Eye, Brain, and Vision
Home
Book
Illusions
Biography
Publications
David Hubel's
Home
Book
Illusions
Biography
Publications
HMS
                                                                                    
       
   
 
Absorption spectra (or sensitivity curves)
differ for the three types of cones. (Spectral-energy curves and absorption curves such as these have their y axes in log units because they operate over such a wide range. The up-and-down position of the x-axis is therefore arbitrary and does not represent zero absorption.)





example, some light may be absorbed and some reflected. For many objects, the relative amount of light absorbed and reflected depends on the light's wavelength. The green leaf of a plant absorbs long- and short-wavelength light and reflects light of middle wavelengths, so that when the sun hits a leaf, the light reflected back will have a pronounced broad peak at middle wavelengths (in the green). A red object will have its peak, likewise broad, in the long wavelengths, as shown in the graph on this page.
An object that absorbs some of the light reaching it and reflects the rest is called a pigment. If some wavelengths in the range of visible light are absorbed more than others, the pigment appears to us to be colored. What color we see, I should quickly add, is not simply a matter of wavelengths; it depends on wavelength content and on the properties of our visual system. It involves both physics and biology.


                               VISUAL RECEPTORS
Each rod or cone in our retina contains a pigment that absorbs some wavelengths better than others. The pigments, if we were able to get enough of them to look at, would therefore be colored. A visual pigment has the special property that when it absorbs a photon of light, it changes its molecular shape and at the same time releases energy. The release sets off a chain of chemical events in the cell, described in Chapter 3, leading ultimately to an electrical signal and secretion of chemical transmitter at the synapse. The pigment molecule in its new shape will generally have quite different lightabsorbing properties, and if, as is usually the case, it absorbs light less well than it did before the light hit it, we say it is bleached by the light. A complex chemical machinery in the eye then restores the pigment to its original conformation; otherwise, we would soon run out of pigment.
Our retinas contain a mosaic of four types of receptors: rods and three types of cones, as shown in the illustration at the top of the facing page. Each of these four kinds of receptors contains a different pigment. The pigments differ slightly in their chemistry and consequently in their relative ability to absorb light of different wavelengths. Rods are responsible for our ability to see in dim light, a kind of vision that is relatively crude and completely lacks color.
Rod pigment, or rhodopsin, has a peak sensitivity at about 510 nanometers, in the green part of the spectrum. Rods differ from cones in many ways: they are smaller and have a somewhat different structure; they differ from cones in their relative numbers in different parts of the retina and in the connections they make with subsequent stages in the visual pathway. And finally, in the light-sensitive pigments they contain, the three types of cones themselves differ from each other and from rods.
The pigments in the three cone types have their peak absorptions at about 430, 530, and 560 nanometers, as shown in the graph on this page; the cones are consequently loosely called "blue", "green", and "red", "loosely" because (i) the names refer to peak sensitivities (which in turn are related to ability to absorb light) rather than to the way the pigments would appear if we were to look at them; (2) monochromatic lights whose wavelengths are 430, 530, and 560 nanometers are not blue, green, and red but violet, blue-green, and yellow-green; and (3) if we were to stimulate cones ofjust one type, we would see not blue, green, or red but probably violet, green, and yellowish-red instead. However unfortunate the terminology is, it is now widely used, and

 

 

 

 

 

 

efforts to change embedded terminology usually fail. To substitute terms such as long, middle, and short would be more correct but would put a burden on those of us not thoroughly familiar with the spectrum.
With peak absorption in the green, the rod pigment, rhodopsin, reflects blue and red and therefore looks purple. Because it is present in large enough amounts in our retinas that chemists can extract it and look at it, it long ago came to be called visual purple. Illogical as it is, "visual purple" is named for the appearance of the pigment, whereas the terms for cones, "red", "green", and "blue", refer to their relative sensitivities or abilities to absorb light. Not to realize this can cause great confusion.
The three cones show broad sensitivity curves with much overlap, especially the red and the green cones. Light at 600 nanometers will evoke the greatest response from red cones, those peaking at 560 nanometers, but will likely evoke some response, even if weaker, from the other two cone types.
Thus the red-sensitive cone does not respond only to long-wavelength, or red, light; it just responds better. The same holds for the other two cones.
So far I have been dealing with physical concepts: the nature of light and pigments, the qualities of the pigments that reflect light to our eyes, and the qualities of the rod and cone pigments that translate the incoming light into electrical signals. It is the brain that interprets these initial signals as colors. In conveying some feel for the subject, I find it easiest to outline the elementary facts about color vision at the outset, leaving aside for the moment the threecentury history of how these facts were established or how the brain handles color.



                     GENERAL COMMENTS ON COLOR
It may be useful to begin by comparing the way our auditory systems and our visual systems deal with wavelength. One system leads to tone and the other to color, but the two are profoundly different. When I play a chord of five notes on the piano, you can easily pick out the individual notes and sing them back to me. The notes don't combine in our brain but preserve their individuality, whereas since Newton we have known that if you mix two or more beams of light of different colors, you cannot say what the components are, just by looking.
A little thought will convince you that color vision has to be an impoverished sense, compared with tone perception. Sound coming to one ear at any instant, consisting of some combination of wavelengths, will influence thousands of receptors in the inner ear, each tuned to a slightly different pitch than the next receptor. If the sound consists of many wavelength components, the information will affect many receptors, all of whose outputs are sent to our brains. The richness of auditory information comes from the brain's ability to analyze such combinations of sounds.

   
 



Contents
Next Page





Most colored objects reflect light that is generally richer in some parts of the visible spectrum than in others. The distribution of wavelengths is much broader than that for monochromatic light, however. This graph shows the spectral content of light that would be reflected from a red object, using a broad-band (white) light source.

 
 
 
 
 


Next Page
Contents
Retinal receptors form a mosaic consisting of rods and the three types of cones. This diagram might represent a part of the retina a few degrees from the fovea, where cones outnumber rods.