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.
This article was originally published with the title Ask the Brains.
See what we're tweeting about
7 Comments
Add CommentWe also estimate distance when each eye focuses on a scene.
Reply | Report Abuse | Link to thisI guess that the brain approaches many problems by elaborating a mental simulation of the issue. For example, when you watch a Rubik's cube, your brain starts automatically simulating the possible movements of pieces. In this special case, as there are trillions of possible positions, what the cube problem does is probably just saturating and overloading the system, such in a computer with too much hard disk full. There are tasks that may be just like a bug for the brain, as the memory capacity of brain is not infinite
Reply | Report Abuse | Link to thisThe problem of solving a rubik's cube is that each move changes all the other colour combinations exponentially and it is hard for the mind to keep track of the constantly changing locations. The only way to effectively solve it is to memorise set moves to manipulate the colours to different locations, however this can only become implicit after many hundreds of hours of practice.
Reply | Report Abuse | Link to thisThere is hardly anything new or revealing in what Terry Sejnowski has said. Anyone having focused attention or inner observation of the workings of our brain could see it
Reply | Report Abuse | Link to thisRajnish Roy
http://rewiringthebrain.net/
The evolutionary background to our brain calculations can plausibly explain the origins of our 3D vision in full-colour inter-active "movie" mode! Which means: fruit & berry picking up in the trees brought us our superb spatial expertise! This truth was brought home to me when I picked some handfuls of moreish mulberries from an overhanging & windswept tree in our backlane - with varying success! For evolutionary diet concepts see also :Youthevity.com
Reply | Report Abuse | Link to thisA long time ago I noticed that if I watched a movie or television with only one eye I would get more of a sensation of depth than when watching with both eyes. This seemed to make sense to me because I figured that the brain uses many ways to determine depth (size, occlusion, movement, etc) but probably gives priority to stereo vision. Closing one eye removes stereo vision from the equation, allowing the other cues to come to the fore. Try it some time -- especially when there is a scene involving a lot of relative movement -- a school of fish swimming is a good example. People tend to laugh when I tell them this theory -- but my ophthalmologist agreed with me.
Reply | Report Abuse | Link to thisThat makes sense zselway. Why only use three dimensional processing to look at something that is really two dimensional. I find that looking at everything from many angles gets a better 'view', like listening to an engine also, not just looking at the body, when evaluating an automobile or looking inside the engine. Using one eye tells your mind to use other clues and methods. And as for people, if you think what everyone else thinks, that's group think, almost always a mistake.
Reply | Report Abuse | Link to this