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Content: It never reversed in normal subjects (Martens, 2015). In persons with lateral canal BPPV, the lateral canal is tilted in the head so debris can roll one way when they are down, and another way when they are sitting. Bitorsional nystagmus (BTN)
This refers to a twisting of the eyes elicited by having the person lie supine. It is nearly always geotropic -- twisting so that the fast phase is downward in space. The origin of this common pattern is unclear. It is sometimes seen in persons with pontine injuries, sometimes in persons with bilateral BPPV, and most comonly in persons with Migraine associated vertigo. In the central forms, presumably it is associated with abnormal processing of otolithic signals. Cervical Nystagmus (CVN)
There are several movies concerning cervical vertigo on thelist of resources. Cervical vertigo basically means dizziness that is a function of the position of the head on the trunk (i.e. neck). Cervical nystagmus is  a nystagmus that always goes the same direction no matter how the head is oriented to gravity.