Is it true that when we drive, walk or reach for something our brain performs calculations? Is this ability learned or innate?
—Helena Larks, San Francisco

Computational neuroscientist Terry Sejnowski of the Howard Hughes Medical Institute at the Salk Institute and the University of California, San Diego, answers:

Our brain is wired to perform calculations that let us judge how far away an object is when we walk or jump around or reach for a container of milk. Although this task may seem easy, it turns out that calculating depth is surprisingly complex.

When we look at an object, our eyes project the three-dimensional structure onto a two-dimensional retina. To see the three dimensions, our brain must reconstruct the three-dimensional world from our two-dimensional retinal images. We have learned to judge depth using a variety of visual cues, some involving just one eye (monocular vision) and others involving both eyes (binocular vision).

Binocular vision provides more precise perception of depth, allowing us to judge small differences between the images on both retinas, whereas monocular vision gives us a larger field of view. Occlusion, a monocular cue whereby an object that is closer partly obstructs the faraway one, enables the brain to judge relative distances. When one object occludes another, the observer can rank the relative distances of these objects.

Another monocular cue is motion parallax, which occurs when an observer moves his or her body (or just the head) to provide hints about the relative distance between objects. By moving the head back and forth, the motion allows you to see the objects from slightly different angles. A nearby object will move more quickly along the retina (creating a larger parallax) than a distant object, allowing you to determine which object is closer. When you are driving a car, for instance, nearby things pass more quickly, and faraway objects appear stationary.

Although our brain circuits are genetically programmed to judge depth from such visual cues, it takes experience to calibrate them. Initially children are bad at judging distance, but over time they train their brain to calculate distance. By adulthood we have become experts at judging depth but only with regard to objects in familiar environments. In unfamiliar territory, such as a new mountainous trail, automatic depth judgment fails because our brain has not yet calibrated new clues in the environment. In these new scenarios, we have to retrain our brain to compute distance.

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