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called accommodation If the lens focuses the image on a point in front of the retina, you see

.
near objects clearly but not distant objects. This nearsightedness — myopia — can be remedied
with glasses, contact lenses, or surgery. (Farsightedness — seeing distant objects better than
near objects — occurs when the lens focuses the image on a point behind the retina.)
For centuries, scientists knew that an image of a candle passing through a small open-
ing will cast an inverted mirror image on a dark wall behind. If the image passing through
the pupil casts this sort of upside-down image on the retina, as in F igure 1.6-8 how can we

,

see the world right-side up? Eventually, the answer became clear: The retina doesn’t “see”
a whole image. Consider the four-tenths of a second a baseball batter takes to respond to a
pitcher’s fastball. The retina’s millions of receptor cells convert the particles of light energy
into neural impulses and forward those to the brain, which reassembles them, right-side up,
into what the batter perceives — incoming fastball! Visual information processing percolates
through progressively more abstract levels, all at astonishing speed.

Copyright © Bedford, Freeman & Worth Publishers.
Figure 1.6-8
Lens Retina
The eye
Pupil
Light rays reflected from a candle
pass through the cornea, pupil,
and lens. The curvature and Fovea
thickness of the lens change to (point of
bring nearby or distant objects central focus)
into focus on the retina. Rays from
the top of the candle strike the Pascal Goetgheluck/Science Source
bottom of the retina, and those Iris
from the left side of the candle Optic nerve
to brain’s
strike the right side of the retina. Cornea visual cortex
The candle’s image on the retina Blind spot
thus appears upside down and
reversed. Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
Information Processing in the Eye and Brain

accommodation the process 1.6-5 How do the r ods and cones pr ocess information, and what is the path
1.6-5 How do the rods and cones process information, and what is the path

by which the eye’s lens changes information travels from the eye to the brain?
information travels from the eye to the brain?
shape to focus images of near or
far objects on the retina.
How do we make meaning out of the light energy constantly striking this marvelous organ,
rods retinal receptors that the eye? Let’s examine how our eyes and our brain work together to enable our visual expe-
detect black, white, and gray, and
are sensitive to movement. Rods rience of the world.
are necessary for peripheral and
twilight vision, when cones don’t The Eye-to-Brain Pathway
respond.
Imagine that you could follow behind a single light-energy particle after it reached your
cones retinal receptors that retina. First, you would thread your way through the retina’s sparse outer layer of cells. Then,
are concentrated near the reaching the very back of your eye, you would encounter the retina’s buried photoreceptor
center of the retina and that

function in daylight or in well-lit cells, the rods and cones ( F igure 1.6-9 ). There, you would see the light energy trigger chem-
conditions. Cones detect fine ical changes. That chemical reaction would spark neural signals in nearby bipolar cells. You
detail and give rise to color could then watch the bipolar cells activate neighboring ganglion cells, whose axons twine
sensations. together like the strands of a rope to form the optic nerve After a momentary stopover at

.

optic nerve the nerve that the thalamus, the information would fly on to its final destination, your visual cortex, in the
carries neural impulses from the occipital lobe at the back of your brain.
eye to the brain.
The optic nerve is an information highway from the eye to the brain. This nerve can send

blind spot the point at which nearly 1 million messages at once through its nearly 1 million ganglion fibers. (The auditory

the optic nerve leaves the eye, nerve, which enables hearing, carries much less information through its mere 30,000 fibers.)
creating a “blind” spot because

,
no receptor cells are located We pay a price for this high-speed connection. Your eye has a blind spot with no receptor

there. cells, where the optic nerve leaves the eye ( Figure 1.6-10 ). Close one eye. Do you see a black
hole? No — because without seeking your approval, your brain fills in the hole.
126 Unit 1 Biological Bases of Behavior
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