Unsupervised learning is closely related to the problem of density estimation in statistics. However unsupervised learning also encompasses many other techniques which seek to summarise and explain key features of the data.
One form of unsupervised learning is clustering. Among neural network models, the Self-Organizing Map and Adaptive resonance theory (ART) are commonly used unsupervised learning algorithms. The ART model allows the number of clusters to vary with problem size and lets the user control the degree of similarity between members of the same clusters by means of a user-defined constant called the vigilance parameter. ART networks are also used for many pattern recognition tasks, such as automatic target recognition and seismic signal processing. The first version of ART was "ART1", developed by Carpenter and Grossberg(1988).