Structure of the Eye
The eye is the sense organ responsible for vision. The structure of the eye is critical to understanding how light is converted into a neural signal. The innermost layer of the wall of the eye, the retina, contains several layers, including pigmented cells, photoreceptors, and neurons. Light reflects off the surface of an object and enters the eye through the cornea, a transparent outer cover on the eye that allows light to enter. Next, light passes through the pupil, an opening in the eye formed by the iris that changes diameter to control the amount of light entering the eye. The iris is the ring of colored muscle tissue surrounding the pupil that controls the size of the pupil. Finally, light passes through the lens, a curved transparent structure found behind the iris that bends light to focus on the retina. The point of clearest focus in the retina is called the fovea.
The retina is made of many layers, including blood vessels and cells to aid in waste removal. Sensing light involves three neural cell layers. The innermost layer, which is actually the first layer to process incoming light, is made up of photoreceptors. Photoreceptors are sensory receptors that respond to light and are one of two types: cones and rods. A cone is a photoreceptor that responds best in high-light conditions to detect color and detail. They are concentrated within the fovea of the retina. A rod is a photoreceptor that responds well in low-light conditions, used to detect shape and motion. They are concentrated in the retina outside of the fovea. The middle layer includes bipolar cells, which relay signals from photoreceptors to the outermost retinal layer. The outermost layer is made up of retinal ganglion cells, a type of neuron that receives inputs from photoreceptors.Light sensation begins when light reflected by an object passes through the open pupil. The cornea and lens help focus that light on the retina. Next, photoreceptors (rods and cones) convert that light energy into a neural signal (transduction). The neural signal is relayed from photoreceptors to the bipolar cells and then to the retinal ganglion cells. The axons of the retinal ganglion cells make up the optic nerve, a cranial nerve that transmits visual information from the retina to the brain for processing.
White light is made up of many different wavelengths of light. Each wavelength corresponds to a specific color. Objects in the environment absorb some wavelengths of light and reflect others. Information about the color of an object is determined by the wavelength of the light reflected off the object and entering the eye. A block reflecting only blue wavelengths of light will appear blue. An object that reflects all wavelengths of light will appear white. One that absorbs all wavelengths of light will appear black.Humans are able to detect color because different photoreceptors in the eye respond to different wavelengths of light. Short wavelength cones (S-cones) detect short wavelengths corresponding to purple to blue light. Medium wavelength cones (M-cones) detect green to yellow light, and long wavelength cones (L-cones) detect orange to red light. There is some overlap in the wavelengths of light detected by different types of cones. Color perception depends on the relative activity of each type of cone. This is known as the trichromatic theory of color processing, or the Young-Helmholtz theory. Rods respond best to medium wavelengths of light but are not involved in color processing.
Relative Sensitivity of Photoreceptors
The opponent processing theory is good at explaining color afterimages. For example, staring at a red circle for a long time causes red-sensing cones to undergo sensory adaptation (a decrease in response after continuous stimulation). Looking away from the red circle to a white wall will create the perception of a green circle. This occurs because red-sensing cones have reduced their activity while green-sensing cones have increased their activity. This phenomenon occurs for complex images as well. If people stare at an American flag drawn with opposing colors (turquoise, yellow, and black), when they shift their gaze to a blank space they will see the flag in the usual red, white, and blue. Both the trichromatic theory and opponent processing theory are needed to fully explain color vision.