Invitation to Psychology

196 Chapter 6 Sensation and Perception

Why the Visual System

is Not a Camera Lo 6.10

Although the eye has often been compared with a

camera, the visual system, unlike a camera, is not a

passive recorder of the external world. Neurons in

the visual system actively build up a picture of the

world by detecting its meaningful features.

Ganglion cells and neurons in the thalamus

of the brain respond to simple features in the en-

vironment, such as spots of light and dark. But in

mammals, special feature-detector cells in the visual

cortex respond to more complex features. This

feature-detector cells
Cells in the visual cor-
tex that are sensitive to
specific features of the
environment.

axons of the ganglion cells converge to form the

optic nerve, which carries information out through

the back of the eye and on to the brain. Where

the optic nerve leaves the eye, at the optic disk,

there are no rods or cones. The absence of recep-

tors produces a blind spot in the field of vision.

Normally, we are unaware of the blind spot be-

cause (1) the image projected on the spot is hit-

ting a different, “nonblind” spot in the other eye;

(2) our eyes move so fast that we can pick up the

complete image; and (3) the brain fills in the gap.

You can find your blind spot by doing the Get

Involved exercise on the facing page.

table 6.1 Differences between rods and Cones

rods Cones

how many? 120–125 million 7–8 million

Where most concentrated? Periphery of retina Center (fovea) of retina

how sensitive? High sensitivity Low sensitivity

Sensitive to color? No Yes

Optic nerve

(to brain)

Rod

Cone

Photoreceptor

cells

Retina

Eyeball

Optic nerve

Direction of

neural impulse

Direction of

neural impulse

Bipolar

Ganglion neurons

cells

Light

Light

Light

Optic

disk

Figure 6.4 The Structures of the retina

For clarity, all cells in this drawing are greatly exaggerated in size. To reach the receptors for vision (the rods and

cones), light must pass through the bipolar and ganglion cells as well as the blood vessels that nourish them (not

shown). Normally, we do not see the shadow cast by this network of cells and blood vessels because the shadow always

falls on the same place on the retina, and such stabilized images are not sensed. But when an eye doctor shines a

moving light into your eye, the treelike shadow of the blood vessels falls on different regions of the retina and you may

see it—a rather eerie experience.