Monkeys use different part of the brain for depth perception than human beings

 (Image: Pixabay © CC0)

(Image: Pixabay © CC0)

Our brain relies on 2D information to create a 3D image of the world. Human beings use a different part of their brain to do this than monkeys, says an international team of researchers led by neurophysiologists Marcelo Armendariz and Wim Vanduffel (KU Leuven & Harvard Medical School) in collaboration with a team from Cambridge.

Our 3D perception of the world is enabled by a chain of processes in our brain. After all, the retina of our eyes, which registers the images, is a 2D structure. To reconstruct these 2D signals into a 3D image of the world, our brain uses a whole range of cues, including information about perspective, shadow, and texture.

Other important depth cues are the apparent position of moving objects or when we move, and the difference between the images received by our left and right eye. As our eyes are slightly apart, our two retinas see a slightly different image. That difference - known as ’binocular disparity’ - gives our brain information to calculate depth. The same applies for what is known as ’motion parallax’: when you move to the right, a nearby object appears to move in the opposite direction very quickly. A faraway object, by contrast, seems to slowly move in the same direction.

Our results confirm that area MT in monkey brains plays the most important role in integrating these depth cues. This confirms that there is a difference between human beings and monkeys.

- neurophysiologist Wim Vanduffel

Much research has already been done into each of the depth cues separately, both in human beings and in monkeys, says Professor Wim Vanduffel. "What we haven’t figured out in detail yet is how our brain puts together all pieces of the puzzle. How does our brain integrate all depth cues from binocular disparity, motion parallax, perspective, and so on?"

"In human beings, the area known as V3B/KO plays a vital role. In monkeys, it’s probably a different part of the cerebral cortex, known as area MT. Human beings have that area as well, but we don’t use it to integrate depth cues."

"The problem with previous studies was that the measuring techniques were not yet the same for human beings and monkeys. In other words: is there truly a difference, or are we comparing apples and oranges?"

The team decided to use the same technique in monkeys that is used in human subjects: an fMRI scan (functional Magnetic Resonance Imaging - ed.) . This brain scan reveals which parts of the brain are active as the subject carries out a specific task, such as looking at images.

"We simultaneously examined this for binocular disparity and motion parallax. Our results confirm that area MT in monkey brains plays the most important role in integrating these depth cues. This confirms that there is a difference between human beings and monkeys. Ample evidence has already shown that area MT is very similar in monkeys and human beings, especially regarding the processing of visual motion signals, but our results show that this does not apply when it comes to fusing depth cues."

Why is a different part of the brain responsible for depth perception in human beings and monkeys’ It’s probably due to evolutionary differences: "Human beings and monkeys have been evolving separately for dozens of millions of years. We walk up straight; monkeys swing from trees. That leads to different needs in terms of vision, so that the functions of different parts of the brain may change as well."

The full text of the study "Areal difference in depth cue integration between monkey and human" by M. Armendariz, H. Ban, A. Welchman, and W. Vanduffel was published in the journal PLOS Biology .