It should be noted that there is variability in the adaptation of rats to foot shocks. This may require that the subjects be grouped as sensitive or resistant to foot shocks. For example, see Chen, B. T., Yau, H-J, Hatch, C., Kusumoto-Yoshida, I., Cho, S. L., Hopf, F. W., & Bonci, A. (2013). Rescuing cocaine-induced prefrontal cortex hypoactivity prevents compulsive cocaine seeking. Nature, 496, 359. doi:10.1038/nature12024