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Any walker or hiker can tell you that an uphill path is more challenging than a flat one. But does it literally look longer, too? A study in the May issue of Attention, Perception, & Psychophysics suggests that’s the case. Participants in the study said that the same distance looked farther when it was on a hill than when it was on flat land.

In past research, people had also reported that uphill distances looked farther, compared to equal distances on level ground. However, the reason for this discrepancy was still up for debate. Two competing explanations had been advanced:

  • Possibility 1: Response bias. People see the two equal distances as the same. After all, their brains are receiving the same input from their eyes in both cases. Study participants only say the distances “look” different because of some post-perceptual process, such as trying to please the experimenters.
  • Possibility 2: Action-specific perception. People actually see the two equal distances as different. That’s because, in addition to getting input from the eyes, the brain receives input from the body about energy and fatigue. All this information comes together in the brain to influence visual perception.

The new study’s findings offer strong support for the second viewpoint. It adds to the growing evidence that your visual perception of spatial properties, such as distance, is shaped in part by your body’s ability to act in the situation at hand.

How far will I have to walk?

For this study, lead author Nate Tenhundfeld, M.S., now a researcher at the U.S. Air Force Academy, teamed up with his doctoral advisor, Jessica Witt, Ph.D. Witt is an associate professor of psychology at Colorado State University and a prominent researcher on this topic. Her talk on “How Our Bodies Shape How We See” was one of the highlights of the Association for Psychological Science annual meeting in May.

Tenhundfeld and Witt devised a series of experiments, which were conducted in a grassy field including both a flat expanse and a hill. They used three methods to test how people perceived the same distance depending on the terrain:

  • Verbal estimates. Cones were placed at set distances on the hill and on flat ground, and study participants were asked to estimate how far away they were. This method has often been used in the past, but there’s a catch: People are terrible at it. “We would have people looking at a 9-meter [29.5 feet] distance and estimating something like 7 feet, which is incredibly far off,” says Tenhundfeld.
  • Visual matching. One cone was placed at a set distance up the hill. A second cone was on the flat ground, and study participants instructed an experimenter to move this cone until it appeared to be as far away as the one on the hill. Using this method, people didn’t have to assign numbers to the distances.
  • Blind-walking. A cone was placed at a set distance on either the hill or the flat ground. Study participants first looked at the cone, noticing how far away it was. Then they closed their eyes and attempted to walk the same distance in another direction. As an action-based measure, this seemed like a good way to study an action-specific effect.

All three measures showed the same thing: Distances on hills looked farther away than the same distances on level ground. “This convergence across measures is one way scientists have traditionally demonstrated that something is a true perceptual effect.” says Tenhundfeld.

How steep is that hill up ahead?

Other research has suggested that distance perception isn’t the only spatial property affected by bodily feedback. The way you see a hill’s steepness may change, too, depending on how much it would tax your energy to walk up it. In a classic study, hills appeared steeper when people were wearing a heavy backpack, already fatigued, or out of shape.

Can two walkers ever truly see eye to eye?

I’m more than 30 years older than Tenhundfeld. So, does that mean he and I would see the same hill differently? Most likely, yes. “What’s hitting your eyes and what’s hitting my eyes is identical,” says Tenhundfeld. “Traditional theories about vision would suggest that you and I are seeing the same thing. But we now understand that this isn’t the case. Our different action capabilities make it so that you and I are not perceiving the situation the same way.”

That’s his tactful way of saying that I might have more difficulty hauling my older body up the hill. At the same time as my brain is receiving optical input, it’s also getting bodily feedback about my levels of energy and fatigue. Tenhundfeld believes that my visual perception adjusts to my physical capabilities, which helps me gauge the demands of walking up the hill more realistically for me.

Now that I know what’s happening, can I consciously undo this calibration to bring my perception in line with objective reality? Most likely, no. Tenhundfeld says that people seem to be unable to override action-based effects on visual perception. He likens it to an optical illusion: Even after you’ve been clued in to the illusion, the way you see it remains unchanged.

Is my perception sabotaging my confidence?

Although you may not be able to control how you see a walking path or hiking trail, Tenhundfeld says you can choose what to do with that information. Rather than regarding it as cause for discouragement, think of it as a reminder to be prepared.

Get your body ready for a challenging trek by sleeping enough, drinking plenty of water, and fueling up on nutritious food. Lace up a good pair of walking shoes. Then pace yourself based on your physical capabilities. Perceiving a path as long or steep doesn’t necessarily mean you shouldn’t take it.

Linda Wasmer Andrews specializes in writing about health, psychology, and especially the intersection of the two. Connect with her on Twitter or Facebook.


Tenhundfeld, N. L., & Witt, J. K. (2017). Distances on hills look farther than distances on flat ground: Evidence from converging measures. Attention, Perception, & Psychophysics, 79, 1165-1181.

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