Some people believe that they perceive the world as it actually is. There are many experiments that can disprove this notion. For instance, take a look at Edward H Adelson’s checker shadow illusion. To me, the two squares A and B look so obviously different that if I didn’t know it was an optical illusion, I would never pause to think that they might be exactly the same color.
Similar experiments can demonstrate that your hearing is just as subjective. There are tone mixes that can be played that some people will hear as ascending tones, some as descending tones. The sense of touch can be fooled too. I’ve not heard of any demonstrations of the subjectiveness of smell-based perception, but I don’t doubt that it could be done.
Nevertheless, we like to think that we see things as they are. Sure, maybe the colors aren’t always right, or the angles look distorted, but the basic details are correct, or so we assume. You were probably taught, like me, that the eye works like a camera—the scene before it is focused by the lens onto the retina, and the signals from the rods and cones are transmitted to the brain where they are processed into an image, right?
Wrong, it turns out.
In April’s Scientific American magazine there was a fascinating article that described how the eye actually works. Further details from the same researchers were published in Nature.
It turns out that the rod and cone cells connect to 10 different kinds of neurons known as bipolar cells. The bipolar cells have long axons which extend into one of 10-12 different layers of what’s known as the plexiform layer. Also connecting into those layers are 12 different types of ganglion cells, which are the cells that actually transmit to the optic nerve. There are also at least 27 types of amacrine cells, which can affect signal transmission between the layers and change propagation of signals within a layer.
Basically, the eye isn’t at all like a camera. It’s more like a chunk of brain tissue that has been wrapped around the inside of the eyeball. (Stranger still, all the sensor cells are on the outside, and all the wiring is on the inside, so light has to pass through everything else to get to the sensors. Intelligent design? I don’t think so.)
Anyway, the sensory data from the rod and cone cells is processed by this retinal tissue into 12 separate streams of information, all of which are sent to the brain in parallel.
One signal stream consists of only the edges detected in the scene; another is only the moving edges. One has just the shadows. One detects and emphasizes highlights. One seems to notice changes of brightness with respect to time. One detects large uniform areas. Another seems to detect backgrounds around central figures. And so on.
The 12 streams of data are sent to different parts of the brain. The brain then somehow uses all of these special purpose signals to work out a mental model of the external world, which it uses to “color in” the photo-like perception we imagine we have.
This is, of course, why optical illusions work. Given the nature of the signals sent to the brain, the bigger mystery is how the brain mostly does such a good job of fooling us into thinking we have cameras for eyes.
Tagged: brain, Edward H Adelson, eyes, perception, science, vision
4 Responses to “The eyes have it”
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June 4th, 2007 at 13:19 -0600
I don’t see why this means why the eye is not like a camera. It’s just that a lot of processing is done inside the camera. It’s a bit like the CCD of a digital camera having built-in JPEG compression. It’s still a camera.
June 4th, 2007 at 14:40 -0600
It’s a little like a 3CCD video camera, in that you have multiple independent streams of time-dependent image signal that are recombined later…but not really.
For the eye, there’s no signal saying “Here is the blue intensity at each point”, “Here is the red intensity at each point” and so on, like there is in any digital camera. Rather, the R G B measurements of the three types of cone seem to be encoded into red-green and blue-yellow channels. Red-green is specific to monkeys and humans, which helps explain the phenomenon of red-green color blindness as a genetic mutation.
But those red-green and blue-yellow signals aren’t straightforward “red-green intensity at this point is X” signals either. As the many color-based optical illusions show, they’re the same kind of weird preprocessed signals that emphasize edges or detect highlights and shadows. And outside of the central few degrees of the retina, there isn’t even any color data to start with; the brain seems to paint in the color in the rest of the scene based on memory and guesswork.
So for human vision, color is an artifact of the brain, rather than an external property of light directly measured and represented in the signal. That makes it very different from a digital camera in my view.
June 4th, 2007 at 19:59 -0600
I’ve been reading Dawkins’ Climbing Mount Improbable recently and was amazed to learn that eyes have evolved in different ways in different species something like 40-60 times during the course of evolution. Furthermore, although many eyes show similariites, there are striking differences as well.
June 5th, 2007 at 11:50 -0600
I see what you’re saying, but to me a camera is a dark box with a hole in it (and probably a lens in that hole) and some kind of screen opppsite the hole, so the eye is a camera, except that it’s not a box. Oh, you know what I mean!