4 - ILLUSIONS Flashcards
illusionary perceptions - slightly incorrect definition
= ‘consistent and persistent discrepancy between the perceived and the real physical properties of a stimulus’
- consistent = ‘invariably occur when a particular stimulus is presented’
- persistent = don’t change when you know the truth and ‘strongly resistant to efforts to suppress them’
- definition doesn’t work for colour illusions (or for taste and smell)
two shortcomings from the previous definition
1 - a discrepancy between perceived properties and physical properties could be because our perceptual systems did not detect/respond to certain things and therefore were not perceived
- these aspects would’ve been absent from conscious experience and this would not count as an illusion
2 - could be due to optical effects in stimulation and would not be to do with how perception works
- this could count as an illusion but not a perceptual one
- eg when a stick in placed in a glass of water, it appear to break
- this is an optical illusion, not perceptual
- eg oxide daisies look fully yellow under daylight but under UV they’re darker on the inside
- this is a discrepancy in the appearance that our eyes fail to detect, so not an illusion
perceptual illusion (correct definition from james)
‘a consistent and persistent discrepancy between a sensory perception and the distal stimulus that evoked the percept (and that percept represents) such that the observer is deceived as to the nature of the distal stimulus. the discrepancy relates only to those (distal) stimulus properties which can be detected by the sensory systems and it occurs as a result of the operation of the processes involved in detecting and processing sensory stimulation’
colour illusion
‘the perceived colour of a surface doesn’t correctly represent the kind of spectral reflectance function that the surface possesses’
the coloured cubes with the yellow and blue washes
- the blue squares with yellow wash and yellow squares with blue wash have exactly the same wavelengths of light
- remove everything else and both would appear grey
- this is an perception illusion
- the colours are the illusion, not the grey
- the the images are printed, the squares have a mixture of pigments so therefore they have a spectral reflectance
- this is therefore a colour illusion as the spectral reflectance does not match the perceived colour of the square ‘correctly’
- the spectral reflectances of the yellow and blue squares on the cube should be flat
- taking the rest of the cube away causes the illusion to also go away
why do colour illusions occur?
- large areas of the retinal image are used to determine the colour of each aspect of that image, therefore the background can influence the perceived colour of one object and how the illusion can go away when the rest of the image is removed
how the visual system processes the cube illusion
- it looks as if a yellow wash has been put over the image, almost as if the cube is being illuminated by a yellowish light
- therefore the visual system will try to discount what this illumination spectrum contributes to the retinal image of the cube in order to perceive the colour of the cube
GREY WORLD THEORY:
- averaging the light from the whole picture and using this to estimate the illuminant
- this estimate is then divided from the light reaching the eye from every region of the picture
- so for the sake of an example, if we assume that this does result in the average light of the picture providing an accurate estimate of the illuminant, then when this is divided out from the square’s (achromatic) reflected light, then the spectral reflectance of the square resembles that which is usually perceived as blue
- this is because the reflected light is equal to the spectral reflectance multiplied by the illuminant spectrum so the resulting calculation provides a spectral reflectance characteristic of surfaces typically perceived as blue
- in order to do this calculation however, one must take the values from across the reflected light spectrum curve on a graph and divide by the corresponding point on the illuminant curve, thus then provides the spectral reflectance for the object in accordance to the whole image
HOW ITS AN ILLUSION:
- if the image is printed, then the spectral reflectance are actually flat for the blue squares, and would therefore look grey
- this is an illusion as the visual system interprets the yellow wash as an illumination, however it’s just part of the picture, not an illuminant
- therefore the visual system accounts for this illumination which isn’t actually correct, discounts it and sums an incorrect reflectance for other parts of the image
‘the dress’
- majority of people see it as blue and dark grey (57%)
- some see white and gold
- some see blue and brown
- it’s actually blue and black
what doesn’t cause the perception differences in ‘the dress’
1 - COLOUR DEFICIENCIES
- only affects 10% of population, majority male
- so cannot be explained by this
2 - CHROMATIC ADAPTATION
- ‘can differ substantially between individuals’
- this occurs when one is exposed to a specific waveband of light for longer than 30 seconds, resulting in the visual system (cones in particular) becoming desensitised to these wavelengths
- this results in the colours similar to that perceived from those wavelengths appear less vivid and paler than normal
- this can also result in COMPLEMENTARY COLOUR AFTER-IMAGES (opponent colours) but they dissipate quickly
- but these aren’t illusions, they’re changes in perceived colour
- only an illusion to those who see the image as white
- people have been exposed to different things in different places which would result in individuals being more or less chromatically adapted
- however studies have concluded that this does not explain perception differences for the dress
3 - UNDEREXPOSURE and NON-SIG BACKGROUND
- makes the dress look bleached out
- makes the image ambiguous
- means that the colour could be due to the illumination or the reflectance
- reflectance = perceive as blue
- illuminant = if you average the light to create an estimate of the illuminant spectrum, then most of it is coming from the dress as it takes up the majority of the photo, therefore the estimated illuminant spectrum will be similar to the spectrum of reflected light from the blue parts
- so the reflectance function would be achromatic and see the blue parts as white
- same for seeing the grey as yellowish because of the yellowish illumination from the background
how to make a grey object seem a different colour
- would need to discount an illuminant with a reverse spectrum to the desired colour in order to create a spectral reflectance estimate
- when dividing out the reverse illuminant spectrum, you would create a false sense of the opposite illumination (eg like a wash on the image)
- so if you wanted the object to look red, you would create a false sense of a bluish green illumination
poggendorf illusion
putting something in front of a straight line makes it look broken/not straight
visual illusions
‘perceptual illusion in the visual morality’
optical v visual illusions
- different
optical = NOT due to processes in the eye or nervous system so therefore are NOT visual illusions
pencil in water illusion
optical illusion due to the water and air refracting light differently
how to make a grey object seem a different colour
- would need to discount an illuminant with a reverse spectrum to the desired colour in order to create a spectral reflectance estimate
- when dividing out the reverse illuminant spectrum, you would create a false sense of the opposite illumination (eg like a wash on the image)
- so if you wanted the object to look red, you would create a false sense of a bluish green illumination
