CCN Seminar with Magnus Tournoy (University of Chicago)

Date & Time

Please join us for a CCN Seminar with Magnus Tournoy, Postdoctoral Fellow at the University of Chicago and a candidate for an FRF position.  To schedule a meeting with Magnus during his visit, please be in touch with Jessica Hauser (


Title: A Toy Model for Uncovering The Structure of Nonlinear Neural Networks


With the experimental advances in the recording of large populations of neurons, theorists are in the humbling position of making sense of a staggering amount of data. One question that will become more into reach is how network structure relates to function. But going beyond explanatory models and becoming more predictive will require a fundamental approach.


In this talk we’ll take the view of a physicist and formulate exact results within a simple, yet general, toy model called Glass networks. Named after its originator Leon Glass, they are the infinite gain limit of well-known circuit models like continuous-time Hopfield networks. We’ll show that, within this limit, stability conditions reduce to semipositivity constraints on the synaptic weight matrix.


Having a clear link between structure and function in possession, the consequences of multistability on the network architecture can be explored. One finding is the factorization of the weight matrix in terms of nonnegative matrices. Interestingly this factorization completely identifies the existence of stable states. Another result is the reduction of allowed sign patterns for the connections. A consequence hereof are lower bounds on the number of excitatory and inhibitory connections. At last we will discuss the special case of “sign stability”, where stability is guaranteed by the topology of the network. Derivations of these results will be supplemented by a number of examples.



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