perceptual mechanism proposed by Bingham et al (1989), which proposed that both size and weight changes affect hefting and throwing the same way. So how are people perceiving this affordance?

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 affordance. Bingham et al then identified an invariant relation between the timing of the motions of the wrist and elbow when people hefted the balls they chose as optimal for throwing, and showed that this kinematic pattern specified a peak in the function which determined how much kinetic energy was transferred to the ball. They suggested that this relation in the joint movements served as information for the dynamic property which led to a maximum distance throw, and that this is how hefting was able to provide information about throwing. They suggested that this was a smart perceptual mechanism for perceiving the affordance property.

That was where things stood until Zhu & Bingham (2008) ran an extensive replication and extension of the original study, to test the specific smart perceptual mechanism proposed by Bingham et al (1989). 

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 people perceive this combination ahead of time? More specifically, can people identify the object which affords throwing to a maximum distance, and if so, how?

Bingham, Schmidt & Rosenblum (1989) is the first paper investigating this question. It is a bear of a paper; I've stripped a lot of the methodological detail out in my summary so I can focus on the bigger picture. That bigger picture is this; Bingham et al first check whether people can identify objects that afford throwing to a maximum distance by hefting them ahead of time (they can). They then investigate the kinematics of hefting to identify an invariant relation in the timing of the wrist and elbow velocities and relate that invariant to the dynamics of throwing (specifically how it maximises the transfer of kinetic energy from the torso muscles to the projectile). They propose that using this invariant reflects a smart perceptual solution (Runeson, 1977) to the problem of selecting objects to throw to a maximum distance - future work (Zhu & Bingham, 2008) will actually show that this specific smart mechanism doesn't hold up, although the replacement is smart too.

The Task Dynamics of Throwing to a Maximum Distance

In my last post I went over the formal concept of task dynamics as a way of analysing a task to identify the affordances in that task. This post will examine the task dynamics of projectile motion and relate these to throwing to a maximum distance.This version of the task has been studied in detail over the last few years. There is another version of the task, namely throwing to hit a target (same dynamic, different parameters, therefore same task) and we will get to that later; we're working now on data from this task.

Part of my goal here is to lay out the research programme you should be following, if you want to study anything to do with perception and action. If you are interested in movement, and you aren't doing this kind of analysis as part of your work, then, I will suggest, you are doing it wrong. As we will see in future posts, this level of detail isn't just playing with numbers; a formal understanding of the underlying dynamics governing the task we are studying is utterly crucial if we want to be able to understand what people are doing, rather than simply describe their behaviour. 

Also it's fun :)

Task Dynamics And The Information They Create

Over the next weeks I want to turn my attention to a detailed account of the process by which you go about studying affordances (formalised as task dynamics) and the perception of affordances (via the kinematic consequences of those task dynamics) using throwing for maximum distances and for accuracy as the task. This post will introduce the basic research programme. Future posts will work through papers from my colleagues Qin Zhu & Geoff Bingham in order (I've done a couple already), as well as work from the animal literature because I want to find ways to use the analyses we're developing to answer questions about throwing and weight perception there.

These posts will do a few things. First, it's important to be as clear as possible about what affordances are, how we might possibly perceive them and how we can do the relevant science within the ecological approach to answer those two questions. Sabrina is developing ways to apply these methodological principles to the study of language, and we have both been working on the issue of information and how it comes to have meaning for us. Being clear about how this all unfolds in the perception-action literature is vital, because this is the foundation for what comes next. Second, I'm working on some throwing data right now and I need to work through the key papers in detail anyway. Third, I'm going to be developing an undergraduate perception-action class for 2014, and this will help me develop course material by laying out the form of the analysis and getting feedback on how well it's coming across. One of my goals is to look at all my collated and edited notes and realise I've accidentally written a text book :)

I'm going to talk about throwing because it's utterly fascinating. It's a complex task but it's one centred around a core dynamic (that of projectile motion) that physics has a pretty good handle on. This is letting us run detailed simulations of the task to identify the affordance structure of the task and see how throwers are operating with respect to those. Throwing entails perception of object and target affordances and the coordination of multiple body segments into precisely timed actions controlled by that perception. It also connects to all kinds of things in our evolutionary history (including, possibly, the origins of spoken language in the form we know) and our psychology (including the size-weight illusion and issues of the psychologist's fallacy). It's close to being that grail of psychology, something only humans do (other animals throw but rarely if ever for the kinds of distances and accuracy we can manage with ease). And most of all, it is endlessly interesting. The deeper I get into this, the cooler it gets.