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A Neural Network Model of Subliminal Priming

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Prof. Phil Barnard (Cognition and Brain Sciences Unit) Contents. Connectionism Now ... Visual Illusions [Carey,01] movements (e.g. grasp apertures) resist ... – PowerPoint PPT presentation

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Title: A Neural Network Model of Subliminal Priming


1
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)
2
Contents
  • Connectionism Now
  • Subliminal Priming
  • Eimer and Schlaghecken Pattern Masked Priming
  • Lateral Inhibition and Opponent Processes
  • The Model
  • Predictions and Future Directions

3
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.

4
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!

5
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

6
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).

7
Theoretical Background
  • Two central theoretical issues
  • the role of conscious control in visuomotor
    performance
  • the cognitive levels at which inhibition
    functions

8
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
9
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
10
Support for Direct Parameter Specifiction (2)
  • More direct evidence provided by masking
    experiments. Two varieties,
  • Metacontrast masked priming
  • Pattern masked priming

11
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

12
Neumann and Klotz Metacontrast Priming Paradigm
Neumann Klotz,94
13
Results
  • Positive compatibility results,
  • compatible trials yield behavioural benefits
    (both errors and reaction times)
  • incompatible trials yield behavioural costs

14
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

15
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??
16
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) Baddeleys central executive
  • Shallices Supervisory Attentional System

Inhibition as a (pre)frontal lobe function?
17
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?

18
Eimer and Schlagheckens Pattern Masked Priming
Task
A subliminal priming paradigm
  • response buttons under left and right index
    fingers
  • basic stimuli -
  • neutral stimuli - ltgt and gtlt and
  • mask - superimposition of gtgt and ltlt

gtgt (right response) and ltlt (left response)
(other masks explored, e.g. classic pattern masks)
19
16 ms
NOTE Prime is subliminal (verified by
forced choice blocks, which follow RT-blocks.)
gtgt
100 ms
PRIME
gtgt
gtgt
100 ms
MASK
gtgt
Time
TARGET
20
Conditions
21
Eimer 99 - Results
22
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.

23
Eimer 99 - LRPs
24
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.

25
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.

26
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

27
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.
28
Further LRP Data
Schlaghecken and Eimer data
29
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

30
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
31
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

32
An Opponent Circuit
Excitatory Link
Response Node
(Opponent) OFF Node
Inhibitory feedback can be threshold gated
Inhibitory Link
OFF node just an inhibitory interneuron
33
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

34
The Network
Perceptual Pathways apply time averaging to
perceptual input
Response Selection difference between response
node activation
Mask/ Neutral
Example (left compatible) 1 cycle of ltlt 6
cycles of the mask 6 cycles of ltlt.
1
ltlt
LEFT ON
5
7
OFF
14
2
gtgt
8
6
OFF
15
3
RIGHT ON
Perception Layer
Perceptual Pathways
Response Selection Layer
35
Formal Parameters
Time averaging activation function
input to node on cycle i
regulates time averaging
activation on cycle i1
Sigmoidal, (squashes activation into range 0 to
1)
t set to 0.3
36
Basic Results
  • difference between response nodes gives
    separation
  • similar pattern to LRP.

37
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!
38
Reduced Strength Prime
  • Prime induced response activation does not cross
    opponent circuit threshold
  • reproduces basic switch to positive compatibility

39
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.

40
Observations (ii)
  • explanation of forced choice results,
  • selection criteria is the models 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

41
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).

42
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

43
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 Libets work and implicit memory
literature
44
Further Conclusions
  • Effects not restricted to specific
    stimulus-response connections Neumann
    Klotz,94
  • rapidly modified through instruction cues
  • cannot be explained through automaticity

45
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.
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