How to Build a Valid Measure of Behaviour

One of the main problems facing psychology as a science is the issue of validity - what is the relationship between what you measured and what you are actually interested in? One of the things I like about studying movement is how straight-forward this issue is - we're interested in the control of action, so I just measure the action! The most common directly measured kinematic variable is displacement, or position over time; you can then derive (via differentiation) the various rates of change of the previous variable (velocity, acceleration, jerk, and, I kid you not, snap, crackle, and pop). In human movement we never tend to go past jerk, and you can do pretty well with just position and it's rate of change, velocity.

This post will discuss how we start from these basic kinematics and derive a measure of coordination that is  entirely valid, covers the entire space of possible states and provides a unique number for every possible state within that space. Psychology doesn't have a lot of these kinds of variables, but you need to be able to characterise your state space to do the kind of modelling I've been describing and advocating.

Stuff on the Internet (3 December 2010)

Some things that have come our way recently that are somewhat on topic for the blog.

Science

Ed Yong has a (paywalled) piece in the New Scientist about how birds visually perceive magnetic fields. He followed up on Not Exactly Rocket Science with interviews with two scientists working on this fascinating question: Klaus Schulten and Thorsten Ritz. 

NPR covers an interesting study on the fact that blindfolded humans tend to walk in circles. This fairly robust result is not due to handedness or any obvious biomechanical asymmetry, but seems to be the result of sensorimotor drift accumulating in the system (link to paper). Without calibration by vision, our sense of direction becomes increasingly noisy. This relates nicely to some data from a study some colleagues and I really need to write up, in which we got people to misperceive which finger was receiving vibrations after an extended period without vision of the hands.

The biomechanics of pterodactyl flight. 

Baroness Greenfield continues her war on the neurological consequences of modern technology.

Cool toys...er, I mean, educational stuff

Microsoft's Kinect motion tracker uses structured infra-red light arrays to passively track multiple moving objects. Deformations in the field are robust enough to detect game-specific motions as well as faces, etc. There's apparently a lot of people using the IR fields in their art (e.g here, via BoingBoing; mildly NSFW depending on where you work) and I think these could be a fun way to teach optic arrays to students. All I need is a Kinect and an IR sensitive camera...dear Microsoft...

Hi-speed video taken from a train.

Media

I caught up on some Rationally Speaking podcasts while recuperating from surgery on my wrist; these are the ones that caught my eye:

• Experimental Philosophy - to be honest, I have yet to hear an argument why this isn't just psychology done by people who don't really know how to do psychology properly.
• Thought Experiments - they worry about basically the same things I do, but are more generally optimistic about their utility.
• Evolutionary Psychology - here they were a little less generous, which I think Sabrina would be ok with.

Newton TV: I haven't had a chance to really poke around, but what could be wrong with science videos hosted by scientists?

Blogging
There's a new blog  on being a science blogger called The Science of Blogging. We're looking forward to some good advice for broadening our reach a bit.

This editorial about the dangers of science blogging is almost entirely wrong.