Download Bin_Rivalry

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
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
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 => A model for studying the
neural basis of conscious visual awareness.
Blake and Logothetis, Nat Rev Neuro, 2002, Vol3
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).
Spatial Features
• Inter-ocular grouping during dominance => Not just suppression
of an eye. (Also, figural grouping during vision rivalry)
• Transitions between phases not instantaneous, but spread in a
wave-like fashion
Where in the visual pathway is rivalry expressed?
Map
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.
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 (F&S) procedure (which rules out inter-ocular rivalry).
18 Hz On-Off flicker of orthogonal
monocular gratings
+
Perceptual Dominance
Durations of 2.0 sec
Swapping gratings between eyes
at 1.5 Hz
Logothetis, et al., Nature (1996), 380, 621-624
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
…
IVbin
IHbin
EVbin
EVleft
EHbin
EHleft
IVleft
IHleft
EVright
EHright
IHright
IVright
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
Ref: Lecture 3
• 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, Emax=100,
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
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
Results:
EVleft
F&S stimulus
Monocular Neurons
cannot generate a
competitive response
alteration
Dominance period of 2.2
sec
Stronger Inhibition at
binocular stage is the
determining factor
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
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)
Conductance Model :
Output of layer 1
Normal Stimulus
Left
Right
F&S Model
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
Bifurcation Diagram for single-level Rivalry Model :
g
Need more inhibitory
strength to produce rivalry
with F&S stimulus.
h
Experimental and Model Results
Positives :
• Gamma distribution of dominance durations is obtained.
• Results for F&S 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 -> for dominance durations?
• Excitatory Feedback – Is it strong enough?
Conclusions and Predictions
Predictions
• Maximum stimulus size for unitary rivalry should increase under F&S conditions.
• fMRI – Blind-spot conditions : No modulation of signal during F&S.
• 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 “
The Naka-Rushton Function
A good fit for V1 spike rates
Steady state firing rate in response to a visual stimulus of contrast P: