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Masking occurs when the perception of a brief stimulus (the target) is impaired by another stimulus (the mask) presented in close temporal and spatial.

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Presentation on theme: "Masking occurs when the perception of a brief stimulus (the target) is impaired by another stimulus (the mask) presented in close temporal and spatial."— Presentation transcript:

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2 Masking occurs when the perception of a brief stimulus (the target) is impaired by another stimulus (the mask) presented in close temporal and spatial contiguity Masking occurs in two main flavours: a) by integration (camouflage) b) by interruption: i) metacontrast masking ii) pattern masking iii) object-substitution masking Why study masking? Because it opens a window on the mind/brain Let’s start with integration…

3 Masking by temporal integration CW: Masking by temporal integration is mediated by visible persistence (iconic memory) Visible Persistence: an image that remains visible for a brief period after the display has been turned off. (NOT a retinal afterimage)

4 Sensory store Storage model of visible persistence (hydrodynamic metaphor)

5 sensory store visible persistence Storage model of visible persistence (the metaphor can also be electrodynamic) time (10 ms)

6 How can we measure the duration of visible persistence? Task : name the matrix location of missing dot Present the matrix in two sequential frames, separated by an ISI ISI F1 F2

7 Short ISI F1F2 time ISI time F1 F2

8 Long ISI time ISI F2 F1 time F1 F2 No overlap No integration

9 VDL JH BJ DR ISI (ms) Percent correct mat1 noarg space time ISI F1F2 ISI F2F1 ISI F2F1

10 VDL RGB SKP Duration of F1 (ms) Percent correct F1F2 ISI mat1 noarg space F1 ISI F2 time ISI F1 F2 Charging the sensory store (increasing the duration of F1)

11 F1F2 F1 F1 duration = 16 ms F1 duration = 300 ms (ISI = 0) mat1 noarg space

12 F2 F1 When F1 is long, integration breaks down. F1 and F2 become segregated In fact, when F1 is long, F1 and F2 become segregated even when displayed concurrently Demo: mat

13 Visible persistence is time-locked to the onset of the stimulus, not to its offset This also means that visible persistence is not based on a decaying sensory store. This means that temporal integration follows an SOA law, not an ISI law

14 Visible persistence is linked to a burst of neural activity time-locked to stimulus onset (Duysens, Orban, & Maes, 1985).

15 In brief, The duration of visible persistence varies inversely with stimulus duration (inverse-duration effect) F2 time F1 F2 F1F2

16 The onset of a stimulus creates an image of itself (opens an object file) that accepts new data for a very brief period (the critical period of integration) All these effects (integration at short SOAs and segregation at long SOAs) can be explained based on ideas of von Holst (1954) and MacKay (1957) After that, the image acts as a filter that passes only new stimuli

17 The critical period is up: the object is complete. No more parts can be added. Short SOA: F1F2

18 The critical period is up: the object is complete. No more parts can be added. Long SOA with ISI: F1F2 ISI

19 F1 F2 Long SOA, no ISI:

20 Applying the principles of temporal integration to forward masking by pattern mask target

21 mask target Demo: int1-4 Masking by temporal integration msk time tgt

22 mask mask + target Demo mask time tgt In the matrix task a long F1 produced segregation. A long leading mask produces unmasking.

23 seg1-4 mask time tgt

24 DURATION OF LEADING MASK (ms) tgt msk

25 DURATION OF LEADING MASK (ms) tgt msk

26 (Long SOA) mask msk+tgt An account based on ideas of von Holst (1954) and MacKay (1957) mask tgt

27 On the basis of this evidence, one might conclude that forward masking never occurs when the leading mask is displayed for longer than about 100 ms But note the effect of task switching (e.g. conventional display sequence in masked priming)

28 55 Reaction Time (ms) Stream No Stream C C No stream No forward masking in either case when no backward mask is presented RT differences mediate more backward masking in the Stream condition Not due to onset transient Leading Distractors GridNoiseDigitScript Reaction Time (ms)

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30 Masking occurs in two main flavours: a) integration (camouflage) b) interruption: i) metacontrast masking ii) pattern masking iii) object-substitution masking

31 Metacontrast masking No spatial overlap between target and mask When target-mask SOA is short, the target’s visibility is reduced (but not when SOA is equal to zero)

32 TARGET Conventional demonstration of metacontrast masking (visibility ratings)

33 MASK Conventional demonstration of metacontrast masking (visibility ratings)

34 SOA = 0 No masking (10 ms)

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37 Stimulus sequence in metacontrast masking SOA

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40 Target Mask Target + Mask Visibility ratings are subjective

41 SOA = 0 No masking (10 ms) Target (10 ms) Mask

42 sim

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46 Target Mask Stimulus sequence in metacontrast masking SOA ISI

47 seq

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51 U-shaped curve

52 % correct responses SOA (ms) No ISI With ISI tgtmsk SOA

53 Effect of varying the SOA

54 SOA = 0

55 SOA = 100

56 SOA = 200

57 SOA = 300

58 SOA = 500

59 Effect of target-mask separation (SOA = 100 ms)

60 Sep = 1

61 Sep = 2

62 Sep = 3

63 Sep = 4

64 Sep = 5

65 Reducing the mask’s contours (SOA = 100 ms)

66 4 dot 1

67 4 dot 2

68 4 dot 3

69 4 dot 4

70 % correct responses SOA (ms) No ISI With ISI tgtmsk ISI tgt msk With ISI No ISI Function of SOA, not ISI (SOA law) NOTE: no masking when SOA = 0

71 A theory of metacontrast masking: Cross-Channel Inhibition (Breitmeyer & Ganz, 1976, 2005)

72 1.A brief display generates activity in two visual channels: a) Transient channels (short latency; short duration; low SF; mediate perception of stimulus onset) b) Sustained channels (long latency; long duration; high SF; mediate perception of stimulus identity) 2. Activity in the transient channels suppresses activity in the sustained channels mask target time ++ no masking when SOA = 0

73 target mask time 1.A brief display generates activity in two visual channels: a) Transient channels (short latency; short duration; mediate perception of stimulus onset) b) Sustained channels (long latency; long duration; mediate perception of stimulus identity) 2. Activity in the transient channels suppresses activity in the sustained channels

74 Support for the cross-channel inhibition hypothesis inhibition vanishes in scotopic viewing (von Békésy) and so does metacontrast masking Light-adaptedDark-adapted

75 Masking of a pattern by parts of itself reveals a new form of masking: Object substitution F1 F2 (30 ms) (300 ms) Demo: mat mat (vary overlap) F2 F1 F2

76 This is unquestionably a form of masking but: The mask consists of a portion of the target The target-”mask” SOA is equal to zero Remember: no metacontrast masking when SOA = 0 So, let’s turn this into a classical metacontrast paradigm, and see if masking occurs when SOA = 0

77 targetmask Metacontrast paradigm time SOA = 0

78 0 demo

79 1x

80 2x

81 3x

82 4x

83 The finding that metacontrast masking occurs when T-M SOA = 0 is inconsistent with accounts based on inhibitory contour interactions ++

84 In addition, common-onset masking occurs without substantial masking contours (four-dot masking)

85 1dotx

86 2dotx

87 3dotx

88 4dotx

89 Common-onset masking occurs with overlearned geometrical shapes

90 The target can be seen easily when the entire display is presented briefly (10 ms)

91 os16-0a

92 os16-0b

93 os16-0c

94 os16-0d

95 Display sequence for object-substitution masking (320 ms) (10 ms)

96 os16-600a

97 os16-600b

98 os16-600c

99 os16-600d

100 How does object substitution come about?

101 ?

102 Object-substitution masking does not occur when set size = 1 (320 ms) (10 ms)

103 os1-600a

104 os1-600b

105 os1-600c

106 os1-600d

107 Object substitution masking does not occur when attention is directed to the target location

108 os16-600a X

109 os16-600b X

110 Expectation based on learned regularities (occlusion of far objects by near objects) influences OSM Kahan & Lichtman (P&P, in press) 30 ms time 600 ms

111 Expectation based on learned regularities (occlusion of far objects by near objects) influences OSM Kahan & Lichtman (P&P, in press) near far Masking Effect % C (common - delayed offset) n-nf-ff-nn-f Target-mask depth

112 Common-onset masking has two components Inhibitory contour interactions early photopic independent of attention requires substantial masking contours does not occur at SOA = 0 Object-substitution relatively late scotopic critically dependent on attention requires minimal masking contours occurs at SOA = 0 ?

113 Duration of mask (ms) % correct responses Set Size (No. elements) Photopic Scotopic Separating the two components of common-onset masking: (inhibitory contour interactions and object substitution)

114 Separating the two components of common-onset masking: (inhibitory contour interactions and object substitution) photopic scotopic

115 Masking occurs in two main flavours: a) integration (camouflage) b) interruption: i) metacontrast masking ii) pattern masking iii) object-substitution masking

116 Pattern masking is probably the most commonly used form of masking but, in my opinion, it is the least interesting Depending on viewing conditions, it includes aspects of both integration and object substitution

117 Here is one way in which the processes of integration and interruption (object substitution) may combine to yield masking (Michaels & Turvey, 1979)

118 … e così via… … und so weiter … … and so on … Ciao!

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