Week 9 & 10 Flashcards

Question

What is the Treisman & Schmidt (1982) study?

Answer 1

Treisman & Schmidt (1982) showed that such illusory conjunctions occur They presented character strings very briefly (95-168 ms) followed by noise mask. The primary task was to report the two numbers. Then O’s were asked to report coloured letters. O’s often associated the wrong colour with the wrong letter Such incorrect bindings are known as illusory conjunctions

Answer 2

A prediction of FIT is that if attention is inhibited, features from different objects will be incorrectly bound together.

Answer 3

RM is a patient who has parietal lobe damage. As a result, has a condition called Balint’s syndrome. When multiple objects are present, RM has difficulty focusing attention on a single object. When shown two letters, each with a different colour, reported the wrong letter colour combinations on 23% of the trials, even when allowed to view the letters for as long as 10 seconds! So, RM very prone to experiencing illusory conjunctions because he could not focus attention on just a single object.

Answer 4

Some forms of visual search require binding to occur. For example, binding is required if the target contains the same features as the distractors. If the target differs from the distractors only by its particular conjunction of features then that is a conjunction search FIT predicts that in conjunction searches attention needs to be applied to each object in turn (i.e. one at a time) to determine whether or not the attended object is the target Thus, these searches are predicted to be very slow

Answer 5

Here the target contains a feature (red) that the distractors do not contain - This is known as a feature search FIT predicts that because binding does not need to occur, attention does not need to be applied to each item in turn. Thus, such searches are predicted to be fast.

Answer 6

Attention can also determine what we remember. If you don’t attend to it, chances are you won’t remember it. You are now going to be shown two images in succession. Your task is to spot the two differences between the two images.

Answer 7

You can remember only a few parts of a scene at one time. If one of those parts change, you notice the change If some other part of the scene changes, chances are you won’t notice the change – change blindness

Answer 8

Answer: Because changes usually generate motion transients that draw attention to the location change, thereby making it easy to spot the change. In the first demonstration, a blank screen was inserted between images. This meant that when the second image was shown, motion transients occurred for every part of the image - not just the parts that changed. This meant that motion transients did not guide attention to the change.

Answer 9

Change blindness doesn’t just happen in static pictures. It can also occur in the real world

Answer 10

Perception seems effortless but it is much harder than it seems. One way to appreciate the difficulties in perceiving objects and scenes is to try to get a computer to do it. It turns out that computers are worse at recognising objects than humans… …and fail in very unpredictable ways.

Answer 11

Currently, the state-of-the-art computer object recognition systems use artificial neural networks. Athalye et al. (2018) investigated what sort of images these object recognition systems would misclassify. Based on what they discovered, they then designed images that would fool these systems. Seemingly bizarre misclassifications such as this are unsettling and fairly common. In fact, you don’t have to use specially-generated images to fool an image classifier. Misclassifications commonly occur with natural images if they are presented at unexpected orientations (Alcon, 2019)

Answer 12

False Object perception is very hard. Our best computer algorithms are still quite bad at it.

Answer 13

Several factors, but the three most important ones are: - The stimulus on the retina is ambiguous - Objects can be hidden or blurred - Objects look different from different viewpoints and in different poses

Answer 14

Similarly, 2D retinal images are also ambiguous in that multiple stimuli can give rise to the same 2D retinal image

Answer 15

Most likely a machine would have difficulty recognising my glasses because they are partially occluded. For example: A pair of glasses is blocked by another object like a book

Answer 16

Machines find it hard to recognise objects when they appear in unexpected poses or are viewed from unexpected angles.

Answer 17

We start by discussing two competing schools of thought: - Structuralism - Gestaltism

Answer 18

Structuralism was proposed by Edward Titchener, based on his studies under Wilhelm Wundt. Structuralism distinguishes between sensations and perceptions Structuralism claims that sensations combine to form perceptions. In other words, according to Structuralism, conscious awareness is the sum of these elementary sensations.... …and contains nothing that was not already present in these elementary sensations.

Answer 19

Sensations: elementary processes occur in response to stimulation

Answer 20

Perceptions: Conscious awareness of objects and scenes

Answer 21

Gestaltism directly contradicts Structuralism. The Gestaltists claim that conscious awareness is more than the sum of the elementary sensations. In other words, conscious awareness can have a characteristics not present in any of the elementary sensations.

Answer 22

There are two main pieces of evidence that support the claim that conscious awareness can be more than the sum of the elementary sensations These two pieces of evidence are: - Apparent motion - Illusory contours motion can be perceived when there is no motion in the image (e.g. apparent motion) contours can be seen when there are no contours in the image (e.g. illusory contours)

Answer 23

In apparent motion an observer sees two stationary dots flashed in succession. Although each of the dots is stationary, the observer perceives motion In other words, the conscious awareness has a character (i.e. motion) not present in the elementary sensations (because they were both stationary). The conscious percept of motion was constructed and was not present in the elementary sensations. The physical stimulus itself is not moving.

Answer 24

Illusory contours are a second example of where the conscious awareness has a characteristic not present in the elementary sensations. Illusory contours are seen in locations where there are no physical contours. The conscious awareness of the illusory contour is constructed – there is no physical contour at these locations.

Answer 25

According to Gestaltism, humans are able to perceive objects and scenes because of perceptual organisation. In other words, humans are able to make sense of a visual image because they can perceptually organise it into the constituent objects.

Answer 26

Perceptual organisation is achieved by the processes of grouping and segregation. Grouping is the process by which parts of an image are perceptually bound together to form a perceptual whole (e.g. the perception of an object) Segregation is the process by which parts of a scene are perceptually separated to form separate wholes (e.g. the perception of separate objects). Together, grouping and segregation allow a scene to perceptually organised into its constituent objects thereby allowing observers to make sense of the scene

Answer 27

Grouping is governed by 5 key principles. The more of these principles that apply, the more likely components of an image will be grouped together to form a perceptual object. The Original Gestalt principles: - Good continuation - Prägnanz - Similarity - Proximity - Common fate Two additional ones (added later) - Common region - Uniform connectedness

Answer 28

Remember we mentioned that occlusions can make object recognition difficult. The principle of good continuation can help. Aligned (or nearly aligned) contours are grouped together to form a single object. This is why contour A is grouped with contour B, instead of with contours C or D.

Answer 29

Literally German for “Good figure”. Also known as “principle of good figure” or “principle of simplicity” Essentially, groupings occur to make the resultant figure as simple as possible. In the figure to the right you see a panda, not a collection of unconnected splotches.

Answer 30

The more similar objects are, the more likely they will be grouped together. In a), all the dots are the same colour so it is unclear whether things are organised vertically or horizontally. In b), colour similarity groups the dots into columns.

Answer 31

The closer the dots are, the more likely they are to be grouped together. In b), grouping by proximity forms horizontal rows.

Answer 32

Things that are moving in the same way are grouped together.

Answer 33

Elements that are within the same region of space tend to group together (Palmer, 1992)

Answer 34

Connected regions with the same visual characteristics (e.g. colour) tend to group together (Palmer & Rock, 1994)

Answer 35

The stimulus on the retina is ambiguous Objects can be hidden or blurred Objects look different from different viewpoints and in different poses

Answer 36

It is not enough to group components of an image together to form an object, you also need to segregate the different objects in the scene from each other… …and also segregate the objects from the background. If you did not do this, you would perceive the entire image as just a single object… …which would be very confusing.

Answer 37

Regions of the image are more likely to be seen as figure if: They are in front of the rest of the image They are at the bottom of the image They are convex They are recognisable.

Answer 38

Regions of an image in front of the rest of the image tend to be seen as figures (i.e. they are seen as objects) The Rubin vase is ambiguous – it can be perceived as either a vase or two faces. It is therefore not clear what the figure is – two faces or one vase. If the vase is brought in front of the image it is then seen as the figure. If the two faces are brought in front of the image, they are then seen as the figure. This shows that depth ordering affects figure perception.

Answer 39

Most people perceive image (a) as a red object in front of a green background. This is because lower areas are more likely to be seen as figures (i.e. are more likely to be perceived as objects) However, there is no left-right bias. Consequently, in which red and green are side by side, It is not clear which side is the figure and which side is the ground.

Answer 40

Peterson & Salvagio (2008) showed that if you see a single border, there is a slight tendency to perceive the convex region as figure. However, if you see multiple convex regions, each with the same colour, you are more likely to perceive those regions as figure. Take home message: Convex regions are assumed to be figures (i.e. objects) *refer to image in week 9, lecture 2, slide 60 to understand

Answer 41

People also used past experience to segregate overlapping objects Experience also affects orientation of an image: As a) is in a familiar orientation it is easier to segregate it from the background than in b)

Answer 42

When scenes are flashed rapidly in front of an observer, she may not be able to identify all the objects in the scene. Nevertheless, she get an overall impression of what the scene is about. For example, she might think that the image shows “a crowded cafe” That “overall impression” is what is known as the “gist” of the scene.

Answer 43

Potter (1976) studied gist perception using the following paradigm. In each trial, the observer was cued with a particular scene description. Then she saw 16 randomly chosen scenes, each for 250 ms. Then she was asked if any of the scenes fitted the description. Observers were at near 100% accuracy. This showed that observers can rapidly perceive a scene’s gist.

Answer 44

Fei-Fei investigated what the minimum scene exposure time is needed to perceive a scene’s gist. Observers were presented with just a single scene, followed by a mask Observers were then asked to describe what they had seen. Fei-Fei et al reported that the longer the stimulus presentation time, the more detailed and accurate the description. People could start to perceive aspects of the scene at about 27 ms, but the perceptions were not very detailed

Answer 45

Although observers can extract the gist of a scene very rapidly, the gist they extract is not very detailed. The longer observers view a scene, the more detailed the gist they extract. 27 ms is enough time to extract some gist, and very accurate perception can be achieved in just 250 ms

Answer 46

Motion perception can - Help break camouflage - Help attract attention - Help segregate objects from the background. - Help us interpret events. - Help us determine the structure of objects - Help us determine what actions people are performing

Answer 47

Motion can also allow us to interpret events By seeing how objects interact, you can infer causality relationships and even social relationships. In the following video a large triangle gets into an argument with a small triangle and a small circle. The large triangle chases off the other two shapes before destroying the house. The entire story is conveyed using motion information

Answer 48

It can help us determine the shape of a moving object This is sometimes referred to as the “kinetic depth effect” In the following video the 3D shape of a number of objects will be revealed by their motion. Note that when these objects are stationary, their 3D shape information is unclear. Motion allows us to infer 3D shape.

Answer 49

Motion can also allow us to interpret actions While static poses are often ambiguous… …when someone moves, their actions and intentions are often made clear. This phenomenon is typically demonstrated using point-light walkers

Answer 50

Point-light walkers are created by placing lights on a person’s joints and having them perform an action (e.g. walking!) They are videoed so that only the lights can be seen. Humans are so good at processing motion stimuli, that they can guess what action the person was performing.

Answer 51

Sometime due to either disease or trauma, a patient will suffer damage to a part of the brain responsible for motion perception. Consequently, the person may no longer be able to perceive motion.

Answer 52

L.M. had difficulties - Pouring a cup of tea - Crossing the street - Following speech Essentially, L.M. could see that things had moved but couldn’t see them moving.

Answer 53

- Real motion (something actually moving) Illusory motion (nothing actually moving)- such as Static image (e.g. rotating snakes illusion - Kitaoka & Ashida, 2003) and Apparent motion - Motion aftereffects - Induced motion such as a Moving background (or a moving object) causes a stationary object to appear to move

Answer 54

- We don’t really know why this particular static illusion gives the impression of motion - Clearly, the percept of motion is caused by the contrast between the colours – but it is unclear why these contrast difference cause the percept of motion *For our current best guess, see Faubert, J. & Herbert, A.M. (1999). The peripheral drift illusion: A motion illusion in the visual periphery. Perception, 28, 617–622.

Answer 55

Apparent motion occurs when a series of stationary images are presented in succession to give the impression of motion. For example, if I alternate between the two slides below, you wouldn’t see two separate dots. Instead, you would see a single dot moving from left to right.

Answer 56

Apparent motion works only if the dots are sufficiently close together If they are too far apart and the alternation rate is too fast, motion perception ceases to occur. Instead, the percept will be of two separate flashing dots For apparent motion to occur, as separation increases alternation rate needs to decrease. This is known as Korte’s Third Law of Apparent Motion (Korte, 1915)

Answer 57

False apparent motion is mostly insensitive to colour changes However colour can be used to disambiguate ambiguous apparent motion

Answer 58

Image of Fall of Foyers near Loch Ness in Scotland where Robert Adams (1834) first experienced the waterfall illusion. After staring at the waterfall for several minutes, he then shifted his gaze to the rockface which appeared to move upwards.

Answer 59

Induced Motion: A nearby object (usually a large one) either affects the perceived motion of a second object (usually a small one) or causes a second object to appear to move.

Answer 60

Although motion can make things more visible (e.g. breaking camouflage), it can also cause things to disappear, as in motion induced blindness (Bonneh, Cooperman & Sagi, 2001) Another motion-related phenomenon we don’t have a good explanation for…

Answer 61

Motion can also make it harder to notice changes In the following demo, when the dots are stationary, it is very easy to notice them changing colour. When they move, it is much more difficult to notice that they change colour.

Answer 62

Normally colour changes attract attention because of the transient signals associated with the change (i.e. the “flicker”) However, when things are moving, there are transient signals associated with all objects – not just the ones that are changing Thus, attention is no longer drawn preferentially to the changing objects, so the changes are not noticed.

Answer 63

Motion illusions can inform us of the processes underlying motion perception

Answer 64

For example, the Footsteps Illusion shows us that contrast affects motion perception. As the yellow and blue rectangles traverse the striped background, they appear to speed up and slow down asynchronously. As the rectangles traverse the striped background the contrast at the lead edges vary. When the contrast is high, they appear to move faster. When the contrast is low, they appears to move slower. When the contrast the leading edge of the yellow rectangle is high the contrast at the leading edge of the blue rectangle is low (and vice versa) Consequently, the yellow and blue rectangles appear to speed up and slow down asynchronously.

Answer 65

The Footsteps Illusion shows us that contrast affects motion perception. Higher contrast objects appear to move faster and lower contrast objects appear to move more slowly This could explain why people often underestimate the speed of cars moving through fog… …a phenomenon that sometimes causes accidents

Answer 66

If you can’t see the ends of a line, the movement of a line is ambiguous. Consequently, the motion of the line viewed through an aperture is ambiguous and is “captured” by the movement of the terminators (the points where the line joins the aperture) When the terminators move horizontally, the line appears to move horizontally. When the terminators move vertically, the line appears to move vertically. When we can see that actual ends of the line, we can see its actual motion

Answer 67

It also demonstrates the aperture problem In this illusion, lines curve around a cylinder creating virtual terminators that move vertically. Thus, the lines themselves appear to move vertically

Answer 68

It has been suggested that colour vision evolved primarily to help humans search for things. For example, foraging for berries. Colour perception can also make judgments. - eg. Seeing if a fruit is ripe - Poisonous animals are often coloured - Animals often use colours to attract mates

Answer 69

Visible light is electromagnetic radiation with wavelengths varying from about 400 nm to about 700 nm White light is a mixture of all these wavelengths

Answer 70

An opaque object is an object that light cannot pass through. It is not at all transparent. The colour of an opaque object is determined by the light that it reflects If an opaque object absorbs blue light but reflects red light, it looks red. If an opaque object reflects all light, it will appear white.

Answer 71

The colour of a transparent object is determined by the colour it transmits. If an object absorbs blue but transmits red, then it will appear red.

Answer 72

Blue paint absorbs red light but reflects blue and green light. Yellow paint absorbs blue light but reflects red and green light. Mixing blue and yellow paint results in a mixture that absorbs both red and blue light but reflects green light – so looks green.

Answer 73

Blue = short (S) wavelength light Green = medium (M) wavelength light Red = long (L) wavelength light

Answer 74

Mixing red and green light makes yellow light. If you mix blue light with yellow light (i.e. light that contains both green and red), you will get light that contains blue, green and red. This light will look white…. …because that is what what white light is - a mixture of all three colours.

Answer 75

In the Munsell colour system colours are categorized according to: Value (lightness) Hue (colour) Chroma (saturation)

Answer 76

In the retina, there are photoreceptors known as rods and cones. Rods cannot distinguish between colours and are active only at low light levels. In normal light conditions, they are not active. In normal light conditions, only cones are active.

Answer 77

There are three types of cones. Each type responds maximally to a different wavelength S cones – 419 nm (blue) M cones – 531 nm (green) L cones – 558 nm (red)

Answer 78

By comparing the relative activities of these three types of cones, humans are able to distinguish between colours. For instance, if the L cones are most active, the light is primarily red Alternatively, if the S cones are the most active, the light is primarily blue.

Answer 79

Consider a pure green light that only contains a wavelength of 500 nm It will strongly activate the M cones and the L cones but only weakly activate the S cones It follows that if a patch of light were to activate the S, M and L cones to the exact same extent it would look identical to the test field. By adjusting the amount of blue, green and red light we can exactly match the cone activations caused by 500 nm test field… …even though the light patches themselves will be physically different (i.e. one is pure 500 nm light while the other is a combination of red, green and blue light). Physically different stimuli that appear the same are called metamers.

Answer 80

There are two main types of colour deficiency Monochromatism Dichromatism

Answer 81

Monochromatism affects about 1 in 100,000 people Monochromats usually have no functioning cones Only have functioning rods Thus, they are truly colour blind and see the world only in shades of gray Very sensitive to light – need to wear dark glasses during the day.

Answer 82

Dichromats are lacking one of the three types of cones Three types of dichromats: Protanopes Deuteranopes Tritanopes

Answer 83

Protanopia is a type of dichromatism: Absent in L cones They can't distinguish Red and green They can distinguish Blue and green/ Blue and red Matching lights needed: 420 nm, 560 nm

Answer 84

Deuteranopia is a type of dichromatism: Absent in M cones They can't distinguish red and green They can distinguish blue and green/ Blue and red Matching lights needed: 420 nm, 640 nm

Answer 85

Tritanopia is a type of dichromatism: Absent in S cones They can't distinguish between blue and green They can distinguish between red and green/ blue and red Matching lights needed: 560 nm, 640 nm

Answer 86

These people have normal trichromatic vision in one eye but dichromatic vision in the other eye Consequently, they can give use insight into what dichromats actually experience. Protanopes and deuteranopes (who cannot distinguish between red and green) see the world in shades of blue and a yellowish-green. Tritanopes (who cannot distinguish between green and blue) see the world in shades of blue and red.

Answer 87

What colour we perceived is determined not only by the cones in our retina The signals from these cones are processed by the cortex where they are combined into three colour opponent channels: Red-green Blue-yellow White-black There is direct physiological evidence for the existence of these colour channels But we will focus on the psychophysical evidence: Afterimages Impossible colours

Answer 88

When viewing a red stimulus, the L cones are more stimulated than the M cones. Consequently, the “Red” percept is more activated than the “Green” percept, which is subsequently suppressed. People, therefore see red Over time, the L cones habituate and become less sensitive to the stimulus. When a white stimulus is then shown, the green cones respond more strongly than the red cones Consequently, the “Green” percept is more strongly activated than the “Red” percept. Another example: When viewing a yellow stimulus, the L and M cones are more stimulated than the S cones. Consequently, the “Yellow” percept is more activated than the “Blue” percept, which is subsequently suppressed. Over time, the L and M cones habituate and become less sensitive to the stimulus. When a white stimulus is then shown, the blue cones respond more strongly than the red and the green cones Consequently, the “Blue” percept is more strongly activated than the “Yellow” percept. Therefore, a blue afterimage is seen.

Answer 89

The second behavioural evidence for the opponent-process theory of colour vision comes from the fact that some colour combinations are impossible. It is possible to have a bluish red (i.e. purple), a yellowish red (i.e. orange), a yellowish green and a bluish green. However, we can’t perceive a bluish yellow or a reddish green So, what happens if you add blue light to yellow light? Eventually it becomes white – you don’t get a bluish yellow. Likewise adding red light to green light creates yellow – not greenish red. The colour bluish yellow is impossible because blue and yellow are opponent colours Similarly, the colour reddish green is impossible because red and green are opponent colours

Answer 90

So far, you have been told that the colour of an opaque object is determined by the colour of the light that it reflects. Thus, a green sweater looks green because it absorbs red and blue light, reflecting only green light. The colour of light an object reflects is determined by not only by its reflectance but also by the colour of light shining on it. If white light is shone onto a green sweater, the green sweater will predominantly reflect green light. However, if the light shone on the sweater is predominantly red, the sweater will reflect predominantly red light Even though green objects absorb red light, if you shine enough red light on them, they will reflect some red. If you only shine red light on them, the only colour they can reflect is red.

Answer 91

The key point is that the light reflected by an object is determined by the product of its reflectance and the illumination Reflectance x Illumination = Reflected light

Answer 92

Part of the solution is that we habituate to the colour. Habituation = becoming less sensitive to that colour If the entire scene is under red light, we habituate to red, so everything appears greener than it otherwise would. However, it turns out that habituation is not large enough to explain colour constancy.

Answer 93

Thus, there are at least two ways the visual system achieves colour constancy Habituation Discounting the illuminant

Week 9 & 10 Flashcards

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