I realize that this is in the Diving Medicine area, and I have no backgroud to answer a medical question, but I thought I'd share my experience. I have had the same issue numerous times, and for me it seems to happen if one ear "clears" before the other on the ascent. Usually the other ear catches up shortly and the feeling goes away immediately. Once I figured out why it was happening I found it very easy to manage, although the first time it happened I was quite freaked out by it. Now when it happens, I slow the acsent until the other ear catches up, and then continue to ascend. Hope that hepls.
Jason
Yes, you have rediscovered a basic principle of vestibular physiology - dizziness often follows when the left and right inner ears are stimulated differently!
The usual explanation for this is that the vestibular system has evolved as two organs set in solid bone, so when you move your head, they experience symmetric but opposite inputs, and your brain is used to interpreting that as motion in three dimensional space. However, there are some circumstances in which one inner ear is stimulated differently than the other, and your brain doesn't know what to do with this input. Among other things, it can cause the subjective impression of vertigo.
There are two ways of asymmetrically stimulating the inner ear in diving: Altenobaric vertigo and the cold caloric response.
Altenobaric vertigo is caused by one ear being ventilated (through the Eustachian tube) better than the other. The caloric response comes from cold water in the outer ear canal stimulating the inner ear (the semicircular canal) through the bone. So you can have dizziness even with perfect ear clearing if, for example, you have earwax or a poorly fitting hood that protects one ear canal from cold water more than the other. In fact, one test of inner ear function is to squirt cold water into one ear, which should result in nystagmus (oscillation of the eyes) or dizziness.
One more point about diving related vertigo - as we said, vertigo results when the brain gets confusing inputs. Your brain monitors your position in space by three inputs: muscle tension receptors in the spine, your visual perception of your environment and your inner ears. So if you sit in the cabin of your dive boat and try to read, the book, your hands and your eyes are all moving the same way, so your eyes are telling your brain that you are sitting still. However your inner ears are disagreeing with that input due to the motion of the whole boat on the ocean, "confusing" the brain and resulting in vertigo! So the reason that staring at the horizon helps with seasickness is that it lets your brain "realize" that the signals that it is getting from the eyes and the inner ears are consistent with each other.
Mike
