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.