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DESCRIPTIONBINOCULAR 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 - PowerPoint PPT Presentation
BINOCULAR RIVALRYA HIERARCHICAL MODEL FOR VISUAL COMPETETIONComputational Evidence for a rivalry hierarchy in visionWilson, PNAS (2003), Vol 100 (24), 14499-14503Shantanu JadhavComputational NeurobiologyUCSD
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 limitationsLecture 1: Benefits of Computational Models- New explanations for cognitive phenomena- Tie explanations of cognitive phenomena to the biological mechanisms
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.BINOCULAR RIVALRY
Blake and Logothetis, Nat Rev Neuro, 2002, Vol3
Perceptual CharacteristicsTemporal 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: MTV1 V2V4
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 visionWilson, 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 +Swapping gratings between eyes at 1.5 HzPerceptual Dominance Durations of 2.0 secLogothetis, et al., Nature (1996), 380, 621-624
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 competition0 ms333 ms666 ms0 ms333 ms666 msLeftRightStimulus :
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 EHrightHVleft: 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 levelh (hyperpolarizing current strength) = 0.47,Excitatory input gain from monocular to binocular level = 0.75Recurrent excitation = 0.02
Results:Stimulus = Continuous vertical grating to left eye, horizontal grating to right eye.Vertical grating responseHorizontal grating responseAlterations in dominance and suppression in both stages.Dominance period of 2.4 secEVleftEHright
F&S stimulusMonocular Neurons cannot generate a competitive response alterationDominance period of 2.2 secStronger Inhibition at binocular stage is the determining factor
Conductance-based model:Simplified equations forMembrane Potential V, Recovery Variable R, inward Ca2+ current conductance T, slow Ca2+ dependent K+ hyperpolarizing conductance HWilson HR, J. Theor. Biol. (1999), 200, 375-388Simplified equations reproduce spike shapes, firing rates and spike-frequency adaptation for human neocortical neurons
Monocular stage: 12 neurons8 excitatory, 2 each for each eye for each grating4 inhibitoryBinocular stage: 6 neurons4 excitatory, 2 each for each grating2 inhibitoryParameters: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 F, TT = 50 msec, TH = 900 msecAfter-hyperpolarizing current:gT = 0.1, gH =2.5 (exc)gT = 0.25, gH = 0 (inh no spike-frequency adaptation)
Conductance Model :Normal StimulusF&S ModelLeftRightOutput of layer 1
Gamma Distribution for Dominance DurationsA Spiking Neuron Model for Binocular Rivalry, Laing and Chow, J. Comp. Neuro. (2002), 12, 39-53Variable Strength Input
Bifurcation Diagram for single-level Rivalry Model :Need more inhibitory strength to produce rivalry with F&S stimulus.hg
Experimental and Model ResultsPositives : 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 predictionsNegatives : 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 PredictionsPredictions 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 ConclusionConsciousness is a characteristic of extended neural circuits comprising several interacting cortical levels throughout the brain
The Naka-Rushton FunctionA good fit for V1 spike ratesSteady state firing rate in response to a visual stimulus of contrast P: