Which of the following describes one characteristic of nystagmus?

Nystagmus is characterized by involuntary eye movements that can be rhythmic and may occur in various planes. One key aspect of nystagmus is the fast phase, which indeed determines the direction of the nystagmus. This fast phase indicates the direction in which the eyes move quickly before slowly drifting back to a primary position, creating the oscillation effect that defines nystagmus. For instance, if the fast phase is directed to the right, the nystagmus is often described as "right-beating" nystagmus.

The other characteristics mentioned do not accurately reflect the behavior of nystagmus. Nystagmus can occur in multiple planes, not limited to vertical, and is not exclusive to horizontal gaze direction. Additionally, while it often presents in both eyes, there are instances where one eye may exhibit different nystagmus patterns than the other, particularly in certain pathologies. Thus, the defining feature of the fast phase correlating with the direction of nystagmus is an essential aspect of its characterization.