Paper on synaptic delay learning

[Walter Senn]

Dear Connectionists

The following paper (to appear in Neural Computation) is available at: 
http://www.cns.unibe.ch/publications/ftp/paper_Delay.pdf 

"Activity-dependent selection of axonal and dendritic delays or, why synaptic
transmission should be unreliable" 

Walter Senn, Martin Schneider and Berthold Ruf 

Abstract: 
Systematic temporal relations between single neuronal activities or population
activities are ubiquitous in the brain. No experimental evidence, however,
exists for a direct modification of neuronal delays during Hebbian type
stimulation protocols. We show that, in fact, an explicit delay adaptation is
not required if one assumes that the synaptic strengths are modified according
to the recently observed temporally asymmetric learning rule with the
downregulating branch dominating the upregulating branch. During development,
slow unbiased fluctuations in the transmission time together with temporally
correlated network activity may control neural growth and implicitly induce
drifts in the axonal delays and dendritic latencies. These delays and latencies
become optimally tuned in the sense that the synaptic response tends to peak in
the soma of the postsynaptic cell if this is most likely to fire. The nature of
the selection process, however, requires unreliable synapses in order to give
`successful' synapses an evolutionary advantage upon the others. Without
unreliable transmission, the learning rule would equally modify all synapses
with the same local time difference between the pre- and postsynaptic signal,
irrespective whether the corresponding total axonal and dendritic delay supports
the postsynaptic firing or not. Stochastic transmission may resolve this
ambiguity by restricting the modification process to the active synapses only,
giving those synapses a higher chance to be strengthened which contribute to the
postsynaptic activity. The width of the learning window does also implicitely
determine the preferred dendritic delay and the preferred width of the
postsynaptic response. Hence, the learning rule may implicitly determine whether
a synaptic connection provides precisely timed infromation or rather
`contextual' information. 


Download from homepage: http://www.cns.unibe.ch/~wsenn/#pub
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Walter Senn                   Phone office: +41 31 631 87 21
Physiological Institute       Phone home:   +41 31 332 38 31 
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