S. Singh, A. Gupta and A. Efros. Unsupervised discovery of mid-level discriminative patches, ECCV, 2012.
And additionally:
L. Bourdev and J. Malik. Poselets: Body Part Detectors Trained Using 3D Human Pose Annotations, ICCV, 2009.
Y. Li, L. Liu, C. Shen, A. Hengel. Mid-level Deep Pattern Mining, arXiv preprint arXiv:1411.6382 (2014).