Understanding Psychology (10th Ed)

 Rods 
are thin, cylindri-
cal receptor cells that are highly sensitive to light.
 Cones 
are typically cone-shaped, 
light-sensitive receptor cells that are responsible for sharp focus and color perception, 
particularly in bright light. The rods and cones are distributed unevenly throughout 
the retina. Cones are concentrated on the part of the retina called the fovea . The fovea 
is a particularly sensitive region of the retina. If you want to focus on something of 
particular interest, you will automatically try to center the image on the fovea to see 
it more sharply.
 
The rods and cones not only are structurally dissimilar but they also play dis-
tinctly different roles in vision. Cones are primarily responsible for the sharply 
focused perception of color, particularly in brightly lit situations; rods are related to 
vision in dimly lit situations and are largely insensitive to color and to details as 
sharp as those the cones are capable of recognizing. The rods play a key role in 
peripheral vision —seeing objects that are outside the main center of focus—and in 
night vision. 
Rods and cones also are involved in dark adaptation , the phenomenon of adjusting 
to dim light after being in brighter light. (Think of the experience of walking into a 
dark movie theater and groping your way to a seat but a few minutes later seeing 
the seats quite clearly.) The speed at which dark adaptation occurs is a result of the 
rate of change in the chemical composition of the rods and cones. Although the cones 
reach their greatest level of adaptation in just a few minutes, the rods take 20 to 30 
minutes to reach the maximum level. The opposite phenomenon— light adaptation , or 
the process of adjusting to bright light after exposure to dim light—occurs much 
faster, taking only a minute or so.