several strange effects arising from perceptual grouping

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Several Strange Effects Arisingfrom Perceptual Grouping in Vision

Psychonomic Society

November 2002

James R. Pomerantz

Department of PsychologyRice University

Houston, Texas, USA

November 2002 Pomerantz, Psychonomic Society 2

AcknowledgmentsStudent Collaborators

Apu Agrawal Martha JeongAnna Ahrens Steve Jewell

Mia Cox Hana KhanCathy Carson Sandra Lozano

Ekanem Ebinne Jane PittsMary Portillo Edward Pristach

Preview of Conclusions

By using stimuli with strong perceptualgrouping tendencies, we can take many

standard findings in visual perception and

stand them on their heads.

Preview of Conclusions

• If we fail to take perceptual organization

into account, we can encounter some odd-looking results in our experiments.

• These odd results stem from a mismatch

between how the experimenter defines the

stimulus and how the perceiver organizes

the stimulus.

What are the Familiar Findings?

1. Irrelevant or information-free ‘noise’ contexts hurt

2. Search slopes range from nearly flat to positive

3. Redundant information often helps, seldom hurts

4. When targets pop out, we know their location

5. When texture boundaries are apparent, we knowtheir correct location too

6. “Same” judgments are generally easy and fast

7. Incongruent flankers are almost always harmful8. Dual-task performance is at least as hard as single.

Familiar Findings: Positive to

Flat Search Slopes

Very inefficient search>30 msec. / element

E.g., conjunction search

Efficient search or popout

~ 0 msec. / element

E.g., vertical among horizontals

Display Size# elements in field

0 Many

Familiar findings…

Incongruent Flankers Hurt

Task: Is center letter an X or an O?

O X O Eriksen & Eriksen, et al.

Other Familiar Findings in Visual

Information Processing

• Irrelevant information hurts performance,unless it can be filtered out or unless it

boosts arousal, etc.

• Redundant information often helps

performance

• When a target pops out, its location isquickly and accurately known.

Common Findings, continued

• Judging whether two things are the same or different is a basic process that is fast and

accurate.

• Attending to two stimuli at once is harder

than attending to just one.

What are the Strange Effects?

1. Facilitation from information-free contexts2. Steeply negative search slopes

3. Redundancy losses4. Perverse (mislocalized) pop out

5. Mislocalized texture boundaries

6. Missing flanker incongruity effects

7. Better dual- than single-task performance

8. Spurious sameness and dubious difference9. Dimensional blindness?

10. Underestimated numerosity?

1. Facilitation from information-free contexts

2. Steeply negative search slopes

Q: When does adding “noise” to adisplay improve performance?

A: When it produces emergentfeatures that lead to configural

superiority effects.

Facilitation from Information-Free Contexts:

Configural Superiority Effects

Configural Superiority, continued

Target Context

(all identical)

Composite

Important Control:

Configural Inferiority Effects

Pomerantz, Sager, & Stoever, 1977

Classes of CSEs

Familiar CSEs – line patterns

Line slope differences (e.g., arrows and triangles)

Line curvature differences (e.g., parentheses)

New CSEs – dot patterns

Dot orientationDot proximity

Parentheses are just an example...

A New CSE:Orientation

The bottom discrimination is

easier than the one in the upper

left, indicating another CSE, probably based on the emergent

feature of orientation.

Another newCSE:

Proximity

Here, the same target dots

combine with shifted

context dots to create a

CSE based on dot proximity.

Other CSEs…?

There are probably atleast a dozen CSEs

For illustrative purposes,the parenthesis pairswill be used, but theyare just a convenientexample.

Graphic from Pomerantz, OPAM 2001

Steeply Negative Search SlopesDoubling the number of items in a display

from 16 to 32 reduces search time by ~1000 msec.

( ( ( (

( ( ( (( ( ) (

( ( ( (

( ( ( (( ( ( (

( ( ( (

( ( ( (( ( ( (

( ( ( (

( ( ( (( ( ) (

( ( ( (

DS=16 Add 16 more

distractors

Pomerantz, Sager, & Stoever, 1977

Negative Search Slopes

Display Size

# elements in field

Very inefficient search

>30 msec. / element

E.g., conjunction search

Efficient search or popout

~ 0 msec. / element

E.g., vertical among horizontals

Steeply negative slopes,

nearly -200 msec. / element

Causes of Negative Search Slopes• Grouping can lead to errors in reporting the

effective display size (e.g., reporting 32when we should report 16), but this cannot

change the sign of the slope.

• Thus, negative slopes are due to somethingelse, namely, emergent features.

Redundancy Losses

Task 1: Control task, with second (right) channel of information irrelevant and uninformative:

Discriminate ( ) from ) )

Task 2: Correlated task, with information on second

channel redundant so stimuli should be “twice asdifferent”:

Discriminate ( ( from ) )

Result: Performance is worse on Task 2Pomerantz & Garner, 1973

h h ff

3. Redundancy losses

4. Perverse (mislocalized) pop out

Perverse (mislocalized) Pop Out

We have no problem here either spotting the odd

element or knowing where it is in the display.

Question: What if the target item were to group

with one of the background items? What, if

anything, will pop out then?

An example…

Here, mislocalization is common…

Demonstration: Mislocalized pop out

In the following slides, call out quickly the

numerical position of the odd item. E.g.:

X O X => “2”

O X X => “1”, etc.

If an element at a texture boundary groupswith another element on the other side of

that boundary, that should lead to a

misperception of the boundary’s location.

Spot the Texture Border

Fairly easy task…

Easy with color differences too…

Texture Borders with Grouping:

Problems Arise:

T t B d ith G i

Parenthesis Pairs

T t B d ith G i

Arrows & Triangles

Te t re Borders ith Gro ping:

Arrows & Triangles

d i h i

Texture Borders with Grouping: Ks

Texture Borders with Grouping: Ks…

Missing flanker incongruity effectsWe find no flanker (‘Stroop’) interference with

parenthesis pairs:

in speeded classification tasks with one elementrelevant, responses to pairs incongruent neighbors

are no slower than with congruent:Congruent: ( ( and ) )

Incongruent: ( ) and ) (

Note: in contrast,Garner interference is very strong.

Better dual- than single-task

performanceCondensation (conjunction) task is easier than

filtering (feature) task with parentheses:

( ( ) ( ( (

) )( ) ) )

Harder Easier

Pomerantz & Pristach, 1989

Spurious Sameness & Dubious Difference

Idea: if two items are grouped perceptuallyinto one, it should be difficult for perceivers

to judge those items as same or different.Q: Same or different: “( )” ?

This could be a difficult task.Reason 1: there is only one thing

Reason 2: symmetry; convexity/concavity

Pilot data: The hypothesis may be right…

Dimensional blindnessWith some multidimensional stimulus sets, it’s

hard to identify the underlying dimensions

Easy Easy Hard

Dimensional blindnessEven color coding the rightmost panel fails to help:

the nominal dimensions are masked by emergent

features…

Easy Easy Hard

6. Missing flanker incongruity effects7. Better dual- than single-task performance

Underestimated numerosityKey idea:

If items in a display group into a singleconfiguration (or clusters of configurations),

then estimates of the number of elements inthe display should be systematically low.

Evidence: experiments not yet designed…

Implications• Remembering the “Experience Error”

• Conducting and Reporting our Experiments

The Stimulus and Experience ErrorsThe Stimulus and Experience ErrorsIn psychology we have often been warned againstthe stimulus error , i.e., against the danger of confusing our knowledge of the physicalconditions of sensory experience with thisexperience as such. As I see it, another mistake,which I propose to call the experience error , is

just as unfortunate. This error occurs when certaincharacteristics of sensory experience are

inadvertently attributed to the mosaic of stimuli”(Kohler 1929/47, p. 95).

Implication: The Stimulus Error Implication: The Stimulus Error • Denying seeing stripes in the rainbow because

we know they are not there.• Confusing brightness with intensity, or

entering a room lit with one candle,illuminating a second candle, and reportingthat the room is now twice as bright.

The Experience Error The Experience Error

• Seeing stripes in the rainbow and

presuming they are actually there.• Similarly, believing that other

organization resides in the stimulus.• Often ignored today, but given

prominent mention in Palmer’s text.

The Experience Error The Experience Error

Presuming what the stimulus is:

• We experience a percept organizedinto regions, objects, and parts.

• We then assume this organization

exists in the distal world and isavailable in the proximal (retinal)

image, rather than having to becomputed.

A Possible Example of theA Possible Example of the

Experience Error?Experience Error?Object-based Attention:

Easier reporting 2attributes of 1 object than

1 attribute of 2 objects.

Issue: Who says the box is

one object and the line

another?(From Duncan, 1984)

Easy Hard

Why don’t we call the upper

component of the ‘box’one object, and the lower

another? Then there would

be two objects and no‘gaps’ at all!

What about Elements that Group?What about Elements that Group?

Parenthesis Pairs

Group StronglySo Are They A Single

Object or Two Objects?

Implication: The Design of StimuliImplication: The Design of Stimuli• Researchers in object perception must be as

careful as those in color vision not tomisrepresent the stimulus.

• Like ‘red’, ‘object’ is a psychologicalconcept.

• E.g., experimenters must distinguish how

many elements they say are in the targetarray from how many our perceivers see.

In ClosingIn Closing……

• Not an overstatement to say it is our ‘dirtylittle secret’ that none of us knows what an

object is, or what a part or a feature is.• We often guess right, but we’re still guessing.

• Once we understand when and how groupingworks, we will better to know what objects,features, and parts are, and so better know how

attention is allocated to them.

Thanks for Listening.

several strange effects arising from perceptual grouping

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