Effects of directional migration on prisoner’s dilemma game in a square domain Hongyan Cheng, Qionglin Dai, Haihong Li, Xiaolan Qian, Mei Zhang and Junzhong Yang Eur. Phys. J. B, 86 4 (2013) 127 Published online: 01 April 2013 DOI: 10.1140/epjb/e2013-40076-5