Do you see what I see?June 2012
Lesson 1: What do you see?
THE EYE AND COLOR VISION
“Rods Rule and Cones Colorize!”
The main function of the visual system is to process patterns of light into information that is useful to the organism.
A very brief journey of light in the visual system (See Figure 1):
Light passes through the cornea, the anterior chamber, lens, and the vitreous chamber before reaching the retina, the light sensing tissue in the eye (the red layer in image above). The lens focuses an image, which is made up of different shades and colors of light, onto the retina the first stage of image processing. Information about the image is transmitted from the optic nerve to the brain through electrical signals. These signals are sent to the thalamus and then to the cortex. Primary visual information is sent to the occipital lobe (at the back of the brain) and then to the other lobes for higher processing.
Our focus will be on the anatomy and the physiology of the retina (See Figure 2):
The retina is a layered sheet of neural cells and photoreceptors lining the back of the eyeball. Study Figure 2 and note that light travels through many layers before reaching photoreceptors at the back of the retina.
The retina is vascularized to provide nutrients to and carry away waste from the neurons.
The retina is organized in a way that enables linear flow of information. The cells that respond directly to the light are called the photoreceptors. When light hits the photoreceptors, information is transmitted to bipolar cells to ganglion cell. The axons of the ganglion cell make up the optic nerve. There is also lateral processing that occurs through the horizontal and the amacrine cells. These two types of cells form inhibitory circuits across the retina, refining the signal flowing through the retina thereby changing visual information getting to the brain.
After passing through the lens, light passes through the vitreous fluid in the center of the eye, and then through the layers of the retina, finally reaching the rod and cone photoreceptors. The layers before the photoreceptors are fairly transparent, so light travels to the photoreceptors relatively easily.
For maximal sharpness in the center of the gaze, there is a specialization in the center of the retina that looks like a pit, called the fovea (See Figure 3). In the fovea, all retinal layers except the photoreceptor layer are shifted aside so that the light strikes the photoreceptors directly without the other tissue impeding its path.
The photoreceptors contain light absorbing chemicals called photopigments that convert, or transduce, light energy into neural energy when they absorb light. These signals travel to the bipolar cells and then to the ganglion cells. The ganglion cells then send the signals via the optic nerve to the brain. The horizontal and amacrine cells integrate information across and between neighboring sites on the retina that reflect neighboring points in our field of vision.
If there were receptors sensitive to every distinguishable color we would need to have at least 200 kinds of receptors, but this sounds impossible! How do we perceive colors?
Thomas Young was the first to tackle with this problem. In 1801 he wrote:
Now, as it is impossible to conceive each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unision with every possible undulation, it becomes necessary to suppose the number limited, for instance, to the principal colours, red, yellow and blue. (Gregory, 122)
Thomas Young was almost right. He continued to tackle with this idea. Writing a little later, he still kept the number of “principle colours” as three, but he changed them from ‘red, yellow and blue’ to red, green and violet, which is accepted today.
We have 2 kinds of photoreceptors: cones and rods. Both of these generate neural signals in response to light. Cones are less sensitive than rods and are used in the daylight vision. Rods are more sensitive than are cones so they are used under dim light, for example at night.
Rods and cones differ in their distribution in the retina: cones are predominantly in the center of the gaze in the fovea while rods are more in the periphery (See Figure 4).
While the visual acuity or visual resolution is much better with the cones, the rods are better motion sensors. Since the rods predominate in the peripheral vision, it enables you to see dimmer objects in your peripheral vision. Test this out: Locate a dim star in the sky. Look directly at it. What happens?
It will likely disappear. Look slightly away from the star, placing it in your peripheral vision. Does it reappear?
It disappears when you look directly at it because you move the image to the cone-rich fovea region which has high color sensitivity but low detection ability. Rods are rich outside the fovea (See Figure 4) allowing you to detect low light objects.
Photoreceptors do not signal color; they only signal the presence of light in the visual field. Most of us have three different kinds of cones. Some people may lack one or more cone cells, which is referred to as color-blindness. Refer to the Figure 5 to see the effects of lacking a type of cone cell on visual system (normal- having three types of cone cells; missing long wavelength- red color blindness; etc.). The visual system perceives colors by comparing responses across a population of photoreceptors (i.e. the three different cones with differing absorption spectra). Because we need at least two classes of photoreceptors to detect color and since we only have one kind of rod, we cannot distinguish colors under very dim light. The three cone types are long-, middle-, and short-wavelength cones (refer to Figure 6 for the corresponding wavelengths). They are also referred to as red, green and blue cones, respectively. The response of each cone type depends on both the wavelength of light and its intensity.
The ratio of responses from cones gives an unambiguous code for the wavelength of light (i.e. when retina is stimulated by red light, the red cones will be activated more than the green cones).
If we had more than three cone types we would be able to tell the difference between mixtures of light and unmixed, spectral colors.
For example, if we had separate cone types peaking in the red, green and yellow ranges, we would be able to distinguish a mixture of red and green light from a pure yellow.
There are many more rods than cones in the human eye. There are ~120 million rods and ~6 million cones in the human retina. As mentioned, rods are more light-sensitive than the cones. However, they are not sensitive to color. They are responsible for our dark-adapted, or “scotopic”, vision. The rods are incredibly efficient photoreceptors. Always remember: Rods Rule and Cones Colorize.
References:
Livingstone, M. (2002). Vision and Art: The Biology of Seeing. New York, NY: Abrams.
Gregory, R.L. (1997) Eye and Brain: the psychology of seeing (5th ed.). Princeton, NJ: Princeton University Press.
Some of the images were taken from: Simple Anatomy of the Retina. Retrieved from on 06.11.2010.
Purves, D., Lotto, R. B. (2003). Why We See What We Do? : An empirical theory of vision. Sunderland, MA: Sinauer.
Images:
Figure 1: Retrieved on July 7, 2010
Figure 2: Retrieved on July 7, 2010
Figure 3: Retrieved on July 7, 2010
Figure 4: Retrieved on July 7, 2010
Figure 5: Livingstone, M. (2002). Vision and Art: The Biology of Seeing. New York, NY: Abrams.
Figure 6: Livingstone, M. (2002). Vision and Art: The Biology of Seeing. New York, NY: Abrams
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