Invitation to Psychology

Chapter 6 Sensation and Perception 195

contains only cones, clustered densely together.

From the center to the periphery, the ratio of rods

to cones increases, and the outer edges contain

virtually no cones.

Rods are more sensitive to light than cones

are and thus enable us to see even in dim light.

(Cats see well in dim light in part because they

have a high proportion of rods.) Because rods

occupy the outer edges of the retina, they also

handle peripheral (side) vision. But rods cannot

distinguish different wavelengths of light so they

are not sensitive to color, which is why it is often

hard to distinguish colors clearly in dim light. To

see colors, we need cones, which come in three

classes that are sensitive to specific wavelengths

of light. Cones need much more light than rods

do to respond, so they don’t help us much when

we are trying to find a seat in a darkened movie

theater. (These differences are summarized in

Table 6.1.)

We have all noticed that it takes some time

for our eyes to adjust fully to dim illumination.

This process of dark adaptation involves chemical

changes in the rods and cones. The cones adapt

quickly, within 10 minutes or so, but they never

become very sensitive to the dim illumination.

The rods adapt more slowly, taking 20 minutes

or longer, but are ultimately much more sensitive.

After the first phase of adaptation, you can see

better but not well; after the second phase, your

vision is as good as it ever will get.

Explore the Concept Virtual Brain: The Visual

System at MyPsychLab

Rods and cones are connected by synapses

to bipolar cells, which in turn communicate with

neurons called ganglion cells (see Figure 6.4). The

dark adaptation A

process by which visual

receptors become maxi-

mally sensitive to dim

light.

ganglion cells Neurons

in the retina of the eye,

which gather informa-

tion from receptor cells

(by way of intermediate

bipolar cells); their axons

make up the optic nerve.

and dark to the brain area that regulates biological
rhythms, as discussed in Chapter 5.) In a develop-
ing embryo, the retina forms from tissue that pro-
jects out from the brain, not from tissue destined
to form other parts of the eye; thus, the retina is
actually an extension of the brain. As Figure 6.3
shows, when the lens of the eye focuses light on
the retina, the result is an upside-down image.
Light from the top of the visual field stimulates
light-sensitive receptor cells in the bottom part of
the retina, and vice versa. The brain interprets this
upside-down pattern of stimulation as something
that is right side up.
About 120 to 125 million receptors in the
retina are long and narrow, and are called rods.
Another 7 or 8 million receptors are cone-shaped,
and are called, appropriately, cones. The center
of the retina, or fovea, where vision is sharpest,

rods Visual receptors

that respond to dim light.

cones Visual receptors

involved in color vision.

Retinal

blood vessels

Fovea

Optic

nerve

Optic disk

(blind spot)

Lens

Iris

Pupil

Cornea

Retina

Light

Figure 6.2 Major Structures of the eye
Light passes through the pupil and lens and is focused
on the retina at the back of the eye. The point of
sharpest vision is at the fovea.

Fovea

Optic nerve

Cornea

Lens

Figure 6.3 The retinal image
When we look at an object, the light pattern on the retina is upside down. René Descartes (1596–1650) was probably
the first person to demonstrate this fact. He cut a piece from the back of an ox’s eye and replaced the piece with
paper. When he held the eye up to the light, he saw an upside-down image of the room on the paper! You could take
any ordinary lens and get the same result.