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The eye’s photoreceptive cells, rods and cones, convert light into electrical signals sent to the brain. Humans have 125 million rods and 6 million cones. The fovea is a highly condensed area of photoreceptor cells in the center of the retina responsible for clear vision. The blind spot is where the optical fiber connects to the back of the retina, and the brain fills it in. The visual cortex isolates useful regularities in visual data, with the topmost layer responsible for the conscious experience of seeing.
The experience of vision begins when photons from the world strike the lens of our eye and focus on a small patch of photoreceptive cells on a part of the eye called the retina. These cells are of two types: rods and cones. Cones are for color sensing, work well in bright light, and rods are more sensitive but also color blind. Humans have about 125 million rods and 6 million cones. Some species have many more rods, especially those adapted to live at night. Some owls have 100 times more acute night vision than we are used to.
Rods and cones perform a function called phototransduction, which simply means converting incoming light into electrical signals to be sent to the brain, making vision possible. All of these cells contain photoreceptive proteins with various pigment molecules. In rod cells these are called rhodopsin. Various pigments can be found in the cones, which allow the eye to distinguish between different colors. When the light associated with the pigment hits the photoreceptor cell, it sends a signal along the optical fiber, otherwise it doesn’t. Photoreceptor cells and the ability to see are extremely ancient evolutionary innovations, dating back to the Cambrian period over 540 million years ago.
There are two notable structural features of the human retina. The first is the fovea, a highly condensed area of photoreceptor cells located in the center of the retina. The cell density here is many times greater than in the periphery, which explains why when we look directly at something it is much clearer than looking at it out of the corner of our eye.
The fovea is also responsible for behavioral adaptations that cause us to quickly turn our heads and stare at something if it scares us. If the fovea didn’t exist and photoreceptor density was uniform across the surface of the retina, we wouldn’t need to do this: we’d just need to turn our heads slightly so that the event at least falls within our field of view. The foveal area is a relatively small portion of the visual field, about 10 degrees wide.
The second notable structural feature in the retina is our blind spot. This is where the optical fiber connects to the back of the retina to obtain visual information, precluding the existence of photoreceptors in a small spot. Our brain automatically fills in our blind spots for us, but various visual exercises can prove it’s there.
Once the light is converted into electrical impulses and sent along the optical fiber, it travels all the way to the back of the brain (after making a few stops), where the visual cortex is located. In the visual cortex, a hierarchy of sensor cells isolates useful regularities in visual data, discarding superfluous information. A cell layer detects things like lines and curves.
A layer higher would detect regularities such as movement and 3D shapes. The topmost layer is where the gestalts appear, the general symbols responsible for the conscious experience of seeing under normal circumstances. The visual cortex is among the best understood brain areas, with a voluminous neuroscience literature.
