Quantitative analysis of sex differences in hormone accumulation in the zebra finch brain: methodological and theoretical issues.

The autoradiographic method was used to compare the numbers of hormone accumulating cells in several brain regions in male and female zebra finches (Poephila guttata) after injection of tritiated testosterone. The brain regions examined were the caudal nucleus of the hyperstriatum ventrale (HVc), magnocellular nucleus of the anterior neostriatum (MAN), robust nucleus of the archistriatum (RA), nucleus intercollicularis of the midbrain (ICo), the tracheosyringeal hypoglossal motor nucleus (nXIIts), and periventricular magnocellular nucleus of the anterior hypothalamus (PVM). All but the last of these regions are thought to be involved in the control of vocalizations in passerine song birds. Males have significantly more labelled cells in HVc and MAN. In RA, there is no difference in total percentage of labelled cells, but there is a sex difference in size distribution of labelled cells. No sex difference was detected in other brain regions. These differences are found when using a criterion for cell labelling which is based on the Poisson distribution, and the relative merits are evaluated of various quantitative criteria used in the analysis of steroid autoradiograms. The magnitude of the observed sex difference may be influenced by several biasing factors, yet the sex difference persists when corrections are applied to eliminate the biases, indicating that the sex difference is not an artifact of autoradiographic procedure. The magnitude of the sex difference in hormone accumulation has certain implications for the process of sexual differentiation of the brain.