Techreport (elahmad2011robustness)
El Ahmad, A. S.; Yan, J. & Tayara, M.
The Robustness of Google CAPTCHAs
School of Computer Science, Newcastle University, UK, 2011

Inproceedings (zhu2010attacks)
Zhu, B. B.; Yan, J.; Li, Q.; Yang, C.; Liu, J.; Xu, N.; Yi, M. & Cai, K.
Attacks and design of image recognition CAPTCHAs
ACM, 2010, 187-200

Inproceedings (yan2007efficient)
Yan, R.; Natsev, A. & Campbell, M.
An efficient manual image annotation approach based on tagging and browsing
ACM, 2007, 13-20

Inproceedings (985733)
von Ahn, L. & Dabbish, L.
Labeling images with a computer game
ACM, 2004, 319-326

Book (lynch1992image)
Lynch, K.
The image of the city
MIT Press, 1992