Is the brain reversing the pictures?

“When you look at a scenery, the picture is printed upside down on your retina”.
Very often, when I read this statement, it is immediately followed by a reassuring “but fortunately your brain flips back the picture to be upright” (for example in [1], [2] or [3]). I’m very puzzled by this last statement: What does it mean? What if the brain wasn’t performing this supposed flipping? Would I see everything upside down? So, I may ask, what is “up” and what is “down”? Let’s define the place were I see my feet when I stand to be the “down”. If my brain wasn’t performing the supposed flipping, would I see my feet “up”, i.e. near my head? It’s impossible, since I rightly defined my feet to be the “down”. Would I see people upside down in the streets, walking on their head? That’s less than probable: if my feet are the “down”, that would mean that the head of the peoples are near my feet, which is seldom the case. So, all in all, I think this inversion performed by the brain doesn’t exist.

Yet, the picture is printed upside-down on my retina. On a movie screen, the pictures could also be displayed upside-down, and I would otherwise perceive that through my eyes. But my eyes are not a movie screen: they are the sensor from which I see. Beneath my eye’s retina, there are only neurons: a neuron has no sense of up and down, it’s just a processing and transmission machine. Let’s continue to suppose that the brain is inverting the picture: where would this process be done? It could be done very simply by the optic nerve: the various nerves coming from different places of the retina could reshuffle themselves to put back the picture in the right order. Of course, that doesn’t make sense either: the various neuron networks to which the optic nerve end is connected doesn’t care about the orientation of the bunch of nerves arriving. If you still own a tower PC computer, it would be like putting the tower upside-down, or twist the cables: it will have no impact on the processes and the picture output.

My eyes are the sensors: I don’t usually “see” the picture printed on my retina, I sense through it. “Seeing” is the whole process including eyeball + brain. If I could look at that picture printed on my retina, for example through a mirror mechanism, I would indeed see an upside-down picture. I think that’s the usual misconception: when I’m being told that the picture on my retina is printed upside down, I picture myself “looking at” this upside down picture. But that’s not the case: I “sense” through it, and the orientation of the organs involved doesn’t matter.

[1] http://www.mind.ilstu.edu/curriculum/vision_science_intro/vision_science_intro.php
[2] http://isee.nei.nih.gov/parts/visualsystem.asp
[3] http://www.physlink.com/education/askexperts/ae353.cfm

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