It looks like they aren't accessible to blind or low-vision people, so I'm going to discourage you from using them in any case. I might add color-blind too for some of them. Clever, but inaccessible.
Agreed on all points (clever point on color-blindness). We have no plan to use them, but are developing a CAPTCHA platform; I'm just curious about their perceived/posited effectiveness against being cracked. They do not strike me as being particularly difficult in relation to Google/reCAPTCHA.
FYI: (a little outdated) http://www.w3.org/TR/turingtest/
Anne Taylor at NFB.org in Baltimore can suggest an accessible one. I'm afraid I don't know which is considered best right now. If you have something really new I'd love to get a look at it from the accessibility perspective.
Thanks for the referral to Anne Taylor. I'll definitely connect. Can you email me off list -- I'd love to get your feedback on our upcoming release,