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Month: January 2014

Neural Networks, Part 2: The Neuron

A neuron is a very basic classifier. It takes a number of input signals (a feature vector) and outputs a single value (a prediction). A neuron is also a basic building block of neural networks, and by combining together many neurons we can build systems that are capable of learning very complicated patterns. This is part 2 of an introductory series on neural networks. If you haven’t done so yet, you might want to start by learning about the background to neural networks in part 1.

Neurons in artificial neural networks are inspired by biological neurons in nervous systems (shown below). A biological neuron has three main parts: the main body (also known as the soma), dendrites and an axon. There are often many dendrites attached to a neuron body, but only one axon, which can be up to a meter long. In most cases (although there are exceptions), the neuron receives input signals from dendrites, and then outputs its own signals through the axon. Axons in turn connect to the dendrites of other neurons, using special connections called synapses, forming complex neural networks.

neuron
Figure 1: Biological neuron in a nervous system

Below is an illustration of an artificial neuron, where the input is passed in from the left and the prediction comes out from the right. Each input position has a specific weight in the neuron, and they determine what output to give, given a specific input vector. For example, a neuron could be trained to detect cities. We can then take the vector for London from the previous section, give it as input to our neuron, and it will tell us it’s a city by outputting value 1. If we do the same for the word Tuesday, it will give a 0 instead, meaning that it’s not a city.

aneuron
Figure 2: Artificial neuron

Neural Networks, Part 1: Background

Artificial neural networks (NN for short) are practical, elegant, and mathematically fascinating models for machine learning. They are inspired by the central nervous systems of humans and animals – smaller processing units (neurons) are connected together to form a complex network that is capable of learning and adapting.

The idea of such neural networks is not new. McCulloch-Pitts (1943) described binary threshold neurons already back in 1940’s. Rosenblatt (1958) popularised the use of perceptrons, a specific type of neurons, as very flexible tools for performing a variety of tasks. The rise of neural networks was halted after Minsky and Papert (1969) published a book about the capabilities of perceptrons, and mathematically proved that they can’t really do very much. This result was quickly generalised to all neural networks, whereas it actually applied only to a specific type of perceptrons, leading to neural networks being disregarded as a viable machine learning method.

In recent years, however, the neural network has made an impressive comeback. Research in the area has become much more active, and neural networks have been found to be more than capable learners, breaking state-of-the-art results on a wide variety of tasks. This has been substantially helped by developments in computing hardware, allowing us to train very large complex networks in reasonable time. In order to learn more about neural networks, we must first understand the concept of vector space, and this is where we’ll start.