2. agenda 6.23.15
• function of the eye
• how eye and brain work together to create vision
• the active, constructed nature of vision
3. 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.
4. So this session is a bit
of a departure.
Today we study the eye, the brain
and the dynamic process of visual
perception, to understand how our
perception works.
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.
8.
9. what happens in the retina?
Light is converted to an electrical signal in retinal
photoreceptors via a light-sensitive protein called rhodopsin
Transduction is the process by which electrical impulses are
converted to chemical form. This occurs differently in rods
and cones.
The signalling mechanism is quite sophisticated. It optimizes
the information transmitted from the retina by 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. these signals are transmitted via the optic nerve to
the primary visual cortex
12. what happens in the
visual cortex?
This is where matters get really complicated!
There is evidence that there are THREE separate systems
that process these signals.
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. Please note: this diagram is still a bit misleading, but it’s a whole lot better than
the previous one.
15. This provides an explanation for why black and white
drawings appear every bit as convincing in their
illusion of space as color drawings.
16.
17.
18.
19.
20.
21.
22.
23. 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.)
http://www.brainfacts.org/sensing-thinking-behaving/senses-and-
perception/articles/2012/vision-processing-information/
24. John Singleton COPLEY
Mrs. Ezekiel Goldthwaite
1771
oil on canvas
50 1/8 x 40 1/8 inches
http://www.mfa.org/collecti
ons/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.
25.
26. 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?
27. 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.
36. summary
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 in the visual cortex and coordinated into a
coherent visual field.
37. 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.
38. 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.
39. The eye adjusts for relative brightness/darkness and
motion stabilization.
40. 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
