PERCEPTION
This story is about a Seattle native, and University of Washington graduate
(1975 in psychology and microbiology) who went on to win a Nobel Prize (2004,
Physiology and Medicine). Her award-winning work centered on perception.
In particular, she was recognized for helping us to understand the complexity
of how we smell. Dr. Linda Buck is now back “home” in Seattle – she is a
member at the Fred Hutchinson Cancer Research Center. For more information
on that work as well as Dr. Buck, visit http://www.hhmi.org/research/nobel/buck.html
In Unit 6, we will finish our tour of psychoactive drugs with those drugs whose primary mode
of action is to alter perception – hallucinogenic or psychedelic drugs. First – it can be said that
ALL drugs alter perception. Aspirin, taken at high enough doses, causes a user to experience
ringing in the ears (tinnitus) that does not correspond with any sound stimulus reaching the
ears but internal alterations in perception. Thus many experts will disagree with us here that
cannabis is a depressant and MDMA is a stimulant. These drugs are often categorized as
hallucinogens. And indeed, they alter perception. But because there are a small number of
drugs that do this as their primary action – that account for the reasons people take these drugs
– psychedelics earn a unique placement in this class. Before embarking on a study of these
drugs, we return, tangentially, back to neglected material from unit 1. We will first give an
overview of one function of the nervous system – perception.
Objectives
After successfully completing this lesson you will be able to:

Understand how stimuli (such as light and sound) are interpreted by sensory receptors
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Understand how these sensory cells transmit data to the brain where it can be interpreted
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Understand why sensory information is generally correctly interpreted by the brain and why,
in interesting cases (illusions or hallucinations) the information can be mis-interpreted.

Explain several optical illusions in terms of anatomy, cell biology, and often behavior.
Before you begin!
o
Your ideas

How many senses do we have?
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What are some senses other animals have that we lack?
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PERCEPTION

If a person enters the room with cologne or perfume, you are aware of the smell. What
happens after (s)he has been in the room for a while?
o
Previously learned material

Through which brain structure do sensory signals get routed?
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Hypothalamus
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Thalamus
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Hippocampus

Cerebellum

What is gray matter?

At a synapse, how does the post-synaptic neuron BEGIN an action potential?
Lesson 25: Perception
Guiding Questions
1. How is sensory information acquired and how does it travel to the brain to be perceived?
2. We say that our senses are not objective – that they often tell us incorrect things about our world.
Gather up examples that include:
- Things we miss that ARE there
- Things we detect that are NOT there
- Things we detect differently from how they really are
3. What are sensory neurons and how do they get stimulated?
4. What are some features of how our visual system is designed that explain illusions?
Key Terms





Sensory receptors, chemoreceptors, mechanoreceptors, photoreceptors (rods and cones)
Sensation vs. perception
Blind spot, optic nerve
Sensory adaptation
Synesthesia
Activity One: Web Site Review
Site no longer available
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PERCEPTION
Activity Two: Sensation is not Perception
We have a number of sensory cells which are really only specialized neurons that help us gather in
information about our world and our body. These cells can be categorized into three basic types: lightsensing receptors (which includes only photoreceptor cells), chemical-sensing cells or chemo-receptors
(which detect pain, taste, and smell), and pressure-sensing cells or mechano-receptors (which are
responsible for touch, hearing, balance, and proprioception). When a stimulus reaches a sensory cell,
such as a photoreceptor cell, it triggers chemical reactions that lead to alterations of that neuron’s
electrical activity.
Incorporate image at http://upload.wikimedia.org/wikipedia/commons/6/61/Structure_rods_cones.png
Proprioception may be new to some of you. The “finger-to-nose” test is a test of proprioception.
Proprioception is like balance. Where balance informs you about the position of your body relative to
gravity, proprioception informs you about the relative positions of your body parts. Some people
without proprioceptive abilities have to watch their hands move, for instance, the cup to their mouths in
order to aim correctly.
The stimulation of our sensory cells is called sensation. But only when the information about that
stimulation reaches the brain AND the brain interprets the information can we say perception has taken
place. Thus, perception is a subjective experience, removed from the initial “data” obtained by our
bodies through the relaying of that information to the brain. Recall the example in the “Before you
begin” section. The person with cologne or perfume did not remove the scent when you stopped
noticing it. The scent is still there but your brain stops reacting to the information, “deciding” that is no
longer relevant.
Sensation occurs when a sensory neuron, responding to a stimulus, responds electrically. For
photoreception, rhodopsin in photoreceptor cells changes its shape. In some sensory cells, stimuli will
cause sodium to enter and originate an action potential in these cells. Recall that sodium entry is also the
way a post-synaptic neuron gets “excited”. Other cells respond to stimuli by INHIBITING electrical
activity that was occurring before the stimulus arrived.
TEST OF CONTENT
The signals that report about touch travel by myelin-enclosed axons to the
brain whereas the signals that report about heat do not use myelin wrapping. Which
stimulus do we perceive first?
Close one eye. Describe how your perception has changed.
Activity Three: Focus on Vision
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PERCEPTION
Because this lesson finishes by examining a few illusions, and because most examples of illusions are
optical illusions, we will start with some basic information about vision.
Incorporate image from
http://commons.wikimedia.org/wiki/File:Human_eye_cross_section_detached_retina.svg
Vision begins by light striking photoreceptor cells on the retina (back of the eye ball) called
photoreceptors. Our photoreceptors specialize either in detecting low light (rods) or colored light of
specific wave lengths (red cones, blue cones, and green cones). Rods and cones are named because of
their shape.
Incorporate image from http://commons.wikimedia.org/wiki/File:Cone-response.svg
Photoreceptors are not equally distributed on the retina. Rods are found more peripherally. A
particularly dense area of cones is found near the center-back of the retina. And NO photoreceptor cells
are located in the portion of the retina through which the axons from photoreceptor cells pass – the optic
nerve.
Once photoreceptor cells detect light, they send that information to the visual cortex in the brain, having
been routed through the optic chiasm and thalamus you will recall from the brain dissection.
Incorporate image from http://commons.wikimedia.org/wiki/File:ERP_-_optic_cabling.jpg
TEST OF CONTENT
Examine the image (insert from http://commons.wikimedia.org/wiki/File:Netzhautlkpolarp.jpg ) and determine which photoreceptor cell type is most restricted in location.
Rods
Red cones
Green cones
Blue cones
Activity Four: Tricking your Photoreceptor Cells
Recall that photoreceptors send their signals down axons that leave the eye via the optic nerve. The
optic nerves from your two eyes then cross at the optic chiasm (left visual field information from both
eyes goes one way while right visual field information goes the other). Then the information passes
through the thalamus which recognizes its origin (eye) and therefore directs it to the visual cortex (left
visual field ends up on the right side of the visual cortex). Keep this routing in mind while performing
the next demonstration on yourself.
IMPORTANT – be gentle. There is no need for you to apply more than a little pressure – this activity
should NOT hurt.
After reading the instructions you will
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PERCEPTION
-
Close your eyes
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Swing your eyeballs to the left (as if you were looking at something to your left).
-
Gently press a finger-tip on the right side of the right eye ball.
finger
closed eye
Drawing – Linda MM
What do you “see”? Try it a few times and see if that changes. Try it in a very dark room and see if it
changes. Be ready to report and discuss the observation on the forum coming up in activity 7.
Activity Five: Your Visual Cortex Tricks You!
Remind yourself about the retina. It might help to draw a “map”. The outer edges of your retina circle
will correspond to your peripheral vision and photoreceptor cells that gather visual information from
these outer reaches of vision. Toward the center, you will have a high density of photoreceptor cells.
Think about when you read a book. You would have a hard time making sense of the words if you held
the pages so they were being perceived by peripheral retinal cells. The high density of receptors helps
you resolve small details about an object (it is called the fovea). Now draw on the portion of the retina
that does not have retinal cells – where the photoreceptor cell axons exit the eye. Human eyes are built
strangely. The photoreceptors receive stimuli on the side farthest from light, and transmit the
information on axons that travel back into the center of the eye.
Axon-leaving end
Light-sensing end
Drawing Linda MM (rod cut-out from Audesirk….)
These photoreceptor cell axons are therefore in the way” and must about-face (arrow) to take
information to the brain. They do so as a collective. Their axons group together into the optic nerve
which leaves the retina in an area called the “blind spot”.
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PERCEPTION
Some other creatures have more logical visual systems. The retinal cells of the octopus are arranged
with light-sensing end facing the light and axon-leaving end already pointed in the out-going direction.
To test your blind spot, look at the strip below, with a smiley face on the left and a frowning face on the
right. (this works better if you print the strip first and can hold the paper at various distances from your
face).
Cover or close your left eye and with the right eye, focus on the smiley face. Move your head toward
and away from your monitor (or move the printed strip close and farther from your face). You will be
able to notice the frowning face in your peripheral vision for most of the time (at most distances from
your face), but it disappears at one point. This is the point at which the frowning image falls upon your
blind spot. No photoreceptor cells are available to report about the image and it disappears from view.
We do not walk around in the world with a big hole in our vision (think about why and report this to the
forum in activity 7). You will probably have thought of one reason, but I’ll tell you the other now with
another demonstration.
Perform the same activity with the strip below (close left eye, right eye focuses on line space to the left,
aware of the space on the right).
At one distance, the GAP disappears. This means that you think you are seeing a continuous line. Your
brain has filled in the gap in a logical way – it assumes that the pattern continues through the gap. So
not only does the blind spot cause you to miss some things, it causes you to see things that are not there.
Activity Six: More Optical Illusions
Follow this link to a presentation that shows you a few more optical illusions. For each, we’ll ask you to
consider what explains that illusion and report about it in activity seven.
Activity Seven: Forum Connecting Observations with Explanations
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PERCEPTION
Join the discussion forum about optical illusions. There will be four sections – one each for the activities
four and five, and two for the illusions presented in activity six. In each forum, we’ll ask you to discuss
both what you “see” (or don’t see) and why you see it (or don’t). For the “why” question – focus on
trying to use biological concepts, not describing the benefits of these illusions.
KEY
Activity 4 – different people see different things. Typical reports include a black spot, a hole, or a
glowing halo of light.
Activity 5 – we don’t walk around with a hole in our vision in part because we have two eyes. We had
to cover one to do the blind spot test.
Activity 6
– American flag – constant exposure to a stimulus (like cologne) will result in our senses
adapting and as they do, they stop reporting to the brain. (A similar effect can be seen at
http://michaelbach.de/ot/col_lilacChaser/index.htm ).
– Stepping blocks - motion is a very important aspect of visual information – we have to pay
attention to the “tiger racing toward us” to stay alive. Contrast is important too – we notice
everything in context of surrounding information. Without contrast, we can’t really see the entire
block (when black is over a partially black background) so it seems as if the block is NOT moving –
as if it is paused, then starts moving again when it is over a contrasting light background. (A related
motion illusion can be found at http://michaelbach.de/ot/mot_spokes/index.html ).
Activity Eight: Synesthesia – “Blended Senses”
A hallucination is the distortion of reality – a distortion of perception. Some people with severe
psychoses hallucinate (hear voices). Other people hallucinate while on drugs (seeing auras, or feeling
sensations that are not there). Hallucination is not always a deviation – it can be part of normal life.
After a long exposure to a clock’s chime, you might still “hear” it even when it is not there, or you might
remember your mother and “smell” home baked
cookies.
Some people “hallucinate” regularly as part of
how they live in the world and without
psychosis. These people are synesthesiacs.
One test for the ability of synesthesia is found at
http://www.youtube.com/watch?v=o39TiACe4m
w&feature=related . When viewing the dots, do
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PERCEPTION
you hear any sound? If so, you have one form of synesthesia.
Another test is shown below. Examine the image BRIEFLY (2-3 seconds). Can you quickly report about
how many #2s there are? If not – click on this link to see this image the way a person with synesthesia
does (second – colorful image).
Or try the experiment at http://faculty.washington.edu/chudler/syne.html to see if you have synesthesia.
Don’t panic if you do! About 0.25% of people have some form of synesthesia. It is particularly common
among artists and poets. Watch the video at http://www.youtube.com/watch?v=aIEiOrxhtNQ and see if
you can get a feel for why this special ability might incline a person toward abstract arts.
How does the brain of a synesthesiac work? For a more complete story on that as well as phantom limbs
– an equally relevant and mysterious perceptive anomaly, you may want to listen to Dr.
Ramachandran’s presentation on this at
http://www.ted.com/talks/vilayanur_ramachandran_on_your_mind.html (phantom limb presentation
starts at 9:28, synesthesia presentation starts at 17.45). The brief answer to this question is that when
information arrives at the visual cortex (or other sensory cortexes), we usually think it just ENDS there.
But in some cases, the information is so vigorous it will travel “laterally” to other cells of the visual
cortex (letter recognition leaks over into color recognition) or even from one sensory cortex to another
(images take on an auditory meaning).
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PERCEPTION
Activity Nine: Reading
Required Reading
Any introductory, college level biology textbook
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Chapter on perception
Internet
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http://www.hhmi.org/senses/ (Howard Hughes Medical Institute educational publication)
Supplemental Reading
Internet
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http://www.hhmi.org/research/nobel/buck.html (bio on Dr. Buck’s Nobel work)
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http://faculty.washington.edu/chudler/syne.html (Neuroscience for Kids on synesthesia)
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http://www.youtube.com/watch?v=aIEiOrxhtNQ (test for synesthesia)
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http://www.ted.com/talks/vilayanur_ramachandran_on_your_mind.html (Dr. Ramachandran)
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