I've heard that foot writing can be very ticklish. I had a female pen pal once who told me that the only time her soles were ticklish were if someone wrote on them with a pen. Otherwise, she said her soles weren't ticklish.
Personally, I prefer clean feet to ones that are written on, because I also like foot worship as well as tickling, so I dont think I'd like to go near feet that had writing on them, but, as with any subfetish of our fetish, I can see the attraction in it.
Interesting photos. Thank you, butterfly.
Mitch