Testosterone: a role in the development of brain asymmetry in the chick.

The visual projections from the thalamus to the Wulst of the chick forebrain are asymmetrically organized. The development of this asymmetry is dependent on light stimulation just prior to hatching, and it is present to a greater extent in males than in females. We have shown that administration of testosterone to the developing embryo alters the development of the asymmetry of the projections in both sexes. Following testosterone treatment, the asymmetry normally present in male chicks after hatching was marginally reversed in direction, and that normally present in females was no longer present. Thus the development of these visual projections depends on the interaction of light stimulation and the level of circulating testosterone.