"What neurotoxin? "
There is a theory that motion sickness has evolved because it serves an evolutionary purpose. You get sick on the boat, you run back to land, you stay at home and survive to breed.
No, really. The theory is that most animals
will ingest neurotoxins at some point, from eating spoiled foods, toxic foods, nasty critters, and the like. A common symptom of that will be eye/ear discord and if you react by puking up whatever you just ate--you get rid of the toxins and stay in the gene pool.
Which does make sense, so while it is just a theory, it could be a logical explanation for why our bodies react so violently to "just" a little ear/eye discord.