Fig. 17 Periodic alternating nystagmus (PAN) sequence is depicted in relation to waveform and null angle. Sequence reflects one PAN period (from 0 to T). Period begins in a neutral phase; null is at 0°. As null shifts to the left, jerk right (JR) nystagmus develops and gradually increases in amplitude to maximum when null is at extreme left (e.g., 30°L). Null then shifts back toward 0°, and JR nystagmus decreases, finally stopping and forming the next neutral phase when null reaches 0°. The same sequence of null shifting to right and back accounts for jerk left (JL) phase. The diagram is idealized and does not reflect the asymmetries of the true clinical state. (Daroff RB, Dell'Osso LF: Periodic alternating nystagmus and the shifting null. Can J Otolaryngol 3:367, 1974)