To cite package ‘ggseg.extra’ in publications use:

  Mowinckel A, Vidal-Piñeiro D (2019). “Visualisation of Brain
  Statistics with R-packages ggseg and ggseg3d.” 1912.08200.

A BibTeX entry for LaTeX users is

  @Misc{,
    title = {Visualisation of Brain Statistics with R-packages ggseg and ggseg3d},
    author = {Athanasia M. Mowinckel and Didac Vidal-Piñeiro},
    year = {2019},
    eprint = {1912.08200},
    archiveprefix = {arXiv},
    primaryclass = {stat.OT},
  }