Alternatives to CAPTCHA

I've started searching for references that document useful alternatives to CAPTCHA. The following Master's thesis contains useful material.
Foley, A. (2012). Biometric alternatives to CAPTCHA: Exploring accessible interface options.
Essentially, several alternative means of biometric authentication were provided, which, together, were designed to account for a range of user capabilities. There's a good section of the thesis discussing biometrics, and another that summarizes alternatives to CAPTCHA.
If I understand correctly, this alternative does not preserve the user's anonymity as a CAPTCHA does, which could be problematic in some applications.


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