The Function of Synchrony Marieke Rohde Reading Group DyStURB (Dynamical Structures to Understand...
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Transcript of The Function of Synchrony Marieke Rohde Reading Group DyStURB (Dynamical Structures to Understand...
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The Function of Synchrony
Marieke Rohde
Reading Group DyStURB
(Dynamical Structures to Understand Real Brains)
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Structure
1. Sound recognition by transient synchrony. (Hopfield & Brody)
2. Long distance synchronisation in Human subjects (Rodriguez et. Al.)
3. Discuss!
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1.) Hopfield, Brody: What is a moment? (Puzzle and Answer)
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Mus Silicium
• Short time integration in an artificial organism Biologically plausible model, spiking neurons.
• Auditory task: one syllable recognition – short time integration required to "represent" the world.
• Mastered robustly
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Mus Silicium - Anatomy
• Layer 4:– 50% inhibitory, 50% excitatory– Lots of cells and connections
• no delays, no plasticity• Sensors: cells are frequency
tuned and respond to– Onsets – Offsets– Peaks
• Transient decay of neural activity at different decay rates.
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Mus Silicium - AnatomyThe alpha and beta neurons from „cortical layer 4“ exhibit the same properties as the sensory neurons!
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Mus Silicium: Responses
• Gamma cells: highly specific to learned syllable.
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Mus Silicium: The SolutionGeneral Principle: Transient synchrony of APs to „signal“ recognition• Representation of time of a stimulus by different decay rates• spatiotemporal patterns: Convergence of firing rate of decaying currents. • Same rate neurons (coupled oscillators) tend to synchronise. (set weights
accordingly)• Detection by cell with small time constant• Invariant to time-warping (rescaling in time), delays and salience
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Mus Silicium: The Solution
• 800 lines (different stimuli and decay rates) from area A project on an excitatory and an inhibitory cell
• Training = find set of coinciding neurons on pattern and mutually couple them (excitatory and inhibitory)
• Balance between excitation and inhibition, to assure network input current from outside.
• Connect whole set to a gamma neuron, to yield a reaction.
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Mus Silicium
• Extensions:– reactivation of sensors? (several, probablistic activation)– Negative evidence. Destroy synchrony/detection.
Robustness against noise
– Multiple patterns: Phase transition n infty to general synchrony
• Structure, not weights. Several structures conceivable• Biological plausibility. • Conclusion:
– A „Many are now equal“ operator.– Model spiking networks if you want to explain the brain!
• How could you have guessed it?
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2.) Rodriguez et.al.:
Perception's shadow: longdistance synchronization of human brain activity
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Long Distance Synchrony
• 30-80 Hz oscillations (gamma) synchronise during a cognitive act. (EEG MEG measurements)
• Task: Recognition of a degraded stimulus (Mooney face)
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Long Distance Synchrony: Methods
1. Detect induced gamma response: "wigner ville time frequency transforms“ of single trials and average.• first peak is known (much
stronger in perception condition)
• second new, practically the same for both conditions.
1. Phase synchrony:– the phase synchrony profile is very different
from the gamma activity profile– baseline: shuffled data. – no perception remains close to baseline. – perception: synchronisation, desynchronisation,
synchronisation (zero centered distribution of phase lags).
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Long Distance Synchrony: Conclusions
• biological significance for cogntion confirmed. (refutation to different criticisms)
• High level, rather than low local feature binding
• New finding: desynchronisation to prepare for next synchronisation (destroy old pattern).
• gamma activity != synchrony.
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Discussion• Differences:
– Local vs. Global (+ role of delays)– Detectors vs. Unknown function.– Low level vs. High level
• What methods to detect it in organisms?– Phase lag: 0 or different?– Time spans vs. every spike.
• Synchrony - Asynchrony• What function could synchrony have?
– Attractive state (type of population code)– Internal clock