Activation Functions

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Activation functions are the mathematical gates that determine, at each node in a Neural Network, whether and to what extent to transmit the signal the node has received from the previous layer.

A combination of weights (coefficients) and biases work on the input data from the previous layer to determine whether that signal surpasses a given threshold and is deemed significant. Those weights and biases are slowly updated as the neural net minimizes its error; i.e. the level of nodes’ activation change in the course of learning. They introduce non-linearity into the network, allowing it to learn and approximate complex relationships between inputs and outputs. Activation functions/gates are an essential component of Deep Learning models and are applied at each layer of the Neural Network. The choice of activation function/gate depends on the problem requirements and the characteristics of the data being processed. Different activation functions/gates offer distinct properties and advantages, and researchers continue to explore new variations to improve the performance and efficiency of deep learning models.


When we learn something new, like reading a story, our brain sends signals to tell us how much we should focus on that information. Without activation functions/gates, the AI will get overwhelmed with all the data it gets and not know what to do. Activation functions/gates helps the AI make better decisions, learn from its experiences, and improve its skills.



Types of Activation Functions/Gates

There are several types of activation functions/gates commonly used in AI:

  • Step Function: The step function is a simple activation function that maps input values below a threshold to 0 and values above the threshold to 1. It is not commonly used in deep learning due to its discontinuous nature, which makes it challenging to optimize during training.
  • Linear Function: The linear activation function computes the weighted sum of the inputs without applying any non-linearity. It is primarily used in linear regression models or as the output layer activation function in certain cases.
  • Sigmoid Function: The sigmoid function, also known as the logistic function, maps the input to a smooth S-shaped curve between 0 and 1. It is widely used as an activation function in the hidden layers of neural networks for binary classification problems. The sigmoid function squashes the output of a neuron to a range suitable for probability interpretation.
  • Tanh Function: The hyperbolic tangent (tanh) function is similar to the sigmoid function but maps the input to a range between -1 and 1. It exhibits stronger non-linearity compared to the sigmoid function and is often used in recurrent neural networks (RNNs) and hidden layers of feedforward neural networks.
  • Rectified Linear Unit (ReLU): The rectified linear unit (ReLU) is one of the most widely used activation functions. It returns the input as the output if it is positive, and 0 otherwise. ReLU introduces non-linearity and overcomes the vanishing gradient problem, making it efficient and effective for training deep neural networks.
  • Leaky ReLU: The leaky ReLU is a variation of the ReLU function that allows a small, non-zero gradient when the input is negative. This modification addresses the "dying ReLU" problem where a neuron can become inactive and stop learning.
  • Exponential Linear Unit (ELU): The exponential linear unit (ELU) function is similar to the leaky ReLU but has a negative output when the input is less than zero. ELU helps improve the robustness of the model to noise and can lead to better generalization.



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Why Non-linear Activation Functions are Preferred Over Linear Functions

Non-linear Activation Functions is essential because they enable AI models to handle complex problems, learn hierarchical representations, and approximate a wide range of functions. They provide the flexibility and power needed to build sophisticated AI systems that can tackle real-world challenges effectively. Reference: Gradient Descent Optimization & Challenges#Vanishing & Exploding Gradients

  • Enabling Complexity: Non-linear Activation Functions introduce non-linearities into the AI model, which allows it to capture and represent complex relationships between input data and output predictions. Many real-world problems are inherently non-linear, and using linear Activation Functions would limit the AI model's ability to handle these complexities effectively.
  • Universal Approximation Theorem: One of the fascinating properties of non-linear Activation Functions is that they can approximate any continuous function, given the right parameters. This property is called the Universal Approximation Theorem. It means that by using non-linear Activation Functions, we can build AI models that are capable of approximating almost any function, making them more versatile and adaptable.
  • Learning Hierarchies: Deep learning, a subfield of AI, has become very successful due to the use of deep neural networks with multiple layers. Non-linear Activation Functions play a crucial role in this success. With each layer using non-linearities, the AI model can learn hierarchical representations of the data, capturing abstract features at higher layers and building a rich understanding of the input.
  • Avoiding Vanishing and Exploding Gradients: Linear Activation Functions suffer from vanishing and exploding gradient problems. When gradients become too small (vanish) or too large (explode), it becomes difficult for the AI model to learn effectively. Non-linear Activation Functions help mitigate these issues, making the training process more stable and efficient.
  • Better Decision Boundaries: In classification tasks, AI models with non-linear Activation Functions can create more complex decision boundaries, allowing them to distinguish between classes more accurately. Linear Activation Functions would only allow for linear decision boundaries, limiting the model's ability to handle complex data distributions.

Choosing Activation Functions

The choice of activation function depends on the problem at hand and the characteristics of the data. Some guidelines for selecting activation functions include:

  • For binary classification problems, sigmoid or tanh functions are often used in the hidden layers, while sigmoid or softmax functions are used in the output layer.
  • ReLU and its variants (leaky ReLU, ELU) are commonly employed in the hidden layers of deep neural networks due to their computational efficiency and ability to prevent the vanishing gradient problem.
  • It is important to consider the properties of the activation function, such as differentiability, monotonicity, and range of output values, to ensure compatibility with the optimization algorithms and desired behavior of the network.


Begin with using ReLU function and then move over to other activation functions in case ReLU doesn’t provide with optimum results. (Always keep in mind that ReLU function should only be used in the hidden layers.)


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Threshold (binary step)

More theoretical than practical; e.g. not offered in TensorFlow library. the gradient of the step function is zero. This makes the step function not so useful since during back-propagation when the gradients of the activation functions are sent for error calculations to improve and optimize the results. The gradient of the step function reduces it all to zero and improvement of the models doesn’t really happen.


Identity (linear)

This can be applied to various neurons and multiple neurons can be activated at the same time. Now, when we have multiple classes, we can choose the one which has the maximum value. The derivative of a linear function is constant i.e. it does not depend upon the input value x. This means that every time we do a back propagation, the gradient would be the same.

Sigmoid (logistic)

A smooth function and is continuously differentiable with the result existing between (0 to 1). Therefore, it is especially used for models where we have to predict the probability as an output - anything exists only between the range of 0 and 1. Has vanishing gradient problem.


tanh (hyperbolic tangent)

The range of the tanh function is from (-1 to 1) - offset of Sigmoid to be zero centered. tanh is also s - shaped. Solves our problem of the values all being of the same sign. Has vanishing gradient problem.


ReLU (Rectified Linear Unit)

half rectified (from bottom). f(z) is zero when z is less than zero and f(z) is equal to z when z is above or equal to zero. The main advantage of using the ReLU function over other activation functions is that it does not activate all the neurons at the same time. What does this mean ? If you look at the ReLU function if the input is negative it will convert it to zero and the neuron does not get activated. This means that at a time only a few neurons are activated making the network sparse making it efficient and easy for computation. The ReLU is the most used activation function in the world right now.But ReLU also falls a prey to the gradients moving towards zero. If you look at the negative side of the graph, the gradient is zero, which means for activations in that region, the gradient is zero and the weights are not updated during back propagation. This can create dead neurons which never get activated.

Leaky ReLU (Rectified Linear Unit)

in an attempt to solve the dying ReLU problem, instead of defining the Relu function as 0 for x less than 0, we define it as a small linear component of x. The main advantage of replacing the horizontal line is to remove the zero gradient.

Explained

Parameterised ReLU (Rectified Linear Unit)

Similar to the Leaky ReLU function that has trainable parameter resulting in learning the value of ‘a‘ for faster and more optimum convergence. The parametrised ReLU function is used when the leaky ReLU function still fails to solve the problem of dead neurons and the relevant information is not successfully passed to the next layer.

Keras - Advanced Activation Layers

Softmax

  • Softmax - visit page for more information

Squeezes the outputs for each class between 0 and 1 and would also divide by the sum of the outputs. This essentially gives the probability of the input being in a particular class.

Neural Arithmetic Logic Units (NALU)

Neural networks can learn to represent and manipulate numerical information, but they seldom generalize well outside of the range of numerical values encountered during training. To encourage more systematic numerical extrapolation, we propose an architecture that represents numerical quantities as linear activations which are manipulated using primitive arithmetic operators, controlled by learned gates. We call this module a neural arithmetic logic unit (NALU), by analogy to the arithmetic logic unit in traditional processors. Experiments show that NALU-enhanced neural networks can learn to track time, perform arithmetic over images of numbers, translate numerical language into real-valued scalars, execute computer code, and count objects in images. In contrast to conventional architectures, we obtain substantially better generalization both inside and outside of the range of numerical values encountered during training, often extrapolating orders of magnitude beyond trained numerical ranges.

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The activation functions provide different types of nonlinearities for use in neural networks. These include smooth nonlinearities (sigmoid, tanh, elu, selu, softplus, and softsign), continuous but not everywhere differentiable functions (relu, relu6, crelu and relu_x), and random regularization (dropout). All activation ops apply componentwise, and produce a tensor of the same shape as the input tensor.

Weights

A weight refers to a parameter that determines the strength or importance of a connection between neurons in the network. Neural Networks are composed of interconnected nodes, also known as neurons, organized in layers. Each connection between neurons has an associated weight, which serves as a coefficient that scales the input of the neuron before passing it through an activation function.

In a Neural Network, each neuron receives inputs from the neurons in the previous layer, and these inputs are multiplied by the corresponding weights. The weighted inputs are then summed up, and the result is passed through an activation function to introduce non-linearity into the network. The activation function determines whether the neuron should be activated or not based on the aggregated input. The process of adjusting the weights in a Neural Network during training is called "learning" or "optimization."

The difference between a traditional Neural Network and the transformer architecture lies in their structure and the way they handle sequences. In a traditional feedforward Neural Network, information flows only in one direction, from input to output. These networks are commonly used for tasks such as image classification or Speech Recognition, where the input data is not sequential.

On the other hand, transformers are designed to handle sequential data, such as natural language sentences or time-series data. The transformer architecture utilizes the Attention mechanism, which allows the model to weigh the importance of different parts of the input sequence when processing each element. The Attention mechanism helps the model focus on relevant information and capture long-range dependencies in the sequence efficiently. In transformers, the weights still play the same role as in traditional Neural Networks, determining the strength of connections between neurons. However, in transformers, there are additional learnable weights associated with the Attention mechanism. These weights determine the importance of each element in the sequence when calculating attention scores for each query element. The weighted sum of the value vectors, computed based on the attention scores, provides context-specific information for each element in the input sequence, making transformers highly effective for sequential data tasks.

Weights in Neural Networks

Weights determine the strength of the connections between neurons in different layers of the network, enabling the network to adjust and adapt during the learning process. Weights and activation functions work together in Neural Networks. Weights in Neural Networks:

  • In a neural network, each connection between neurons is associated with a weight. These weights represent the strength of the connection between two neurons and act as parameters that the network adjusts during training to optimize its performance.
  • Initially, the weights in a neural network are assigned random values. As the network is trained on a labeled dataset, these weights are updated using optimization algorithms like gradient descent, which minimizes the error between the network's predicted output and the actual output.
  • The weights determine the contribution of each neuron's output to the input of the subsequent neurons in the network. A larger weight implies a stronger influence, while a smaller weight implies a weaker influence.
  • During training, the network adjusts the weights based on the error signal propagated through the network. This adjustment aims to minimize the overall error and improve the network's ability to make accurate predictions.
  • The final values of the weights after training represent the learned patterns and relationships in the input data, which the network can utilize to make predictions on unseen data.

Interaction between Weights and Activation Functions

  • Weights and activation functions work together in a neural network. The weights determine the strength of the connections between neurons, while the activation functions introduce non-linearity to capture complex patterns in the data.
  • The weights are responsible for controlling the contribution of each neuron's output to the subsequent layers. By adjusting the weights, the network can assign more or less importance to certain features or patterns in the input data.
  • The activation functions, on the other hand, determine the output range and the transformation applied to the network's activations. The non-linear nature of the activation functions allows the network to model complex relationships and learn non-linear decision boundaries.
  • During the forward pass of training or inference, the activation of a neuron is computed by taking the weighted sum of inputs and passing it through the activation function. This transformed activation becomes the input for the next layer of neurons.
  • The weights are updated during the backward pass of training using gradient descent or other optimization algorithms. The updates are based on the gradients of the loss function with respect to the weights, which are obtained through backpropagation.
  • The combination of different weights and activation functions in the network architecture determines the overall learning capacity and flexibility of the neural network. Different choices of activation functions and weight initialization schemes can have a significant impact on the network's performance and convergence.

Weights with Attention Mechanism/Transformer Architecture

In the context of the Attention mechanism and the transformer architecture, the weights represent the importance assigned to different elements in the input sequence during the computation of the Attention scores. The Attention mechanism is a fundamental component of the transformer architecture, which was introduced in the "Attention is All You Need" paper by Vaswani et al. in 2017. It enables the model to focus on relevant parts of the input sequence when processing each element, allowing for more efficient and effective learning. In the transformer architecture, there are three main components related to weights and attention:

  • Query, Key, and Value (Q, K, V) Vectors: For each element in the input sequence, the Attention mechanism computes three vectors: the query vector (Q), the key vector (K), and the value vector (V). These vectors are obtained by projecting the input sequence through learnable linear transformations (weight matrices). The query vector represents the element we want to process, and the key and value vectors represent the elements in the input sequence that provide context and information.
  • Attention Scores: The attention scores are calculated by computing the dot product between the query vector (Q) and the key vectors (K) for each element in the input sequence. The dot product measures the similarity or relevance between the query and key elements. The resulting attention scores indicate how much attention should be assigned to each element in the input sequence.
  • Softmax and Weighted Sum: To make the attention scores interpretable as probabilities, a Softmax function is applied to the computed scores. The Softmax normalizes the attention scores across all elements, ensuring that they sum up to one. Then, the weighted sum of the value vectors (V) is taken using the normalized attention scores as weights. This weighted sum represents the context or information that the model focuses on for the particular query element.

In summary, the weights in the attention mechanism of the transformer architecture are the learnable parameters in the form of weight matrices used to project the input sequence into query, key, and value vectors. The Attention scores are derived by computing the dot product between the query and key vectors, and the Softmax function converts these scores into probabilities. Finally, the weighted sum of the value vectors is calculated using the normalized attention scores, representing the context-specific information for each element in the input sequence. This attention mechanism allows the transformer to effectively capture long-range dependencies and improve performance on various Natural Language Processing (NLP) tasks, among others.

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