Color Perception and the Human Body

An illustration of color perception, by Benjamin Lawless

Part 1: Color Perception and the Retina

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figure 1a. Human retina as seen through an opthalmoscope
The retina of the eye is formed by a layer of cells lining the inside of the eye. It is viewable through the pupil of the eye and is the object of interest when an optometrist examines the eye with a light. Along with the many blood vessels running through the layer, two discrete spots are discernible from this vantage point: the fovea and the blind spot, or optic disk (See Figures 1a, b). The fovea is also referred to as the focal point. It is this slightly indented region, containing high concentrations of cone cells, upon which the lens focuses entering light. All other areas of the retina are responsible for the perception of peripheral vision (1. Kolb, 2005, 2. Silverthorn, 1998).

The blind spot gets its name from the fact that, due to the lack of either photoreceptor (rods or cones), it is literally an area of the retina incapable of detecting light. This is the area in which the long axons of ganglion cells, which carry light information from all parts of the retina, converge to form the optic nerve (the nerve connecting the eye to the brain). It is also the entry and exit point of veins and arteries that feed the retina (1. Kolb, 2005, 2. Silverthorn, 1998).

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figure 1b. Cross section of the eye

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figure 2. Light absorption of visual pigments
Among the layers of cells within the retina, the rods and cones are the only photoreceptive cells; they are the cells that react in response to light (See figure 2). Rods are responsible for night vision. The retina contains around 90 million rod cells which grow in concentration the further the distance from the fovea. Rods are 100 times more sensitive than cones, thus enabling them to function in extremely dim light. Their sensitivity is greatest in the blue area of the spectrum (498 nm), making reds difficult to see at night. Despite their sensitivity, many rods synapse on (communicate with) one bipolar cell, creating a large receptive field. This means that the signal to the brain is not specific as to which particular rod within the group signaled the perception of light. This high rod to bipolar cell ratio is responsible for grainy night vision, in much the same way that the large crystals in high-speed film produce a grainy image (2. Silverthorn, 1998, 3. Rod Cell, 2007).

Alternately, Cones are responsible for the perception of color. There are approximately 4.5 million cones in the human eye, concentrated mainly at the fovea. They are less sensitive to light than rods, but can perceive finer detail and respond more rapidly. The greater acuity is due to the lower ratio of cones to ganglion cells. In fact, at the fovea, many cones answer to only one ganglion cell. This exclusivity allows for much finer pinpointing of the source of stimulus. Three variations of cones exist; they are commonly called red, green and blue cones; however, they are better termed short-wave (which peaks at 420nm in the blue-violet range), medium-wave (534nm in the bluish-green range), and long-wave (564 nm in the yellow-green range). Notice that reds (700nm) are poorly detected by the long-wave cone (not to mention, the rod). Thus well-lit conditions are required in order to perceive reds. Additionally, the ratio of each type of cone is not a set value. Short-wave cones occur in much lower concentration than medium and long, while the ratio of the medium to long varies wildly from person to person (see Figure 3)(2. Silverthorn, 1998, 4. Cone Cell, 2007). Amazingly, these variations have no effect on color perception; a person with an abundance of long-wave cones can pick out a true yellow as easily as one whose medium-wave cones dominate.

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figure 3. Concentrations of red, green, and blue cones in two individuals

Looking at the retina in cross section, one observes that there are many cells positioned between the incoming light and the photoreceptor cells (rods and cones) (See Figure 4). The only exception is at the fovea, where the cells are parted to allow direct access to the cones. Light emerging from the lens of the eye passes through the cell layers and is finally absorbed by the pigmented epithelium that serves to prevent the scattering and reflecting of light. The rod cell is stimulated by light at its cylindrical end where sections of the cell membrane are invaginated into a multitude of layers to form a stacked-pancake appearance. These folded layers create greater surface area to house high concentrations of the molecule Rhodopsin. Upon exposure to light, rhodopsin breaks down into its two component molecules: opsin and retinal (a derivative of Vitamin A). The dissociation of this molecule causes a chain reaction within the cell which will ultimately send a signal to the bipolar cell upon which it synapses. Cones are stimulated by light in a similar manner, except that, rather than housing rhodopsin in their conical stacks of membranes, they posses one of three variants of photopsin. Each photopsin is responsible for the variations in wavelength absorption among the three cone types. Like rhodopsin in rods, photopsin breaks down into two components, opsin and retinal, upon exposure to a sufficient level of light. A chemical chain reaction occurs as a result. In broad daylight, all of the Rhodopsin in rod cells dissociate due to the high sensitivity, leaving only cones functioning. After the exposure, the cells enzymatically re-combine opsin with retinal to re-form rhodopsin in rods and photopsin in cones (2. Silverthorn, 1998 3. Rod Cell, 2007 4. Cone Cell, 2007).

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figure 4. Cross section of the retina

As mentioned earlier, photoreceptors synapse upon bipolar cells, and a bipolar cell may have many photoreceptors that synapse upon it (either rods or cones, but not both). When a photoreceptor is at rest (not stimulated by light), it secretes a constant amount of the neurotransmitter, glutamate, into the synapse between itself and the bipolar cell. When light stimulates the photoreceptor, the chain reaction begun by the breakdown of rhodopsin or photopsin causes a reduction in the amount of glutamate secreted into the synapse. Depending on the type of bipolar cell, it will be either stimulated or inhibited by this change in chemical concentration. In turn, multiple bipolar cells can synapse upon a ganglion cell and either excite or inhibit it. The ganglion and its corresponding photoreceptors create what is termed a receptive (visual) field (6. Photoreceptor, 2007 7. Receptive Field, 2007).

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figure 5. On-center and off-center ganlia

There are two parts to the receptive field, the "center" and the "surround" (Figure 5). There are also two types of ganglia: on-center and off-center. A ganglia which is on-center will be stimulated when light-induced signals are primarily focused on the "center", but inhibited when along the edges or "surround" of it's receptive field. Conversely, an off-center ganglia will be excited by stimulation of the "surround", but inhibited by the "center". Both types of ganglia will give a weak response if light is evenly distributed between the two areas. The Horizontal cell is responsible for the "center" and "surround" signal that is perceived by the Ganglion. These cells, whose activities are poorly understood, connect to all the photoreceptors within the receptive field of the ganglion cell and receive signals from these photoreceptors. Stimulation of the horizontal cell causes it to feedback onto the photoreceptors in a manner which inhibits them. Photoreceptors receiving minimal light become completely inhibited. Photoreceptors receiving the most intense light also experience the inhibitory effects of the Horizontal cells, but the light intensity overrides the signal. In this manner, dimly lit photoreceptors are silenced. This process, termed lateral inhibition, increases contrast and sharpens edges (6. Photoreceptor, 2007 7. Receptive Field, 2007 8. Horizontal Cell).

Amacrine cells are poorly understood, but are thought to work on the bipolar cells in a manner similar to horizontals, adjusting brightness and allowing the detection of motion.

References

  1. Kolb, Helga, Eduardo Fernadez, Ralph Nelson. "Webvision: The Organization of the Retina and Visual System." John Moran Eye Center. 2005. University of Utah. 20 March, 2007 http://webvision.med.utah.edu.
  2. Silverthorn, Dee Unglaub. "Human Physiology, An Integrated Approach." New Jersey: Prentice Hall, 1998.
  3. Rod Cell. "Wikipedia. The Free Encyclopedia." 2 March 2007, 0121 UTC Wikimedia Foundation, Inc. 19 March 2007 http://en.wikipedia.org/wiki/Rod_cell.
  4. Cone Cell. "Wikipedia. The Free Encyclopedia." 19 March 2007, 1448 UTC Wikimedia Foundation, Inc. 19 March 2007 http://en.wikipedia.org/wiki/Cone_cell.
  5. Thompson, L.T. "Sensory Systems II." Aging and Memory Research Center. 14 Dec. 2006 University of Texas at Dallas. 19 Mar 2007 http://www.utdallas.edu/~tres/integ/sen3/display7_09.html.
  6. Photoreceptor. "Wikipedia. The Free Encyclopedia." 16 March 2007, 2051 UTC Wikimedia Foundation, Inc. 19 March 2007 http://en.wikipedia.org/wiki/Photoreceptors.
  7. Receptive Field. "Wikipedia. The Free Encyclopedia." 23 Feb. 2007, 1635 UTC Wikimedia Foundation, Inc. 20 March 2007 http://en.wikipedia.org/wiki/Receptive_fields.
  8. Hubel, David. "Bipolar Cells and Horizontal Cells." Eye, Brain, and Vision. Harvard Medical School, Neurobiology Dept. 20 March 2007 http://neuro.med.harvard.edu/site/dh/b12.htm.
  9. Horizontal Cell. "Wikipedia. The Free Encyclopedia." 13 March 2007, 1500 UTC Wikimedia Foundation, Inc. 19 March 2007 http://en.wikipedia.org/wiki/Horizontal_cell.


Article written by Christine Dahlquist.