Most mothers know that their babies, when they first learn to crawl, will crawl right off the changing table or even a stair landing. Why don’t young infants fear heights? This fearlessness is not due to a lack of depth perception. Indeed, infants develop stereopsis, or the ability to use the two eyes to see in 3D, at 3 to 4 months of age, yet they don’t start to crawl until about 7 or 8 months, and they don’t develop a wariness to heights until around 9 months of age.

A recent study reported in Psychological Science offers an intriguing explanation. Since infants develop a fear of heights after they start moving on their own, self- locomotion must play an important role. When we move forward, objects to our right and left appear to move backward. This is called peripheral laminar optic flow. (You can see a great demonstration of this in a You Tube video of the opening of the movie “Hugo.” Look for the scene in the train station starting at around 42 seconds.) As babies crawl about, they become increasingly aware of the view to either side and learn to use peripheral laminar optic flow to orient and balance themselves. This awareness also leads to a wariness of heights.

In the study just mentioned, infants were placed in a room in which the side walls could move forward and back. The side walls were moved in the direction that creates the optic flow that the infant would see if he were moving forward. Since he would sense himself moving forward, he would adjust by swaying backward. The experimenters found that only infants who had learned to crawl responded with a postural adjustment to the moving walls. Younger babies did not. So, a baby’s own experience with self-locomotion teaches him how to interpret laminar optic flow. What’s more, these were the infants who demonstrated a wariness of heights. You can see videos of the moving wall experiments here.

In a second experiment, the investigators placed infants, who had not yet learned to crawl, in a cart that they could power themselves. In this way, these babies experienced a form of self-locomotion. After 15 days of training, these infants also responded with postural changes to the moving walls and demonstrated a fear of heights. 

Why would an increased awareness of optic flow help infants learn to avoid drop-offs? Imagine yourself as a baby crawling toward the end of a table. To your right and left, you see the table below you. But just as you are about to crawl right off the edge, you poke your head beyond the table’s surface, and your peripheral vision tells you that the table below you has disappeared. All you see is thin air so you stop and go no further.

I was very struck by this study because I experienced something similar when I went through vision therapy. Since I was cross-eyed all my life, my two eyes saw very different views. I dealt with this conflict by attending to what was front and center and ignoring the visual periphery. With vision therapy, I learned to simultaneously aim both eyes at the same place in space, resolving this visual conflict. I learned to see in 3D and also became increasingly aware of the visual periphery and optic flow. For example, about a month after I began to see with stereopsis, I wrote in my journal, “When I walk, I am aware that the objects to the side of me are changing their spatial relationship to me. As I was walking up Mosier Street today, I notice that things to the side of me went backward as I went forward. I hadn’t noticed that before.”

A few months after writing this journal entry, I went on a trip with my family to Hawaii. We visited a scenic viewing spot overlooking a beautiful canyon. I went right up to the protective railing to take in the view. Suddenly, I felt like I was floating unsupported high above the incredibly deep canyon. This sensation was much stronger than I had ever experienced before. While leaning over the safety rail, with my heightened sense of the visual periphery, I was keenly aware that the ground dropped off to my right and left. I quickly backed away from the railing, moving to a safer spot behind other tourists and the view below. Indeed, I am a bit more wary of heights now than I was before gaining stereopsis and a wide angle view of the world.

Eyes on the Brain

A neurobiologist explores the amazing capacity of the brain to rewire itself at any age
Susan Barry by Rosalie Winard

Susan R. Barry, Ph.D., is a professor of neurobiology in the Department of Biological Sciences at Mount Holyoke College and the author of Fixing My Gaze (June, 2009).

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