The document discusses how vision works, beginning with light entering the eye and stimulating the retina, which sends electrical signals to the brain. It notes that contrary to common assumptions, there is no "image" in the eye - vision is an active process constructed by the brain from retinal signals. These signals are processed in the visual cortex into perceptions of shape, color, movement, and spatial relationships. The processes of lateral inhibition and comparing light intensities allow inferences of depth, size and distance. While the eye functions like a camera in some ways, the brain's role in constructing vision based on these signals is what allows for complex tasks like object recognition. Artists exploit these visual illusions through manipulating light and shadow to depict three-dimensional forms in

1. ART 100
UVC
Class 2.1 VISUAL PERCEPTION
& THE ACTIVE BRAIN

2. agenda
function of the eye
how eye and brain work together to create vision
the active, constructed nature of vision
comparing fine art and folk art

3. Briefly, we will
study the eye and the dynamic process
of visual perception, to understand how
our brains create the images we see.

4. But in general in this course, we are interested not
in the “nature” of vision, but in its culture; in
other words, how humans have developed
languages of visual communication given our
status as sighted creatures.

5. schematic diagram of how vision works
(please note: this diagram is WRONG)
Most people assume that vision works as pictured in the diagram below.
Put in words: our vision is just what our eye sees and reports to the brain.

6. why is the eye/camera
idea wrong?
There is no “image,” no picture in the eye
at all.
In the eye, light admitted through the pupil and focused
through the lens differentially stimulates the neuron-rich
tissue at the back of the eye (the retina), sending patterns of
electrical impulses to the brain (specifically to the visual
cortex), where the signals must be processed and
interpreted to create what we see.

7. “the eye is like a camera”
This analogy holds up to a point.
The point at which it no longer holds
is the retina.
Please note: this diagram is TRUE up to a
point and then becomes FALSE.

9. what happens in the retina?
light is converted to an electrical signal in retinal photoreceptors
via a light-sensitive protein called rhodopsin
transduction
electrical impulses are converted to chemical form
active/inactive phases
transduction occurs differently in rods and cones
signalling mechanism is sophisticated and optimizes the
information on site using “inhibition” to reduce the signal in
certain areas, thus boosting the rest

10. cones: large range of intensities, color vision, work quickly,
very sensitive to small changes, concentrated in center
rods: evolutionarily more recent, but outnumber cones 20
to only work in very low light, evenly distributed across the
retina

11. Please note: this diagram is still a bit misleading, but it’s a whole lot better than the
previous one.

12. then
the signal is transmitted to the primary visual cortex

13. animal and human evidence for
3 discrete processing systems
“Although the visual processing mechanisms are not yet
completely understood, recent findings from anatomical and
physiological studies in monkeys suggest that visual signals are
fed into at least three separate processing systems. One system
appears to process information mainly about shape; a second,
mainly about color; and a third, movement, location, and spatial
organization.”
Human psychological studies support the findings obtained
through animal research. These studies show that the
perception of movement, depth, perspective, the relative size of
objects, the relative movement of objects, shading, and
gradations in texture all depend primarily on contrasts in light
intensity rather than on color.”
SOURCE: http://www.brainfacts.org/sensing-thinking-behaving/senses-and-
perception/articles/2012/vision-processing-information/

14. in the third system
(depth/location/movement)
“About 60 years ago, scientists discovered that each
vision cell’s receptive field is activated when light hits
a tiny region in the center of the field and inhibited
when light hits the area surrounding the center. If light
covers the entire receptive field, the cell responds
weakly.”
Another way to put this is: “the visual process begins by
comparing the amount of light striking any small region of
the retina with the amount of surrounding light.”
This process is enhanced by “lateral inhibition,” in which
all but the strongest signals are filtered out by the retina
before even reaching the brain. (Preference for edges.)
SOURCE: http://www.brainfacts.org/sensing-thinking-behaving/senses-and-
perception/articles/2012/vision-processing-information/

15. how is color/brightness processed? (this appears to be an
independent pathway)
how are form/shape processed to produce object
recognition?
how are motion, depth, and spatial relations processed?
when and how are all of these coordinated?

16. object recognition
Humans are capable of instantly recognizing people and
objects in visually cluttered scenes.
Machines cannot do this, yet.
What can machines do? Current research on software
teaching scene recognition.

21. http://vision.stanford.edu/documents/LiFei-
Fei_ICCV07.pdf

25. let’s summarize
The brain constructs your field of vision from electrico-
chemical impulses sent by your eyes.
The eye collects data on:
shape
color
position/location/movement
These elements seem to be processed via discrete
mechanisms.

26. The raw data entering the third system has to do with
differences in light intensity. These signals are enhanced by the
retina through the process of lateral inhibition and are
subsequently interpreted by the visual cortex to produce our
field of vision, which we experience as continuous and
compelling rather than as a series of approximations of
distance, size and depth via contrasts between light and
shadow.

27. Our ability to, judge distance, move through space, avoid
obstacles,—these are all INFERENCES drawn from
information about contrasts between light intensity rather
than actual visual data—even though we perceive them
as properties of our vision.
This is ANOTHER REASON why the eye/camera idea is
completely misleading.

28. John Singleton COPLEY
Mrs. Ezekiel Goldthwaite
1771
oil on canvas
50 1/8 x 40 1/8 inches
http://www.mfa.org/collections/
object/mrs-ezekiel-goldthwait-
elizabeth-lewis-32756
This human perceptual
preference for “edges”—
areas of high contrast between
light and shadow—is also
exploited by artists wanting to
create convincing three-
dimensional illusions in their two-
dimensional art.

29. In the beginning of art school, students learn how to create
illusions of depth and mass through manipulating areas of light and shadow.

30. You start with simple geometrical forms like spheres,
cones and cubes.

31. Then move on to organic objects, which grow progressively trickier.

33. When color is added, the illusion is enhanced, but as you would predict from
your knowledge of visual neuroscience, the illusion is quite strong even without
the addition of color.

34. As students master tonal manipulation, they can create increasingly convincing
illusions of more and more complex subjects.

35. Raphael
(Italian, 1483-1520)
Upraised Right Hand, with Palm
Facing Outward: Study for Saint
Peter
1518-20
in the collection of the Art
Institute of Chicago
The best artists can create these
illusions with an extremely light
touch.

36. Ammi PHILLIPS
Lady in a gold-colored dress
Probably New York,
Connecticut, or
Massachusetts, 1835–1840
Oil on canvas
33 1/2 x 28 1/4 in.
How does this artist
handle light and shadow?
Do you think he studied
art formally?

38. try these at home!
http://serendip.brynmawr.edu/bb/blindspot1.html
http://serendip.brynmawr.edu/bb/blindspot/
http://serendip.brynmawr.edu/bb/latinhib.html
http://dragon.uml.edu/psych/illusion.html

39. In this class we consider…
not the spectrum of light visible
to the human eye,
but our culturally bound sense
of what colors mean.

40. But in what human
beings have made
of vision.
In other words, not what
vision is,

41. M.C. Escher (Dutch, 1898 – 1972), Drawing Hands, 1948, ithograph, 11 1/8 x 13 1/8 in