Path analysis of sex difference, forebrain commissure area and brain size in relation to degree of laterality in selectively bred mice.

Male and female mice from HI and LO lines selectively bred by Collins for strength of lateralization were tested for paw preference and then studied histologically to assess size of forebrain commissures and myelination of the corpus callosum. When compared to LO line mice, HI line mice were more strongly lateralized for paw preference and had larger brains as well as greater cross-sectional areas of the anterior commissure and corpus callosum. A substantial sex difference was found only for body size. Myelination of the corpus callosum did not differ consistently between the lines or sexes. Path analysis indicated that the line difference in the anterior commissure was a consequence of the difference in brain size, but corpus callosum size was actually smaller in the HI line than the LO line when brain size was taken into account. However, the size of the corpus callosum relative to brain size was not related to strength of paw preference, whereas brain size relative to corpus callosum size was positively correlated with strength of paw preference. These results support the hypothesis that the large difference in brain size between the Collins HI and LO lines is an important cause of the difference in strength of behavioral lateralization.