the mechanisms of subliminal stimuli: a meta-analysis and new

Laboratory of Experimental PsychologyKULeuven - Campus Kortrijk

General doctoral seminar 16/04/2008

The mechanisms of subliminal stimuli: A meta-analysis and new experimentsEva Van den BusscheBert Reynvoet & Wim Van den Noortgate

Overview• PART I: the moderators of masked semantic priming

– Study 1: Masked priming effects in semantic categorization are independent of category size

– Study 2: Masked semantic priming: a picture prime study– Study 3: Mechanisms of masked semantic priming: a meta-

analysis • PART II: conscious versus unconscious processing

– Study 4: Conscious and unconscious proportion effects in masked priming

– Study 5: The interaction between consciousness and attention: an empirical study using the priming paradigm

• General conclusions

Part I• PART I: the moderators of masked semantic

priming– Introduction– Study 1: Masked priming effects in semantic

categorization are independent of category size – Study 2: Masked semantic priming: a picture prime

study– Study 3: Mechanisms of masked semantic priming: a

meta-analysis– Conclusions

Introduction• Can unconsciously presented

information influence our behaviour?Subliminal advertising:– “Drink Coca-cola, Eat popcorn” James

Vicary (1957)– McDonald’s

Introduction• Can unconsciously presented information

influence our behaviour?Masked priming paradigm (Marcel, 1983)

– Masked/subliminal/unconscious semantic priming~> an unidentifiable visually masked stimulus (prime) facilitates the subsequent semantic classification of a visible related stimulus (target)

– Priming effect ~> improved responding when primes are congruent/related to targets, relative to when prime-target pairs are incongruent/unrelated

Introduction• Example: is target smaller or larger than 5?

Congruent/related trial

Incongruent/unrelated trial

Target 1 9

Prime 4 4

Both < 5Faster RTs

One <, one > 5Slower RTs

Introduction• Can unconsciously presented

information influence our behaviour? – Masked priming paradigm (Marcel, 1983)– Mid-1990s: subliminal perception exists!

YES!• BUT…

Introduction• But…• How deep can subliminal stimuli be

processed? – Dehaene et al. (1998): semantically

processed semantic account

Introduction• Semantic account, congruent trial:

Smaller than 5!

Response = press left

Congruent faster RT!

Prime4

Target1 Smaller than 5!

Introduction• Semantic account, incongruent trial:

Smaller than 5!

Incongruent slower RT!

Prime4

Target9 Larger than 5!

Response = press right

Introduction• How deep can subliminal stimuli be

processed? – Dehaene et al. (1998): semantic account– Damian (2001): NOT semantically

processed, but due to S-R mappings S-R account

Introduction• S-R account, repeated primes, congruent trial:

Smaller than 5!

Prime4

Target1

Introduction• S-R account, repeated primes, incongruent trial:

Incongruent slower RT!

Prime4

Target9

Response = press RIGHT

Introduction• S-R account, novel primes:

Prime3

Target1

processed? – Dehaene et al. (1998): semantic account– Damian (2001): S-R account– Recent findings: significant priming for

novel primes semantically processed!– Kunde et al. (2003): NOT semantically

processed, but due to action triggers action trigger account

Introduction• Action-trigger account, congruent trial:

Congruent faster RT

Target1

Task: categorize numbers

between 1 and 9 as smaller or larger than 5

Action triggers for: 1,2,3,4 = press left6,7,8,9 =press right

Prime3

Introduction• Action-trigger account, incongruent trial:

Incongruent slower RT

Target9

Task: categorize numbers

between 1 and 9 as smaller or larger than 5

Action triggers for: 1,2,3,4 = press left6,7,8,9 =press right

Prime3

Introduction• Action-trigger account, large categories:

Congruent faster RT

Targetcat

Task: categorize words

as animals or objects

Action triggers for: Dog, cat,…= press leftBall, pot,…= press right

Primecow

processed? – Dehaene et al. (1998): semantic account– Damian (2001): S-R account– Kunde et al. (2003): action trigger account– Recent findings: significant priming for

large categories and large target sets semantically processed!

processed? – ???– Contradictory research findings– Diverging theories

Introduction• Which factors moderate masked priming

effects?- Task- Prime novelty- Category size- Target set size- Nature of primes and targets- Prime duration and SOA- Masking- Prime visibility- Sample size- Population

Aims1. Assess whether subliminal stimuli are

semantically processed

2. Assess the influence of potential moderators on subliminal semantic priming effects

Study 1

Study 1: Masked priming effects in semantic categorization are independent of category size

Van den Bussche & Reynvoet, 2007

Introduction1. Semantic account: priming effects, no matter

what2. S-R account: priming effects only for repeated

primes and small target sets3. Action trigger account: priming effects only for

small categories and small target setsGoal = Clarify the role of category size and target set size on subliminal priming effects using novel primes

Design

Larger than dog

Fig. 2

Incongruent trial

######

Smaller than dog

Fig. 1

Congruent trial

######

Larger than dogSmaller than dog

Design• In all 3 experiments, targets never

appeared as primes (novel primes)• Prime visibility was always objectively

measured “apply same task to prime”d’ measures never differed from 0!

Design• Experiment 1(N = 16): 3 separate parts:

– 1a: categorize numbers as smaller or larger than 5 (small category, small target set)

– 1b: categorize body parts as below or above the pelvis (small category, small target set)

– 1c: categorize animals as smaller or larger than a dog (large category, small target set)

• Experiment 2 (N = 14): categorize words as animals or objects (large category, large target set)

• Experiment 3 (N = 16): categorize words as animals or not (large category, large target set)

Results

Experiment 1a Experiment 1b Experiment 1c

(ms) Congruent

Incongruent

Fig. 3

20 ms, p < .001

25 ms, p < .001

8 ms, p < .05

Experiment 1

Results

Experiment 2: animal targets Experiment 2: object targets

(ms) Congruent

Incongruent

Experiment 3: animal targets Experiment 3: non-animal targets

(ms) Congruent

Incongruent

Fig. 5

Experiment 3

11 ms,p < .01

Fig. 4

Experiment 2

15 ms,p < .001

Conclusions• Significant priming effects for novel

primes, across different tasks, irrespective of category size and target set size!

• Evidence in favor of the semantic account ~> subliminal primes are semantically processed!

Study 2

Study 2: Masked semantic priming: a picture prime study

Van den Bussche, Notebaert & Reynvoet, in preparation

Introduction• Abrams (in press) priming effects of

Van den Bussche & Reynvoet (2007) did not stem from semantic processing of the primes, but from subword processing, since substantial orthographic overlap between primes and targets was present (e.g. been – teen, kat – rat)

Design• N = 21• Task: categorize words as animals or not

(large category, large target set)• To eliminate overlap: use picture primes

and word targets!

Design

Fig. 6

Congruent trial

AnimalNo animal

400ms +######

######

4x13ms

Design

Fig. 7

Incongruent trial

AnimalNo animal

400ms +######

horse4x13ms

4x13ms ######

Results and conclusions• Significant priming effect of 9 ms (F(1,

20) = 15.65, p = .001)Even when overlap is eliminated, significant priming was still observed for a large category and large target setEvidence in favor of the semantic account ~> subliminal primes are semantically processed!

Study 3

Study 3: Mechanisms of masked semantic priming: a meta-analysis

Van den Bussche, Van den Noortgate & Reynvoet, submitted

AimsStatistically combine published and unpublished data using meta-analytic techniques to:

1. Assess whether the literature to date provides clear evidence in favor of semantic processing of subliminal information.

2. Assess the influence of potential moderatorson subliminal semantic priming effects

Moderators• Which factors moderate masked priming

effects?- Task- Prime novelty- Category size- Target set size- Nature of primes and targets- Prime duration and SOA- Masking- Prime visibility- Sample size- Population

Method• Search criteria:

– Published between 1998 – 2006– Language: English, Dutch, French or German– Unconscious semantic priming

• Search string:– (SEMANTIC OR ASSOCIATIVE) AND

(PRIMING OR PRIME) AND (MASKED OR SUBLIMINAL OR UNCONSCIOUS OR AUTOMATIC)

Method• Four databases were searched

– Web of Science– ScienceDirect– PubMed– PsycInfo

• Cross-references, reviews, experts• 477 published articles were selected• Two independent reviewers coded these

articles

Method• Inclusion criteria:

1. Prime-target relation of a semantic nature in the visual domain (e.g. cat - dog)

2. Primes had to be presented subliminally3. Semantic categorization, lexical decision or naming

task4. Standard priming procedure 5. Centrally presented single word or symbol primes 6. Healthy sample 7. Sufficient statistical information to compute an effect

Method• 37 published and 8 unpublished studies

containing 135 separate experiments / conditions

• Effect sizes were computed:

RT (unrelated trials) – RT (related trials)ES =

SD– ES = 0: no effect– ES > 0: positive ES (~ priming effect)

Method

• 3 standard tasks• 2 separate meta-analyses for:

– Conditions using a semantic categorization task

– Conditions using a lexical decision or naming task

Introduction• Semantic categorization

congruent trial: incongruent trial:

Animal!

Primecat

Targetcow Animal!

Animal!

Primecat

Targettable Object!

Congr faster RT

Incongrslower RT

Congr faster RT

Incongr slower RT

More relatedfaster RT

Less relatedslower RT

Introduction• Lexical decision

related trial: unrelated trial:

Primecat

Targetcow Word!

Primecat

Targettable Word!

More relatedfaster RT

Less relatedslower RT

MethodImportant discrepancy in processing of congruent/incongruent trials! Furthermore, different set of moderators2 separate meta-analyses for:

– Conditions using a semantic categorization task

– Conditions using a lexical decision or naming task

Meta-analysis 1

Meta-analysis 1:Semantic categorization conditions

MA1: Descriptives22 studies containing 87 conditions (k = 87)

PopulationStudents: k = 62, Adults: k = 25

Prime duration M = 42, SD = 70.7, range = 17-72

TargetsSymbols: k = 34, Words: k = 37, Both: k = 16

Obj/SubjObj: k = 56, Subj: k = 10, Both: k = 7 (k = 73)

SOA M = 96, SD = 35.0, range = 41-273

MaskingBM and FM: k = 70, BM: k = 1, FM: k = 16

No: k = 14, Yes: k = 73

M = 0.19, SD = 0.20, range = -0.06-0.66 (k = 58)

Visibility measured

N M = 20, SD = 12.1, range = 6-80

PrimesSymbols: k = 32, Words: k = 37, Both: k = 18

NoveltyRepeated: k = 40, Novel: k= 43, Both: k = 4

Category size Small: k = 44, Large: k = 43

Target set size M = 19, SD = 22.8, range = 4-90

MA1: Effect sizes

Fig. 8

Observed effect sizes

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88

Experiment

MA1: Empty model• Overall mean ES = 0.79 (SE = 0.10)• Significant differences between studies• Significant differences between

conditions, within studiesLook for moderators!

MA1: Main effects (1)• Prime novelty (F(2,65.8) = 14.21, p < .0001)

Fig. 9

Observed effect sizes for novel and repeated primes

Repeated primes Novel primes Both

ES = 1.07, p < .0001

ES = 0.70, p = .006

ES = 0.54, p < .0001

MA1: Main effects (2)• Category size (F(1,14.8) = 47.63, p < .0001)

Fig. 10

Observed effect sizes for small and large categories

Small category Large category

ES = 0.35, p = .0003

ES = 1.08, p < .0001

MA1: Main effects (3)

• d’ (β = 1.04, F(1,54.1) = 8.09, p = .006)• Positive slope higher visibility ~ higher

ES• BUT: at intercept (where d’ = 0), still

significant ES (β = 0.44, t(14.3) = 3.73, p= .002)

MA1: Interaction effects and multiple models

• 2-way interaction effects: none were significant (p-values ranging from .20 to .93)

• Add several main effects in one model • Using several strategies, the same optimal

model always emerged: a model containing prime novelty and category size (adding d’slightly improved the model)

MA1: Publication bias• Meta-analysis: published AND unpublished data!

Fig. 11

-1 -0,5 0 0,5 1 1,5 2 2,5 3

MA1: Conclusions

Less priming for novel primes, large categories and completely invisible primesStill, significant priming was always observed, even under circumstances where S-R accounts are less likely (for novel primes and large categories)Subliminal primes can be processed semantically

Meta-analysis 2

Meta-analysis 2:Lexical decision and naming

conditions

MA2: Descriptives24 studies containing 48 conditions (k = 48)

PopulationStudents: k = 38, Adults: k = 10

Task Lexical decision: k = 39, Naming: k = 9

TargetsSymbols: k = 9, Words: k = 39

Visibility measured No: k = 21, Yes: k = 27

d’ M = 0.05, SD = 0.04, range = -0.01-0.08 (k = 4)

Prime duration M = 51, SD = 17.5, range = 13-84

SOA M = 91, SD = 67.0, range = 33-340

BM and FM: k = 26, BM: k = 5, FM: k = 13, none: k = 4

Obj: k = 19, Subj: k = 6, Both: k = 2 (k = 27)

Masking

Obj/Subj

N M = 37, SD = 29.1, range = 11-132

PrimesSymbols: k = 2, Words: k = 46

NoveltyRepeated: k = 2, Novel: k= 46

Category sizeSmall: k = 2, Large: k = 38 (k = 40)

Target set size M = 117, SD = 109.1, range = 6-320 (k = 40)

MA2: Effect sizes

Fig. 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Experiment

MA2: Empty model• Overall mean ES = 0.46 (SE = 0.06)• Significant differences between studies• NO significant differences between

conditions, within studiesLook for moderators!4 variables could not be included (nature of primes, prime novelty, category size and d’)

MA2: Main effects

• Target set size (β = 0.002, F(1,24.3) = 25.24, p < .0001)

• Positive slope more targets ~ higher ES

• BUT: at intercept, still significant ES (β = 0.26, t(15.3) = 4.27, p = .0006)

MA2: Interaction effects and multiple models

• 2-way interaction effects: none were significant (p-values ranging from .60 to .74)

• Add several main effects in one model• Using several strategies, the same

optimal model always emerged: a model containing only target set size

MA2: Publication bias• Meta-analysis: published AND unpublished data!

Fig. 13

-1 -0,5 0 0,5 1 1,5 2

MA2: Conclusions

4 variables (nature of primes, prime novelty, category size and d’) could not be included in the meta-analysis: caution warranted! Significant priming was observed, even under circumstances where S-R accounts are less likely (lexical decision and naming) Subliminal primes can be processed semantically

MA1+MA2: Conclusions• Can unconscious stimuli be processed in depth?

– YES! Priming effects are found in conditions where S-R accounts are unlikely (naming/lexical decision tasks, semantic categorization with large categories and/or novel primes)

– BUT, S-R accounts can also play a prominent role (significantly smaller ES for naming/lexical decision tasks, novel primes, primes from large categories)

These findings reconcile the two reigning theories regarding subliminal semantic priming: when given a chance, automatic S-R effects will clearly boost the priming effects. However, when S-R influences are minimized or avoided, subliminal information can be genuinely semantically processed!

Part I: Conclusions• Can subliminal stimuli influence our behavior?

YES!• Can subliminal stimuli be processed semantically?

YES! Significant priming effects are found in conditions where S-R accounts are unlikely:– Naming and lexical decision tasks– semantic categorization tasks with large categories,

large target sets and novel primes• Priming effects are moderated by several factors for

semantic categorization (prime novelty, category size) and lexical decision/naming (target set size)

Part II• PART II: conscious versus unconscious

processing– Introduction– Study 4: Conscious and unconscious

proportion effects in masked priming – Study 5: The interaction between

consciousness and attention: an empirical study using the priming paradigm

– Conclusions

Introduction• Understand unconscious processing by

contrasting it to conscious processing• Look at the effect of a manipulation at

the conscious vs. the unconscious level on priming effects– Study 4: proportion of notations– Study 5: attention

Study 4

Study 4: Conscious and unconscious proportion effects in masked

priming

Van den Bussche, Segers & Reynvoet, in press

Introduction• Global Neuronal Workspace theory (Dehaene

& Naccache, 2001) 2 assumptions:1. A fully consciously perceived stimulus can be

used strategically to improve task performance- Top-down manipulations can have an effect on

subliminal processing (e.g. spatial and temporal attention, the target set and target notation)

- E.g. Kunde et al. (2003)Hypothesis: A manipulation on target level will influence observed priming effects!

Introduction2. An unconscious stimulus can NOT be

used strategically to improve task performance- Dehaene et al. (2006): unconscious

primes have insufficient strength to result in a bottom-up effect

- Never explicitly investigated!Hypothesis: A manipulation on prime level will NOT influence observed priming effects!

Aims• Investigate the 2 assumptions of the

GNW by manipulating the proportion of Arabic numbers (e.g. 1) versus number words (e.g. “one”): – Consciously (target level): experiment 1– Unconsciously (prime level): experiment 2

• All other task conditions were controlled

74Fig. 14

Congruent trial

#$#$#$

$#$#$#

Larger than 5Smaller than 5

Design

Larger than 5

Fig. 15

Incongruent trial

#$#$#$

$#$#$#

Smaller than 5

Design• Exp1: manipulation of the proportion Arabic

targets vs. number word targets• 3 conditions (N = 46):

– 1a: 50% Arabic / 50% number word targets– 1b: 25% Arabic / 75% number word targets– 1c: 75% Arabic / 25% number word targets

• Proportion Arabic primes vs. number word primes was always identical (50% / 50%)

Design• Exp2: manipulation of the proportion Arabic

primes vs. number word primes• 3 conditions (N = 47):

– 2a: 50% Arabic / 50% number word primes– 2b: 25% Arabic / 75% number word primes– 2c: 75% Arabic / 25% number word primes

• Proportion Arabic targets vs. number word targets was always identical (50% / 50%)

Predictions• Exp1: priming effect for a certain prime

notation (Arabic numbers or number words) will be larger if the proportion of targets in that notation is large as opposed to when the proportion of targets in that notation is small

• Exp2: manipulation of the proportion of Arabic/number word primes will have no effect and thus no differences in are expected between the priming effects observed for the 3 conditions of experiment 2

Results

1a: baseline 1b: 75% numberw ord targets

1c: 75% Arabictargets

2a: baseline 2b: 75% numberw ord primes

2c: 75% Arabicprimes

Condition

Arabic primes Number word primes

F(2,43) = 3.70, p = .03 F(2,44) = 0.22, p = .81

Fig. 16

Conclusions• The results are in line with the 2

assumptions of the GNW theory:– The weak activation evoked by an

unconscious stimulus is insufficient to be used strategically to enhance task performance

– A fully consciously perceived stimulus can be used strategically to enhance task performance

Study 5

Study 5: The interaction between consciousness and attention: an empirical study using the priming

paradigm

Van den Bussche, Hughes & Reynvoet, in preparation

Introduction• Dehaene et al. (2006): Conscious,

preconscious and subliminal processing: a testable taxonomy

• Both bottom-up stimulus strength (conscious vs. unconscious) and top down attention (attended or not) will define these different brain states

Introduction

Fig. 17

Aims• “A testable taxonomy” test it!• Can we distinguish these different brain

states? Do we observe differential priming pattern for these different states?

• Behavioral study using the masked priming paradigm

Design• N = 52• Categorize numbers as smaller or larger than

5 • Stimuli could appear on 2 locations• A cue always indicated the target location• The prime could appear on the same place as

the target (i.e. attended) or on the other place (i.e. not attended)

• The prime could either be presented subliminally or clearly visible

Design

Fig. 18

Congruent trial, subliminal condition

480ms ####

Design

Fig. 19

Congruent trial, conscious condition

480ms ####

Design• This design created 4 conditions:

Unattended Attended

Subliminal Subliminal, unattended

Subliminal, attended

Conscious Conscious, unattended

= “preconscious”

Conscious, attended

Predictions

Unattended Attended

Subliminal Subliminal, unattended

Subliminal, attended

Conscious Conscious, unattended

Conscious, attended

Results

Fig. 20

subliminal

conscious

not attended attended

10 ms*11 ms**

50 ms***

Results1. No priming for subliminal, unattended

primes2. Priming for all other states3. Attention modulates the observed

priming effects 4. Consciousness modulates the observed

priming effects

Conclusions• Completely in line with the taxonomy of

Dehaene and colleagues• Significant priming can be obtained for

attended, subliminal primes• Significant priming can be obtained for

unattended, conscious primes Receiving either attention or being consciously perceived is sufficient for a prime to elicit significant priming

Part II: Conclusions• Top-down manipulations (manipulation of

proportion target notations, attention) influence subliminal priming effects

• Subliminal primes have insufficient bottom-up strength to influence subliminal priming effects

• For subliminal primes, attention is a prerequisite in order to elicit significant priming

General Conclusions• Subliminal primes can be semantically

processed• Several variables moderate the emergence of

subliminal priming effects• Top-down influences have an impact on

subliminal priming, but subliminal primes themselves have insufficient strength to influence priming effects

• Attention is a prerequisite for subliminal priming

Contact Info• Eva Van den Bussche

KULeuven - Campus KortrijkE. Sabbelaan 53 8500 Kortrijk BelgiumTel.: +32 56 246074Fax: +32 56 246052 Eva.Vandenbussche@Kuleuven-kortrijk.beMore info: http://www.kuleuven-kortrijk.be/~u0050367/

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