Your Inner Robot

Have you ever stepped onto a stopped escalator? If so, you'll probably have experienced a strange sensation - a rather unique kind of giddy, whole-body jolt - and you may well have stumbled clumsily for a second or so.

But why? And why is it that the phenomenon happens even if you've seen and understood that the escalator is stopped? (This has happened to me, more than once. It's rather disturbing.)

This is the question that prompted Fukui et al, a team of Japanese psychologists, to write a paper wonderfully titled Odd Sensation Induced by Moving-Phantom which Triggers Subconscious Motor Program. As they put it -

This phenomenon raises an intriguing question as to how implicit motor programming escapes from conscious top-down control and offers an opportunity to study it.

Fukui et al took seven men and asked them to walk onto three objects - a moving up-escalator, a stopped up-escalator, and some wooden steps built to be the same size as the escalator steps. The volunteer's movements were analyzed by placing reflective markers onto their back and feet, and recording their movements using an infra-red camera system. They were also asked to rate to what extent they experienced the odd sensation.

The authors point out that there are three possible explanations for the phenomenon -

1. The odd sensation concurrently but independently occurs with the postural or leg behavioral properties [i.e. it has nothing to do with movement and] simple unfamiliarity with encountering a stopped escalator could induce the sensation.
2. The odd sensation occurs due to the unique height of the steps, in which the first step is shorter than others...
3. The odd sensation results from an inappropriate action inconsistent with the current situation despite the proper understanding of the situation. Stepping onto a moving escalator is a highly habituated action, so the habitual motor program for a moving one would emerge even when we step onto a stopped one. The subconscious emergence of the habitual escalator-specific motor program leads to the inappropriate motor behavior, which leads to the odd sensation.

And their results point to the third explanation. The volunteers reported a strong "odd sensation" stepping onto the stopped escalator, but not the equivalent wooden steps. This rules out the possibility that the step height is to blame. Further, walking onto the stopped escalator felt weird even when it was done immediately after two goes on the wooden steps.

The results of the body movement analysis were also enlightening. When you step onto a (moving) up-escalator, you need to walk faster and tilt your body forwards just before you step onto it, in order to maintain balance. And this is what the volunteers did, with the moving escalator. With a stopped escalator, they didn't, which is entirely appropriate. In other words, their movements started perfectly normally.

But when they actually stepped onto the stopped escalator, things suddenly went wrong. Their walking velocity decreased, they swayed forwards, and their feet decelerated too soon on the downward movement - as if they were "expecting" the steps to be higher than they were.

This suggests that, in the stopped-escalator situation, a habitual escalator-specific motor program anticipating the step elevation emerged regardless of the full awareness that the escalator was stopped. The actual downward movement of the heel was therefore too short to arrive at the step, so corrective lower limb movement was required.

The authors interpret these results to mean that the visual stimulus of stepping onto an escalator triggered an escalator-specific movement program, despite the fact that the volunteers consciously knew that this was not required.

The idea of movement program is not new. After all, we all know that when walking, driving, speaking, or doing anything else, a lot of what we do is "unconscious". We're not consciously aware of every single little movement. (At the moment I'm aware of typing, not of moving one finger, then another, then another...) But in this experiment, the movement program was not just unconscious but flew in the face of consciousness - it occurred despite conscious awareness that it was wrong.

But that's not all. The strength of the odd sensation seemed to correlate most strongly with the degree of inappropriate upper-body movement, rather than leg movement. Although this wasn't true in every single subject, and although the mathematics of the correlations are a little complex, the authors make a fascinating suggestion to explain why this is -

[In everyday life] limb movement (e.g., lower limb movement for foot clearance) is controlled by a voluntary component (although an automatic component also exists) with action intention, while posture is controlled mainly in an automatic fashion

In other words, the reason why the automatic-but-wrong leg movements were not felt as "odd" might be that they were felt as part of the conscious act of walking - although of course we know that they can't have really been consciously planned, because the volunteers knew that the escalators were stopped.

But if that's true, and if our minds can trick us in this way, we face the rather mind-bending possibility that a lot of other things we do in life might be only felt to be consciously planned. This is not the first experiment to raise this idea, but it is one of the simplest and most elegant. Philosophers and others with an interest in the question of "free will" should pay close attention next time they step onto an escalator.

Fukui, T., Kimura, T., Kadota, K., Shimojo, S., & Gomi, H. (2009). Odd Sensation Induced by Moving-Phantom which Triggers Subconscious Motor Program PLoS ONE, 4 (6) DOI: 10.1371/journal.pone.0005782