A phenomenon: left-biassed asymmetrical eye-opening in artificially reared rat pups

From 4 or 5 postnatal days, rat pups were either reared by their mothers (MR) or artificially reared (AR) without their mothers by infusion of 'milk' through a gastric cannula. In AR, but not MR, pups the left eye was more likely to open first than the right eye (P less than 0.001). The left-biassed eye-opening of AR pups may be a manifestation of stress-induced lateralisation of their brains.