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 Why study masking? Because it opens a window on the mind/brain Masking occurs in two main flavours: a) by integration (camouflage) b) by interruption: i) metacontrast masking ii) pattern masking iii) object-substitution masking Let’s start with integration… 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) Storage model of visible persistence (hydrodynamic metaphor) Sensory store Storage model of visible persistence (the metaphor can also be electrodynamic) sensory store visible persistence time (10 ms) 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 Short ISI F1 ISI F2 time F1 F2 time Long ISI ISI F1 F2 time No overlap No integration F1 F2 time F1 ISI F2 time F1 ISI F2 F1 ISI F2 100 VDL 80 Percent correct JH BJ DR 60 40 20 0 40 80 120 160 200 mat1 noarg space ISI (ms) Charging the sensory store (increasing the duration of F1) F1 ISI F2 time F1 ISI F2 F1 ISI F2 100 VDL 80 RGB Percent correct SKP 60 40 20 0 10 40 80 120 160 200 Duration of F1 (ms) mat1 noarg space F1 duration = 16 ms F1 F2 F1 duration = 300 ms F1 F2 (ISI = 0) mat1 noarg space 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 F2 F1 Demo: mat2 20-1-0 Visible persistence is time-locked to the onset of the stimulus, not to its offset This means that temporal integration follows an SOA law, not an ISI law This also means that visible persistence is not based on a decaying sensory store. Visible persistence is linked to a burst of neural activity time-locked to stimulus onset (Duysens, Orban, & Maes, 1985) . In brief, The duration of visible persistence varies inversely with stimulus duration (inverse-duration effect) time F1 F2 F1 F2 F1 F2 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) 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) After that, the image acts as a filter that passes only new stimuli Short SOA: F1 F2 The critical period is up: the object is complete. No more parts can be added. Long SOA with ISI: ISI F1 F2 The critical period is up: the object is complete. No more parts can be added. Long SOA, no ISI: F2 F1 Applying the principles of temporal integration to forward masking by pattern mask target Masking by temporal integration mask target msk tgt time Demo: int1-4 In the matrix task a long F1 produced segregation. A long leading mask produces unmasking. tgt mask time mask mask + target Demo tgt mask time seg1-4 msk tgt DURATION OF LEADING MASK (ms) tgt msk DURATION OF LEADING MASK (ms) An account based on ideas of von Holst (1954) and MacKay (1957) (Long SOA) tgt mask mask msk+tgt 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) No Stream stream 440 5 5 420 C C Reaction Time (ms) 7 400 440 5 380 420 9 Reaction Time (ms) 400 2 360 380 3 7 360 340 340 320 320 300 Grid Noise Digit Script 300 Leading Distractors No Stream Stream No forward masking in either case when no backward mask is presented RT differences mediate more backward masking in the Stream condition Masking occurs in two main flavours: a) integration (camouflage) b) interruption: i) metacontrast masking ii) pattern masking iii) object-substitution masking 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) Conventional demonstration of metacontrast masking (visibility ratings) TARGET Conventional demonstration of metacontrast masking (visibility ratings) MASK SOA = 0 No masking (10 ms) (10 ms) Stimulus sequence in metacontrast masking SOA Visibility ratings are subjective Target Mask Target + Mask SOA = 0 No masking (10 ms) Target (10 ms) Mask sim sim sim sim Stimulus sequence in metacontrast masking ISI Target SOA Mask seq seq seq seq U-shaped curve SOA tgt msk 100 100 % correct responses ondition tch. Gap 80 80 varied ed 10 60 60 ed 100 40 40 Average 10 Br. Mtch. No ISI 0 100 20 20 With ISI Gap 0 40 80 120 160 SOA (ms) Effect of varying the SOA SOA = 0 SOA = 100 SOA = 200 SOA = 300 SOA = 500 Effect of target-mask separation (SOA = 100 ms) Sep = 1 Sep = 2 Sep = 3 Sep = 4 Sep = 5 Reducing the mask’s contours (SOA = 100 ms) 4 dot 1 4 dot 2 4 dot 3 4 dot 4 Function of SOA, not ISI (SOA law) With ISI ISI tgt msk No ISI tgt msk 100 100 % correct responses ondition tch. Gap 80 80 varied ed 10 60 60 ed 100 40 40 Average 10 Br. Mtch. No ISI 0 100 20 20 With ISI Gap 0 40 80 120 160 SOA (ms) NOTE: no masking when SOA = 0 A theory of metacontrast masking: Cross-Channel Inhibition (Breitmeyer & Ganz, 1976, 2005) 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 no masking when SOA = 0 time target + + mask 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 time target mask Support for the cross-channel inhibition hypothesis inhibition vanishes in scotopic viewing (von Békésy) and so does metacontrast masking Light-adapted Dark-adapted Masking of a pattern by parts of itself reveals a new form of masking: Object substitution F1 F2 (30 ms) (300 ms) F2 F2 Demo: F1 mat2 0-2-0 mat2 0-2-20 (vary overlap) 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 Metacontrast paradigm target mask time SOA = 0 demo 0 1x 2x 3x 4x The finding that metacontrast masking occurs when T-M SOA = 0 is inconsistent with accounts based on inhibitory contour interactions + + In addition, common-onset masking occurs without substantial masking contours (four-dot masking) 1dotx 2dotx 3dotx 4dotx Common-onset masking occurs with overlearned geometrical shapes The target can be seen easily when the entire display is presented briefly (10 ms) os16-0a os16-0b os16-0c os16-0d Display sequence for object-substitution masking (10 ms) (320 ms) os16-600a os16-600b os16-600c os16-600d How does object substitution come about? ? Object-substitution masking does not occur when set size = 1 (10 ms) (320 ms) os1-600a os1-600b os1-600c os1-600d Object substitution masking does not occur when attention is directed to the target location X os16-600a X os16-600b Expectation based on learned regularities (occlusion of far objects by near objects) influences OSM Kahan & Lichtman (P&P, in press) 30 ms 600 ms Expectation based on learned regularities (occlusion of far objects by near objects) influences OSM Kahan & Lichtman (P&P, in press) far near % C (common - delayed offset) 10 8 Masking Effect 6 4 2 0 n-n f-f f-n n-f -2 Target-mask depth 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 Separating the two components of common-onset masking: (inhibitory contour interactions and object substitution) Photopic Scotopic 100 100 % correct responses 80 80 60 60 Set Size (No. elements) 40 1 2 4 8 20 16 0 80 160 240 320 0 80 160 240 320 Duration of mask (ms) Separating the two components of common-onset masking: (inhibitory contour interactions and object substitution) photopic scotopic Masking occurs in two main flavours: a) integration (camouflage) b) interruption: i) metacontrast masking ii) pattern masking iii) object-substitution masking 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 Here is one way in which the processes of integration and interruption (object substitution) may combine to yield masking (Michaels & Turvey, 1979) Ciao!
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