Light experience and asymmetry of brain function in chickens

Abstract

Asymmetry of brain function has been demonstrated for several non-human species¹, most clearly in the avian brain^2,3. In the chicken, a single treatment of the left forebrain hemisphere during the first week of life after hatching with either cycloheximide or the putative neurotransmitter, glutamate, subsequently causes retarded visual and auditory learning, and elevated attack and copulation responses^3–5. Similar treatment of the right hemisphere is without effect. The cellular mechanisms by which these pharmacological agents may alter brain development and so reveal lateralization have already been discussed in detail^6,7. It is commonly assumed that lateralization of brain function in humans and other species is inherited either genetically^1,8 or cytoplasmically⁹. However, Rogers and Anson have suggested previously³ that light experience may have an important role in establishing lateralization in the chicken forebrain, because after day 17 of incubation the embryo is oriented in the egg such that the left eye is occluded by the chicken's wing and body while the right eye is next to the air sac and exposed to light input (refs 10,11 and J. V. Zappia and L.J.R., in preparation). Because the optic nerves decussate completely and most of the information reaching each tectum is processed by its ipsilateral forebrain hemisphere, it is possible that light entering the right eye stimulates developmental processes in the left hemisphere in advance of the right³. If so, chickens hatched from eggs incubated in darkness should fail to show functional asymmetry of the forebrain. I now report that this is indeed the case for asymmetrical control of attack and copulation.