Interestingly, no statistically significant hemispheric differences could be identified at this fine structural level that were corroborated in both brains (paired one-sided t-tests, P < 0.01, Benjamini–Hochberg (BH)-corrected). Although surprising given well described lateralization of function, this finding is consistent with a recent study of developing human neocortex that failed to identify hemispheric differences despite extensive efforts using microarrays and quantitative PCR11. It may be that the basis for lateralization of function involves more subtle changes in specific cellular components, differences in relative area rather than type of functional domains between hemispheres, or is more related to functional connectivity patterns than molecular differentiation. Given this observation and to increase statistical power, samples from the two hemispheres for each structure were pooled for all subsequent analyses. In each brain independently, 84% of unique transcripts on the microarrays (29,412, referred to as genes for this manuscript) were found to be expressed in at least one structure (91.4% overlap in expressed gene sets between brains), consistent with the percentage of genes expressed in mouse brain by ISH (80%; ref. 1) and fetal human
