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Rafal Bogacz

 

Wednesday 23rd March 2016

Time: 4.00pm

 

Ground Floor Seminar Room

25 Howland Street, London, W1T 4JG

 

An approximation of the error back-propagation algorithm in a predictive coding network with local Hebbian synaptic plasticity

 

 

To efficiently learn from feedback, the cortical networks need to update synaptic weights on multiple levels of cortical hierarchy. An
effective and well-known algorithm for computing such changes in synaptic weights is the error back-propagation. It has been
successfully used in both machine learning and modelling of the brain's cognitive functions. However, in the back-propagation
algorithm, the change in synaptic weights is a complex function of weights and activities of neurons not directly connected with the
synapse being modified. Hence it has not been known if it can be implemented in biological neural networks. This talk will discuss
relationships between the back-propagation algorithm and the predictive coding model of information processing in the cortex, in
which changes in synaptic weights are only based on activity of pre-synaptic and post-synaptic neurons. It will be shown that when the
predictive coding model is used for supervised learning, it performs very similar computations to the back-propagation algorithm.
Furthermore, for certain parameters, the weight change in the predictive coding model converges to that of the back-propagation
algorithm. This suggests that it is possible for cortical networks with simple Hebbian synaptic plasticity to implement efficient
learning algorithms in which synapses in areas on multiple levels of hierarchy are modified to minimize the error on the output.

 

Biography:
Rafal Bogacz did his undergraduate in Computer Science in Poland, where he is originally from. He conducted his PhD at the University of Bristol jointly in the Departments of Computer Science and Anatomy, and next he worked as a postdoctoral researcher at Princeton University jointly in Departments of Applied Mathematics and Psychology. In 2004 he came back to Bristol where he worked as a lecturer and then a reader. He moved to University of Oxford in 2013, where is an associate professor in the MRC Brain Network Dynamics Unit.