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