3D revisited

A comment to the post Time and Space started me thinking about whether our perception is more 2D and less 3D than we realize.

The following excerpt is slightly off the subject of the comment, but still interesting in this regard. It is from a Scientific American interview by Lehrer of Sue Barry. She learned to see stereoscopically after 40 years of seeing two dimensionally. (here)

LEHRER: What was it like to see the world in 3-D? Could you describe your first reactions?

BARRY: Many people tell me that the world looks about the same to them whether they look with one eye or with two. They don’t think stereovision is all that important. What they don’t realize is that their brain is using a lifetime of past visual experiences to fill in the missing stereo information. Seeing in 3-D provides a fundamentally different way of seeing and interpreting the world than seeing with one eye. When I began to see in stereo, it came as an enormous surprise and a great gift.

For the first time, I could see the volumes of space between different tree branches, and I liked immersing myself in those inviting pockets of space. As I walk about, leaves, pine needles, and flowers, – even light fixtures and ceiling pipes – seem to float on a medium more substantial than air. Snow no longer appears to fall in one plane slightly in front of me. Now, the snowflakes envelope me, floating by in layers and layers of depth. It’s been seven years since I gained stereovision, but ordinary views like these still fill me with a deep sense of wonder and joy.”

Her description implies that although she had not automatically perceived three dimensions in her life, she definitely thought in 3D. She did not have to learn how to use this extra ingredient in her perception although it was definitely novel and surprising.

Another recent research report in ScienceDaily gives indications of where and how 3D calculations are done. ( here )

“They found, surprisingly, that 3-D motion processing occurs in an area in the brain—located just behind the left and right ears—long thought to only be responsible for processing two-dimensional motion (up, down, left and right). This area, known simply as MT+, and its underlying neuron circuitry are so well studied that most scientists had concluded that 3-D motion must be processed elsewhere. Until now…

For the study, Huk and his colleagues had people watch 3-D visualizations while lying motionless for one or two hours in an MRI scanner fitted with a customized stereovision projection system…The fMRI scans revealed that the MT+ area had intense neural activity when participants perceived objects (in this case, small dots) moving toward and away from their eyes. Colorized images of participants’ brains show the MT+ area awash in bright blue…

The tests also revealed how the MT+ area processes 3-D motion: it simultaneously encodes two types of cues coming from moving objects…There is a mismatch between what the left and right eyes see. This is called binocular disparity… For a moving object, the brain calculates the change in this mismatch over time. Simultaneously, an object speeding directly toward the eyes will move across the left eye’s retina from right to left and the right eye’s retina from left to right. “The brain is using both of these ways to add 3-D motion up,” says Huk. “It’s seeing a change in position over time, and it’s seeing opposite motions falling on the two retinas.”

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