Greg Prof. Francis
1/5/11
Visual perception
Visual perception
Signals sent to area V1 in cortex
Greg Francis
Light enters eye
PSY 200
Neural networks tuned to
brightness
Lecture 09
color
form
motion
texture
Why you see color
afterimages.
depth...
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Neurophysiology
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On-center, off-surround
How we see things is largely
determined by the properties of
receptive fields
center-surround cells tend to not
respond well to homogeneous
light that covers both excitatory
and inhibitory parts
on-center, off-surround
simple cells
complex cells
Characteristics of cell receptive
fields force additional properties
of the visual system
=> percepts of the middle of an
object is derived from the edges
And by network interactions among
cells
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On-center, off-surround
Characteristics of cell receptive
Brightness contrast
fields force additional properties
of the visual system
Edge responses are influenced by the surrounding
light
both center squares have the same light intensity
center-surround cells tend to not
respond well to homogeneous
light that covers both excitatory
and inhibitory parts
=> percepts of the middle of an
object is derived from the edges
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PSY 200: Intro. to Cognitive Psychology
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1
Prof. Greg Francis
1/5/11
Brightness contrast
Two receptive fields inside the middle square
receive the same excitatory and inhibitory signals
Little response
Brightness contrast
Receptive fields on the corner
Receive the same excitation at the center
differ in the amount of inhibition in the surround
less
inhibition
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Brightness contrast
more
inhibition
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Brightness contrast
Thus, the visual system computes brightness as
something like local contrast
Its a property of the center-surround cells
Our percept of brightness is determined by the responses
of cells at contrast edges
As a result, things that have equal physical intensities can
look dramatically different (next slide)
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Brightness contrast
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PSY 200: Intro. to Cognitive Psychology
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Brightness contrast
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2
Prof. Greg Francis
1/5/11
Brightness contrast
Hermann grid
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Filling-in
Hermann grid
We see color and brightness inside objects
It sometimes gets things messed up
Water color effect
so edge information must fill-in to the interior
Seems related to on-center,
off-surround cells
Cells at intersections receive
more inhibition than cells at
single roads
How do we explain the other
version?
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Filling-in
Brightness
information
spreads across
surfaces
Filling-in
Carefully fixate
the pink center
Craik-OBrienCornsweet
effect
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PSY Intro. 200: to Cognitive Psychology
If you keep your
eyes very still, it
will disappear
The yellow fills-in!
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3
Prof. Greg Francis
1/5/11
Color system
competition between
opposite colors
Color system
red-green
red-green
blue-yellow
blue-yellow
black-white
competition between
opposite colors
black-white
habituating gate
offset of one color leads to
rebound in other
Initial balance
Neither color wins
competition
Gated dipole circuit
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Color system
competition between
opposite colors
Color system
competition between
opposite colors
red-green
red-green
blue-yellow
blue-yellow
black-white
black-white
Extra input to green
Green wins competition
Extra input to green
Fading of green signal
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Color system
competition between
opposite colors
Color system
competition between
opposite colors
red-green
blue-yellow
black-white
red-green
blue-yellow
black-white
Offset of green
Rebound of red signal
Recovery of green pathway
Disappearance of rebound
Return to initial state
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PSY 200: Intro. to Cognitive Psychology
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4
Prof. Greg Francis
1/5/11
Orientation competition
Color afterimage
competition between
orthogonally tuned cells
habituating gate
offset of horizontal leads to
rebound in vertical
Same kind of gated dipole
circuit
Principles of neural
computation!
Baseline response
Due to tonic input
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Orientation competition
With additional input to
Orientation competition
horizontal pathway,
horizontal channel wins
With additional input to
horizontal pathway,
horizontal channel wins
competition
competition
But as horizontal gate
habituates, horizontal
signal weakens
It still wins the
competition, though
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Orientation competition
At offset of horizontal
Orientation competition
As the horizontal gate
input, the gated horizontal
signal is weaker than the
vertical signal
recovers, the system
returns to baseline and
the vertical after response
A vertical rebound
disappears
appears
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PSY 200: Intro. to Cognitive Psychology
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5
Prof. Greg Francis
1/5/11
Oriented afterimages
Oriented
reset signals
Conclusions
Visual perception
brightness
are also
implicated in
an unusual
color
form
type of
afterimage
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Largely determined by the receptive fields
and network structure of visual circuits
Neurophysiology strongly determines
what we see!
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Next time
Review for Exam 1, then
Visual dynamics
Flicker
Persistence
Motion perception
CogLab on Apparent motion due!
Why computer monitors work.
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PSY 200: Intro. to Cognitive Psychology
6