The Function of Synchrony Marieke Rohde Reading Group DyStURB (Dynamical Structures to Understand Real Brains)
Jan 05, 2016
The Function of Synchrony
Marieke Rohde
Reading Group DyStURB
(Dynamical Structures to Understand Real Brains)
Structure
1. Sound recognition by transient synchrony. (Hopfield & Brody)
2. Long distance synchronisation in Human subjects (Rodriguez et. Al.)
3. Discuss!
1.) Hopfield, Brody: What is a moment? (Puzzle and Answer)
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
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.
Mus Silicium - AnatomyThe alpha and beta neurons from „cortical layer 4“ exhibit the same properties as the sensory neurons!
Mus Silicium: Responses
• Gamma cells: highly specific to learned syllable.
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
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.
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?
2.) Rodriguez et.al.:
Perception's shadow: longdistance synchronization of human brain activity
Long Distance Synchrony
• 30-80 Hz oscillations (gamma) synchronise during a cognitive act. (EEG MEG measurements)
• Task: Recognition of a degraded stimulus (Mooney face)
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).
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.
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