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)
Sensory store Storage model of visible persistence (hydrodynamic metaphor)
visible persistence sensory store (10 ms) Storage model of visible persistence (the metaphor can also be electrodynamic) time
Task: name the matrix location of missing dot F1 F2 ISI How can we measure the duration of visible persistence? Present the matrix in two sequential frames, separated by an ISI
time time F1 F2 Short ISI F1 F2 ISI
F1 F2 ISI No overlap time time No integration F1 F2 Long ISI
F1 F2 ISI F1 F2 ISI 100 F1 F2 ISI time 80 VDL JH 60 BJ DR Percent correct 40 20 0 40 80 120 160 200 ISI (ms) mat1 noarg space
time F1 F2 ISI F1 F2 ISI 100 80 F2 F1 ISI 60 VDL RGB SKP 40 Percent correct 20 0 10 40 80 120 160 200 Duration of F1 (ms) Charging the sensory store (increasing the duration of F1) mat1 noarg space
F1 F2 F1 F2 F1 duration = 16 ms F1 duration = 300 ms (ISI = 0) mat1 noarg space
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: mat2 20-1-0
Visible persistence is time-locked to theonset of the stimulus, not to itsoffset This means that temporal integration follows anSOAlaw, not anISIlaw This also means that visible persistence isnotbased on a decaying sensory store.
Visible persistence is linked to a burst of neural activity time-locked tostimulus onset (Duysens, Orban, & Maes, 1985).
time F1 F2 F1 F2 F1 F2 In brief, The duration of visible persistence varies inversely with stimulus duration (inverse-duration effect)
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
F1 F2 Short SOA: The critical period is up: the object is complete. No more parts can be added.
F1 F2 ISI Long SOA with ISI: The critical period is up: the object is complete. No more parts can be added.
F2 F1 Long SOA, no ISI:
mask target Applying the principles of temporal integration to forward masking by pattern
mask target time msk tgt Masking by temporal integration Demo: int1-4
tgt mask + target mask time mask Demo In the matrix task a long F1 produced segregation. A long leading mask produces unmasking.
tgt time mask seg1-4
DURATION OF LEADING MASK (ms) msk tgt
msk DURATION OF LEADING MASK (ms) tgt
tgt mask An account based on ideas of von Holst (1954) and MacKay (1957) (Long SOA) 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 5 5 440 440 420 2 400 420 380 Not due to onset transient 400 Reaction Time (ms) 360 340 380 320 Reaction Time (ms) 360 300 Grid Noise Digit Script Leading Distractors 340 320 300 No Stream Stream C C 7 5 9 3 7 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)
SOA Stimulus sequence in metacontrast masking
Mask Target Target + Mask Visibility ratings are subjective
SOA = 0 No masking (10 ms) Target (10 ms) Mask
ISI Target SOA Mask Stimulus sequence in metacontrast masking