https://www.sciencedirect.com/science/article/pii/S1053811907004247 (Publisher version)
Lohmann, G., Volz, K., & Ullsperger, M. (2007). Using non-negative matrix factorization for single-trial analysis of fMRI data. NeuroImage, 37(4), 1148-1160. doi:10.1016/j.neuroimage.2007.05.031.