Can neural net weights be negative?

I have a question on neural net weights. My understanding is that the default is they add up to one. Can they be negative? (If they can't be negative, it seems to me I would need to make sure to make input data generally proportional to the target output. If they can be negative, then I can just let the system adapt to the proportionality.) Thanks!

Answer 1

[Shashank Prasanna]

Weights can be whatever the training algorithm determines the weights to be. If you take the simple case of a perceptron (1 layer NN), the weights are the slope of the separating (hyper)plane, it could be positive or negative.

Take a look at some examples in the neural network toolbox:

http://www.mathworks.com/help/nnet/index.html

Answer 2

[Michael]

Thanks Shashank, I'll check it out.