Are there any textual CAPTCHA challenges which can fool AI, but not human? Are there any modern techniques of generating textual CAPTCHA (so person needs to type the right text) challenges which can easily fool AI with some visual obfuscation methods, but at the same time human can solve them without any struggle?

For example I'm talking about plain ability of recognising text embedded into image (without considering any external plugins like flash or java, image classification, etc.) and re-typing the text that has been written or something similar.

I guess adding noise, gradient, rotating letters or changing colours are not reliable methods any more, since they can be quickly broken.

Any suggestions or research has been done?
A method that could possibly work is utilising optical illusions such as one where two lines down a hallway are identical but one seems longer to the human eye, then they could be prompted with a multiple choice question as to the state of the line, which to our eyes looks longer, but to a computer, is still the same length of line. Of course, there is always the issue of people with eye based disabilities not being able to complete them, but different illusions could be used to accommodate that.

Example