This Sunday, I’ll be taking part in a little stroll around London.

I’ve been training for this for a few months now, and I’ve noticed something in my longer runs. I become very bad at estimating how long it will take to pass a given point. Like many long distance runners, I tend to resort to distraction techniques as the fatigue kicks in. I pick a paving slab or pavement and “bet” myself how long it will take to pass that point. I find that I consistently overestimate how long it will take to reach that point – a lamppost that I think is 10 seconds away tends to be reached in seven. This effect gets more pronounced the longer I run. I’ve thought of several reasons why this may be:

  1. I consistently round up times to the nearest block of five seconds.
  2. I count seconds too slowly.
  3. I’m bad at estimating the distance to a landmark, especially when tired.

Hypothesis three seemed plausible, especially when I remembered reading a nice study by Proffitt et al (2003) that demonstrated (inter allia) a significant increase in perceived distance when laden with a backpack than when not. Participants (n=24) were asked to give a visual reckoning of distance from themselves to a cone in a flat, grassy field. Half of the participants were unladen for the duration of the task; the other half wore a backpack throughout. Proffitt et al found that all participants underestimated distance, but the laden participants gave significantly higher estimates of distance than the unladen ones.

Perception of distance is usually thought of as being due to a combination of visual cues, such as occlusion, perspective, relative size and height, parallax, texture and brightness. However, there is a model that suggests the perception of egocentric distance is also functional.That is, what we really estimate is some function of the effort it will take to travel from “here” to “there”. Poffitt et al summarise this nicely:

“Berkeley concluded that perception of distance must be augmented by sensations that arise from
eye convergence and from touch. For egocentric distances, tangible inforformation arises from the effort required to walk a distance, and thus, effort becomes associated through experience with visual distance cues…”

They contrast the “functional” model of distance perception with the “geometric” model of visual perception:

“[In] complex, natural environments viewed with both eyes by moving observers, there is suf´Čücient information… to specify egocentric distance. Thus, a role for effort in perceiving distance seems unnecessary if the goal of perception is to achieve a geometrically accurate representation”.

Which, of course, it isn’t. The “goal” of perception (or at least, the reward associated with perception) is the ability to interact effectively with the environment. The backpack result presented by Proffitt et al supports the argument for a functional model of distance perception: what is estimated is not distance directly, but the effort required to cover that distance.

So far so plausible. But results in Hutchinson and Loomis, (2006a) and Woods and Philbeck, (2006) appear to cast doubt on the reproducibility of the backpack effect.

Hutchinson and Loomis made two departures from the original method. The first difference was the use of a within-participants setup (each participant made estimates in the laden and unladen condition). The second difference was that participants were told that they would have to estimate the size of the target, rather than walk to it (Proffitt et al, 2006). Hutchinson and Loomis (2006b) state that they failed to find any effect in the between-participant comparisons, and that the original Proffitt et al study did not prompt the participants to anticipate walking to the target.

Woods and Philbeck (2007) also fail to find an effect. Their method also differs from the original, in that they asked some participants to rate their (anticipated) effort on the Borg CR10 scale. However, levels of anticipated effort were also not significantly different between laden and unladen conditions. Which, to me, raises the question of whether or not the backpacks were sufficiently heavy. There certainly doesn’t seem to be anything in that result that invalidates the functional model of distance perception: no difference in effort = no difference in perceived distance.

What can be made of these differences? There are several other studies, many by Proffitt and Bhalla that support the functional model of distance (and slope) estimation. A functional model of distance perception is prima facie plausible – experiential learning is more likely to build a model of distance based on exertion (which is directly accessible to the organism) than a geometric measure of distance (not directly accessible, despite the invention of the ruler). However, the backpack results at least have not gone unquestioned.

So where does this leave me on Sunday? I haven’t had time to carry out a complete literature review of the effects of glucose depletion/aerobic exercise/fatigue/endorphin release on perception- but if someone wants to sponsor me to run next year’s marathon, I’ll happily participate in psychophysics experiments en route. And to be on the safe side, I’ll be running this year’s Marathon carrying as little as possible.

Hutchison, J. J., Loomis, J. M. (2006a). Does energy expenditure affect the perception of egocentric distance? A failure to replicate Experiment 1 of Proffitt, Stefanucci, and Epstein (2003) [Abstract]. Journal of Vision, 6(6):859, 859a, doi:10.1167/6.6.859.

Jeffrey J. Hutchison, Jack M. Loomis (2006b), Reply to Proffitt, Stefanucci, Banton, and Epstein, The Spanish Journal of Psychology, Vol. 9, No. 2, 343-345, [PDF]

Proffitt, D.R., Stefanucci, J., Banton, T., Epstein, W. (2003). The role of effort in perceiving distance. Psychological Science, 14(2), 106-112. DOI: 10.1111/1467-9280.t01-1-01427

Dennis R. Proffitt, Jeanine Stefanucci, Tom Banton, and William Epstein (2006) A Final Reply to Hutchison and Loomis, The Spanish Journal of Psychology, Vol. 9, No. 2, 346-348, [PDF]

Woods, A. J., & Philbeck, J. (2007). Does perceived effort influence verbal reports of distance? [Abstract]. Journal of Vision, 7(9):421, 421a, doi:10.1167/7.9.421.