After staring at the flag for 30 seconds and then looking at the dot, I see a faint purple color, which is the opponent color of yellow.

1. Introduction to Sensation
and Perception: Vision
(Module 14)

2. How do we construct our
representations of the world?
Our sensory receptors detect physical energy from the
environment and translate it into neural signals. This is
called sensation.
At this level of detection, sensory analysis uses bottom-up
processing (…is sensory analysis that begins at the
entry level, with information flowing from the sensory
receptors to the brain).
However, we also select, organize, and interpret our
sensations—a process called perception.
During perception, our mind processes what our senses
detect. At this level, sensory analysis uses top-down
processing (… is information processing guided by
higher-level mental processes, as when someone
constructs perceptions drawing on experience and
expectations).

3. Bottom-up Processing
Analysis of the stimulus begins with the sense
receptors and works up to the level of the brain
and mind.
Letter “A” is really a black blotch broken down into
features by the brain that we perceive as an “A.”

4. Top-Down Processing
Information processing guided by higher-level
mental processes as we construct perceptions,
drawing on our experience and expectations.
THE CHT

5. Sensory and perceptual processes form a continuum.
Failures occurring anywhere between sensory detection
and perceptual interpretation can distort our view of the
world.
EX: Patient E.H. suffered from propopagnosia, which is the
inability to recognize and connect with the outside the
world. Complete sensation was present but perception
was incomplete. If shown an unfamiliar face, there was
no response. If shown a familiar face, the autonomic
system reacted by perspiring. If shown her own face,
she still could not recognize it. This is what is known as
a lack of top-down processing. She is unable to relate
her stored knowledge to the sensory input.

6. Making Sense of Complexity
Our sensory and perceptual processes work
together to help us sort out complex images.
“The Forest Has Eyes,” Bev Doolittle

7. Sensing the World
Senses are nature’s gifts that suit an organism’s needs.
Because of particular receptor cells in a frog’s eyes, a frog
can detect the motion of insects and can therefore thrive. A
frog could be surrounded by motionless insects and starve
to death!
A male silkworm moth is sensitive to female sex-attractant
odor and can be affected up to a mile a way!
We as human beings are sensitive to sound frequencies that
represent the range of human voice.

8. Psychophysics
A study of the relationship between physical
characteristics of stimuli (such as intensity) and
our psychological experience with them.
Psychological
Physical World
World
Light Brightness
Sound Volume
Pressure Weight
Sugar Sweet

9. Sensation
Sensing the World:
Some Basic Principles
 Thresholds
 Sensory Adaptation

10. Exploring the Senses
What stimuli cross our threshold for
conscious awareness?
(Absolute Threshold)

11. Absolute Threshold
To some kinds of stimuli we are especially sensitive. If we
were to stand on top of a mountain (on a dark, clear
night), most of us would be able to see a candle flame
on top of another mountain 30 miles away.
Most of us would be able to feel the wing of a bee on our
cheek.
Most of us would be able to smell a drop of perfume in a 3
room apartment.
Our awareness of these faint stimuli illustrates our absolute
thresholds— which is the minimum stimulation
necessary to detect a particular light, sound, pressure,
taste, or odor 50% of the time.
EX: A hearing specialist exposes an individual to varying
sounds in order to figure out their absolute threshold for
hearing certain pitches.

12. Absolute Threshold
Absolute Threshold: Minimum stimulation needed
to detect a particular stimulus 50% of the time.
Proportion of “Yes” Responses
0.50 1.00
------Subliminal Threshold
below the threshold of
0.00
conscious perception
0 5 10 15 20 25
Stimulus Intensity

13. Subliminal Threshold
Have you ever tried a cessation program that delivered you
messages subliminally?
For example, listening to audiotapes that are supposed to
help you lose weight, stop smoking, or improve your
memory?
These audiotapes are usually masked by soothing ocean
sounds and unheard messages such as “I am thin”,
“Smoke tastes bad”, or “I do well on tests because I have
total recall of information”.
These subliminal messages are all in an attempt to
influence our behavior.
We can unconsciously sense subliminal stimuli and without
our awareness can to some extent be affected by it.

14. A stimulus is Subliminal if it is below your
absolute threshold, meaning that you detect
it less than 50% of the time. For instance, a
microscopic cell is subliminal to you because
you cannot see it with your naked eye.
Subliminal advertisements (Drink Coke,
eat popcorn etc.), do have an affect on you,
(they prime you—predisposing you to making a
particular decision) but do not persuade you.
The final statement of subliminal messages is that
much of our information processing occurs
automatically, out of sight, and off the radar
screen of our conscious mind.

15. Subliminal Threshold
Subliminal Threshold:
When stimuli are below
one’s absolute threshold for
conscious awareness.
Kurt Scholz/ Superstock

16. Difference Thresholds
How does the magnitude of a stimulus
influence our threshold for detecting
differences?
The Difference Threshold (a.k.a the just
noticeable difference or jnd) is the lowest
difference you can detect between any
two stimuli 50% of the time. For example,
you are just able to notice the difference
between 1kg and 1.02kg half the time.

17. Weber’s Law
Two stimuli must differ Constant
Stimulus
by a constant (k)
proportion (rather than Light 8%
a constant amount), to
be perceived as Weight 2%
different. Weber Tone 3%
fraction: k = ∆I/I.

18. Sensation
Sensing the World:
Some Basic Principles
 Thresholds √√
 Sensory Adaptation

19. Sensory Adaptation
Diminished sensitivity as a consequence of
constant stimulation.
Put a band aid on your arm and after awhile
you don’t sense it.

20. Sensory Adaptation: What purpose
does it serve?
Although sensory adaptation reduces our
sensitivity, it offers an important benefit:
Freedom to focus on informative changes in our
environment without being distracted by the
constant chatter of uninformative background
stimulation.
Our sensory receptors are alert to novelty; bore
them with repetition and they free our attention
for more important things.
This reinforces a fundamental lesson: We
perceive the world not exactly as it is, but as it is
useful for us to perceive it. (ex: smokers, body
scents, etc.)

21. Now you see, now you don’t
(a) A projector mounted on a contact lens makes the projected image move with the
eye. (b) Initially the person sees the stabilized image, but as the sensory system
becomes fatigued, she begins to see fragments fading and reappearing.

22. Sensation
Vision
 The Stimulus Input: Light Energy
 The Eye
 Visual Information Processing
 Color Vision

23. Vision

24. Transduction
The transformation of stimulus energy (sights,
sounds, smells) into neural impulses.

25. The Stimulus Input: Light Energy
Both Photos: Thomas Eisner
Visible
Spectrum
Light is composed of electromagnetic waves with Wavelengths (distance from
one peak to another peak on a wave) and Amplitudes (height of the wave).

26. Physical Characteristics of Light
1. Wavelength (hue/color)
3. Intensity (brightness)

27. Wavelength (Hue)
Hue (color) is the
dimension of
color determined
by the
wavelength of the
light.
Wavelength is the
distance from the
peak of one wave
to the peak of the
next.

28. Wavelength (Hue)
Violet Indigo Blue Green Yellow Orange Red
400 nm 700 nm
Short wavelengths Long wavelengths
Different wavelengths of light result
in different colors.

29. Intensity (Brightness)
Intensity:
Amount of
energy in a
wave
determined by
the amplitude.
It is related to
perceived
brightness.

30. Intensity (Brightness)
Blue color with varying levels of intensity.
As intensity increases or decreases, blue color
looks more “washed out” or “darkened.”

31. The Eye

32. Parts of the eye
• Cornea: Transparent tissue where light enters
the eye.
• Iris: Muscle that expands and contracts to
change the size of the opening (pupil) for light.
• Lens: Focuses the light rays on the retina.
• Retina: Contains sensory receptors that process
visual information and sends it to the brain.

33. The Lens
Lens: Transparent
structure behind the
pupil that changes shape
to focus images on the
retina.
Accommodation: The
process by which the
eye’s lens changes shape
to help focus near or far
objects on the retina.

34. Retina
Retina: The light-
sensitive inner
surface of the eye,
containing receptor
rods and cones in
addition to layers of
other neurons
(bipolar, ganglion
cells) that process
visual information.

35. Optic Nerve, Blind Spot & Fovea
Optic nerve: Carries neural impulses from the eye to the
brain. Blind Spot: Point where the optic nerve leaves the
eye because there are no receptor cells located there.
Fovea: Central point in the retina around which the eye’s
cones cluster.
http://www.bergen.org

36. Test your Blind Spot
Try is on your own time with the photo in your
book. Close your left eye, and fixate your right
eye on the black dot. Move the page towards your
eye and away from your eye. At some point the
car on the right will disappear due to a blind spot.

37. Photoreceptors
E.R. Lewis, Y.Y. Zeevi, F.S Werblin, 1969

38. Bipolar & Ganglion Cells
Bipolar cells receive messages from
photoreceptors and transmit them to ganglion
cells, which converge to form the optic nerve.
Cone
-- Rod
Neural impulse

39. Visual Information Processing
Optic nerves connect to the thalamus in the
middle of the brain, and the thalamus connects to
the visual cortex.

40. How is visual information
processed in the brain?
The retina processes information before
routing it via the thalamus to the brain’s
cortex.
After the rods and cones process the visual
input, the information travels to the
ganglion cells (whose axons make up the
optic nerve) and then to the visual cortex
in the brain (located in the occipital lobe).

41. Feature Detection
Feature Detectors: Nerve cells in the visual
cortex that respond to specific features, such as
edges, angles, and movement.
Ross Kinnaird/ Allsport/ Getty Images
Electrodes record how individual cells in this monkey’s visual cortex respond to
different visual stimuli. Hubel and Wiesel won a Nobel prize for their discovery that
most cells in the visual cortex respond only to particular features—for example, to the
edge of a surface or to a bar at a 30-degree angle in the upper right part of the field of
vision. Other cells integrate information from these simpler ones.

42. Shape Detection
Specific combinations of temporal lobe activity
occur as people look at shoes, faces, chairs and
houses.
Ishai, Ungerleider, Martin and Haxby/ NIMH
During an fMRI, different areas of the brain “light up” when looking at certain
objects.

43. Visual Information Processing
Processing of several aspects of the stimulus
simultaneously is called parallel processing. The
brain divides a visual scene into subdivisions such
as color, depth, form, movement, etc.

44. Sometimes parallel processing is hindered in some way.
For example: Mrs. M was a woman who suffered stroke
damage to both sides of her brain. She became unable
to perceive movement. She was unable to see people
move yet they would miraculously appear in different
places although she could not see how they had gotten
there.
There is also a phenomenon called Blindsight. This is when
people who cannot consciously perceive can still
remarkably locate objects but are consciously unaware
of how they knew where the objects were or what their
orientation was.

45. From Sensation to Recognition: A Simplified
Summary of Visual Information Processing
Tim Bieber/ The Image Bank

46. Color Vision
The Young-Helmholtz Trichromatic (three color)
theory and the Opponent-Process theory help
contribute to our understanding of color vision.
We know (or we think we know) that a tomato is
red.
Ask yourself this: If a tree falls in the forest and no
one is around to hear it, does it make a sound?
Now ask yourself this: If no one sees the tomato,
is it red?

47. The answer is NO.
The tomato is everything but red, because it
rejects (or better yet, reflects) the long
wavelengths of red.
Also, the tomato’s color is our mental
construction.
Light rays are not colored. Color does not
reside in an object; it resides within our
brain; hence our ability to dream in color.

48. Color Vision
Trichromatic theory: Young and von Helmholtz
suggested that the eye must contain three receptors
that are sensitive to red, blue and green colors, which
when stimulated can produce the perception of any
color.
Standard stimulus
Comparison stimulus
Max Medium Low
Blue Green Red

49. Color Blindness
Genetic disorder in which people are blind to green or red
colors. This supports the Trichromatic theory. They will
not be able to perceive the number within the design.
Show color blindness video: http://www.youtube.com/watch?v=CWyrp3hu4KE
Ishihara Test

50. Color Vision
Opponent Process Theory:
Hering, a physiologist, found that when you stare
at a green square for a while and then look at a
white sheet of paper, you see red, which is
green’s opponent color.
If you stare at a yellow square and do the same,
you will see blue, which is yellow’s opponent
color.
The Opponent Process theory is the theory that
opposing retinal processes (red/green,
yellow/blue, white/black) enable color vision.

51. Opponent Colors
Gaze at the middle of the flag for about 30
seconds. When it disappears, stare at the dot and report
what you see. You should see the opponent colors.