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Visual Neuroscience > W7 - Perceptual Illusions > Flashcards

W7 - Perceptual Illusions Flashcards

1
Q

What are the main differences between sensory and perceptual illusions?

A
  1. Sensory illusions = hardware fault / limitation in how the cells encode information
  2. Perceptual illusions = sensory representation is fine, but illusions occur when we interpret the retinal image incorrectly

it is NOT a limitation in how cells code the information, but how they are processed / interpreted

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2
Q

How is the dalmation an example of a perceptual illusion?

A

The visual system is trying to make sense of image from getting a global interpretation of the isolation elements to form a coherent stable percept

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3
Q

How is the vase/face illusion or necker cube an example of the perceptual illusion?

A
  1. Both are bistable percepts = such that what you perceive depends on how your interpret what is the foreground/background, or how the image is interpreted at a perceptual level instead of a sensory level
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4
Q

Many researchers use bistable percepts to get to brain areas of consciousness. If a brain area changes in phase with change in percept of bistable percept, it is argued that that brain area is more involved in consciousness than other areas

What might be a limitation using bistable percepts to tap consciousness?

A
  1. Some people have strong emotional reactions (either positive or negative) towards bistable percepts and this may alternate perception between the two ways of viewing the object
  2. These emotional responses have been shown to be linked to other areas of cognition less related to consciousness, including:
  • cognitive empathy / TOM
  • negative affect
  • intolerance of uncertainty (gone after controlling for CE and NA)
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5
Q

What is the other supporting evidence that suggests bistable percepts might actually be measuring emotional processing instead of consciousness?

A
  1. Wide distribution of emotional responses of bistable percepts, peak of having no emotional responses to disliking to enjoying them -4 to +4
  2. Getting people to do cognitive tests can also put people into a negative emotional state (eg. Laybourn et al., 2022)
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6
Q

Why do we see the tables as different lengths despite being the same?

A
  1. We use perceptual size constancy to interpret the size and shape of the retinal image to create a percept of the world
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7
Q

What are the 4 stages of visual processing?

A
  1. Object - image projection
  2. Retinal image
  3. Sensory representation is encoded by brain cells
  4. Perceived object from internal representation
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8
Q

Example: Why do we have to actively interpret the retinal image?

A
  1. because the retinal image is constantly changing when we move in relation to an object
  2. Example - As hand moves closer that it is half the original distance from the eye -, linear distance of image has doubled and surface area of image has increased 4x

Halving the distance of the object makes it appear 4x as big as what it originally was

Thus we have to actively interpret the retinal image

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9
Q

What would happen if percept of size is based upon how big the retinal image is?

A

If percept of size is based upon how big the retinal image is, the percept of size would be constantly changing in the real world, such that when we moved closer to objects, they would appear to grow in size, and when we moved further away, objects would appear to shrink

objects would appear to change as your view of them changed, thus the visual system must actively interpret the changing sensory retinal image to construct a stable construct

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10
Q

How can we tell that we have perceptual constancy?

A

the perceived physical attributes of an object typically do not change as our viewing conditions change, the perceived colour, size and shape stay the same, exceptions of this, like inflating a balloon changing the perceived size

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11
Q

What is sensation vs. perception?

A

Sensation = representation formed by sense organs / hardware
Perception = how sensory input is interpreted in the neural representation in the brain / software

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12
Q

What is the only example of optical illusions?

A

Optical illusions = basically none, would be due to the optics of the eye or the wavelengths themselves, apart from mirages in the desserts, due to density differences in the air

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13
Q

What 3 things can SENSORY illusions tell us about the brain?

A
  1. What brain cells are tuned to what information cortical areas process
  2. How brain cells interact with each other
  3. How information is transformed from one brain area to another
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14
Q

What 3 things can PERCEPTUAL illusions tell us about the brain?

A
  1. How the brain interprets sensory representation
  2. What assumptions it makes about the outside environment encoded by brain cells
  3. How they are wired together with patterns of excitation and inhibition between them
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15
Q

What are the 4 main types of perceptual constancies?

A

sizes, shapes, colour and luminance
(Shape is a variation of size)

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16
Q

How is The Dress an example of individual differences in colour constancy?

A

Colour constancy is ability to perceive the same colour regardless of changes in the combination of wavelengths reflected from off the object

  1. The dress illusions occurs as light source changes, the distribution of the wavelengths reflecting off the object changes, but having the full spectrum of colour retains your perception of the colour

The conflict of the dress illusion is probably due to individual differences in colour constancy mechanisms, whereby different people interpret the dress as different colours in different lighting conditions due to discrepancies in their colour constancy mechanisms

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17
Q

Is the dress a bistable percept?

A

It’s not a bistable percept, but some people can flip the percept

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18
Q

How is the checker shadow illusion an illusion of luminance constancy?

A

It has the same physical luminance, but seen as distinct since B is portrayed in shadow which is interpreted by the brain, and A and B are interpreted as separate to be compatible with the chequered pattern

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19
Q

Are perceptual constancy mechanisms illusions themselves?

A

Not really - the illusions occur when the PC mechanisms get things wrong

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20
Q

What information does the brain use for interpreting perceptual constancy?

A

The size of the retinal image
An estimation of viewing distance to the object

INTERPRETATION = scaling the retinal image since through estimating how far away the object is

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21
Q

In eye tests, why would just stating the size of the letters not be sufficient?

A

1) the size of the letter
2) how far away the person is trying to resolve it

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22
Q

What measure includes both size object and distance of object?

A

visual angle

Example = object size H at a distance of X produces an angle that is related to how big the image is on the retina
Measure visual angle in degrees of arc / degrees, minutes, seconds

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23
Q

How does viewing distance change visual angle?

A
  1. Person closer to the object produces a larger visual angle and thus a larger retinal image, vise versa for further away
  2. Person is closer than tree but is producing the same size retinal image, as the two different sized heights at different differences substend the same visual angle
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24
Q

Holway & Boring (1941) -
What did they test and what was their aim?

A
  1. Testing visual angle, removed depth cues: “By removing depth cues, Holway and Boring (1941) concluded that the only remaining cue, the size of the retinal image, would have to determine the perceived size.”
  2. To determine whether we take into account the viewing distance of the object to generate a percept?
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25
Q

Holway & Boring (1941) -
What was the method?

A

IV = Tests disks of various sizes at various distances that always produced the same sized retinal image
As disk got further away, it became larger: the disk size increased with viewing distance - such that the visual angle of the disk remained constant

Control IV = This was compared with a comparison disk at a fixed distance

DV = match the size of the comparison disk with the test disk

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26
Q

What is the most common error in misperceiving an object size?

A

a misestimation of distance

(“The greater the estimate of distance, the bigger we will scale up the retinal image”)

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27
Q

What pattern of results would support the idea that viewing distance is not used to scale perceived size?

A

Participants couldn’t tell the difference between the test and comparison disks, it would mean that only the object size was involved in perceived size

If they are not distance to scale size of retinal image, and if you’re not doing perceptual size constancy, the only information left to go on would be the size of the retinal image, the percept of size should stay the same”

28
Q

What pattern of results would support the idea that viewing distance is used to scale perceived size?

A

if participants were could tell the difference between the test and comparison disks, using other cues of distance like motion parallax and shadows, it would mean that both the object size and perceived viewing distance are involved in perceived size

29
Q

On a graph that plots size of disk to viewing distance, what would the results look like if viewing distance was NOT being used to perceive size?

A

Graph would be a flat line

30
Q

On a graph that plots size of disk to viewing distance, what would the results look like if viewing distance WAS being used to perceive size?

A

Graph would go up, positive correlation between size of disk to viewing distance

31
Q

Since, viewing distance is used to scale perceived size, how could you reduce the ability of the brain to do this? (4)

A

Get rid of cues that infer distance:

  1. Perceptual cues, light, shadows
  2. a background for context
  3. Block one eye - reduce depth perception / binocular cues for estimating distance of objects”
  4. Get rid of surrounding objects
32
Q

What were the overall findings of the Holway & Boring (1941) experiment?

A

As the experiment reduced brain’s ability to scale an object using viewing distance, the participants relied more on object size alone, and the graph that plotted disk size and distance got flatter.

The comparison disk was perceived as more constant in size

only the size of the object / the retinal image was being used to make a judgement of size, without perceptual constancy

33
Q

Using Emmert’s Law, can we use a retinal afterimage to show that we use an estimate of viewing distance to generate the perception of size?

A

The size of the afterimage remains the same as the object

  1. First, look at a shape that directly produces an afterimage of the identical size
  2. Then Look at a table to trick the brain into thinking its a real object, this should produces a smaller afterimage
  3. Using Emmert’s Law: “If the test distance is closer than the adaption distance, the perceived size of the afterimage will be smaller than that of the adaption stimulus”
34
Q

What is Emmert’s law?

Why does it occur?

A

If the test distance (table) is closer than the adaption distance (original scale), the perceived size of the afterimage will be smaller than that of the adaption stimulus”

If you view the afterimage on the table, and the table is closer to you, the brain unconsciously scales down the retinal image up to account for the closer viewing conditions, such that the image appears constant.

This indicates that we take viewing distance into account when viewing an object through perceptual constancy mechanisms - as the perceived size of the object appears to change when viewed under different viewing conditions.

35
Q

What other things help Perceptual size scaling scale up images that are further away?

A

The brain scales UP the size of the retinal image more when the person is FURTHER AWAY from the influence of other visual cues like

  1. depth to the object and
  2. background cues for context
36
Q

Why are the 2 arrow illusion lines are the same but are perceived to be different?

A

The angle of these arrow lines is different:

one looks like its extending away from you
the other is projecting towards you

due to perceptual size scaling, the line extending outward is going to be scaled by a greater amount than the right one

Other influences may also be the global precedence effect? We have a precedence to incorporate all features of object and take longer to isolate features to make judgements

37
Q

In aircraft, people, houses and cars look tiny, why does that occur?

A

We don’t perceive things in isolation, we do contrast comparison by magnifying differences between objects,

However, with aircraft, we are NOT doing perceptual size scaling or scaling size of retinal image because there to NO distance cues available, must rely solely on retinal image size

38
Q

Is size constancy innate or do we have to learn to apply it? What sorts of experiments could we do?

A
  1. Cross-cultural studies for studies like the Ames Room, idea that people assume room is rectangular over stability of people size
  2. Habituation study: babies look at stimulus / preferential looking / sucking response to new stimuli, then responses will decrease and indicate habituation, eg. habituation on circle and introduce new test circles, if they preferentially look at the new disk A, they are perceiving it as a larger new object, babies do show size constancy mechanisms
  3. Vision case studies = People who lived with milky corneas for decades, then underwent cataract surgery have to relearn how to see, as tuning properties not done in critical periods
39
Q

Apart from viewing distance and object size, what other factors that affect perceived size?

A
  1. Relative size to other surrounding objects, eg. Ebbighaus / Titchener illusion is due to size construct effect
  2. Shape/object constancy (foreshortening) -
    distances along the line of sight get shorter with increasing viewing distance more rapidly than orthogonal distances, eg. frontal plane projection
  3. Regression to the real = the tendency to perceive the actual shape of the object rather than the retinal shape: eg. a plate viewed at an angle and a distance yields a proximal stimulus that is elliptical and small, but the perception is somewhat rounder and larger
40
Q

Recap = what is foreshortening?

A

occurs when distances along the line of sight get shorter with increasing viewing distance more rapidly than distances further away (orthogonal distances)

Example = frontal plane projection

41
Q

What is a foreshortening example with shapes?

A

When 2d circles are viewed 3 dimensionally, the image shows the retinal shape produced: the circle gets distorted into an ellipse, square becomes a trapezoid

Why? because distances/objects in our line of sight get smooshed/compressed, but distances orthogonal to those stay constant

42
Q

What is a good way of observing the real retinal image (instead of the interpreted percept)?

A

To see the retinal image, can observe in children’s drawings: Children’s drawings show exaggeration of shapes instead of ellipse produced by the retina

43
Q

Why did it take artists so long to use perspective information (retinal shape distortions) in painting (took until the Renaissance)?

A

Prior to photographs, people couldn’t figure out perceptual perspective distortions because you couldn’t see them on the retina, thus perceptually the people will appear the same size in paintings regardless of how far away they were in a scene and in relation to surrounding objects, eg. trees, castles

44
Q

How did people first learn about perspective cues?

A
  1. In Alberti’s Window / pane of glass, close on eye and outline the objects, like a “manual photograph” in 1436
45
Q

What is constructivism? (One approach to studying vision)

A

brain must construct percept of outside world based upon the retinal image as it is impoverished and must be actively interpreted (anti-Gibsonian)

The aim of visual processing is to build an internal representation of the outside world, focus was a constructive process

46
Q

Ames: what is the theory of a hierarchy of assumptions in vision?

A

What assumptions of the outside world are more important than others in vision?

Example = What is processed over the other: the assumption of rooms are rectangular OR the assumption that people stay the same size as they move?

47
Q

How did Ames test for the hierarchy of assumptions in vision?

A
  1. Built the Ames room =
    a trapezoidal zoom but arranged s produce the same retinal image as a rectangular room under monocular viewing conditions
  2. The effect looks like someone moving from A to B, the visual angle of the person is getting a lot bigger / the retinal image produced of the person becomes significantly greater than that of a normal room
48
Q

What are the 2 interpretations of the Ames room?

A
  1. Room is rectangular but the person changed size: the size of the retinal image is getting bigger but the distance stays the same and person got bigger
  2. Room is not rectangular and the person didn’t change size: The size of the retinal image is getting bigger as the people gets closer to you
49
Q

What were the overall findings of the Ames room?

A
  1. In the hierarchy of assumptions, the brain seems to place more importance on the constancy of rectangular rooms over people staying the same size
50
Q

What is the main limitation of the Ames Room?

A
  1. there are more visual cues telling you that the room is rectangular over people being the same size, stacked artificially against the second assumption
51
Q

When viewing a real Ames room, why do you have to close one eye to get the illusion?

A

You need to view the Ames room with one eye to remove the depth perception cues provided by the binocular abilities of both eyes

After binocular depth is removed, you can’t perceive correct shape of the room, and thus interpret people changing size

However, being consciously aware of that fact that the room is non-rectangular, you will still get the illusion, because the perception is not encoded at a cognitive level but at a cellular level through patterns of excitation and inhibition

52
Q

How do we test whether the illusion of the Ames Room an innate or learnt assumption?

(the rectangular assumption overrides the one that people don’t change size as they move)

A

We can test whether these assumptions are innate or learnt by studying different populations, such as people who do not live in rectangular rooms to see if they are still susceptible to the illusion

It was found that people who live in NON-rectangular rooms STILL get the illusion - because we want to form a stable percept, and a slanted object will always produce a distortion on the retina at a cellular level, if object could be any rhombic shape, the slant and shape is unknown

52
Q

Why might we still be susceptible to the Ames Room illusion?

A

The visual system might assume that things are rectangular to avoid the unknowns of slant and shape without stereopsis

a slanted object will always produce a distortion on the retina at a cellular level, if object could be any rhombic shape and the slant and shape is unknown

53
Q

How does the Ames Room use subconscious inferences?

A
  1. There are inferences made in interpreting sensory input to create percept occurs at a subconscious level, such that we are not aware of these assumptions or the hierarchy of these assumptions at a cognitive level, only aware of the final percept
  2. These are encoded via patterns of excitation and inhibition between cells (Marr-poggio model for the correspondence problem and stereo depth perception)
54
Q

What is an example of conscious reasoning interact with subconscious inferences and draws the opposite effect?

A

The Moon illusion

the full moon looks huge on the horizon but looks small in the sky, but the size of the retinal image doesn’t change as it moves up in the sky as shown in the picture

55
Q

What mechanism is involved in the Moon illusion?

A

perceptual size constancy

56
Q

What 2 things do we use to perceive the moon illusion?

A
  1. Size of the retinal image
  2. Distance cues
  3. We perceive that the moon is larger in the horizon because it is perceived to be further to us, and thus we scale up the retinal image of the moon on the horizon to correspond to our distance cues.
  4. The moon in the sky is perceived to be closer and thus the retinal image is scaled down, even when both images produce the same size upon the retina, thus the brain interprets the moon on horizon as closer.
56
Q

Why might we perceive the moon to be bigger on the horizon than in the sky?

A
  1. There are fewer cues to distance available to us high up in the sky, while there are often more cues around the horizon, (buildings, trees) to compare to the moon size.

We do not interact with things high in the sky, thus celestial bodies is difficult to estimate the distance to,

From an evolutionary perspective, things up in the sky like birds would be typically closer than things on the horizon, thus the brain may assume the horizon moon is larger than the sky moon despite producing the same retinal image upon the retina

57
Q

What is the logic behind perceptual constancy?

A

There are an infinite number of different sized objects that can create the same size retinal image -

so to perceive the object as it really is, the brain scales the SIZE of the retinal image by the estimate of DISTANCE

  1. if 2 objects produce the same sized retinal image, the object perceived to be further away will be scaled up and perceived as larger than the object perceived to be closer
58
Q

Why do you still get the Ames Room illusion with a video clip with 2 eyes open?

A

because your eyesight is limited to a single image from the scene, you have stripped away binocular cues through the limited single image provided by the video screen

59
Q

If you walk to a physical Ames room, you can figure out that the room is not rectangular, and then close one eye, would you get the illusion?

A

YES, even that you know the real shape, it doesn’t change the assumptions that the brain is making when its processing the Ames room when depth information is not there

Thus the assumption that the room is rectangular is not represented at a cognitive level that CANNOT be modified by conscious awareness

60
Q

What opposite effect did Boring find about the moon illusion when he asked people which one was closer?

A

Boring found that people estimated the distance to the bigger-perceiving horizon moon as being CLOSER to themselves than the smaller-perceived moon in the sky as FURTHER AWAY

61
Q

After Boring found that people think the horizon moon is CLOSER rather than the perceptual constancy logic that it should be more further away, does that mean perceptual constancy logic is wrong?

A

No = Counter argument to this: conscious awareness don’t change / assumptions that generate the percept occur at a subconscious level and conscious interprete may actually draw the opposite conclusions to assumptions that occur at a subconscious level

62
Q

How can we decrease the magnitude of the moon illusion? (2)

A
  1. By decreasing cues to distance: eg. Holway and Boring (1941) after stripping cues to distance, eg. trees, cityscape features, perceptual size scaling was lessened and the only thing left is to interpret is the size of the retinal image
  2. Upside down: If you invert the image of the moon illusion, you remove some cues to distance and thus the moon illusion is lessened
63
Q

What are 2 Other theories to explain the moon illusion (pretty much INCORRECT)?

A
  1. Angle of regard = if you look up, it makes you see things smaller than if you look at something smaller
    —–Operationalisation = change the angle the person is looking at an object and estimate size, try to get moon illusion
  2. Size comparison to objects on the horizon, perceptual contrast effect eg. ebbinghaus illusion, the moon on horizon looks bigger as it is surrounded by other things buildings/trees (might play a small role)
    ——-Operationalisation = compare how the moon looks on the horizon with and without other objects, eg. ocean/open field vs. cityscape

Visual Neuroscience (6 decks)

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