Lecture 3: Illusions as Key to Reality
Course co-ordinator: Johannes M. Zanker, j.zanker@rhul.ac.uk, (Room W 246)
the guiding question towards the understanding of the visual system is:
what are the fundamental steps of information processing to convert the outside
(physical) world into internal (psychological) events
an early description of the fundamental principles of optical projection, (neural) transmission and (cortical) mapping can be found in Descartes (1664)
questions to ask:
answers to these questions can be given by PSYCHOPHYSICS (defined as discipline by Fechner, 1898)
some hints come from looking at visual illusions: what you see is not what you have
illusions play a major role in the scientific study of sensory systems
illusions are most favourite tools in a painter’s bag of tricks
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identify various depth
cues in Tiepolo’s
(1758) ‘An
allegory with Venus and Time’ (National Gallery London) |
animated arts of the 21st century: cinema creating big, fast, loud illusions (e.g. Harry Potter, Lord of the Rings, The Golden Compass, ...) |
Exclusive behind the
scenes video that features the creatures of Middle-earth and interviews
with Richard Taylor and the cast Click here to View the Quicktime Video |
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Illusions and other
visual puzzles are frequently used in advertising: have a look at the
Droste
effect |
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so what can we learn from illusions about visual information processing ?
there are two fundamental approaches to understand illusions:
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the 3D environment is projected to flat two-dimensional (2D) images in the two eyes >>> from there the information is
in the brain an internal 3D representation of objects & events is formed
From the visual field to the retinotopic map ('cortical images'): the 3D environment is captured as flat two-dimensional (2D) images in two eyes and then the information is processed along the visual stream.
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visual scene > left and right eye > optic nerve > optic chiasm > optic tract > LGN (lateral geniculate nucleus) > optic radiation > striate cortex > association cortex |
the approach here is to look at the visual stream with the eyes of an engineer, in the framework of cognitive psychology, to understand the mechanisms of visual information processing - the key processing concepts are: parallel and serial data analysis (feature extraction) and mapping in a complex network of brain regions
serial processing –
one processing step is taken after the other
information processing hierarchy
parallel processing – the same processing step is taken simultaneously on a complete data set (such as mapping through a broad data bus)
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white to black: how many shades of grey can you discriminate ? (8 bit computer screen: 256 grey levels > 5000 perceptual)
the action of sensory neurons is often described as filter - what does this mean ?
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the filter mechanism is interpreted as a property emerging from the receptive
field structure of sensory neurons
parallel sets of filters for other properties: colour, orientation, velocity,
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what you have  what you see |
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at each level of the luminance steps the grey seems to be brighter at the left
boundary than at the right boundary
- why is this happening ? -
at each luminance step the receptive field (the simplified light counter model)
works like this:
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no response in dark
areas: excitation and inhibition cancel each other |
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negative response at dark side of boundary: more inhibition than excitation |
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positive response at bright side of boundary: more excitation than inhibition |
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no response in bright areas: excitation and inhibition cancel each other |
opponent filtering (centre-surround receptive fields) :
generates minimum and maximum at luminance boundaries
and discards average luminance
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receptive fields (spatial filters) can account for
this illusion !
these grey spots are believed to be the result of opponency filtering (centre-surround
receptive fields, leading to contrast enhancment)
at the intersections larger parts of the inhibitory surround are stimulated,
leading to an apparent reduction of brightness:
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obviously this is a very idealised picture, but the main result is essential: because the intersection regions would generate a lower score than the bar regions, they would be perceived as darker !
also have a look at the Scintillating
Grid Illusion, and compare various versions here
for some explanations see Schrauf et al.1997.
when we revisit the simple receptive field outputs at different locations of
the black squares in the Hermann grid we make the following observation:
central regions should be perceived as brighter than boundary regions, but
each square looks plain black !
encoding of spatial changes
through opponent processes removes 'redundant' image components (i.e.
those which do not contain information): image
compression
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what you have
what you should see |
| however, the world does not look like
the output of opponency filters (closed areas of grey are perceived homogenous
in brightness) this raises another question: how is the average intensity level reconstructed in the brain ? |
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to reconstruct the average intensity in regions without intensity change, there are special cortical mechanisms by which regions enclosed by clear boundaries assume the same intensity (e.g. Grossberg & Pessoa 1998)
a cortical mechanism called ‘filling in’ reconstructs redundant image regions by assuming that regions enclosed by clear boundaries originally have the same intensity
image compression and reconstruction leads to the prediction that certain luminance profiles should be perceived as similar although they are physically rather different !!!!
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physical luminanceprofile
perceived brightness profile |
for an impressive demonstration of this effect, click here
Brightness illusions
(please note that these images give only a rough idea, because digital image
compression like JPEG and other interferes with the actual luminance)
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the perceived brightness at a given location depends on its neighbourhood (simultaneous contrast effects) |
 closed Koffka ring open Koffka ring Craik-Cornsweet |
the perceived brightness
of a clearly outlined surface tends to be uniform (filling in) |
how do you identify objects ?? |
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colour adds a lot of information to an image !! |
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physical |
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perceptual
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spectral composition is to be converted into hue & saturation
some Historical Systems of colour description (Hans Irtel)
many observations in colour vision are based on the fact that a colour is defined by 3 components (e.g. RGB in colour monitors)
trichromatic theory of colour vision advanced by Thomas Young and H. v. Helmholtz: (Young-Helmholtz trichromatic theory)
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additive colour
mixing:
superposition of coloured spotlights produces new colours (and can generate colour shadows) note: subtractive colour mixing in paints is quite a different thing, because pigments in paints remove comonents from the light spectrum
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three neural mechanisms: short-, medium-, long-wave sensitive photoreceptors:
this is the physiological basis of trichromatic processes |
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simultaneous contrast is the enhancement of colour differences in space, as consequence of opponent processing similar to that in the perception of brightness (see Jameson & Hurvich 1964)
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the cross lines appear grey in front of the yellow and yellow in front of the grey field, but they are the same colour (look at the top centre where they touch!) Josef Albers, The Interaction of Color, 1963 |
… strawberries look most tempting
when embedded in green leaves … |
such observations lead to another theory of colour vision:
opponent colours !
opponent-process theory of colour vision, advanced by E. Hering, is suggesting that two pairs of opponent colours that exclude each other (red-green and blue-yellow) deternmine the perceptual space of colour vision.
NOTE: these ‘opposing’
theories do not exclude each other !!!
they reflect different coding strategies at different stages of the visual stream
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Munker-White (1979) effect : assimilation of colour! (this is a change of perceived colour in the opposite direction of simultaneous contrast - in this demonstration coloured stripes are getting closer to their surrounding: assimilation instead of contrast)
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physically, the two red squares have the same colour
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There are also longlasting effects that affect perceived colour, such as the McCullough effect (Vladusich and Broerse, 2002)
Other factors that influence perceived colour include: illumination (red sunlight, blue artificial light), tinted glass, colour blindness.
For instance, colour constancy further affects our perception of colour: surface colours are often perceived independent of illumination !
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fixate the black dot in the centre for 60 seconds ... … and then look at a the black dot in the right panel ! |
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what do you see?
afterimages are the consequence of encoding stimulus change in time:
(1) adaptation :
As a natural property of sensory neurones, after the onset of the green-red stimulus the neural response jumps to a maximum value and then gradually returns to resting levels.
NOTE: this 'adaptation' means that only changes are encoded (remove redundancy in time)
(2) opponency :
According to the opponent theory of colour vision, stimuli of opposite quality (red-green) are subtracted from each other, and perceived colour is determined by the difference between green and red activation.
NOTE: this is conventional feature contrast enhancement (<< this also means that there is some kind of temporal filling in because colours are not fading with time >>)
(3) aftereffect :
At the end of the adaptation period locations stimlated by green light are less sensitive to green than to red, and vice versa. After stimulus offset, a grey region (grey = red + green light) appears red where the green light was shown before (and vice versa), because a larger neural signal is generated in the non-adapted red channel than in the adapted green channel: this imbalance leads to a preceived opponent colour as result of the opponency processing.
NOTE: this creates contrast enhancement in the time domain
(for more about this, see Jameson & Hurvich 1964, Anstis et al. 1978)
main textbook : Zanker, J.M. (2010) Sensation, Perception, Action - an evolutionary perspective. Palgrave (152.1 ZAN) : chapter1
additional textbook : Goldstein, E.B. (2007) Sensation and Perception (7th ed.) Wadsworth-Thompson (152.1 GOL), (in particular first pages of chapter1)
to download a pdf copy of lecture slides, click here
back to course
outline
last update
28-01-2015
Johannes
M. Zanker
