Horizontal direction-changing
Horizontal-canal BPPV — direction-changing on the roll test.
What the animation does not show. This clip shows one horizontal beat and does not localise the affected ear. Side is decided by comparing the strength of the two roll positions at the bedside, which a single-position animation cannot show. Read the direction here, not a side (Bhattacharyya 2017).
Purely horizontal nystagmus that reverses direction as the head is rolled to each side while supine. The common geotropic form beats toward the ground in both positions. Treated with a barbecue-roll or Gufoni maneuver, not the Epley.
The supine roll test turns the head fully to one side, then the other. Horizontal-canal BPPV beats horizontally in both positions, but the direction reverses between them — that reversal is what names it. In the geotropic form the beat is toward the ground (the dependent ear) in each position (Bhattacharyya 2017).
Which ear is affected is inferred by comparing the two positions, not from a single view: in geotropic disease the stronger beat is usually with the affected ear down. That comparison — stronger side versus weaker side — is a bedside judgement made across both roll positions.
It stays peripheral and treatable. The maneuvers are a barbecue-roll or a Gufoni, not the Epley, which is a posterior-canal treatment (Bhattacharyya 2017; Lempert; Gufoni).
Source: AAO-HNS BPPV clinical practice guideline (Bhattacharyya 2017), which covers the lateral canal; the Lempert roll and Gufoni maneuver are named there.