Converting Light into Images

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Chapter 3 Sensation and Perception
years, accommodation becomes more difficult. Converging light rays may come into
focus either before or after they reach the retina, causing images to be blurry. This is
why most older people become “farsighted,” seeing distant objects clearly but needing
glasses for reading or close work. A more common problem in younger people is “nearsightedness,” in which close objects are in focus but distant ones are blurry. This condition has a genetic component, but it can also be influenced by environmental factors
such as reading habits (Quinn et al., 1999; Zadnik, 2001).
Converting Light into Images
The conversion of light energy into neural activity takes place in the retina, which contains neurons that are actually an extension of the brain. The word retina is Latin for
“net,” and the retina is in fact an intricate network of cells (Masland, 2001).
Rods and Cones Specialized cells in the retina called photoreceptors convert light
energy into neural activity. There are two main types of photoreceptors: rods and cones.
Rods and cones are retinal cells that are named for their shapes and that contain chem-
photoreceptors Specialized cells in
the retina that convert light energy into
neural activity.
rods Photoreceptors in the retina that
allow sight even in dim light but that
cannot discriminate colors.
cones Photoreceptors in the retina
that are less light-sensitive than rods
but that can distinguish colors.
dark adaptation The increasing ability
to see in the dark as time passes.
fovea
A region in the center of the
retina.
optic nerve A bundle of fibers that
carries visual information to the brain.
blind spot The point at which the
optic nerve exits the eyeball.
icals that respond to light. When light strikes these chemicals, they break apart, creating a signal that can be transferred to the brain.
The process of rebuilding these light-sensitive chemicals after they break down
takes a little time. This explains why you cannot see when you first come from
bright sunshine into a dark room (Mahroo & Lamb, 2004). In the dark, as your rods
build up their light-sensitive chemicals, your ability to see gradually increases. The
increasing ability to see in the dark over time is called dark adaptation. You
become about 10,000 times more sensitive to light after about half an hour in a
darkened room.
There are three kinds of light-sensitive chemicals in cones, and they provide the
basis for color vision. Rods have only one kind of chemical, so they cannot discriminate colors. However, rods are more sensitive to light than cones. Rods allow you to
see in dim light, as on a moonlit night, but they don’t allow you to see colors. It’s only
at higher light intensities that the cones, with their ability to detect colors, become
most active. As a result, you might put on what looked like a matched pair of socks
in a darkened bedroom, only to go outside and discover that one is dark blue and the
other is dark green.
Cones are concentrated in the center of the retina, in a circular region called the
fovea, which is where the eye focuses incoming light. Differences in the density of
cones in the fovea probably account for differences in various people’s visual acuity, or
ability to see details (Beirne, Zlatkova, & Anderson, 2005). There are no rods in the
human fovea. With increasing distance from the fovea, though, the number of cones
gradually decreases, and the proportion of rods gradually increases. So, if you are trying to detect a weak light, such as the light from a faint star, it is better to look slightly
away from where you expect to see it. This focuses the weak light on the very lightsensitive rods outside the fovea. Because cones do not work well in low light, looking
directly at the star will make it seem to disappear.
If the eye simply transferred to the brain the
images it focused on the retina, we would experience something like a slightly blurry
TV picture. Instead, the eye first sharpens visual images. How? The key lies in the interactions among cells of the retina.
Light rays pass through several layers of retinal cells before striking the rods and cones.
Signals generated by the rods and cones then go back toward the surface of the retina,
making connections with bipolar cells and ganglion cells, which allow the eye to begin analyzing visual information even before that information leaves the retina. Ganglion cells
in the retina have axons that form the optic nerve, which then goes to the brain. Because
there are no receptors for visual stimuli at the point where the optic nerve exits the eyeball, a blind spot is created, as Figure 3.8 demonstrates.
From the Retina to the Brain
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Seeing
This
electron
microscope
view
of
by
rods (blue) and cones (aqua)
shows what your light receptors look
like. Rods are more light-sensitive, but
they do not detect color. Cones can detect
color, but they require more light in order
to be activated. To experience the difference in how these cells work, try looking
at an unfamiliar color photograph in a
room where there is barely enough light
to see. This dim light will activate your
rods and allow you to make out images
in the picture. But because there is not
enough light to activate your cones, you
will not be able to see colors in the
photo.
doing
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learn
RODS AND CONES
After leaving the retina, about half the optic nerve fibers cross over to the opposite side of the brain, creating a structure called the optic chiasm. (Chiasm means
“cross” and is pronounced “KYE-az-um.”) Fibers from the inside half of each eye,
nearest to the nose, cross over. Fibers from the outside half of each eye do not. So
no matter where you look, all the visual information about the right half of the
visual world goes to the left hemisphere of your brain and all the visual information from the left half of the visual world goes to the right hemisphere (Roth, Lora,
& Heilman, 2002).
The optic chiasm is part of the bottom surface of the brain. Beyond this chiasm,
optic fibers extend into the brain itself. The axons from most of the retina’s ganglion cells
form synapses in the thalamus. Neurons there send the visual input to the primary
visual cortex in the occipital lobe at the back of the brain. The primary visual cortex sends visual information to many association areas of the brain for processing
(see Figure 2.10).
FIGURE 3 .8
Find Your Blind Spot
There is a blind spot where the
optic nerve exits the eye. To
“see” your blind spot, cover
your left eye and stare at the cross inside
the circle. Move the page closer and
farther away, and at some point the dot
to the right should disappear from view.
However, the vertical lines around the dot
will probably look continuous, because the
brain tends to fill in visual information at
the blind spot. We are normally unaware
of this “hole” in our vision because the
blind spot of one eye is in the normal
visual field of the other eye.
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by
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Fovea
Optic disk
(blind spot)