**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.