Human Interactive Proofs (HIPs)

Characters or Faces: A User Study on Easy of Use for HIPs . Yong Rui, Zicheng Liu, Shannon Kallin, Gavin Janke, Cem Paya. HIP 2005, Lecture Notes in Computer Science 3517, Henry S. Baird and Daniel P. Lopresti (eds.) pp53-65.

ARTiFACIAL: Automated Reverse Turing test Using FACIAL features. Yong Rui, Zicheng Liu, Multimedia Systems Journal , No. 6, Vol. 9, pp. 493--502, 2004.