Depth Perception; a Human Eye Story
It’s Sunday and I am sitting with a bunch of friends discussing the possibility of a movie to become the next big hit in Bollywood, when someone points out how much our cinema has grown, from making black and white films to now using multiple drones to capture the perfect angle.
Thinking all about cameras and movies, I stumbled upon a question which had once troubled me in 9th standard and I had never bothered to ask or look that up on the internet back then. Today was going to be different and I opened my browser and entered the words, “How does human eye perceive depth”. What I found, amazed me quite a bit, which is why I thought to write about the same. I hope you enjoy reading it too.
The fundamental question that I will address here is, “How do we see in 3D when the projection formed on our retinas in our eyes is in 2D? ”
The short answer lies in the fact that we have 2 eyes which are located at different positions, due to which an image of the same object as seen from left and right eye are located at different positions. The brain uses this disparity to create a stereo vision, which helps in depth perception. Certain other factors like shadowing and shading also help the brain.
Before I jump into the long answer I do have a fun activity to describe and I want you to perform. Pull up the picture of your favourite fruit from the internet and hold your finger right in the middle of the screen and your eye, now try to look at the fruit. Does the finger block your view or does your opaque finger suddenly became transparent and you can still see the whole fruit on the screen? Interesting, right.
Let’s make it more interesting, focus now on your finger and you will see 2 images of your favourite fruit in the background. This one might be a little harder but give it a few seconds and you will be able to enjoy 2 fruits at the price of one. By now I think now I must have captured your curiosity to know the answer about the magic behind your vision so enough experimenting.
The long answer adds many layers to the short answer, which I will try to put together. A point to keep in mind is that scientists still are not 100% sure as to how the disparity between 2 eyes is used to create a sensation of depth but there have been many different studies which try to explain this process.
Let’s start by answering, what makes vision possible. The visual stimuli, which is basically light, enters our eyes through photoreceptors, present in the eye which is then projected at the retina. At the retina, the light stimuli are changed to neural impulses which are then transferred to the visual cortex in the brain through neurons. Doesn’t this does seem an awful lot like a camera projection, so is it that the images we see are just direct mapping onto our brains like a projector?
Well, the answer is not that simple because we clearly can perceive the world around us in 3D and also get the sense of objects which are further from us than the objects which are close to us, which wouldn’t have been possible with just direct projections. To help with these distinctions, both monocular view (single eye view) and binocular view (double eye view) are used. Monocular vision helps in determining the size of the objects we see in relation to the other object but is bad with extracting different features of the same object, while binocular vision helps us get more information about the same object.
The perception of depth happens when both our eyes converge on the same object. This object when seen from different eyeballs have slightly different shape and size which combined with texture and shape are understood as features describing a more complete view of the object. This is done by the brain.
When the input from both eyes is perceived and compared it is known as stereopsis.
Stereopsis: The perception of depth produced by the reception in the brain of visual stimuli from both eyes in combination; binocular vision.
Although primary visual cortex drives the main sensory understanding of vision in our brain, the depth perception or the stereo vision does require other parts of the brain. Dorsal visual cortex located just above the primary visual cortex is particularly tuned for disparity recognition and for texture recognition. It has also been seen that sometimes people with damage to the visual cortex have remarkable depth perception which further suggests that it is not only a single spot in the brain responsible for 3D vision.
We have come far in understanding the brain but there is still much we don’t understand, but we do have the answer to the question about depth perception and simply put it is a dirty trick our brain has evolved to learn where it takes two, 2 dimensional images of the same object and then extrapolates it to create a 3-dimensional view of the world.
I have an important question for you to ponder on, do animals like deer’s who have their eyes located on different sides on the face have Stereopsis, can they perceive depth as humans can. Try and think of the reason and let me know in the comments below. Before I go, I have a very cool optical illusion to share with you, have a look :
https://www.youtube.com/watch?v=eelAdmh8PdU
Keep reading and supporting. Cheers!
