July 2010


Perception and Short-Term Unconscious Adaptation of Human Locomotion

Many activities that most of us take for granted are actually very complex with a lot of interesting science behind them. It's easy to see evolutionary advantages to even the simple ability of shadow interpretation; if the shadows cast on another person's face prevents you from perceiving their emotions, the two of you won't easily relate with one another, hindering cooperation (and a sabertooth cat may pick you off as a mid-day snack).

Take stepping over a brick in the sidewalk. Do you get down there with a ruler; measure the height, length, and width of the brick; and carefully move your leg up just high enough to avoid it?

No, you probably just step over it, with little if any thought. If you're me though, you'll probably trip over it anyway, but I'm just about the clumsiest person in the world, so forget about me for the moment.

It's actually not known whether, or now much, explicit perception is required for human locomotion; previous studies of this topic have given conflicting results. As discussed later on, this may have evolutionary implications.

Christopher Rhea (Purdue University, United States) and coworkers have re-examined this issue. They have found that human locomotion is not necessarily under explicit conscious control (and maybe my subconscious self is the one who's a total klutz).

Navigating over an obstacle.

The scientists' goal was to check for a relationship between perceptual differences in obstacle height and actual locomotion. They studied 15 adults in this work: 2/5 of them were male, and they were approximately 25 years old, 5'8" tall, and 162 pounds (the greatest varability was in the participants' weight).

They wanted the participants to navigate over an obstacle placed on a short walkway, and to minimize extraneous visual cues as to the height of the obstacle. They consequently illuminated the grey walkway with one 40 watt light bulb placed roughly 10 feet behind the start.

They set up two kinds of obstacles: a "full obstacle" and a "perimeter obstacle." Neither kind was more than a foot long.

The full obstacle was completely covered with glow-in-the-dark tape. In contrast, only a thin outline of the top and side edges of the perimeter obstacle was covered in this manner.

This results in an illusion of the full obstacle being larger than the perimeter obstacle. Nevertheless, as mentioned, they were actually of the exact same dimensions.

The participants were asked to navigate both (full and perimeter) obstacles (assigned randomly to do one or the other first), and to perform three tasks in order. The first task was to estimate the height of the obstacle; the second was to step over the obstacle (50 consecutive times), and the third was to again estimate the height of the obstacle.

The participants estimated obstacle height with a ruler along the wall to their left. The height of the obstacle relative to the height of the participants' toes (i.e. actual obstacle clearance) was measured from the right of the obstacle with a laser.

Perception and adaptation.

The participants thought that the full obstacle was slightly larger than the perimeter obstacle, as expected (remember that they were actually the same dimensions). There was no statistically significant difference between pre-navigation height estimate and post-navigation height estimate.

In contrast, the participants' toe elevation over the full obstacle was higher than the perimeter obstacle for the first five of fifty steps, but not statistically higher for the last five of fifty steps. Toe elevation decreased over time for the full obstacle.

What this shows is that the participants still thought that the full obstacle was larger than the perimeter obstacle after fifty trials. However, after fifty trials, their physical clearance over the full obstacle approached that of the perimeter obstacle.

Therefore, perception remained unchanged, but physical navigation brought locomotion back into alignment for both types of obstacles. This says that the human locomotor system can adapt over time without our being aware of it, even if explicit perception does not adapt.


Why are these results interesting? They strongly suggest that perception of actions and control over actions are guided differently.

From a practical standpoint, it may be something to keep in mind when designing artificial limbs. It may not be possible to design an artificial limb that operates as smoothly as the lost limb, if subconscious thought guides its movement.

I think it would be interesting to extend this study to non-consecutive trials. Is there subconscious locomotor adaptation to tasks that may be performed a bunch of times, but not in close sequence, such as reaching for the tube of toothpaste every morning?

I also wonder how long subconscious training lasts; for minutes, hours, days? Does how long it lasts depend on how much time was spent learning it in the first place?

Although only short-term adaptation was demonstrated in this research, it's reasonable to suspect that long-term subconscious locomotor adaptation would provide a strong evolutionary advantage. If you can perform routine tasks without having to think much about it, you have more time to think about new and more complex situations, possibly standing a better chance at survival.

ResearchBlogging.org for more information:
Rhea, C. K., Rietdyk, S., & Haddad, J. M. (2010). Locomotor Adaptation versus Perceptual Adaptation when Stepping Over an Obstacle with a Height Illusion PLoS ONE, 5 (7) DOI: 10.1371/journal.pone.0011544