Friday, May 27, 2005

Radial Basis Functions, Neural Net Visualization

Two new things...

1. I've been experimenting with a layer of radial basis functions (RBFs) for the input layer to my neural nets. Before I would just use a single real value for each input. For example, the pendulum angle was a single real value which used a single input neuron. Now I separate each input into an array of RBFs. It seems that a lot of people use RBFs when using neural nets for motor control. They increase the "locality" of learning: only the connection weights associated with a particular part of the state space are affected, so it helps the neural net retain old knowledge better. Also, it's neat to think about RBFs in terms of biology (think of the arrays of sensory nerves on the skin, the retina, the vestibular system, etc.)

2. I have always had a hard time visualizing what's going on with the neural networks while they learn. The combination of learning rates, eligibility traces for connections, neuronal firing rates, etc. makes it hard to understand what's going on. I had already been plotting various parameters on graphs after the simulations end, but I really wanted to watch how the parameters change in real time. So I wrote some code to see the neural nets floating in 3D space beside the agent's physical body. It shows me the neuron firing rates, connection weight sign/magnitudes, and eligibility traces, making it much easier to see what's going on.

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