There is an alternative to the popular Google reCaptcha (https://www.google.com/recaptcha) that I have used 10+ years ago, called Text Captcha. (Try http://api.textcaptcha.com/cp.json for a demo) It is simple language type questions that require a very minimal effort to follow…but somehow I prefer greatly over those terrible images I can’t seem to understand.
The way to “challenge” this sort of captcha would be through natural language processing. The varied options aren’t that hard, that I’m sure it wouldn’t be too challenging to break. I was thinking however of combining this with the now defunt Google image labeler (http://en.wikipedia.org/wiki/Google_Image_Labeler). I used to play this once in a while many years ago. Let’s take the game and the logic to the next level. Imagine if we have a series of three images when put together would convey a theme. For instance: turkey + leaves + football = Thankgiving. Yeah that was corny, but that is not a simple thing to crack. That involves understanding what each image may possible mean (in all contexts) and then limit that understanding to the perceived intent with the accompanying images.
An alternate type could be the complete the sequence. Like having a ‘horse and buggy’, ‘train’, ?, that would be an automobile. The sequence of course is forms of ground transportation going from the past to the most modern.
The two options I have given are not friendly to those who are visually impaired, but that doesn’t rule it out as a possibility for a majority of the population. I think that a more complex text captcha is also possible, but that will still be left to a simpler NLP attack.