vHIT: What are the terms RALP, LARP and laterals in reference to?

The video Head Impulse Test (vHIT) produces a rotational stimulus to the head and then assesses the resulting eye movement via the Vestibulo-Ocular Reflex (VOR). The rotation stimulus is applied in the anatomical planes of the respective semi-circular canals hence each stimulus is best detected by one pair of canals; each canal is a coplanar pair with another canal in the other ear.

“Lateral” refers to a head rotation in the horizontal plane, usually with the head bowed forward by approximately 30°. This best stimulates the horizontal canals of each ear (which are a co-planar pair).

“RALP” means Right Anterior, Left Posterior and refers to a head rotation in an orientation which best stimulates this co-planar pair. “LARP” means Left Anterior, Right Posterior and refers to a head rotation in an orientation which best stimulates this co-planar pair. The below figure is a schematic showing a top-down view of the head, which visualises the orientation of the semi-circular canals of each ear.