# Neural Networks

This post is part of the series Neural Networks.

Devising a mathematical model of a system requires careful thought so that it accurately represents the system. In relatively simple applications, for example in calculating the trajectory of a ball launched with a certain velocity, the underlying physics can be represented directly by the model. In such an example, the equations of Newtonian mechanics can be integrated to calculate the path of the ball.

As the underlying system becomes more complex, for example when modelling a biological system, simplifications and approximations must be made, as the system becomes too large to model explicitly.

In machine learning, the physics or relationships underlying the data may not be known at all - in fact, it is usually our goal to discover these relationships through our analysis of the data with no or limited a priori knowledge. Deciding on a suitable mathematical model to fit to the available data can then be a difficult task - the choice of an unsuitable model may unfairly bias the results of the analysis.

## Inspiration from Neurology

Originally devised to model and study the mechanisms of the brain, computational neural networks have increasingly found utility as a powerful machine learning technique. The brain is modelled as a large network of neurons, each of which is considered to be a simple computational unit taking a number of inputs $$\{x_i\}$$ and providing a single output (or “activation”) $$a$$. The activation is calculated by applying weights $$\{w_i\}$$ and a bias term $$b$$ and passing to an activation function $$g$$:

$a = g \left( \sum_i w_i x_i + b \right)$

A common choice for $$g$$ is the sigmoid function:

$g(x) = \sigma(x) = \frac{1}{1 + e^{-x}}$

Neurons are often depicted as below, receiving a number of inputs from the left and providing an output to the right:

A more complex model can then be constructed by assembling a network of such neurons, where multiple layers of neurons have their inputs and outputs connected together:

More complex structures with reentrant positive feedback loops are also possible and may offer additional power, however here we concentrate on the simpler unidirectional, layer-arranged structure as above. Note that while some neurons in the above network appear to provide different outputs, the different arrows leaving a neuron in fact correspond to the same output being sent to multiple neurons in the next layer of the network.

## Intuition

By combining together a network of simple computational networks, it is possible to form a network capable of producing a wide range of mathematical representations. A set of inputs $$\{x_i\}$$ can be provided to the input layer, processed through the neural network, and a useful output or outputs $$\{a_i\}$$ then generated. This is how real neural networks in the brain are thought to work, and the application of the concept to machine learning turns out to be a powerful technique.

## Further Detail

For further detail on the implementation and application of neural networks to a handwritten digit recognition system, see the list of other posts in this series below.