FoundationalConceptsNeuroscience

200-+-

g^1

100-4

(thouands

per

square

mm)

8 1

Photoreceptor

cell

density

11109 8765432103123 45 6 7 8 9 1011

Distance from fovea (mm)

Figure 14.3. Distribution of cones (solid line) and rods (dotted line) in the
human retina. Cones are concentrated at the fovea and are sparse elsewhere in
the retina. Conversely, rods are sparse at the fovea and more densely distrib-
uted elsewhere in the retina, with peak density a short distance away from the
fovea. There are no cones or rods in the blind spot, the place where the axons
forming the optic nerve exit the eye.

Because the cones at the fovea require relatively bright light to
become activated, very dim light is best detected when you are not
looking directly at it. You can experience this when the first stars
begin to twinkle in the sky at twilight. If you look directly at sucha
star, it may flicker into invisibility; look slightly away and it returns.
When you look away from the star, its dim light is focused on a region
of the retina that is not the fovea but a bit off into the periphery. Away
from the fovea there are many rod cells, the cells that are more sensi-
tive to dim light.
A short distance from the fovea there is a small region of the retina