Information encoding and processing via spatio-temporal spike patterns in cortical networks Misha Tsodyks, Dept of Neurobiology, Weizmann Institute, Rehovot, Israel Thanks to: Alex Loebel, Omri Barak, Asher Uziel, Henry Markram

Rate coding (V1)

Y. Prut, …, M. Abeles 1998

W. Bair & C. Koch 1996

DeWeese, …, Zador 2003

Open questions: How do precise spike patterns emerge in the cortex? How can they be robust in the presence of random firing of surrounding neurons? What is the relation between the spike patterns and the stimuli that they are coding for? How can the information carried by spike patterns be processed?

Open questions: How do precise spike patterns emerge in the cortex? How can they be robust in the presence of random firing of surrounding neurons? (Synfire chains? – I don’t like it!) What is the relation between the spike patterns and the stimuli that they are coding for? How can the information carried by spike patterns be processed?

Tsodyks et al 2000 Recurrent networks with dynamic synapses (unstructured)

Wang Yun et al 1998

Modeling Time-Dependent Release 4 Synaptic Parameters  Absolute strength  Probability of release  Depression time constant  Facilitation time constant

Population spikes

Origin of Population Bursts

Temporal Correlations

Network response to stimulation

i JJ Simplified model (no inhibition, uniform connections, rate equations)

The rate equations  Two sets of equations representing the excitatory units firing rate, E, and their depression factor, R : Loebel & Tsodyks 2002

Population spikes in the simplified model

Adiabatic approximation (except during the population spike)

Adiabatic approximation Population spike: (except during the population spike)

Adiabatic approximation Population spike: Higher spontaneous activity – lower propensity for population spikes.

Response to excitatory pulses Inputs: Response: Population spike No population spike

Inputs: Response: Population spike No population spike

Inputs: Response: Population spike No population spike

Response to tonic stimuli The tonic stimuli is represented by a constant shift of the {e}’s, that, when large enough, causes the network to burst and reach a new steady state

Interaction between stimuli

Open questions: How do precise spike patterns emerge in the cortex? (Synfire chains?) How can they be robust in the presence of random spontaneous and evoked firing of surrounding neurons? What is the relation between the spike patterns and the stimuli that they are coding for? How can the information carried by spike patterns be processed?

Extended model Loebel & Tsodyks 2006

The model response to a ‘ pure tone ’

Constraining the propagation of the PS along the map

Rotman et al, 2001 Forward suppression

Network response to complex stimuli

Open questions: How do precise spike patterns emerge in the cortex? (Synfire chains?) How can they be robust in the presence of random spontaneous and evoked firing of surrounding neurons? What is the relation between the spike patterns and the stimuli that they are coding for? How can the information carried by spike patterns be processed?

Processing spike patterns: Tempotron (Guetig and Sompolinsky, 2006) Barak & Tsodyks, 2006 Learned patterns vs background patterns

Variance-based learning where

Cost function for learning

Learning rules for spatio-temporal patterns Gradient descent: Correlation-based:

Convergence of learning

Performance of the tempotron

Measuring the tempotron performance

Robustness to time warps

Conclusions 1. Networks with synaptic depression can encode spatio-temporal inputs by precise spike patterns. 2. Random spontaneous activity could play a crucial role in setting the sensitivity of the network to sensory inputs (top-down control, attention, expectations, …?) 3. Coding by spike patterns is highly nonlinear. 4. Effective learning rules for recognition of spike patterns in tempotron-like networks can be derived.