Chapter 2 Summary
1. This chapter provided some insight into the complex journey that is required for
us to see stars and other spots of light. The path of the light was traced from a
distant star through the eyeball and to its absorption by photoreceptors and its
transduction into neural signals. In subsequent chapters we’ll learn how those
signals are transmitted to the brain and translated into the experience of
2. Light, on its way to becoming a sensation (a visual sensation, that is), can be
absorbed, scattered, reflected, transmitted, or refracted. It can become a sensation
only when it’s absorbed by a photoreceptor in the retina.
3. Vision begins in the retina, when light is absorbed by rods or cones. The retina is
like a minicomputer that transduces light energy into neural energy.
4. Retinal ganglion cells have center–surround receptive fields and are concerned
with changes in contrast (the difference in intensity between adjacent bits of the
5. The retina sends information to the brain via the ganglion cells; neurons whose
axons make up the optic nerves.
6. The visual system deals with large variations in overall light intensity by (a)
regulating the amount of light entering the eyeball, (b) using different types of
photoreceptors in different situations, and (c) effectively throwing away photons
we don’t need.
7. Retinitis pigmentosa (RP) is a family of hereditary diseases characterized by the
progressive death of photoreceptors and degeneration of the pigment epithelium.
In the most common form of the disease, patients first notice vision problems in
their peripheral vision and under low light conditions, situations in which rods
play the dominant role in collecting light.
Chapter 2 Study Questions
A Little Light Physics
1. Describe the two ways used to conceptualize light.
2. Describe the difference between light that is reflected and light that is transmitted.
1. One way is to think of it as a wave that travels through a medium. Another is to
think of it as a stream of photons, tiny particles, each consisting of one quantum
www.notesolution.com 2. Reflected light occurs when a ray of light strikes a light-colored surface and then
bounces back towards its point of origin. Transmitted light occurs when light is
neither reflected nor absorbed by a surface. An example is a transparent window;
light passes through the surface and is transmitted to the other side.
Eyes That See Light
3. What is the purpose of the cornea?
The cornea is a transparent surface on the exterior of the eye. It protects the eye from
the outside world. Being transparent, it allows light to be transmitted through it and
into the eye.
4. What is the purpose of the retina?
The retina is a light-sensitive membrane in the back of the eye that contains rods and
cones, which receive an image from the lens and send it to the brain through the optic
5. How does the process of accommodation take place in the eye?
Accommodation takes place in the lens of the eye. The lens changes its refractive
power by changing its shape. This causes the eye to be able to focus on a given object,
whether it is near or far.
6. What is astigmatism and how can it be fixed?
Astigmatism is a visual defect caused by the unequal curving of one or more of the
refractive surfaces of the eye, usually the cornea. It can be fixed by wearing lenses
that have two focal points (that provide different amounts of focusing power in the
horizontal and vertical planes).
7. Why are photoreceptors important in the process of seeing?
Photoreceptors are the cells that make up the backmost layer of the retina. They are
sensitive to light, and as soon as they sense it, they can cause neurons in the
intermediate layers to fire action potentials. Photoreceptors are important in the
process of seeing because they transduce the physical energy of light into neural
energy that our brains can analyze
Retinal Information Processing
8. What are rods and cones?
Rods and cones are photoreceptors present in the retina. Rods are specialized for
night vision, while cones are specialized for daylight vision, fine visual acuity, and
9. Explain what happens in the process of hyperpolarization.
www.notesolution.com Hyperpolarization is an increase in membrane potential in where the inner
membrane surface becomes more negative than the outer membrane surface. This
process is one in a sequence of events that occur once light is sensed by the
10. Why can’t rods signal differences in color?
Rods cannot signal differences in color because they only have one type of
photopigment. Cones, on the other hand, have three types of photopigments,
which help them differentiate between colors.
11. What is the role of horizontal cells?
Horizontal cells are specialized retinal cells that contact both photoreceptors and
bipolar cells. They produce lateral inhibition, which allows the signals that reach
retinal ganglion cells to be based on differences in activations between nearby
photoreceptors rather than absolute levels of activation.
12. What is visual acuity?
Visual acuity is a measure of the finest detail that one can resolve.
13. What is the difference between an “ON” midget bipolar cell and an “OFF” midget
An “ON” midget bipolar cell is a small cone bipolar cell that depolarizes in
response to an increase in light intensity. An “OFF” midget bipolar cell is a small
cone bipolar cell that depolarizes in response to a decrease in light intensity.
These two cells have opposite reactions to light.
14. What is a receptive field?
A receptive field is the region on the retina in which stimuli will activate a neuron.
Receptive fields vary in size, shape, and complexity.
15. Why is the center–surround organization of retinal ganglion cells so important?
The center–surround organization of retinal ganglion cells is important because it
allows for sensitivity to contrast rather than absolute illumination levels. Ganglion
cells are most sensitive to differences in the intensity of light in the center and in
the surround, and they are relatively unaffected by the average intensity of light.
This is useful because the average intensity of light falling on the retina will be
quite variable, depending on whether the observer is indoors, outdoors, etc., but
contrasts of light are relatively constant.
16. What is a filter and how is it important in vision?
A filter is an acoustic, electrical, electronic, biological, or optical device,
instrument, or computer program that allows the passage of some frequencies or
www.notesolution.com digital elements and blocks others. Filter is important in vision because it allows
the transformation of the raw image into a representation by the brain. The filter
highlights certain important visual information while it eliminates other
unimportant information. The center–surround receptive fields of retinal ganglion
cells are filters.
17. What are some consequences of the differing sizes of M ganglion cell and P
ganglion cell receptive fields?
P ganglion cells have smaller receptive fields than M ganglion cells at all
eccentricities. This allows the M ganglion cells to respond to a larger portion of
the visual field. In addition, they are much more sensitive to visual stimuli under
low lighting conditions than P ganglion cells. P ganglion cells, on the other hand,
provide finer resolution (greater acuity) than M ganglion cells can, as long as
there is enough light for them to operate.
Whistling in the Dark: Dark and Light Adaptation
18. Explain how the pupil adapts to dark and light conditions.
The pupil has the ability to dilate and constrict, depending on amount of light. For
example, under well-lit conditions, the pupil tends to constrict to let less light into
the eye. Under dark conditions, the pupil dilates to allow more light into the eye.
19. Explain why it is that we are generally not bothered by variations in overall light
We are generally not bothered by variations in overall light levels because we have
several mechanisms for regulating how much light enters the eye. One mechanism
is the pupil size. Another is the regeneration rates of pigments in our
photoreceptors. Yet another is the rod/cone dichotomy—cones operate at
moderate and high light levels while rods take over for low light levels. Finally,
the neural circuitry of the retina itself helps stabilize external light variations by
emphasizing contrasts in luminance rather than absolute light levels.
The Man Who Could Not See Stars
20. Describe the family of diseases known as retinitis pigmentosa.
Retinitis pigmentosa is a family of hereditary diseases that involves the
progressive death of photoreceptors and degeneration of the pigment epithelium.
The rods are usually affected before the cones. Therefore, people suffering from
this disease first notice vision problems in their periphery and under low light
conditions (where rods play the dominant role in collecting light).
21. Describe what happens to the visual field of a patient with retinitis pigmentosa.
www.notesolution.com Patients suffering from retinitis pigmentosa typically exhibit an overall shrinkage
of their visual fields, as well as “ring scotomas,” which are bands of blindness
between the relatively normal central fields and the periphery.
Chapter 3 Summary
1. In this chapter we followed the path of image processing from the eyeball to the
brain. Neurons in the cerebral cortex translate the array of stars perceived by
retinal ganglion cells into the beginnings of forms and patterns. The primary
visual cortex is organized into thousands of tiny computers, each responsible for
determining the orientation, width, color, and other characteristics of the stripes in
one small portion of the visual field. In Chapter 4 we will continue this story by
seeing how other parts of the brain combine the outputs from these minicomputers
to produce a coherent representation.
2. Perhaps the most important feature of image processing is the remarkable
transformation of information from the circular receptive fields of retinal
ganglion cells to the elongated receptive fields of the cortex.
3. Cortical neurons are highly selective along a number of dimensions, including
stimulus orientation, size, direction of motion, and eye of origin.
4. Neurons with similar preferences are often arranged in columns in primary visual
5. Selective adaptation provides a powerful, noninvasive tool for learning about
stimulus specificity in human vision.
6. The human visual cortex contains pattern analyzers that are specific to spatial
frequency and orientation.
7. Normal visual development requires normal visual experience. Abnormal visual
experience early in life can cause massive changes in cortical physiology that
result in a devastating and permanent loss of spatial vision.
Visual Acuity: Oh Say, Can You See?
1. What is visual acuity and how can it be measured?
Visual acuity is the smallest spatial detail that can be seen accurately. It can be
measured by doing a visual acuity test, which requires looking at figures from a
distance and identifying them.
2. Explain what happens during the phenomenon of aliasing.
www.notesolution.com Aliasing is the misperception of a grating due to undersampling. When looking at
gratings, the visual system “samples” the grating discretely via the array of
receptors at the back of the retina. If the receptors are spaced such that the lightest
and darkest parts of the grating fall on separate cones, the observer can detect the
grating. However, if the lightest and darkest parts of the grating both fall on the
same cones, then the grating will be aliased and appear gray.
3. Explain the meaning of being able to see 20/20.
Being able to see 20/20 means that the observer can identify an object at 20 feet as
well as a “normal” observer would be able to identify it at 20 feet. If the
observer’s vision is 20/40, that means that the observer can see at 20 feet what
somebody with normal vision can see at 40 feet (meaning the observer needs
4. What can we infer from the contrast sensitivity function?
The contrast sensitivity function describes our window of visibility. Any object whose
spatial frequencies and contrast fall within the region specified by the contrast
sensitivity function will be visible. Those objects outside the region are outside our
window of visibility. We can infer from this function that sensitivity to contrast
depends on the spatial frequency of the stimulus.
Retinal Ganglion Cells and Stripes
5. Explain how retinal ganglion cells respond to stripes.
Each ganglion cell responds to certain types of stripes or gratings. For instance, an
ON ganglion cell responds to gratings with spatial frequencies and phases that make
the lightest part of the grating fall on the center of the cell and the darkest part of the
grating fall on the surround. When the spatial frequency of the grating is too low, the
ganglion cell responds weakly because part of the bar of the grating lands in the
inhibitory surround, damping the cell’s response. Similarly, when the grating’s spatial
frequency is too high, the ganglion cell responds weakly because both dark and bright
stripes fall within the receptive field’s center and surround, washing out the response.
When the frequency is just right, the cell responds vigorously.
The Lateral Geniculate Nucleus
6. What is the role of the lateral geniculate nucleus?
The lateral geniculate nucleus is a nucleus in the midbrain that shares connections
with both the retina and visual cortex.
7. What are the two types of layers of the LGN and how are they different from each
www.notesolution.com The two types of layers of the LGN are the magnocellular layers and the parvocellular
layers. The magnocellular layers are the two bottom layers of the LGN, and contain
neurons that are physically larger than those in the parvocellular layers. Neurons in
these layers respond to large, fast-moving objects. The parvocellular layers are the top
four layers of the LGN. They contain neurons that respond to details of stationary
8. Explain the notion of topographical mapping.
Topographical mapping is the orderly mapping of the world in the lateral geniculate
nucleus and the visual cortex. Points of light that are near each other in the world will
be processed by neurons that are near each other in the brain. This orderly
representation provides us with a neural basis of knowing where things are in space.
9. What are two important features of the visual cortex? Explain.
One important feature of the visual cortex is topographical mapping, which is the
orderly mapping of the world in the brain. The second feature is the dramatic scaling
of information from different parts of the visual field. Objects on or near the fovea are
processed by neurons in a large part of the striate cortex, while objects imaged in the
periphery are allocated a much smaller portion of the striate cortex. This feature is
known as cortical magnification.
Receptive Fields in Striate Cortex
10. What is orientation tuning?
Orientation tuning is the tendency of neurons in striate cortex to respond optimally to
certain orientations, and less to others.
11. In what way do striate cortex neurons function as filters?
Each striate cortex neuron responds to a particular location and is tuned to a particular
spatial frequency, orientation, and phase. These narrow tuning functions mean that
each striate cortex neuron functions as a filter for the portion of the image that excites
12. What is ocular dominance?
Ocular dominance is the property of the receptive fields of striate cortex neurons by
which they respond more vigorously when a stimulus is presented in one eye than
when it is presented in the other.
13. What is the difference between simple and complex cells?
Simple cells are cortical neurons with clearly defined excitatory and inhibitory
regions, while complex cells are neurons whose receptive field responds to any
properly oriented bar of light, regardless of whether it is light or dark.
www.notesolution.com 14. What is the role of end stopping?
End stopping refers to a property of certain cortical neurons in which they respond
vigorously when the end of a bar of light falls within their receptive field. It plays an
important role in our ability to detect luminance boundaries and discontinuities.
Columns and Hypercolumns
15. What does a hypercolumn contain?
A hypercolumn contains at least two sets of columns, each covering every possible
orientation, with one set preferring input from the left eye and one set preferring input
from the right eye.
16. What is the enzyme cytochrome oxidase used for?
This enzyme is used to reveal the regular array of “CO blobs,” which are spaced
about 0.5 mm apart in the primary visual cortex. These blobs have been implicated in
processing color, motion, and spatial structure.
Selective Adaptation: The Psychologist’s Electrode
17. How can adaptation provide insights into the properties of cortical neurons?
Adaptation is the diminishing response of a sense organ to a sustained stimulus. It is
helpful in learning about the properties of cortical neurons because, by exposing an
observer to a particular stimuli for an ext