constancy

Question 1

what is meant by perceptual constancy

Answer 1

the extent to which objects appear unchanging despite changes in the conditions of observing

Question 2

what is size constancy

Answer 2

the tendency for objects to appear constant in size despite changes in the size of the retinal image caused by changes in viewing distance

Question 3

what is shape constancy

Answer 3

the tendency for objects to appear constant in shape despite changes in the shape of the retinal image caused by changes in viewing angle

Question 4

what is emmert’s law

Answer 4

the apparent size of the after image increases in direct proportion to viewing distance

the size of the retinal image of real objects depends on viewing distance

in contrast after images are fixed in size on the retina so apparent size changes with the distance it is projected (viewing distance)

Question 5

how can we measure perceptual constancy

Answer 5

two modes: questions about sensations, or judgements of the real world

will produce different results

whichever method is used, less constancy will be obtained as contextual cues are reduced

participants make systematic errors in judgements about an image that matched their retinal image
tend towards the properties of the object rather than the properties of the retinal image

Question 6

what did Thouless (1931) mean by phenomenal regression to the real world

Answer 6

when making judgements about an image people make systematic errors

Question 7

what is colour constancy

Answer 7

the tendency for objects to appear unchanging in colour despite changes in the spectral composition of the illuminant

Question 8

how is light reflected and absorbed from the surface of an object

Answer 8

as spectral energy as a function of wavelength

thus reflected light = e(𝞴).

Question 9

what is reflected light

Answer 9

illumination l(𝞴) x surface reflectance R(𝞴)..

Question 10

how can we mathematically represent cone signals

Answer 10

SML cone spectral sensitivity x surface reflectance x illumination (integrated over all wavelengths)

S = ∫ s(λ)R(λ)I(λ)dλ

M = ∫ m(λ)R(λ)I(λ)dλ

L = ∫ l(λ)R(λ)I(λ)dλ

note we perceive object properties rather than the properties of the retinal image

Question 11

what is the effect of a change in illumination

Answer 11

changes in illumination change the reflected light and change the cone signals

despite this we tend to perceive a colour that is associated with surface reflectance thus maintaining its constant appearance

Question 12

what are the three components of colour constancy

Answer 12

the calculations required to undo the effect of an illuminant change

the parameters of the calculations, determined by the image

the perceptual apparatus where these transformations are implemented

Question 13

what happens to cone signals when the light changes

Answer 13

scaling factors are required to compensate for illuminant changes

different illuminant changes produce different scaling factors

the perceptual system can only draw on prior knowledge of scenes it has already viewed

Question 14

what is von kries’ coefficient rule (1878-1905)

Answer 14

light adaption influences colour appearance by

turning the signal from each cone class up or down by a multiplicative coefficient

these changes within each cone class are independent at each retinal location such that signals from M S cones do not influence L cones

for each cone class the gains are set in inverse proportion to the spatial average of the photon caught by cones of the same class

the size and shape of a photon varies with the photon energy or wavelength

perceived colour is determined by the ratios of the scaled cone signals (rather than raw cone signals)

Question 15

can the effect of spectral changes in the illuminant be summarised by a multiplicative scaling of the cone signals

Answer 15

This is not mathematically true for all lights, surfaces and photoreceptor sensitivities.
This is true for the sets of real lights and surfaces likely to be encountered by a visual system with particular photoreceptors.
There is regularity in the physical world that the perceptual system can exploit.

Question 16

what is the ives transform (1912)

Answer 16

First mechanism for constancy under an illuminant change

Demonstrated that the multiplicative factors which transform the coordinates of incandescent carbon illumination to those of a reference illuminant, also transform the coordinates of surfaces to approximately their coordinates under the reference illuminant.

BUT in practice the visual system rarely has direct access to the cone-coordinates of the illuminant and instead the illuminant has to be estimated from cues in a complex scene.

This could operate by assuming that the brightest illuminant is white.

Question 17

what is the argument that we assume ‘the brightest is white’

Answer 17

the brightest surface in a scene is likely to reflect all wavelengths in the illuminant spectrum without alteration, appearing white

we are good at identifying white surfaces

this only uses one data - much more robust to have many

Question 18

what it the global mean argument

Answer 18

global mean = mean of all data points
if constant, can be used to estimate the gradient of the line
however, normalising the global mean will only provide constancy if the surfaces in a scene remain constant in their average properties

Question 19

what is the grey world hypothesis?

Answer 19

assumes that the average reflectance from a group of surfaces is neutral and consistently reflects the illuminant

normalising the mean for a green biassed collection of surfaces under sunlight overcorrects the L cone signal

normalising the mean of a red biassed collection of surfaces under skylight under corrects the L cone signal

we wrongly attribute the bias of these surfaces to the illuminant resulting in under or over corrections

GWH says that the set of surfaces stay constant and on average are grey.
BUT statistically this does not work as we do not average things out to be grey.
Better to argue for a constant world hypothesis or a set of biases that over a particular amount of time might be constant.

Question 20

what is the local mean

Answer 20

Kries suggested that the signal in each cone class was normalised to the local mean for that cone class

this achieves constancy but eliminates differences between surfaces

normalising to the local mean allows corrected sunlight images to be the same as corrected skylight images, but you lose the structure and variation across the original scene

normalising to the global mean is a more useful operation which may provide an estimate of the illuminant

Question 21

what is Smithson & Zaidi’s 2004 temporal mean

Answer 21

in an experiment of perceived colour constancy

participants were presented with a series of frames with a central square every 1.5s, and were asked to judge the colour of the square

illumination was manipulated such that the entire scene was illuminated by skylight or sunlight, or the frame and square were illuminated differently

they found that when the entire scene was illuminated uniformly, constancy was high

but when the test patch was illuminated locally by a different cast light to the background, its colour was perceived correctly to the local illumination (rather than the background illumination)

We normally make judgements about colour constancy relative to the surroundings and from a single view of a test patch our visual system cannot deduce that it is rendered in a different light from the surround

this suggests that the local illuminant was estimated over successive trials

eye movements operate in natural scenes to help average out different biasses in different parts of a scene and extract a reliable temporal estimate of the average properties of the illuminant

thus the global mean does not have a big effect on the colour appearance of the test patch

rather than a spatial average we consider the recent history of the surfaces we’ve seen

Question 22

what is Kraft & Brainard’s 1999 multiple cue proposal

Answer 22

in an experiment of perceived colour constancy one perceptual cue was systematically silenced to infer the relative influence each one has on colour constancy

tested the contribution of three classic hypotheses:
local adaption
adaption to the spatial mean
adaption to the most intense image region

actual illuminated surfaces were used and independently manipulated

observers used computer controls to make an achromatic scene which informs the experimenters of what the observer perceives the illuminant light to be (white surfaces perfectly reflect the illuminant light)

local surrounds were equated to the test patch so that they gave no information about the different illuminants (grey cardboard for neutral illumination, blue cardboard for orange-red light)

equated maximum flux so that the brightest area is the same colour in both the neutral illuminant and yellow light to silence the brightest is white cue (frame of yellow cardboard, neutral illumination; frame of magenta cardboard, yellow light)

found that constancy gets worse as the number of available cues decreases (i.e. judgements are closer to the retinal stimulus)

there is individual variation in the way that multiple cues are used suggesting neither of the classic theories are sufficient

multiple cues may be used in combination and other uncontrolled cues may be playing a role as performance never falls to failure of constancy

Question 23

how can we explain ‘the dress’

Answer 23

individual differences in illuminant estimation

the photo does not have many cues so constancy breaks down somewhat as it is difficult to correct for the illuminant

we focus on irrelevant cues giving false information

we can manipulate perception by manipulating the context its seen in

need to deduce the relative contributions of the illuminant colour spectrum and the surface reflection to the overall perception

Either the dress is in bluish light/ shadow.
In this scenario the only reflectance that can be multiplied is flare and therefore is a white stripe.
Or the dress is in yellow light
In this scenario the light must be hitting a blue surface that reflects a lot of short wavelength but not long wavelength light and is therefore a blue stripe.
Depending on which judgements are made the perception of the dress will change.

Question 24

how can we measure colour constancy

Answer 24

matching a standard display
matching to an internal standard
colour naming

matching to a standard display: simultaneous asymmetric matching (Adjust the colour of the central square in the righthand display to match the colour of the central
square in the left-hand display.), successive asymmetric matching (Adjust the colour of the central square in the second
display to match the colour of the central square in
the first display), haploscopic matching (Adjust the colour of the central square in the righthand display to match the colour of the central
square in the left-hand display)

matching to an internal standard: achromatic setting (Adjust the colour of the central square so that it looks
grey or colourless.), colour boundaries (Does the central square appear reddish or greenish?
Does the central square appear yellowish or bluish?)

colour naming (What colour is the central square?)

Question 25

why does it matter what question you ask

Answer 25

Arend & Reeves (1986) presented a clear demonstration of the influence of instructions in a colourmatching task.
When observers were asked to make a match to ‘look as if it were cut from the same piece of
paper’, they showed relatively good constancy compared with conditions where they were asked
to match ‘hue and saturation’.
The appearance-based constancy obtained in the second case is a demonstration of
“phenomenal regression to the real object” (Thouless 1931).

Question 26

what is the neural implementation of colour constancy

Answer 26

scaling of cone signals supports a high level of constancy and these transformations should happen from the level of the photoreceptor to later signals

Question 27

summarise colour constancy

Answer 27

Perceptual constancy is all about recovering the properties of objects in the world, and not the properties of the retinal image.
In measuring perceptual constancy, the result can be influenced by the way the question is asked. However, it is clear that with fewer cues to the viewing circumstances, constancy is reduced.
In understanding constancy, we must consider three components of the problem:
What calculations are required to “undo” the effect of the change in viewing circumstances?
How are the parameters of the calculations determined by the image?
Where in our perceptual apparatus are these transformations implemented?
For colour constancy, a multiplicative change in cone signals provides a good approximate solution to the problem. The multiplicative factors may be set by the mean (over an appropriate set of elements) or by the brightest elements.
It is likely that several image features are used in combination to set the correction, and implemented at multiple stages in the visual system.