Rods and cones differ in a 
number of ways. The most 
important difference is in their 
relative sensitivity: rods are 
sensitive to very dim light, 
cones require
 much brighter light. I have 
already described the 
differences in their 
distribution throughout the 
retina, the most notable being 
the absence of rods in the 
fovea. They differ in shape: rods 
are long and slender; cones are 
short and tapered. Both rods 
and cones contain light-
sensitive pigments. All rods 
have the same pigment; cones 
are of three types, each type 
containing a different visual 
pigment. The four pigments are 
sensitive to different 
wavelengths of light, and in the 
case of the cones these 
differences form the basis of 
our color vision.

    The receptors respond to 
light through a process called 
bleaching. In this process a 
molecule of visual pigment 
absorbs a photon, or single 
package, of visible light and is 
thereby chemically changed 
into another compound that 
absorbs light less well, or 
perhaps differs in its 
wavelength sensitivity. In 
virtually all animals, from 
insects to humans and even in 
some bacteria, this receptor 
pigment consists of a protein 
coupled to a small molecule 
related to vitamin A, which is 
the part that is chemically 
transformed by light. Thanks 
largely to the work in the 1950s 
of George Wald at Harvard, we 
now know a lot about the 
chemistry of bleaching and the 
subsequent reconstitution of 
visual pigments.

    Most ordinary sensory 
receptors--chemical, thermal, 
or mechanical--are depolarized 
in response to the appropriate 
stimulus, just as nerves become 
depolarized in response to an 
excitatory stimulus; the 
depolarization leads to release 
of transmitter at the axon 
terminals. (Often, as in visual 
receptors, no impulse occurs, 
probably because the axon is 
very short.) Light receptors in 
invertebrates, from barnacles 
to insects, behave in this way, 
and up to 1964 it was assumed 
that a similar mechanism--
depolarization in response to 
light--would hold for vertebrate 
rods and cones.