PSYC 212 Lecture Notes - Lecture 8: Principles Of Grouping, Photopic Vision, Monochromacy
6 Nov 2018
School
Department
Course
Professor
Perceptual committees
Committee rules:
Put together what goes together (ex.: Gestalt grouping
rules)
§
Avoid accidents (ex.: accidental viewpoint)
Accidental viewpoint: a viewing position that
produces some regularity in the visual image that is
not present in the world
□
§
Honour physics (exclusion)
§
Reach consensus (ex.: ambiguous figures)
§
Separate what should be separated (ex.: figure vs ground)
Figure vs ground assignment: the process of
determining that some regions of an image belongs
to a foreground object (figure) and other regions are
part of the background (ground)
□
Gestalt figure-ground assignment principles
Surroundedness: the surrounding region is
likely to be ground
®
Size: the smaller region is likely to be figure
®
Symmetry: a symmetrical region tends to be
seen as figure
®
Extremal edges: if edges of an object are
shaded such that they seem to recede in the
distance, they tend to be seen as figure
®
□
§
○
Not that simple
Structuralism + committee rules = object perceived
§
Face inversion effect: we are better at recognizing faces
that are upright
§
Logically, it seems that object recognition should follow
figure-ground assignment, but then it shouldn't matter if
the face is up or down…
§
Object recognition also helps with figure-ground
assignment
§
It cannot all be a feed-forward process, there has to be
reciprocal communication between object recognition and
figure-ground assignment
§
○
•
How do we recognize objects?
Naïve template theory: the proposal that the visual system
recognizes objects by matching the neural representation of the
image with a stored representation of he same "shape" in the
brain\
Don't work: there are too many 'A's to recognize that
don't fit a particular template
§
Structural description: maybe what we recognize is a
particular organization of simpler features
Biederman (1987) proposed that any object could
be made of a finite set of simpler geons ("geometric
irons")
□
Geons and relationships between geons (ex. "geon
A is on top of geon B") are viewpoint invariant
□
Minor changes in shape won't alter the structural
description
□
The fact that viewpoint invariance isn't perfect is a
problem for structural-description theories
□
§
○
•
Colour
Three steps to colour perception
Detection: wavelengths of light must be detected in the first
place
1.
Discrimination: we must be able to tell the difference between
one wavelength (or mixture of wavelengths) and another
2.
Appearance: we want to assign perceived colours to lights and
surfaces in the world and have those perceived colours be stable
over time, regardless of different lighting conditions
3.
•
Colour detection
Three types of photoreceptors
S-cones detect short wavelengths
§
M-cones detect medium wavelengths
§
L-cones detect long wavelengths
§
○
More accurate to refer to them as short, medium, and long
rather than blue, green, and red, since they each respond to a
variety of wavelengths
The L-cone's peak sensitivity is 565 nm, which corresponds
to yellow, not red
§
○
Photopic: light intensities that are bright enough to stimulate
the cone receptors and bright enough to "saturate" the rod
receptors to their maximum responses
Sunlight and bright indoor lighting are both photopic
lighting conditions
§
○
Scotopic: light intensities that are bright enough to stimulate the
rod receptors but too dim to stimulate the cone receptors
Moonlight and extremely dim indoor lighting are both
scotopic lighting conditions
§
○
•
Discrimination
A single photoreceptor show different responses to lights of
different wavelengths but the same intensity
○
Lights of 450 nm and 625 nm elicit the same responses from this
particular photoreceptor
○
If you decrease the intensity of the lights of 490 and 600 nm will
also elicit the same response
○
The principle of univariance: an infinite set of different
wavelength-intensity combinations can elicit exactly the same
response from a single type of photoreceptor
Therefore, only one type of photoreceptor cannot make
colour discriminations based on wavelength
§
○
With three cone types, we can tell the difference between lights
of different wavelengths
For every wavelength, there is a particular combination of
activity across the three type of cones that remains
constant across different intensities
§
Now imagine that you don't have one type of cone (say M-
cones), every frequency above S-cone limit is only encode
by L-cones and can therefore produce the same response
for different wavelength/intensity combination
§
However, colours to the left will be preserved
§
○
About 8% of male population and 0.5% of female population has
some form of colour vision deficiency: "colour blindness"
○
Colour-anomalous: a term for what is usually called "colour
blindness." most "colour-blind" individuals can still make
discriminations based on wavelength. Those discriminations are
just different from the norm
Deuteranope: due to absence of M-cones
§
Protanope: due to absence of L-cones
§
Tritanope: due to absence of S-cones
§
Cone monochromat: has only one cone type; truly colour
blind
§
○
Rods are sensitive to scotopic light levels
All rods contain the same photopigment molecule:
rhodopsin
§
All rods have the same sensitivity to various wavelengths
of light
§
Therefore, rods obey the principle of univariance and
cannot sense differences in colour
§
○
Trichromacy: the theory that the colour of any light is
determined in our visual system by the relationships of three
number, the outputs of three receptor types now known to be
the three cones
○
Thomas Young (1773-1829) and Hermann von Helmholtz
(1821-1894) independently discovered the trichromatic nature
of colour perception using a colour-matching technique
developed by James Maxwell (1831-1879)
○
Three colour were required to match any other colour. Two
were sometimes insufficient
○
Generally, we don’t see 'pure' wavelengths, but rather mixtures
of wavelengths
○
When mixing two 'pure' wavelengths (say green and red), the
combination of M- and L-cone activity is identical to the
combination resulting from a 'pure' wavelength in the yellow
frequency. We can't distinguish the two - they will all look
yellow
○
Metamers: different mixtures of wavelengths that look identical;
more generally, any pair of stimuli that are perceived as
identical In spite of physical differences
○
Additive colour mixing: a mixture of lights
If light A and light B both reflected from a surface to the
eye, in the perception of colour, the effects of those two
lights add together
Blue and yellow make white□
§
○
Subtractive colour mixing: a mixture of pigments
If pigment A and pigment B mix, some of the light shining
on the surface will be subtracted by A and some by B. only
the reminded contributes to the perception of colour
Blue and yellow make green□
§
Take 'white' light that contains 2 broad mixtures of
wavelengths
1.
Pass it through a filter that absorbs shorter wavelengths.
The result will look yellowish
2.
Pass that through a bluish filter that absorbs all but a
middle range of wavelengths
3.
The wavelengths that make it through both filter will be a
mix that looks greenish
4.
○
•
Lecture 8
Thursday, February 1, 2018
1:07 PM
Perceptual committees
Committee rules:
Put together what goes together (ex.: Gestalt grouping
rules)
§
Avoid accidents (ex.: accidental viewpoint)
Accidental viewpoint: a viewing position that
produces some regularity in the visual image that is
not present in the world
□
§
Honour physics (exclusion)
§
Reach consensus (ex.: ambiguous figures)
§
Separate what should be separated (ex.: figure vs ground)
Figure vs ground assignment: the process of
determining that some regions of an image belongs
to a foreground object (figure) and other regions are
part of the background (ground)
□
Gestalt figure-ground assignment principles
Surroundedness: the surrounding region is
likely to be ground
®
Size: the smaller region is likely to be figure
®
Symmetry: a symmetrical region tends to be
seen as figure
®
Extremal edges: if edges of an object are
shaded such that they seem to recede in the
distance, they tend to be seen as figure
®
□
§
○
Not that simple
Structuralism + committee rules = object perceived
§
Face inversion effect: we are better at recognizing faces
that are upright
§
Logically, it seems that object recognition should follow
figure-ground assignment, but then it shouldn't matter if
the face is up or down…
§
Object recognition also helps with figure-ground
assignment
§
It cannot all be a feed-forward process, there has to be
reciprocal communication between object recognition and
figure-ground assignment
§
○
•
How do we recognize objects?
Naïve template theory: the proposal that the visual system
recognizes objects by matching the neural representation of the
image with a stored representation of he same "shape" in the
brain\
Don't work: there are too many 'A's to recognize that
don't fit a particular template
§
Structural description: maybe what we recognize is a
particular organization of simpler features
Biederman (1987) proposed that any object could
be made of a finite set of simpler geons ("geometric
irons")
□
Geons and relationships between geons (ex. "geon
A is on top of geon B") are viewpoint invariant
□
Minor changes in shape won't alter the structural
description
□
The fact that viewpoint invariance isn't perfect is a
problem for structural-description theories
□
§
○
•
Colour
Three steps to colour perception
Detection: wavelengths of light must be detected in the first
place
1.
Discrimination: we must be able to tell the difference between
one wavelength (or mixture of wavelengths) and another
2.
Appearance: we want to assign perceived colours to lights and
surfaces in the world and have those perceived colours be stable
over time, regardless of different lighting conditions
3.
•
Colour detection
Three types of photoreceptors
S-cones detect short wavelengths
§
M-cones detect medium wavelengths
§
L-cones detect long wavelengths
§
○
More accurate to refer to them as short, medium, and long
rather than blue, green, and red, since they each respond to a
variety of wavelengths
The L-cone's peak sensitivity is 565 nm, which corresponds
to yellow, not red
§
○
Photopic: light intensities that are bright enough to stimulate
the cone receptors and bright enough to "saturate" the rod
receptors to their maximum responses
Sunlight and bright indoor lighting are both photopic
lighting conditions
§
○
Scotopic: light intensities that are bright enough to stimulate the
rod receptors but too dim to stimulate the cone receptors
Moonlight and extremely dim indoor lighting are both
scotopic lighting conditions
§
○
•
Discrimination
A single photoreceptor show different responses to lights of
different wavelengths but the same intensity
○
Lights of 450 nm and 625 nm elicit the same responses from this
particular photoreceptor
○
If you decrease the intensity of the lights of 490 and 600 nm will
also elicit the same response
○
The principle of univariance: an infinite set of different
wavelength-intensity combinations can elicit exactly the same
response from a single type of photoreceptor
Therefore, only one type of photoreceptor cannot make
colour discriminations based on wavelength
§
○
With three cone types, we can tell the difference between lights
of different wavelengths
For every wavelength, there is a particular combination of
activity across the three type of cones that remains
constant across different intensities
§
Now imagine that you don't have one type of cone (say M-
cones), every frequency above S-cone limit is only encode
by L-cones and can therefore produce the same response
for different wavelength/intensity combination
§
However, colours to the left will be preserved
§
○
About 8% of male population and 0.5% of female population has
some form of colour vision deficiency: "colour blindness"
○
Colour-anomalous: a term for what is usually called "colour
blindness." most "colour-blind" individuals can still make
discriminations based on wavelength. Those discriminations are
just different from the norm
Deuteranope: due to absence of M-cones
§
Protanope: due to absence of L-cones
§
Tritanope: due to absence of S-cones
§
Cone monochromat: has only one cone type; truly colour
blind
§
○
Rods are sensitive to scotopic light levels
All rods contain the same photopigment molecule:
rhodopsin
§
All rods have the same sensitivity to various wavelengths
of light
§
Therefore, rods obey the principle of univariance and
cannot sense differences in colour
§
○
Trichromacy: the theory that the colour of any light is
determined in our visual system by the relationships of three
number, the outputs of three receptor types now known to be
the three cones
○
Thomas Young (1773-1829) and Hermann von Helmholtz
(1821-1894) independently discovered the trichromatic nature
of colour perception using a colour-matching technique
developed by James Maxwell (1831-1879)
○
Three colour were required to match any other colour. Two
were sometimes insufficient
○
Generally, we don’t see 'pure' wavelengths, but rather mixtures
of wavelengths
○
When mixing two 'pure' wavelengths (say green and red), the
combination of M- and L-cone activity is identical to the
combination resulting from a 'pure' wavelength in the yellow
frequency. We can't distinguish the two - they will all look
yellow
○
Metamers: different mixtures of wavelengths that look identical;
more generally, any pair of stimuli that are perceived as
identical In spite of physical differences
○
Additive colour mixing: a mixture of lights
If light A and light B both reflected from a surface to the
eye, in the perception of colour, the effects of those two
lights add together
Blue and yellow make white□
§
○
Subtractive colour mixing: a mixture of pigments
If pigment A and pigment B mix, some of the light shining
on the surface will be subtracted by A and some by B. only
the reminded contributes to the perception of colour
Blue and yellow make green□
§
Take 'white' light that contains 2 broad mixtures of
wavelengths
1.
Pass it through a filter that absorbs shorter wavelengths.
The result will look yellowish
2.
Pass that through a bluish filter that absorbs all but a
middle range of wavelengths
3.
The wavelengths that make it through both filter will be a
mix that looks greenish
4.
○
•
Lecture 8
Thursday, February 1, 2018 1:07 PM
Document Summary
Accidental viewpoint: a viewing position that produces some regularity in the visual image that is not present in the world. Figure vs ground assignment: the process of determining that some regions of an image belongs to a foreground object (figure) and other regions are part of the background (ground) Surroundedness: the surrounding region is likely to be ground. Size: the smaller region is likely to be figure. Symmetry: a symmetrical region tends to be seen as figure. Extremal edges: if edges of an object are shaded such that they seem to recede in the distance, they tend to be seen as figure. Face inversion effect: we are better at recognizing faces that are upright. Logically, it seems that object recognition should follow figure-ground assignment, but then it shouldn"t matter if the face is up or down . It cannot all be a feed-forward process, there has to be reciprocal communication between object recognition and figure-ground assignment.
