This is a subset of the full data set from Mehta et al., which contained fourteen litters. The litters were selected to have an equal sample size across all conditions for illustrative purposes. The complete data can be found in the supplementary material of Lazic and Essioux (2013).
Lazic SE, Essioux L (2013). Improving basic and translational science by accounting for litter-to-litter variation in animal models. BMC Neuroscience 14:37.