Key points are not available for this paper at this time.
Mary J. Lindstrom, Douglas M. Bates, Newton-Raphson and EM Algorithms for Linear Mixed-Effects Models for Repeated-Measures Data, Journal of the American Statistical Association, Vol. 83, No. 404 (Dec., 1988), pp. 1014-1022
Lindstrom et al. (Thu,) studied this question.