CAPTCHAs play a significant role in differentiating humans and machines in any Web-based authentication process. With technological advances in text recognition and image extraction, it's now possible to extract the characters shown in a CAPTCHA with satisfactory accuracy. CAPTCHAs were introduced to cope with the immense threat to online authentication systems, but interpreting them becomes harder for regular Internet users every day. The authors' alternative CAPTCHA provides a variety of mathematical, logical, and inference problems that only humans can understand and answer correctly. The proposed framework supports question diversity and a user-friendly interface. The authors' informal user study evaluates the developed system's performance with different backgrounds. The study shows the efficacy of the implemented system with a good level of user satisfaction.
Saha, S., Nag, A., & Dasgupta, D. (2015). Human-cognition-based CAPTCHAs. IT Professional, 17 (5), 42-48. https://doi.org/10.1109/MITP.2015.79