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I'm studying and trying to implement convolutional neural networks, but I suppose this question applies to multilayer perceptrons in general.

The output neurons in my network represent the activation of each class: the most active neuron corresponds to the predicted class for a given input. To consider a cross-entropy cost for training, I'm adding a softmax layer at the end of the network, so that each neuron's activation value is interpreted as a probability value.

My question is: should the neurons in the output layer apply a non-linear function to the input? My intuition is that it is not necessary:

  • if the input to the $i$-th output neuron is the dot product $x^T\theta_i$ between a vector $x$ (coming from the previous layer) and the weights $\theta_i$ for that neuron,
  • and if I employ a monotonic non-linear function like the sigmoid or the ReLU
  • then the larger activation output will still correspond to the largest $x^T\theta_i$, so from this point of view the non-linear function would not change the prediction.

Is something wrong with this interpretation? Are there some training factors that I'm overlooking which make the output non-linearity necessary?

And if I'm right, would anything change if instead of using the sigmoid function I use the ReLU $$\max(0,x^T\theta_i)$$ function, which is not strictly monotonic?

EDIT

With reference to Karel's answer, whose answer basically was "it depends", here is a more detailed description of my network and doubt:

Suppose I have N hidden layers, and my output layer is just a softmax layer over a set of neurons representing classes (so my expected output is the probability that the input data belongs to each class). Assuming the first N-1 layers have nonlinear neurons, what is the difference between using nonlinear vs linear neurons in the N-th hidden layer?


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  • $\begingroup$ Are the layers N-2, N-3,.. 1 linear or nonlinear? $\endgroup$ – Karel Macek Aug 5 '15 at 10:19
  • $\begingroup$ Layers from 1 (closest to the input) to N-1 are nonlinear. Layer N is the last (closer to the output) hidden layer. The softmax layer is layer N+1. $\endgroup$ – rand Aug 6 '15 at 15:50
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    $\begingroup$ What about a BN layer right before the softmax? is that non-linearity ok? (does it count as a non-linearity?) $\endgroup$ – Charlie Parker Mar 7 '18 at 20:47
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You should not use a non-linearity for the last layer before the softmax classification. The ReLU non-linearity (used now almost exclusively) will in this case simply throw away information without adding any additional benefit. You can look at the caffe implementation of the well-known AlexNet for a reference of what's done in practice.

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    $\begingroup$ What about a BN layer right before the softmax? is that non-linearity ok? (does it count as a non-linearity?) $\endgroup$ – Charlie Parker Mar 7 '18 at 20:47
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You might want to send a negative value into the softmax function, to indicate that an event has low probability. If you pass the input values into a relu, then the network isn't going to pass any gradient through the units where the input to the relu is negative. So while the expressive power of the softmax isn't changed, it will probably make learning a lot harder.

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The answer is not yes or no. It strongly depends on your expectation you have about your network. I assume that you want to have a good classifier, possibly applicable to a wide range of problems. Therefore, the non-linearity can be helpful to capture non-trivial classes. The non-linearity might be included either in the last layer before the soft-max layer or it can be in the preceding layer.

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  • $\begingroup$ Can you have a look at my edit? $\endgroup$ – rand Aug 5 '15 at 10:15

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