In 40 cases of benign paroxysmal positional nystagmus, the author observed a characteristic change in the type of nystagmus with change in direction of gaze. This typical reaction consisted of vertical nystagmus when looking away from and rotary nystagmus when looking toward the lowermost precipitating ear. The fast component was usually up in the vertical nystagmus and toward the lower ear in the rotary form. Stimulation of a posterior semicircular canal, possibly by loose otoconia, may account for these findings.
Harbert F. Benign Paroxysmal Positional Nystagmus. Arch Ophthalmol. 1970;84(3):298-302. doi:10.1001/archopht.1970.00990040300007