I made a blind test with my kids on a chat interface. They gave me so unexpected results because it was just a idea, I had to try it out a bit further.
When you browse a BBS 'oldskool' with a ASCII terminal you scroll up and down not sideways, so I simply replaced "previous / next" in the form of << and >> with a distinct image with arrow UP or Arrow DOWN, with the text " UP " and "DOWN in the same size.
Page up and Page down. It's not logical to scroll sideways when you have a scrollbar up/down, or?
It's like having a UI with a MAJOR scrollbar underside to jump forward in messages.....
/X