The term bicommissural line refers to a reference line in the midline sagittal plane that defines the horizontal plane for brain stereotaxis. For ventriculography In the human it is defined as the line passing through the upper border of the anterior commissure and the lower border of the posterior commissure ( Talairach-1957 ). The same definition was commonly used in MRI studies for many years. More recently some authors recommend that it be defined as the line passing through the centers of the anterior commissure and the posterior commissure ( Kretschmann-2004 }. The latter definition is commonly used to define the bicommissural line and the horizontal plane in the macaque ( Martin-1996; Paxinos-2000 ).