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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

Collaborators:

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)

contents
Contents
  • 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

Specification

Hypothesis

[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
results
Results
  • 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
implications
Implications
  • “.. 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

function?

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)

slide19

16 ms

NOTE:

Prime is subliminal

(verified by forced

choice blocks, which

follow RT-blocks.)

>>

100 ms

PRIME

>>

>>

100 ms

MASK

>>

Time

TARGET

implications22
Implications
  • 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?

NOTE:

Signal-detection

theory not used.

further lrp data
Further LRP Data

Schlaghecken

and Eimer

data

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.

excitatory

inhibitory

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

Response

Node

(Opponent)

OFF Node

Inhibitory feedback

can be threshold

gated

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

Mask/

Neutral

Example (left

compatible):

1 cycle of <<;

6 cycles of

the mask;

6 cycles of <<.

1

<<

LEFT ON

5

7

OFF

14

2

>>

8

6

OFF

15

3

RIGHT ON

Perception

Layer

Perceptual

Pathways

Response Selection

Layer

formal parameters
Formal Parameters

Time averaging activation function:

input to node on cycle i

regulates time

averaging

activation

on cycle i+1

Sigmoidal,

(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)

Assuming:

(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.

NOTE:

RTs currently

very

approximate!

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
conclusions
Conclusions
  • 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.