I have no expertise in this area, but how about patterns? Since we're already parsing so much text, why not take strings of legible text (with a minor visual distortion), and strip a connecting word such as a preposition? The user is then prompted to enter the connecting word. Synonyms could be matched.
For example: "I can't stand all ____ your lies" (of)
or: "This is all too ____ for me to handle!" (much)
Sure, some strings would be hard to solve, but I feel like I'd be improved over time. Plus, captchas already take a few tries to solve anyway. I suppose it may impose on foreigners though.
For example: "I can't stand all ____ your lies" (of)
or: "This is all too ____ for me to handle!" (much)
Sure, some strings would be hard to solve, but I feel like I'd be improved over time. Plus, captchas already take a few tries to solve anyway. I suppose it may impose on foreigners though.