El Ahmad, A. S.; Yan, J. & Tayara, M. (2011), 'The Robustness of Google CAPTCHAs' , Technical report, School of Computer Science, Newcastle University, UK .
Zhu, B. B.; Yan, J.; Li, Q.; Yang, C.; Liu, J.; Xu, N.; Yi, M. & Cai, K. (2010), Attacks and design of image recognition CAPTCHAs, in 'CCS '10: Proceedings of the 17th ACM conference on Computer and communications security' , ACM, New York, NY, USA , pp. 187--200 .
Yan, R.; Natsev, A. & Campbell, M. (2007), An efficient manual image annotation approach based on tagging and browsing, in 'MS '07: Workshop on multimedia information retrieval on The many faces of multimedia semantics' , ACM, New York, NY, USA , pp. 13--20 .
von Ahn, L. & Dabbish, L. (2004), Labeling images with a computer game, in 'CHI '04: Proceedings of the SIGCHI conference on Human factors in computing systems' , ACM, New York, NY, USA , pp. 319--326 .
Lynch, K. (1992), The image of the city , MIT Press .