**Convolutional Layers**

Convolutional layers are the major building blocks used in convolutional neural networks.

A convolution is the simple application of a filter to an input that results in an activation. Repeated application of the same filter to an input result in a map of activations called a feature map, indicating the locations and strength of a detected feature in an input, such as an image.

The innovation of convolutional neural networks is the ability to automatically learn a large number of filters in parallel specific to a training dataset under the constraints of a specific predictive modelling problem, such as image classification. The result is highly specific features that can be detected anywhere on input images.

**Activation function**

Activation function decides, whether a neuron should be activated or not by calculating weighted sum and further adding bias with it. The purpose of the activation function is to introduce non-linearity into the output of a neuron.

Neural network has neurons that work in correspondence of weight, bias and their respective activation function. In a neural network, we would update the weights and biases of the neurons on the basis of the error at the output. This process is known as back-propagation. Activation functions make the back-propagation possible since the gradients are supplied along with the error to update the weights and biases.

**1) Linear Function:**

- Equation: Linear function has the equation similar to as of a straight line i.e. y = ax
- No matter how many layers we have, if all are linear in nature, the final activation function of last layer is nothing but just a linear function of the input of first layer.
- Range : -inf to +inf
- Uses: Linear activation function is used at just one place i.e. output layer.
- Issues: If we will differentiate linear function to bring non-linearity, result will no longer depend on input “x” and function will become constant, it won’t introduce any ground-breaking behavior to our algorithm.

**2) Sigmoid Function:**

- It is a function which is plotted as ‘S’ shaped graph.
- Equation:

A = 1/(1 + e-x)

**Nature**: Non-linear. Notice that X values lies between -2 to 2, Y values are very steep. This means, small changes in x would also bring about large changes in the value of Y.- Value Range: 0 to 1
**Uses**: Usually used in output layer of a binary classification, where result is either 0 or 1, as value for sigmoid function lies between 0 and 1 only so, result can be predicted easily to be 1 if value is greater than 0.5 and 0 otherwise.

**Tanh Function**: The activation that works almost always better than sigmoid function is Tanh function also knows as Tangent Hyperbolic function. It’s actually mathematically shifted version of the sigmoid function. Both are similar and can be derived from each other.

**Pooling Layer**

The pooling or downsampling layer is responsible for reducing the spacial size of the activation maps. In general, they are used after multiple stages of other layers (i.e. convolutional and non-linearity layers) in order to reduce the computational requirements progressively through the network as well as minimizing the likelihood of overfitting.

The key concept of the pooling layer is to provide translational invariance since particularly in image recognition tasks, the feature detection is more important compared to the feature’s exact location. Therefore the pooling operation aims to preserve the detected features in a smaller representation and does so, by discarding less significant data at the cost of spatial resolution.

**Fully connected**

Fully connected layers connect every neuron in one layer to every neuron in another layer. It is the same as a traditional multi-layer perceptron neural network (MLP). The flattened matrix goes through a fully connected layer to classify the images.

Fully connected neural networks (FCNNs) are a type of artificial neural network where the architecture is such that all the nodes, or neurons, in one layer are connected to the neurons in the next layer.

While this type of algorithm is commonly applied to some types of data, in practice this type of network has some issues in terms of image recognition and classification. Such networks are computationally intense and may be prone to overfitting. When such networks are also ‘deep’ (meaning there are many layers of nodes or neurons) they can be particularly hard for humans to understand.