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CHAPTER 2: THE BEGINNINGS OF PERCEPTION
Chapter Outline
I.
Preview Questions
II.
Starting at the Beginning
III.
Light and Focusing
A. Light: The Stimulus for Vision
1. Electromagnetic spectrum
2. Wavelength and visible light
3. Photons
B. The Eye
1. Pupil
2. Cornea
3. Lens
4. Retina
5. Rods and cones
6. Optic nerve
C. Light is Focused by the Eye
1. Light waves bent by cornea and lens
2. Accommodation increases lens focusing power
a. Demonstration: Becoming aware of what is in focus
b. Accommodation and the near point
D. Loss of Accommodation with Increasing Age
1. Presbyopia
2. Solutions
E. Myopia (Nearsightedness)
1. Refractive and axial myopia
2. Solutions
a. Glasses and contact lenses
b. LASIK surgery
F. Hyperopia (Farsightedness)
IV.
Receptors and Perception
A. Transforming Light Energy into Electrical Energy
1. Transduction begins at the receptors
a. Role of the outer segment for rod transduction
i. Opsin and retinal
b. Isomerization results in electrical signal
i. Psychophysical evidence (Hecht et al, 1942)
ii. Physiological evidence (Enzyme cascade)
B. Adapting to the Dark
1. Distribution of rods and cones
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a. Macular degeneration
b. Retinitis pigmentosa
c. Blind spot
i. Demonstration: Becoming aware of the blind spot
ii. Demonstration: Filling in the blind spot
2. Measuring the dark adaptation curve
a. Method: Measuring the dark adaptation curve
3. Overall dark adaptation curve
4. “Mythbusters” test why pirates wore eyepatches
5. Measuring cone adaptation
6. Measuring rod adaptation
a. Rod monochromats
b. Rod-cone break
7. Visual pigment regeneration
a. Retinal and opsin rejoin
b. Rushton: Time measurement of regeneration
c. Detached retina shows importance of pigment regeneration
C. Spectral Sensitivity
1. Spectral sensitivity curves
2. Method: Measuring a spectral sensitivity curve
a. Rods most sensitive to 500 nm
b. Cones most sensitive to 560 nm
c. Perceptual consequence: Purkinje shift
3. Rod and cone absorption spectra
a. Explains difference between rod and cone curves
b. Evidence for three types of cones
V.
“Test Yourself 2.1"
VI.
Electrical Signals in Neurons
A. Structure of Neurons
1. Cell body
2. Dendrites
3. Axon
4. Illustrated example of an action potential
B. Recording Electrical Signals in Neurons
1. Method: The setup for recording from a single neuron
C. Basic Properties of Action Potentials
1. Propagated response
2. Stimulus intensity affects rate of firing
3. Refractory periods and rate of firing
4. Spontaneous activity
D. Chemical Basis of Action Potentials
1. The role of ions
2. The role of permeability
3. Phases of the action potential
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4. The role of the sodium-potassium pump
E. Transmitting Information Across a Gap
1. Synapse
2. Neurotransmitters
3. Response at the receptor site
a. Excitation/depolarization
b. Inhibition/hyperpolarization
VII.
Neural Convergence and Perception
A. Retinal anatomy
1. Receptors
2. Bipolar cells
3. Ganglion cells
4. Horizontal and amacrine cells
5. Neural convergence
6. Differences between rod and cone convergence
a. Signals from rods converge more than signals from cones
b. Many foveal cones have no convergence
B. Convergence Causes the Rods to Be More Sensitive Than the Cones
1. Illustrated example of rod convergence and increased sensitivity
2. Illustrated example of cone “private lines” and decreased sensitivity
C. Lack of Convergence Causes the Cones to Have Better Acuity Than Rods
1. Visual acuity – ability to see details
2. Demonstration: Foveal versus peripheral acuity
VIII.
Something to Consider: Early Events are Powerful
A. Hubble Space Telescope Faulty Lens and “Eyeglasses”
B. Electromagnetic Energy and Visual Pigments Affect Vision
IX.
Developmental Dimension: Infant Visual Acuity
A. Method: Preferential Looking
B. Infant Perception of Details
C. Rod and Cone Development
X.
“Test Yourself 2.2”
XI.
Think About It Exercises
XII.
Key Terms
XIII.
Media Resources: Virtual Lab/Companion Website
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Learning Objectives
At the end of the chapter, the student should be able to:
1. Discuss how the electromagnetic spectrum is related to vision.
2. Describe how the cornea and lens focus the image on the retina.
3. Discuss why the near point is used as a measure of presbyopia.
4. State the difference between myopia and hyperopia, and treatments for both.
5. Describe the role of visual pigments in transduction.
6. Describe the method for measuring dark adaptation, and the overall results.
7. Discuss the differences between the distribution of the rods and the cones.
8. Name two visual conditions that differentially affect the rods and the cones
9. Explain why the “blind spot” exists, and why we are not usually aware of it.
10. Describe the method for isolating the rod and cone components of the dark
adaptation curve, and the results for each.
11. Explain how visual pigment regeneration is related to dark adaptation.
12. Describe the difference in spectral sensitivity curves between rods and cones.
13. Define the Purkinje shift, and discuss how spectral sensitivity curves explain this
phenomenon.
14. Identify the key components of neurons.
15. Describe the method involved in recording from a single neuron.
16. Define propagated response, and discuss how this is related to measuring activity
in a single neuron.
17. Understand how the “refractory period” and “spontaneous activity” are important
concepts in understanding the action potential.
18. Describe how electrical signals in a neuron are created, using the terms “ions,”
“resting potential,” “action potential,” “permeability” and “selective
permeability.”
19. Describe the process of synaptic transmission.
20. Discuss the difference between excitatory transmitters and inhibitory transmitters.
21. Differentiate depolarization and hyperpolarization.
22. Discuss why inhibition exists.
23. Describe what convergence is, and the difference in convergence between the
rods and cones.
24. Explain why convergence results in greater sensitivity.
25. Define visual acuity.
26. Identify the difference in acuity between foveal and peripheral vision.
27. Explain why the lack of cone convergence results in greater acuity.
28. Discuss the challenges to testing infant perception.
29. Describe the methods used to test infant visual acuity.
30. Discuss how visual acuity develops over the first year of life.
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Chapter Overview/ Summary
Chapter 2 begins with a discussion of the early stages of vision, specifically, how visible light
is transformed by the major structures of the eye and receptors. Visible light (400 nm to 700
nm) is structured by the environment and reflected into the eye. The cornea and the lens focus
the image on the retina. The lens accomplishes this by the process of accommodation,
increasing the focusing power of the lens when viewing near objects. As we age,
accommodation becomes more difficult, a condition called presbyopia. Two additional vision
problems, myopia and and hyperopia, are described. After the visual image reaches the retina,
transduction is accomplished by rods and cones. Exactly how transduction occurs in the rods
and cones is then summarized. The visual pigment molecules are composed of opsin and
retinal. The retinal reacts to light, by changing shape (a process called isomerization), which
results in transduction. Physiological studies showed that isomerizing one visual pigment
molecule results in the enzyme cascade, causing a sequence of chemical reactions within the
receptor. The role of photopigments in the receptors are then discussed. The rods and cones are
different in several ways: their shape, the location of each type on the retina, and overall
number of each in the human eye. As rods and cones have distinct functions, they also can be
associated with different disorders in vision: macular degeneration is a condition that destroys
foveal cones; and retinitis pigmentosa primarily destroys the peripheral rods. Where there are
no receptors (where the optic nerve leaves the eye), a blind spot exists. Rods and cones also
differ in their adaptation to the dark, achieving maximum sensitivity at different times. Rushton
showed that these times correspond to the time needed for pigment regeneration. Spectral
sensitivity is also dependent on characteristics of the visual pigment, specifically, the
absorption spectrum. A demonstration of the difference in spectral sensitivity for rods and
cones is the Purkinje shift – enhanced perctption of short wavelengths during dark adaptation.
After transduction, visual information is represented via an electrical signal. The electrical
signal begins at the receptor and travels through the nerve to the brain where it synapses with
neurons. The major parts of a neuron are listed, and the receptors are discussed as a specific
type of neuron. The role of microelectrodes in recording neural activity is described. The
details of how electrical signals are transmitted are discussed next. Four basic properties of
action potentials are discussed: (1) the action potential is a propagated response; (2) changes in
stimulus intensity increases rate of firing, not the size of the action potential; (3) refractory
periods occur, providing an upper limit to firing rate; and (4) neurons have spontaneous
activity, even when no stimuli are presented. A detailed discussion of the action potential
follows including the role of sodium, chlorine, and potassium ions in the conduction of the
electric signal. The values of the resting potential and how these change during the action
potential are provided, as well as the role of permeability and selective permeability in the
process. Communication between neurons occurs at the synapse: neurotransmitters are released
across the synapse, and taken into the receptor sites. The change in the voltage of the
postsynaptic neuron depends on the amount of excitation and inhibition it receives from the
presynaptic neurons. Goldstein then describes differences in the transfer of information from
rods and cones. He begins with the concept of convergence, then demonstrates the resultant
trade-off between sensitivity and acuity. Convergence (as occurs with the rods) leads to greater
sensitivity, but decreased acuity. “Direct lines” (as occurs with the foveal cones) leads to
decreased sensitivity, but greater acuity. The final portion of the chapter details the challenges,
methods, and results of research on infant perceptual development. Testing infant perceptual
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capabilities can be difficult as infants are non-verbal and because they act like infants.
Therefore, methods like preferential looking (PL) and visual evoked potentials have been
devised to assess infant visual capacities. Using these techniques, it has been determined that
acuity progresses from about 20/400 - 20/600 at 1 month to full acuity after 1 year. The major
reason for this is that the retinal cones and cortical cells have to develop. Contrast sensitivity
functions, in which contrast is used to determine the sensitivity to seeing gratings, show that
sensitivity to higher frequencies develop over the first three months, so fine details are only
perceived from after 3 to 6 months.
Demonstrations, Activities, and Lecture Topics
(1) Classroom Pinhole Camera: Prull and Banks (2005) have developed an
outstanding demonstration to teach about the functioning of the eye. They created a
“classroom-sized pinhole camera” to demonstrate pupil functioning, image
inversion, and focusing (including myopia and hyperopia). Details can be found at:
Prull, M. W., & Banks, W. P. (2005). Seeing the light: A classroom-sized pinhole
camera demonstration for teaching vision. Teaching of Psychology, 32, 103-106.
(2) Online Tests for Visual Disorders: Free online eye “exams” for macular
degeneration, myopia, and hypermyopia can be found online at the Optometric Eye
Site. The St. Luke’s Cataract and Laser Institute website has a page entitled “Eye Q”
which also contains the Amsler grid (can be printed) and information. For the test, the
student would cover one eye and look at the fixation point. While doing this, they
should see if the lines appear wavy or if portions of the grid are missing; either result
would be a potential indicator of an abnormality.
(3) Visual Disorders: The topic of visual disorders always leads to numerous
questions from students. Be prepared for questions about LASIK and other
surgical treatments, macular degeneration, and astigmatism. The websites listed
below contain information, photos, and animations on numerous disorders.
(4) Eye Models: For instructors with larger budgets, Denoyer-Geppert Science
Company offers a range of eye models that provide students with “hands-on
experience.” There is a less-expensive student version, and a more detailed and
expensive “Giant Five-Part Eyeball.” “Rubin’s Eye and Vision Lab” also may be
a good investment, in which you can change lenses to demonstrate focusing
problems. These can be ordered from the website.
(5) Video Suggestion: Denoyer-Geppert also has a video series – “The Special Senses” –
that covers some information presented in Chapter 2. The video reviews each sensory
receptor (vision, hearing, smell, taste). For vision, there is discussion of the eyeball
and the process of refraction.
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(6) Interactive Class Demonstrations: There have been at least three articles that have
addressed how to get the class directly and actively involved in simulating neural
functioning. Some have been used for introductory psychology and physiological
psychology, but certainly would be appropriate for this chapter. Here are the
references for these three:
Bockoven, J. (2004). The pedagogical toolbox: Computer-generated visual displays,
classroom demonstration, and lecture. Psychological Reports, 94, 967-975.
Hamilton, S. B., & Knox, T. A. (1985) The collossal neuron: Acting out physiological
psychology. Teaching of Psychology, 12(3), 153-156.
Reardon, R.R., Durso, F.T., & Wilson, D.A.. (1994). Neural coding and synaptic
transmission: Participation exercises for introductory psychology. Teaching of
Psychology, 21(2), 96-99.
(7) 4) Evolution and Synapses: An interdisciplinary approach to neural functioning was
addressed in a 2008 New York Times article on the relationship between evolution
and synapse formation. The article is entitled, “Brainpower may lie in the complexity
of synapses”.
(8) Preferential Looking and Confounds: A research method issue of experimenter
effects can be highlighted by research involving the preferential looking
technique. Goren, Sarty, and Wu (1975) reported that newborns preferred looking
at “scrambled faces” than “unscrambled faces.” Maurer and Young (1983) were
skeptical about the results, and replicated the study, with two important
differences: (1) they put the infant in a baby seat, instead of the experimeter’s lap;
and (2) made sure that the experimenter could not see the stimulus. These changes
eliminated the preference for the scrambled faces. The implication is that the
experimenters in the first study may have unintentionally influenced the infant’s
looking behavior. The bottom line is that care must be taken in using these
techniques when testing infants.
9) Dog Eye Charts: A humorous spin on the issues involving testing visual acuity of
non-verbal individuals (infants and dogs) is the construction of “dog eye charts.”
An original example of this was used in a “Peanuts” cartoon in which Charlie
Brown takes Snoopy to the “eye doctor,” where Snoopy reads a “Snellen-like”
chart where the stimuli are different-sized paw prints. Local Paper Studio (online
vendor) provides humorous dog and cat “eye charts”.
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10) VEP and Dyslexia: Although the chapter focuses on the use of VEPs to study
infant development of acuity, VEPs can also be used to determine if there are
differences in the visual processing of older children. One area where this has
been tested is to see if there is a difference in early visual processing between
dyslexic and non-dyslexic children. Solan et al. (1990) found that the VEP
amplitude for response to reversing checkerboards was generally higher for
control children than dyslexic children, but the binocular advantage was similar
for the two groups.
Solan, H.A., Sutija, V.G., Ficarra, A. P., & Wurst, S. A. (1990). Binocular
advantage and visual processing in dyslexic and control children as measured by
visual evoked potentials. Optometry and Visual Science, 67, 105-110.
Suggested Websites
ePsych: An Electronic Psychology Text
The site contains many student-friendly tutorials, very readable, informative, and
sprinkled with humor. Click on “The Biological Mind” and begin with “The Neuron”. A
nice introduction to the rationale for studying the biological aspects of vision appears in
the section, “It’s All in Your Brain” and discussion of the structure of the eye, blind spot,
rods and cones, and dark adaptation appear in “The Eye”.
SUNY School of Optometry
This is the website for the SUNY School of Optometry. Students who express an interest
in a career in optometry can check out the admissions policies. Another interesting aspect
of this optometry school is that they maintain a “learning disability unit.” Students with
overlapping interests in vision and developmental/clinical issues should find their work
fascinating.
University of Michigan Kellogg Eye Center
This website represents another optometry school. It also has a great summary of visual
conditions and treatments.
National Eye Institute
The National Eye Institute website has a wealth of information, pictures, and videos.
You can search the site for information about retinitis, macular degeneration, detached
retina, and other conditions discussed in Chapter 2.
Foundation Fighting Blindness
The goal of the foundation represented on this website is to support research of retinal
degenerative diseases. The website contains numerous links, images, and information
sources. News reports are updated frequently. In addition to the factual information about
the conditions, causes, and treatments, the site also addresses the emotional aspect of
coping with visual disorders.
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Jan Worst Research Group: Ophthalmic Studies
This Netherlands-based ophthalmology center has some interesting (if somewhat graphic)
images of corneal surgery, and the surgical equipment used.
Webvision: The Organization of the Retina and Visual System
This website is probably aimed more at graduate students than undergraduates, but
instructors can peruse this outstanding site for many concepts covered in Chapter 2. Find
the Table of Contents (positioned on the right side of the page) and select Part II
(Anatomy and Physiology of the Retina) and Part III (Retinal Circuits) for information
relevant to this chapter.
The Brain from Top to Bottom
McGill University sponsors this informative website. Click on “From the simple to the
complex” to access a nice tutorial on neural functioning or “Vision” for a tutorial on the
structures involved in vision. A great feature of this website is that you can select
“beginner,” “intermediate,” and “advanced” levels of information once you have chosen
a topic area.
Neurons - Animated Cellular and Molecular Concepts
Animations and Powerpoint presentations abound on this website from the University of
Toronto on neuron anatomy, synaptic transmission, and the action potential.
UCLA Baby Lab
The lab studies perceptual and cognitive development in infants. Eye tracking is one of
the techniques they use with infants.
Infant Vision Laboratory at Smith-Kettlewell
An active infant perception lab at Smith-Kettlewell is headed by Andrew Norcia. They
conduct VEP studies of infants. Also click on “What can my baby see?” for a great
summary of helpful information.
“PsyCinema: Perception” Clip
Wild Wild West: (23:16-24:44): In this scene, West and Gordon (Will Smith and Kevin
Kline) attempt to determine the last image a dead man saw. Gordon cites the “retinal
terminus theory,” and projects this image from the dead man’s head (much to the
displeasure of West!). This theory was popular in the mid 1800s (the time the movie is
set in). One of the theory’s major proponents was Kuhne, a physiologist at Heidelberg
University, who claimed he was able to retrieve the “optogram” from the eye of a
guillotined man. Based on the information in Chapter 2, students can critically discuss the
merits (or lack of merits) of this theory. Added bonuses in the film clip: Gordon refers to
aqueous humor and that the image projected is upside-down due to refraction.
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