What is an Activation Function?

An activation (or transfer) function maps a neuron’s weighted inputs plus bias to its output, adding non-linearity so the model can learn complex patterns beyond simple linear ones.

Activation Functions are also known as Transfer Function in the context of Neural Networks.

• Math functions that calculate weighted sum of inputs and adds bias to give non-linearity to output of neuron.
• Decides whether a neuron should be activated (“fired”) or not.
• This helps Neural Network to use important information and suppress not so useful data points.
• Adds non-linearity to Neural Network to tackle complex problems.
• Real-world problems are non-linear. Recognizing cats vs. dogs
• Without activation functions, f(z) = z, linear regression model, multiple linear layers form up to one big linear equation; useless for non-linear problems.

Maxout Function

piecewise linear function

• General-purpose neuron activation function
• Outputs the maximum value from a set of linear functions.
• Used in hidden layers of deep neural networks as an alternative to ReLU
• Generalizes ReLU and leaky ReLU because it can choose among many.
• Handles vanishing gradients better than sigmoids.
• Requires more parameters, which can increase the model size.
• Good for Dropout regularization (Maxout works very well with it).
• Output range (-∞ , ∞)

Maxout - Mathematical Derivation

• accepts an input tensor x
• reshapes the axis dimension into two dimensions
• takes maximum along the axis dimension
• returns: Output variable. The shape of the output is same as x except that axis dimension is transformed from M * pool_size to M.

Let x ∈ R^d and w_i​ and b_i are the weights and biases for the i-th linear function, and k is the number of such functions.

How to apply Maxout?

Assume these inputs, weights, bias and the pool size of 2:

(Shape: 1x3) Input X_i: [3.2, 1.2, 0.5]

(Shape: 3x4) Weight W_i:
[0.2 0.4 0.1 -0.5
0.7 -0.3 0.5 0.2
-0.6 0.8 -0.2 0.9]

(Shape: 4) Bias Vector b = [0.1, -0.2, 0.3, 0.0]

Step 1: Input X dot product W weights

Z₁ = 3.2⋅0.2 + 1.2⋅0.7 + 0.5⋅(−0.6) = 0.64 + 0.84 − 0.3 = 1.18
Z₂ = 3.2⋅0.4 + 1.2⋅(−0.3) + 0.5⋅0.8 = 1.28 − 0.36 + 0.4 = 1.32
Z₃ = 3.2⋅0.1 + 1.2⋅0.5 + 0.5⋅(−0.2) = 0.32 + 0.6 − 0.1 = 0.82
Z₄ = 3.2⋅(−0.5) + 1.2⋅0.2 + 0.5⋅0.9 = −1.6 + 0.24 + 0.45 = −0.91

Step 2: Add the bias

1.18 + 0.1 = 1.28
1.32 + (−0.2) = 1.12
0.82 + 0.3 = 1.12
−0.91 + 0.0 = −0.91

Before Maxout pooling: [1.28, 1.12, 1.12, −0.91]

Pool size = 2

So, 2 groups of 2 units.

Step 3: Group based on pool size

Group 1: [1.28,  1.12]

Group 2: [1.12,  −0.91]

Step 4: Take max value from each

Group 1 output = max(1.28, 1.12)=  1.28

Group 2 output = max(1.12, −0.91)=  1.12

Final Maxout Output: [1.28, 1.12]

Now, let’s try this example in Python Code with NumPy, PyTorch and TensorFlow.

How to implement Maxout Function in Python?

We will write simple code for implementing Softmax activation function in 3 most popular platforms viz. Numpy, PyTorch and TensorFlow.

All code samples are executable in Google Colab easily.

Maxout in Numpy

Maxout in PyTorch

Maxout in TensorFlow

Applications of Maxout

• Object recognition tasks
• Networks dealing with noisy data
• Scenarios needing robustness to vanishing gradients
• Text sentiment analysis

Use case of Maxout

Image Classification Benchmarks

Pairing Maxout with Dropout yields state-of-the-art performance on vision benchmarks such as MNIST, CIFAR-10/100

Text Sentiment Analysis

Doubling convolutional filters (ReLU2x) and using the maxout 3-2 variant achieved the highest sentiment-classification accuracy, outperforming lower-memory activations (LReLU, SeLU, tanh)

Advancements in Maxout Function

Adaptive Piecewise Linear Units

• Unlike Maxout, APLU learns the shape of the activation function during training.
• Offers more expressiveness than Maxout but with fewer parameters.

Mix of Experts (MoE)

• A modular system that picks among multiple expert networks to solve different parts of a task.
• Scalable and adaptive for huge models.

Efficient Approximations

• Efficient approximations are not new functions but rather implementation or architectural strategies to make Maxout more practical
• Reducing computational overhead: Simplifying the Maxout function to require fewer operations.
• Lowering parameter count: Achieving similar performance with fewer learnable parameters.

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