In patients with positional vertigo a persistent positional direction-changing nystagmus (PDCN) of apogeotropic direction (a-PDCN) in the supine yaw plane has been described earlier 1-5. It has been suggested that the cupula in the lateral semicircular canal has a higher specific weight than the surrounding endolymph making the cupula sensitive to gravity. This condition is known as ”heavy cupula”. We have described, in Paper I, a geotropic persistent direction-changing nystagmus (g-PCDN) in patients during vestibular crisis6. In addition, when the patient is in the supine position and the head is turned slowly from one side to the other it is possible to discern a zero zone where the geotropic nystagmus is absent. This is accomplished when the head is turned circa 10-20 degrees laterally. Theoretically this occurs when the longitudinal axis of the affected cupula is aligned with the gravitational vertical 7-9. On the assumption that a position dependent nystagmus such as persistent a-PDCN is caused by a heavy cupula in one of the lateral semicircular canals (LSCC), thus it could be hypothesized that a g-PDCN can be caused by the cupula that is lighter than the surrounding endolymph. We have called this new diagnostic entity “light cupula”. A similar phenomenon of a PDCN is seen in subjects with positional alcohol nystagmus (PAN) 10-18. This phenomenon is based on the” buoyancy hypothesis” 19,20. In order to reproduce a clinical condition where the density of the cupula was lower or higher than the surrounding endolymph we examined the nystagmus pattern in different head positions in unilaterally deafferented patients during the stage of PAN 1 and PAN 2 respectively21,22. We compared results of nystagmus direction during PAN 1 (Paper II) with the findings in patients with light cupula in the LSCC (Paper I). Nystagmus direction in both supine and prone lateral head positions was compatible with that of a light cupula. However, the nystagmus directions at head straightforward in prone and supine position as well ...