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!