We are going to look at the way our brain analyzes the image.
As a reminder, our eyes record images with a sharp, contrasting, colorful center and a very good resolution. As we move away from the center we will lose color information, resolution, but not in light intensity or sharpness. This is what makes it possible to see in black and white in the dark. We see in the dark with our peripheral vision. A simple experiment to do at night is that when you observe the stars, those of very low intensity are better perceived when you do not look at them directly but from the corner of your eye.
What makes me not see life as my eye does?
Well, to spoil the suspense, it is the brain that allows us to see everywhere with a colorful and good quality image.
First of all, the photoreceptors of the retina (cones and rods), when stimulated by a light sensation, send electrical information to the optic nerve. As an aside, the retina has no pain nerve endings. The notable consequence is that if we were to look directly at the sun, we would burn our retina without having the sensation of burning it. So if you do it, you’re going to damage your eyesight, potentially permanently without having been hurt. Don’t do it…
When filming with a digital camera, the image is formed on the sensor. The sensor is formed by a matrix, a grid of photosensitive cells. The number of long and wide sensors determines the resolution of the device. A layer of green, blue and red filter is placed on the sensors to capture the color. Thus a sensor, behind a green filter, does not capture any information, it is either that there is no light intensity at all, or that there is no green light to capture.
Sensors for blue, green and red?
In the same way that we have TV or computer screens, we have pixels for blue, green and red. We have three cones specialized in the capture of these three colors: the cyanolabe cones for the blue, chlorolabe for the green and heritrolabe for the red.
So by aligning blue, green and red receivers we can capture all the colors of the visible spectrum! This arrangement of blue, green and red sensors is called the Bayer matrix. On a sensor of your camera, you can see that there are twice as many sensitive cells for green, and the same amount for blue and red. This is not related to chance, but to our eye which is able to perceive a phenomenal quantity of green shades.
Parenthesis on its creator
Brice Bayer, employed at Kodac, invented this arrangement of blue, green and red sensors and this matrix bears his name: the Bayer matrix or mosaic. The irony of the story is that Kodac never believed in the development of digital and continued to believe in the development of our photos on paper. The company almost disappeared. It has converted to high quality inks and printers.
In a digital camera or your mobile phone, how does it work?
If we take a picture, the sensor transmits each information of each photosensitive cell. Good cameras can transmit up to 22 million pieces of information or pixels. All this information is received and processed line by line, within a few microseconds.
So how does the brain process the information from about 6 million cones and about 100 million rods per eye?
The retina is composed of several layers. On the outermost layer, we find our cones and rods. On the innermost layers there are special neurons, which are called retinal ganglion cells. These are the neurons that will join to form the optic nerve. About 1.2 million neurons per eye. This means, if we do the math, that each of these neurons processes approximately one hundred cones and rods of information.
The optic nerve is composed of more than 1 million nerve fibers that send the entire image captured by an eye at the same time. And all this multiplied by two, since we have two eyes.
For each eye, we can separate this large bundle of nerve fibers into two. The first packet for the left side of our eye, and the second packet for the right side of each eye.
It all goes to the center of our skull below our brain, in a part called the chiasma. At this point, the information will cross. Everything that comes from the right side of the two eyes, goes to the right side of the brain and everything that comes from the left side of the two eyes goes to the left side of the brain. The left side of our brain processes all the information coming from the left side of our two eyes and the same for the right side.
Except that the image that has formed in the back of our eye, exactly as in a dark room, is a reversed image. Everything in your right visual field is processed by the left side of your brain and everything is put there head down and feet up.
Certain fibers of our optic nerve are redirected to other places in our brain in order to have a more immediate processing of visual information. These fibers go to the hypothalamus in the suprachiasmatic nucleus, an area of our brain that manages our internal biological clock. This will make the light intensity captured, even before the image is processed, will give information to this clock on the day and night cycle.
But that’s not all! Some fibers will go to the pretectum which is in charge of all the reflexes of the pupil and the spontaneous movement of the eye. This is where the information comes from that makes the pupil open and close in relation to the light intensity and makes us move our eye very quickly. In this regard, the fastest muscles in your body are not those of your arm or leg, but the muscles that allow you to move your gaze, as if to see the soccer ball coming at you.
There are also 10% of all axons or nerve fibers forming the optic nerve that go to the superior colliculus. It is also located in the center of our cranium under our brain. Its major role is to direct our gaze and attention to objects considered of interest. The image is not really processed yet, it is a spontaneous and immediate reflex. In a sense it completely curtails the role of the visual cortex. This is where the most rapid eye movements take place. Its neighbor is the inferior colliculus which deals with hearing and together they talk to each other.