masking occurs when the perception of a brief stimulus (the target) is impaired by another stimulus...
TRANSCRIPT
Masking occurs whenthe 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 maskingiii) object-substitution masking
Why study masking?Because it opens a window on the mind/brain
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)
sensory store visible persistence
Storage model of visible persistence(the metaphor can also be electrodynamic)
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 F2
time
ISI
timeF1 F2
Long ISI
time
ISIF2F1
timeF1 F2
No overlap
No integration
VDLJHBJDR
ISI (ms)
100
80
60
40
20
040 80 120 160 200
Perc
ent
corr
ect
mat1 noarg space
time
ISIF1 F2
ISI F2F1
ISI F2F1
VDLRGBSKP
100
80
60
40
20
010 80 120 160 20040
Duration of F1 (ms)
Perc
ent
corr
ect
F1 F2ISI
mat1 noarg space
F1 ISI F2
timeISIF1 F2
Charging the sensory store (increasing the duration of F1)
F1 F2
F2 F1
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 segregatedeven when displayed concurrently
Demo: mat2 20-1-0
Visible persistence is time-lockedto the onset of the stimulus, not to its offset
This also meansthat visible persistence
is not based on a decaying sensory store.
This means that temporal integration followsan SOA law, not an ISI law
Visible persistence is linked to a burst of neural activitytime-locked to stimulus onset
(Duysens, Orban, & Maes, 1985).
In brief,The duration of visible persistence varies inversely
with stimulus duration(inverse-duration effect)
F2time
F1
F1 F2
F1 F2
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 explainedbased on ideas of von Holst (1954) and MacKay (1957)
After that, the image acts as a filter that passesonly new stimuli
The critical period is up:the object is complete.No more parts can beadded.
Short SOA:
F1 F2
The critical period is up:the object is complete.No more parts can beadded.
Long SOA with ISI:
F1 F2
ISI
F1
F2Long SOA, no ISI:
Applying the principles of temporal integrationto forward masking by pattern
mask target
mask target
Demo: int1-4
Masking by temporal integration
msk
time
tgt
mask mask + target
Demo
mask
time
tgt
In the matrix task a long F1 produced segregation.A long leading mask produces unmasking.
seg1-4
mask
time
tgt
DURATION OF LEADING MASK (ms)
tgtmsk
DURATION OF LEADING MASK (ms)
tgt
msk
(Long SOA)
maskmsk+tgt
An account based on ideas of von Holst (1954) and MacKay (1957)
mask
tgt
On the basis of this evidence, one might conclude that forward masking neveroccurs 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)
5 5
React
ion
Tim
e (
ms)
300
320
340
360
380
400
420
440
StreamNoStream
C7
59
23
7
C
StreamNo
stream
No forward masking in either case when no backward mask is presented
RT differences mediate more backward masking in the Stream condition
Not d
ue to
onse
t tra
nsient
Leading DistractorsGrid Noise Digit Script
React
ion T
ime (
ms)
300
320
340
360
380
400
420
440
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)
TARGET
Conventional demonstration of metacontrast masking(visibility ratings)
MASK
Conventional demonstration of metacontrast masking(visibility ratings)
SOA = 0
No masking
(10 ms)
(10 ms)
Stimulus sequencein metacontrast masking
SOA
Target Mask Target + Mask
Visibility ratings are subjective
SOA = 0
No masking
(10 ms)
Target
(10 ms)
Mask
sim
sim
sim
sim
Target
Mask
Stimulus sequencein metacontrast masking
SOA
ISI
seq
seq
seq
seq
U-shaped curve
Figure 2
SMG
SOA (ms)
40 80 120 1600
20
40
60
80
100
MIR
40 80 120 1600
VDL 20
40
60
80
100
AVM
ISI:Tgt Dur:
Tgt Lum:Msk Dur:
Msk Lum:
ConditionBr. Mtch. Gap
0 variedvaried 10varied 100
10 10100 100
Per
cent
age
of c
orre
ct r
espo
nse
s
20
40
60
80
100
Gap
AverageBr. Mtch.
Figure 2
SMG SMG SMG
SOA (ms)
40 80 120 1600 40 80 120 1600
20
40
60
80
100
20
40
60
80
100
MIR MIR
40 80 120 1600 40 80 120 1600
VDL 20
40
60
80
100
20
40
60
80
100
AVM AVM
ISI:Tgt Dur:
Tgt Lum:Msk Dur:
Msk Lum:
ConditionBr. Mtch. Gap
0 variedvaried 10varied 100
10 10100 100
Per
cent
age
of c
orre
ct r
espo
nse
s
20
40
60
80
100
20
40
60
80
100
Gap
AverageBr. Mtch.Gap Gap
AverageBr. Mtch.Br. Mtch.
% c
orr
ect
resp
onse
s 100
80
60
20
40
0 40 80 120 160
SOA (ms)
No ISI
With ISI
tgt msk
SOA
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
Figure 2
SMG
SOA (ms)
40 80 120 1600
20
40
60
80
100
MIR
40 80 120 1600
VDL 20
40
60
80
100
AVM
ISI:Tgt Dur:
Tgt Lum:Msk Dur:
Msk Lum:
ConditionBr. Mtch. Gap
0 variedvaried 10varied 100
10 10100 100
Per
cent
age
of c
orre
ct r
espo
nse
s
20
40
60
80
100
Gap
AverageBr. Mtch.
Figure 2
SMG SMG SMG
SOA (ms)
40 80 120 1600 40 80 120 1600
20
40
60
80
100
20
40
60
80
100
MIR MIR
40 80 120 1600 40 80 120 1600
VDL 20
40
60
80
100
20
40
60
80
100
AVM AVM
ISI:Tgt Dur:
Tgt Lum:Msk Dur:
Msk Lum:
ConditionBr. Mtch. Gap
0 variedvaried 10varied 100
10 10100 100
Per
cent
age
of c
orre
ct r
espo
nse
s
20
40
60
80
100
20
40
60
80
100
Gap
AverageBr. Mtch.Gap Gap
AverageBr. Mtch.Br. Mtch.
% c
orr
ect
resp
onse
s 100
80
60
20
40
0 40 80 120 160
SOA (ms)
No ISI
With ISI
tgt mskISI
tgt msk
With ISI
No ISI
Function of SOA, not ISI (SOA law)
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
mask
target
time
+ +
no masking when SOA = 0
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
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)
Demo:mat2 0-2-0mat2 0-2-20(vary overlap)
F2
F1
F2
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
target mask
Metacontrast paradigm
time
SOA = 0
0
demo
1x
2x
3x
4x
The finding that metacontrast masking occurs when T-M SOA = 0is 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 theentire display is presented briefly
(10 ms)
os16-0a
os16-0b
os16-0c
os16-0d
Display sequence forobject-substitution masking
(320 ms)
(10 ms)
os16-600a
os16-600b
os16-600c
os16-600d
How does object substitutioncome about?
?
Object-substitution masking does not occur when set size = 1
(320 ms)
(10 ms)
os1-600a
os1-600b
os1-600c
os1-600d
Object substitution masking does not occurwhen attention is directed to the target location
os16-600a
X
os16-600b
X
Expectation based on learned regularities(occlusion of far objects by near objects)
influences OSMKahan & Lichtman (P&P, in press)
30 ms
time
600 ms
Expectation based on learned regularities(occlusion of far objects by near objects)
influences OSMKahan & Lichtman (P&P, in press)
nearfar
0
4
2
8
6
10
-2
Mask
ing E
ffect
% C
(co
mm
on -
dela
yed o
ffse
t)
n-n f-f f-n n-fTarget-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
?
Duration of mask (ms)
% c
orr
ect
resp
on
ses
0 80 160 240 320
20
40
60
80
100
Set Size(No. elements)
1 2 4 816
Photopic
0 80 160 240 320
60
80
100
Scotopic
Separating the two components of common-onset masking:(inhibitory contour interactions and object substitution)
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 maskingbut, in my opinion, it is the least interesting
Depending on viewing conditions, it includes aspects of bothintegration 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)
… e co
sì via…
… u
nd so w
eiter …
… and so
on …
Ciao!