Subject: Connectionists: New paper about reward-modulated spike-timing-dependent plasticity
From: Pecevski Dejan <[email-masked]>
To: [email-masked], [email-masked]
Date: Wed, 29 Oct 2008 16:21:58 +0100
List-Id: Discussion and announcement of events related to neural computation <connectionists.cs.cmu.edu>

Dear all,

A new paper that provides a theoretical analysis of the functional 
properties of  reward-modulated spike-timing-dependent plasticity is 
available online at:
http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.1000180 

(http://www.igi.tugraz.at/maass/psfiles/183_legenstein_etal_2008.pdf )

The paper also discusses the possible role of spontaneous activity and 
trial to trial variability in cortical networks as an exploration 
strategy during learning with reward-modulated STDP.

Abstract:

Reward-modulated spike-timing-dependent plasticity (STDP) has recently 
emerged as a candidate for a learning rule that could explain how 
behaviorally relevant adaptive changes in complex networks of spiking 
neurons could be achieved in a self-organizing manner through local 
synaptic plasticity. However, the capabilities and limitations of this 
learning rule could so far only be tested through computer simulations. 
This article provides tools for an analytic treatment of 
reward-modulated STDP, which allows us to predict under which conditions 
reward-modulated STDP will achieve a desired learning effect. These 
analytical results imply that neurons can learn through reward-modulated 
STDP to classify not only spatial but also temporal firing patterns of 
presynaptic neurons. They also can learn to respond to specific 
presynaptic firing patterns with particular spike patterns. Finally, the 
resulting learning theory predicts that even difficult credit-assignment 
problems, where it is very hard to tell which synaptic weights should be 
modified in order to increase the global reward for the system, can be 
solved in a self-organizing manner through reward-modulated STDP. This 
yields an explanation for a fundamental experimental result on 
biofeedback in monkeys by Fetz and Baker. In this experiment monkeys 
were rewarded for increasing the firing rate of a particular neuron in 
the cortex and were able to solve this extremely difficult credit 
assignment problem. Our model for this experiment relies on a 
combination of reward-modulated STDP with variable spontaneous firing 
activity. Hence it also provides a possible functional explanation for 
trial-to-trial variability, which is characteristic for cortical 
networks of neurons but has no analogue in currently existing artificial 
computing systems. In addition our model demonstrates that 
reward-modulated STDP can be applied to all synapses in a large 
recurrent neural network without endangering the stability of the 
network dynamics.

-- 
Dejan Pecevski, Dipl.-Ing.
Institute for Theoretical Computer Science
Graz University of Technology
A-8010 Graz, Austria