BINOCULAR RIVALRY

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BINOCULAR RIVALRY
A HIERARCHICAL MODEL FOR VISUAL COMPETETION
Computational Evidence for a rivalry hierarchy in
vision Wilson, PNAS (2003), Vol 100 (24),
14499-14503
Shantanu Jadhav Computational Neurobiology UCSD
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Outline

• What is the Binocular Rivalry the cognitive
  phenomenon
• Characteristics Psychophysical features
• Experimental data and evidence
• The model
• - What it tries to explain
• - Implementation
• - Results
• - Predictions and limitations

Lecture 1 Benefits of Computational Models -
New explanations for cognitive phenomena - Tie
explanations of cognitive phenomena to the
biological mechanisms
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BINOCULAR RIVALRY

• A class of phenomena characterized by
  fluctuating perceptual experience in the face of
  unvarying visual input.
• Bistability as a result of ambiguous information
  dissimilar images presented to the two eyes.
• Competition between the two images for
  perceptual dominance.
• Dissociation between unchanging physical
  stimulation and fluctuating conscious awareness
  gt A model for studying the neural basis of
  conscious visual awareness.

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Blake and Logothetis, Nat Rev Neuro, 2002, Vol3
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Perceptual Characteristics
Temporal Dynamics

• Fluctuations in dominance and suppression are
  not regular.
• No voluntary control over fluctuations
• Stimulus strength, attention and visual context
  influence dominance periods.
• Dominance and suppression rely on distinct
  neural processes.
• Successive durations of perceptual dominance
  conforms to gamma distribution (universal
  phenomenon in bistable percepts).

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Spatial Features

• Inter-ocular grouping during dominance gt Not
  just suppression of an eye. (Also, figural
  grouping during vision rivalry)
• Transitions between phases not instantaneous,
  but spread in a wave-like fashion

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Where in the visual pathway is rivalry expressed?
Map
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NEURAL CORRELATES OF RIVALRY EXPERIMENTAL
EVIDENCE

• fMRI Modulation of activity during dominance
  and suppression phases in V1 (also MEGs and
  VERs)
• Electrophysiology No evidence for rivalry
  inhibition in the LGN
• Modulation in Neural spiking activity in early
  visual cortical areas.
• Increased modulation in successive stages of
  visual areas
• MT
• V1 V2
• V4
• Higher areas Response only to particular
  preferred stimulus stage of processing beyond
  the resolution of perceptual conflict.
• Decrease in visual sensitivity during
  suppression.
• Rivalry involves multiple, distributed processes
  throughout the rivalry hierarchy.

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Computational Evidence for a rivalry hierarchy in
vision Wilson, PNAS (2003), Vol 100 (24),
14499-14503

• A Competitive Neural Model Need at least two
  hierarchic rivalry stages for explaining data.
• Specifically, the model explains the
  observations of a flicker and switch (FS)
  procedure (which rules out inter-ocular rivalry).

18 Hz On-Off flicker of orthogonal
monocular gratings Swapping gratings
between eyes at 1.5 Hz
Perceptual Dominance Durations of 2.0 sec
Logothetis, et al., Nature (1996), 380, 621-624
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Stimulus
Right
Left
0 ms
333 ms
666 ms
0 ms
333 ms
666 ms

• A single phase of perceptual dominance can span
  multiple alternations of the stimuli
• The persistence of dominance across eye-swaps
  depends on temporal parameters of the stimulus
• High temporal frequencies reduce the efficacy of
  recurrent feedback inhibition within a network
• This bypasses an initial competitive
  inter-ocular rivalry stage, and reveals higher
  levels of binocular competition

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IHbin
IVbin
EVbin
EHbin
EVright
EVleft
EHleft
EHright
IVleft
IHleft
IHright
IVright
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Spike-Rate Equations
EVleft Firing rate of an excitatory neuron
responding to a vertical grating presented to the
left eye, Asymptotic firing rate given by
Naka-Rushton function
EVleft drives Inhibitory Neuron Ivleft which
inhibits EHright
HVleft Slow self-adaptation by an
aftehyperpolarizing current
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Ref Lecture 3
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• Monocular Representations of horizontal and
  vertical gratings compete via strong reciprocal
  inhibition.
• The competing sets of neurons self-adapt, giving
  rise to dominance and suppression alterations.
• Spike-frequency adaptation by an Ca2 dependent
  K current.
• The second competitive stage with binocular
  neurons described by similar equations, with
  input from first layer.
• Vleft-bin(t) EVleft(t) EVright(t)
• Parameters
• V 10, Emax100,
• g (inhibitory gain) 45 at monocular level,
  1.53g at higher level
• h (hyperpolarizing current strength) 0.47,
• Excitatory input gain from monocular to binocular
  level 0.75
• Recurrent excitation 0.02

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Results
Stimulus Continuous vertical grating to left
eye, horizontal grating to right eye.
Vertical grating response
Horizontal grating response
Alterations in dominance and suppression in both
stages. Dominance period of 2.4 sec
EHright
EVleft
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FS stimulus Monocular Neurons cannot generate a
competitive response alteration Dominance period
of 2.2 sec Stronger Inhibition at binocular stage
is the determining factor
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Conductance-based model
Simplified equations for Membrane Potential V,
Recovery Variable R, inward Ca2 current
conductance T, slow Ca2 dependent K
hyperpolarizing conductance H
Simplified equations reproduce spike shapes,
firing rates and spike-frequency adaptation for
human neocortical neurons
Wilson HR, J. Theor. Biol. (1999), 200, 375-388
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Monocular stage 12 neurons 8 excitatory, 2 each
for each eye for each grating 4
inhibitory Binocular stage 6 neurons 4
excitatory, 2 each for each grating 2 inhibitory
Parameters TR 4.2 msec (Exc), TR 1.5 msec
(Inh Fast spiking cells with narrow AP) ENa
50 mV, EK -95mV, ECa 120 mV, C 1 µF, TT
50 msec, TH 900 msec
After-hyperpolarizing current gT 0.1, gH 2.5
(exc) gT 0.25, gH 0 (inh no spike-frequency
adaptation)
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Conductance Model
Output of layer 1
Normal Stimulus
FS Model
Left
Right
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Gamma Distribution for Dominance Durations
Variable Strength Input
A Spiking Neuron Model for Binocular Rivalry,
Laing and Chow, J. Comp. Neuro. (2002), 12, 39-53
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Bifurcation Diagram for single-level Rivalry
Model
Need more inhibitory strength to produce rivalry
with FS stimulus.
g
h
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Experimental and Model Results
Positives

• Gamma distribution of dominance durations is
  obtained.
• Results for FS stimulus matched
• - 18.0 Hz flicker 1.5 Hz swap by
  themselves give conventional rivalry
• Dominance durations for variable stimulus
  strength reproduced.
• Excitatory Feedback of max 0.02 results in
  similar dynamics.
• Stronger inhibition at higher stages More
  modulation during traditional rivalry !?
• Makes clear experimental predictions

Negatives

• Inter-ocular grouping not accounted for (?)
• Spatial inhomogenities Spread in a wave-like
  fashion.
• Do we really need two layers -gt for dominance
  durations?
• Excitatory Feedback Is it strong enough?

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Conclusions and Predictions
Predictions

• Maximum stimulus size for unitary rivalry should
  increase under FS conditions.
• fMRI Blind-spot conditions No modulation of
  signal during FS.
• V1 physiology No modulation.

Conclusions

• Rivalry involves multiple, distributed processes
  throughout the visual system hierarchy
• No locus or neural site of rivalry
• Form vision and rivalry implemented through
  similar multiple networks.

Grand Conclusion Consciousness is a
characteristic of extended neural circuits
comprising several interacting cortical levels
throughout the brain
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The Naka-Rushton Function
A good fit for V1 spike rates Steady state firing
rate in response to a visual stimulus of contrast
P
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