I like to run. It is very peaceful to go for a nice long run on a beautiful sunny day. There are days when I feel like I can run forever. My legs just keep moving. On those days, I fix my eyes on a point in the distance and before I know it I am passing what I was looking at. Other days, of course, I feel like I'm running in Jell-O. On those days, the end of the block looks like it is a million miles away.

Of course, that must just be a metaphor, right?

Actually, there is growing evidence that what you are able to do with your body at any given time affects what you see. A nice example of this work comes from a paper by Sally Linkenauger, Jessica Witt, Jeanine Stefanucci, Jonathan Bakdash, and Dennis Proffitt in the December, 2009 issue of the Journal of Experimental Psychology: Human Perception and Performance.

They had people sit at a table and judge the distance between them and a tool that was placed somewhere on the table. For example, in one study, right-handed people sat at a table and a hammer was placed on the table. They were to imagine picking up the hammer from the table. On each trial of the experiment, the hammer was placed some distance away from them. Then, they had to judge the distance between them and the hammer. To make this judgment, the experimenter who was sitting across from them opened up a tape measure (with the numbers facing away from the participant). The participant directed the experimenter to lengthen or shorten the exposed ruler until they thought it was the same length as the distance from them to the hammer.

Now here's the interesting part. On some trials, the hammer was placed with the handle facing to the right, so that it would be easy to pick up the hammer by just reaching straight out for it. On other trials, the hammer was placed with the handle facing to the left. In order to pick up the hammer on these trials, people had to imagine reaching and turning their hand over to grasp the hammer normally. This movement is awkward. So, the hammer would be easier to pick up when it was on the right than when it was on the left.

When the hammer was placed so that it would be easy to grasp, people systematically judged the distances to be shorter than when the hammer was oriented so that it would be hard to grasp. That is, the ease of picking up the hammer affected people's judgments of how far away the hammer was in space.

Interestingly, left-handed people did not show the same pattern of data. The left-handers judged the tool to be the same distance away regardless of whether it was oriented in a way that would make it easy to grasp or hard to grasp. The authors point out that left-handers are usually more ambidextrous than right-handers, and so even though they were asked to imagine picking up the hammer with their left hand only, they may still perceive objects as if they will be grasped in a natural way regardless of how they are oriented.

Finally, I should point out that Dennis Proffitt and his colleagues have done a number of studies of the effects of actions on perception. For example, they also find that people judge hills to be steeper when they are tired or weighed down by carrying a heavy backpack than when they are not.

These findings make clear that our visual system does not just give us a true picture of what is out there in the world. Instead, the human visual system provides information that is useful for acting on the world. It might take us a long time to reason through how likely we are to succeed at climbing a hill or running a distance or picking up a heavy tool. By biasing our judgments of distance and steepness, though, our visual systems provide us with information that is available quickly and gives us a way of estimating whether we will succeed at a course of action.