Linear mixed model: question about t-values

I have a question about the output of linear mixed model fitted in R
using nlme package. In particular, what are the t-values that are given
in an output, how are they calculated and based on what test? I guess it
cannot be a simple Student t-test, otherwise how can the simple Student
t-test test for significance of interactions, right? I cannot find this
information in any of R help resources on linear-mixed models and I also
checked few books.