Sex differences and reversal of brain asymmetry by testosterone in chickens

The study examined the effect of testosterone on the asymmetry of visual discrimination performance of young chicks. Two-week-old chicks were tested on the pebble floor visual discrimination task. Male chicks were found to have brain asymmetry for visual discrimination learning, since chicks tested binocularly, or tested monocularly using their right eye system, have superior learning performance compared to chicks tested monocularly using their left eye system. Control female chicks were not found to have brain asymmetry. Testosterone treatment (12.5 mg of testosterone oenanthate on day 2 posthatch) reversed the pattern of brain asymmetry observed in control males but not females. In addition, asymmetry of visual discrimination learning is demonstrated in both male and female chicks after unilateral intracranial treatment with monosodium glutamate.