Vision is one of the senses that humans rely on most, and humans can perceive light within the range of 400 to 790 Terahertz.
A network of light-sensitive nerve cells (photosensors) at the retina (at the back of the eye) detect light waves and convert them into nerve impulses. The main types of photosensors are rods and cones. Rods are more numerous than cones (about 120 million per retina) and have a lower acuity, but they are more sensitive to incoming light. Rods are more numerous in the periphery of the retina, and detect light in shades of grey. Cones have a higher acuity, and have three main sub-types, sensitive to specific wavelengths of light (the colours red, green and blue). Cones are most densely concentrated at the fovea (central area of the retina), and each retina has about 6 million cones (1).
Light information is processed through several layers of cells in the retina, and is sent to the brain via the optic nerve. This information, which includes colour, visual features (such as edges of objects) and the location of stimuli, is merged from both eyes and processed in the brain, producing a three-dimensional image. An ordered map can be found in the brain, which corresponds to areas of the retina covering specific segments of the visual field (2).
1. Sherwood, L. (2004). Human Physiology: From Cells to Systems, 7th ed. (pp. 197-208), Belmont, CA: Brooks/Cole Cengage Learning.
2. Greenstein, B., Greenstein, A. (2000), Color Atlas of Neuroscience: Neuroanatomy and Neurophysiology (pp. 274-292). New York: Thieme.