一些gassian分布假设的，并假设均值依赖于隐藏层，方差固定的模型

PPCA_probabilistic principal component analysis

M. E. Tipping and C. M. Bishop, “Probabilistic principal component
analysis,”Journal of the Royal Statistical Society, Series B, vol. 61,
pp. 611–622, 1999.

FA_factor analysis

G. Young, “Maximum likelihood estimation and factor analysis,”
Psychometrika, vol. 6, no. 1, pp. 49–53, 1940

ICA_independent component analysis

D. MacKay, “Maximum likelihood and covariant algorithms for
independent component analysis,” 1999.

SC_sparse coding

B. A. Olshausen and D. J. Field, “Sparse coding with an overcom-plete basis set: a strategy employed by v1?”Vision Research, vol. 37,
pp. 3311–3325, 1997.

GRBM_gassian restricted Boltzmann machine

M. Welling, M. Rosen-Zvi, and G. Hinton, “Exponential family har-moniums with an application to information retrieval,” inAdvances
in Neural Information Processing Systems, 2005.