When I started reading Japanese books in my twenties, I was surprised at how frequently my eyes accidentally jumped to the wrong line when moving back to the top of the page. Reading the correct line was completely automatized in my horizontal reading, but for a while when I first started vertical reading I needed to put a notecard or bookmark on the page and use it to mark the line I was on. I got used to it, though, and now it’s automatic again in both horizontal and vertical text.
When I started reading Japanese books in my twenties, I was surprised at how frequently my eyes accidentally jumped to the wrong line when moving back to the top of the page. Reading the correct line was completely automatized in my horizontal reading, but for a while when I first started vertical reading I needed to put a notecard or bookmark on the page and use it to mark the line I was on. I got used to it, though, and now it’s automatic again in both horizontal and vertical text.