Vision is important to most people. We would have difficulty navigating our surroundings and writing articles, like this one, if we did not have it. Millions of people around the world do all they can to improve their sight. Some get contact lenses and glasses, spending hundreds of dollars, while others receive laser-eye-surgery, spending thousands. This question should be of interest to those who want to have good vision: “How do we see images?”
To answer this question, we must first take a quick look at the basic components of the human eye. The human eye is a complex organ, more sensitive than any device created by scientists or engineers. The components of the eye that are visible from the outside include the sclera, the cornea, the iris, the pupil, the lens, and the anterior and posterior chambers. Just like a digital camera, the eye has a dark interior, a diaphragm for contro lling light levels, a sensor for capturing the images projected on it, and a lens that automatically focuses (8). The sclera is the white part of the eye. Like a camera’s interior, the sclera is brown, or dark, on the inside. This allows it to absorb light to keep the images received by the brain from being washed out. Connecting to the sclera and covering the iris is a transparent membrane called the cornea. The cornea is a crystal-clear membrane which consists of five layers, totaling half a millimeter thick (2). It alone accounts for two-thirds of the eye’s focusing power (2).
Lying directly underneath the cornea is the iris, which contains the pigment melanin. Melanin in the iris can produce varying shades of blue, green, and brown, depending on the amount and distribution of the melanin. The iris encircles a hole called the pupil. As the iris sphincter muscle contracts or expands, the iris changes size, causing the pupil diameter to expand to a maximum of 7 millimeters or contract to a minimum of 3 millimeters (6).
The space between the cornea and the iris is called the anterior chamber. This chamber is filled with a thin, watery fluid, called the aqueous humor (1). Lying behind the iris is a delicate structure for focusing light, called the lens. The space between the iris and the lens is called the posterior chamber and is filled with a watery gel called the vitreous humor. This substance also is found within interior of the eye and fills 80 percent of the volume of the eye (5). Mostly composed of water, one percent of the gel consists of salts, acids, sugar, collagen fibrils, and peripheral cells (5).
Immersed in the vitreous humor, the lens is a special structure that can change shape as a ciliary muscle relaxes or contracts. When the muscle relaxes, the lens is stretched and becomes more flattened, and when the muscle contracts, the lens becomes more rounded (4). A rounded lens has a shorter focal length than a flatter lens. Therefore, an eye with the condition of near-sightedness (myopia) has a rounded lens, while an eye with far-sightedness (hypermetropia) has a flattened lens. Both conditions can be easily corrected with glasses or contact lenses.
Once light passes through the lens, it enters a dark space filled with the transparent, gelatinous material called the vitreous humor, mentioned earlier. The brown inside layer of the sclera forms the largest part of the eye, absorbing stray light particles so that the retina at the back of the eye does not receive unwanted light, which might make the image appear washed out. Located at the back of the eye, opposite the lens, is a complex arrangement of light receptors called, collectively, the retina. The retina serves the same purpose as the light sensor grid in a digital camera. In the eye, these receptors have two types: cones and rods. Cones detect bright light and color. We use these receptors when we are reading, jogging, movie watching, or doing most of our hobbies. However, when the light levels are very low ¾ such as in the middle of the night, we use our rod receptors. The images we see with rods tend to be grainy, black and white, or nearly-colorless (4). Though they form grainy images, rods are not primitive sensors. These amazing structures outperform any scientific detection equipment we currently possess, having the ability to detect a single photon (4): a particle much smaller than an atom.
When light particles strike these receptors, their location in the image, their frequency (and thus, color), and their intensity (or brightness) are recorded and transmitted to the visual cortex in the brain. The cones are divided into three categories: cones that absorb long-wavelength (red) light, cones that absorb short wavelength (blue) light, and cones that absorb medium-wavelength (green) light (4). A complex arrangement of neurons transmit the signals, produced by the reception of light, up neural pathways into the primary visual cortex, at the back of the brain (7). In the primary visual cortex, light signals transmitted from the retinas are reconstructed and reverted (7). Since the light coming through the lens is inverted, the signal reaching our brains is of an inverted image. The brain itself reconstructs the images and reverts them in a highly complex process scientists do not fully understand.
While the exact process of how the brain receives, stores, and interprets neural transmissions from the eyes is not entirely understood, researchers at the Johns Hopkins University made a breakthrough in understanding how the brain reconstructs images from neural input. Recording the activity of nerve cells in the visual cortex of macaque monkeys, the researchers found that the ape brains can reconstruct a whole image even thought the ape had been focusing only on a part (3). According to professor Rudiger von der Heydt of the Zanvyl Krieger Mind-Brain Institute, who worked on the project, the brain has “a sophisticated program” for finding and processing relevant information, at any given moment (3). Von der Heydt said that:
An image can be compared with a bag of thousands of little Lego blocks in chaotic order. To pay attention to an object in space, the visual system first has to arrange this bag of blocks into useful 'chunks' and provide threads by which one or the other chunk can be pulled out for further processing. (3)