To cite TractoR (of which 'tractor.base' is part) in publications,
please use:

  Clayden JD, Muñoz Maniega S, Storkey AJ, King MD, Bastin ME, Clark CA
  (2011). “TractoR: Magnetic resonance imaging and tractography with
  R.” _Journal of Statistical Software_, *44*(8), 1-18.
  doi:10.18637/jss.v044.i08 <https://doi.org/10.18637/jss.v044.i08>.

A BibTeX entry for LaTeX users is

  @Article{tractor-paper,
    title = {{TractoR}: Magnetic resonance imaging and tractography with {R}},
    author = {Jonathan D Clayden and Susana {Muñoz Maniega} and Amos J Storkey and Martin D King and Mark E Bastin and Chris A Clark},
    journal = {Journal of Statistical Software},
    year = {2011},
    volume = {44},
    number = {8},
    pages = {1--18},
    doi = {10.18637/jss.v044.i08},
  }