Speculation.
In his 2007 paper "Self-taught learning", Professor Ng talksSelf-taught learning: Transfer learning from unlabeled data, Rajat Raina, Alexis Battle, Honglak Lee, Benjamin Packer and Andrew Y. Ng. In Proceedings of the Twenty-fourth International Conference on Machine Learning, 2007. Available at http://www.cs.stanford.edu/people/ang//papers/icml07-selftaughtlearning.pdf