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A Neural Network Model of Subliminal Priming. Howard Bowman Computing Laboratory, University of Kent at Canterbury. Collaborators : Friederike Schlaghecken (Psychology, Univ Warwick) Adam Aron (Psychiatry, University of Cambridge) Prof. Martin Eimer (Psychology, Birkbeck College)

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a neural network model of subliminal priming
A Neural Network Model of Subliminal Priming

Howard Bowman

Computing Laboratory, University of Kent at Canterbury


Friederike Schlaghecken (Psychology, Univ Warwick)

Adam Aron (Psychiatry, University of Cambridge)

Prof. Martin Eimer (Psychology, Birkbeck College)

Prof. Phil Barnard (Cognition and Brain Sciences Unit)

  • Connectionism Now
  • Subliminal Priming
  • Eimer and Schlaghecken Pattern Masked Priming
  • Lateral Inhibition and Opponent Processes
  • The Model
  • Predictions and Future Directions
connectionism the claim
Connectionism - The Claim
  • Connectionism claims to offer a bridge between cognition and the brain.
  • How complex behaviour emerges from the interaction of a multitude of computationally unsophisticated (one might even say dumb) units.
is this claim justified
Is this claim justified?
  • Arguments against Connectionism
    • Neural networks are not actually biologically plausible, e.g. problems with backpropagation
    • How are computational implementations related to psychological theories? Does the model work because of the theory it realises or because of hidden implementation assumptions?
    • Models can do anything, e.g. backpropagation can learn any computable function!
connectionism now
Connectionism Now
  • Biological plausibility
    • all models are abstractions
    • connectionist abstractions are becoming more grounded, e.g. spiking neurons and biologically plausible learning
      • re-circulation algorithms instead of backpropagation
      • Hebbian learning, sparse representations and sparsification techniques
reducing the degrees of freedom
Reducing the degrees of freedom
  • work closely within context of existing cognitive theory;
  • apply biological constraints;
  • systematic and transparent parameter setting (e.g. do statistics on model);
  • make clear the key mechanisms involved;
  • testable predictions from the models (especially counter-intuitive ones).
theoretical background
Theoretical Background
  • Two central theoretical issues
    • the role of conscious control in visuomotor performance
    • the “cognitive levels” at which inhibition functions
issue 1 consciousness and visuomotor performance
Issue 1: Consciousness and Visuomotor Performance
  • Tight coupling of vision and action
    • actions planned on basis of visual information
    • action execution guided by vision
  • The debate,
    • “To what extent is conscious experience a prerequisite for the control of visuomotor performance?”
    • “Is there a direct, below conscious, link from vision to action?”

Direct Parameter



[Neumann & Klotz,94]

tentative support for direct parameter specification 1
Tentative Support for Direct Parameter Specification (1)
  • Blindsight [Weiskrantz et al,74]
    • above chance visual guided action in absence of visual awareness
  • Visual Illusions [Carey,01]
    • movements (e.g. grasp apertures) resist visual illusions
  • Visual Form Agnosia [Milner et al,91]
    • profound deficits in object recognition, but intact visuomotor performance

Note: dissociation

dorsal (where) stream

ventral (what) stream

but subcortical routes

also significant

support for direct parameter specifiction 2
Support for Direct Parameter Specifiction (2)
  • More direct evidence provided by masking experiments. Two varieties,
    • Metacontrast masked priming
    • Pattern masked priming
metacontrast masked priming
Metacontrast Masked Priming
  • Fehrer & Raab (1962), Neumann & Klotz (1994), Vorberg (2002)
  • For example, [Neumann & Klotz,94]
    • subjects not told of presence of prime
    • subjects either respond to diamonds or squares
    • respond left or right depending upon target position
    • target stimulus metacontrast masks the prime
    • strict criteria for perception of prime - signal detection
  • Positive compatibility results,
    • compatible trials yield behavioural benefits (both errors and reaction times)
    • incompatible trials yield behavioural costs
signal detection blocks
Signal Detection Blocks
  • Signal detection blocks follow reaction time blocks (rules out learning)
  • subjects asked to state whether prime present on 5 point scale, e.g. “I am pretty sure that prime was present”
  • signal detection gives d-prime statistically equivalent to zero, i.e. no phenomenological experience of prime
  • “.. a stimulus can have access to the motor system and activate or even start an intended, planned response without being represented in consciousness.” [Neumann & Klotz,94]
  • Note further, this preconscious processing requires integration of form (diamonds vs squares) and position (left vs right) information. More than just a presence / absence judgement.

what - where dissociation??

issue 2 levels of inhibition
Issue 2 - Levels of Inhibition
  • Inhibitory mechanisms certainly ubiquitous in brain, e.g. GABAergic interneurons throughout cortex and subcortical regions.
  • Typically, psychological theories situate inhibitory mechanisms at level of attentional (executive) function, e.g.
    • (working memory) Baddeley’s central executive
    • Shallice’s Supervisory Attentional System

Inhibition as a

(pre)frontal lobe


inhibition and task set shifting
Inhibition and task / set shifting
  • Neuropsychological example - frontal lobe damage yields perseveration errors (e.g. Wisconsin Card Sorting task) and increased distractibility.
  • Psychological example - negative priming (conscious inhibition of distractors)
  • Inhibition argued to be central to conscious attentional control
  • QUESTION: could inhibition arise at level of direct parameter specification?
eimer and schlaghecken s pattern masked priming task
Eimer and Schlaghecken’s Pattern Masked Priming Task

A subliminal priming paradigm

  • response buttons under left and right index fingers;
  • basic stimuli -
  • neutral stimuli - <> and ><; and
  • mask - superimposition of >> and <<

>> (right response) and

<< (left response);

(other masks explored, e.g. classic pattern masks)


16 ms


Prime is subliminal

(verified by forced

choice blocks, which

follow RT-blocks.)


100 ms




100 ms





  • negative compatibility;
  • behavioural costs on compatible trials and benefits on incompatible trials;
  • candidate explanation: inhibitory processes at work;
  • suppression of response activation (even before response fires);
  • supporting evidence from EEG study.
further implications
Further Implications
  • LRP shows it is more than just sensory priming, i.e. residual perceptual activation. Prime induced activation propagated right through to response systems.
candidate explanation
Candidate Explanation
  • low-level inhibitory mechanism
  • “emergency brake” - suppress response once visual evidence for that response removed
  • possibly part of a larger “clearing-up” mechanism - suppress activation traces of completed responses in order to enable sequences of co-ordinated actions.
data to reproduce
Data to reproduce
  • short mask-target SOAs (0-32ms) yield positive compatibility (target and mask presented together)
  • longer mask-target SOAs (64ms - 150ms (ish)) yield negative compatibility
  • low strength primes yield positive compatibility. Reduce strength by,
    • presenting prime in periphery, or,
    • presenting prime centrally, but overlaid with random-dot degradation
further data to reproduce
Further Data to Reproduce
  • forced choice - at chance
    • forced choice blocks both with and without target
    • forced choice blocks follow reaction time response blocks (thus, learning to detect prime not an explanation)
    • QUESTION?? - how can priming affect target response speeds but not forced choice judgements?



theory not used.

further lrp data
Further LRP Data


and Eimer


mechanism 1 competition and masking
Mechanism (1) - competition and masking
  • masked and masking stimuli compete for shared neural resources, see [Keysers & Perrett,02]
  • neural trace of prime rapidly suppressed when mask presented
  • implementation possibilities,
    • lateral inhibition
    • gating mechanism
mechanism 2 response competition
Mechanism (2) - response competition
  • responses compete in a winner take all fashion.
  • only one response can be executed.
  • sustains (in fact, accentuates) response separation, c.f. M00 condition.
  • implemented through lateral inhibition between response nodes.



response 2

response 1

mechanism 3 opponent processes
Mechanism (3) - Opponent Processes
  • previously investigated in a number of models, e.g.
    • negative priming and inhibition of return [Houghton & Tipper,94]
    • serial order in working memory and in motor action sequencing [Houghton,90]
an opponent circuit
An Opponent Circuit

Excitatory Link




OFF Node

Inhibitory feedback

can be threshold


Inhibitory Link

OFF node just an

inhibitory interneuron

mechanism 4 s r binding
Mechanism (4) - S-R Binding
  • nodes in relevant stimulus-response pathways pre-activated and hence foregrounded from the set of possible S-R bindings
  • called response-set delineation in [Bowman et al,02]
  • implemented by giving backgrounded nodes a strongly negative bias
the network

Excitatory Gating

Links: Links:

Inhibitory Links:

Perceptual Pathways:

apply time averaging

to perceptual input

The Network

Response Selection:

difference between

response node activation



Example (left


1 cycle of <<;

6 cycles of

the mask;

6 cycles of <<.




















Response Selection


formal parameters
Formal Parameters

Time averaging activation function:

input to node on cycle i

regulates time



on cycle i+1


(squashes activation into

range 0 to +1)

t set to 0.3

basic results
Basic Results
  • difference between response nodes gives separation;
  • similar pattern to LRP.
response time comparison
Response Time Comparison

(mean response times)


(i) one cycle corresponds to 16.6666 ms;

(ii) selection criteria -

separation magnitude (absolute value) - 0.4;

(iii) latency of 200 ms compared to Eimer data.


RTs currently



reduced strength prime
Reduced Strength Prime
  • Prime induced response activation does not cross opponent circuit threshold;
  • reproduces basic switch to positive compatibility
observations i
Observations (i)
  • model RTs in right general ball park (parameter optimizations would improve these);
  • RT difference between conditions is good;
  • time course of separation close to LRP profile.
observations ii
Observations (ii)
  • explanation of forced choice results,
    • selection criteria is the model’s analogue of super / subliminality threshold (simplification since just located at action end).
    • while selection criteria not satisfied, no evidence available for decision process, i.e. at chance
    • residual activation from prime only affects outcome if it is built upon (since it influences speed with which threshold (selection criteria) is crossed)
    • one reason for selection criteria is to ensure background fluctuations do not yield overt responses
relationship to houghton and tipper model
Relationship to Houghton and Tipper Model
  • inhibition modulated by high level attentional processes in HT94;
  • selection of target from distractor in negative priming;
  • orienting system in inhibition of return;
  • our model - a direct (low level) link from perception to action - inhibition is a “dumb” mechanism (not directed by high-level attention).
further work
Further Work
  • Biological plausibility - fMRI studies suggest basal ganglia as locus of inhibition
  • broaden scope of model - locate pattern and metacontrast masked priming in same modelling framework
  • modelling error rates
  • Masked priming data fits with a theory of consciousness in which,
    • “when we apperceive the stimulus, we have usually already started responding to it; our motor apparatus does not wait for consciousness, but does restlessly its duty, and our consciousness watches it and is not entitled to order it about.” [Munsterberg,1889]
  • Two largely independent effects of a stimulus,
    • determines a motor response
    • has later effect in consciousness

Also see Libet’s

work and implicit

memory literature

further conclusions
Further Conclusions
  • Effects not restricted to specific stimulus-response connections [Neumann & Klotz,94]
    • rapidly modified through instruction cues
    • cannot be explained through automaticity
discussion points
Discussion Points
  • Why this model?
  • Clearly (even at psychological level) there are many models which could satisfy the data (even infinitely many);
  • In favour of model,

+ Simple and canonical;

+ Opponent networks used to model a number of

inhibitory phenomena;

+ Increasing body of empirical data explained.