Patricia E Cowell

The investigation of regional neuroanatomy along dimensions of interest such as male-female differences is a complex task. It might seem at first glance a simple job to compare the size of a brain region in one group of research subjects to the size of the same region in another group. However, even the most straightforward comparisons of neurobiological measures can be contaminated or influenced by variability in the data that is related to factors outside the purview of the particular research in question. For example, a comparison of temporal lobe size in men and women to examine anatomical sex differences in this part of the brain may be affected by other aspects of the subjects' biographies (e.g., age, medical history) or neurobehavioral profiles (e.g., brain size, hand preference) even though these were not the original or primary foci of the study. When the relationships between such other factors and the research measures of interest are not well understood themselves, the task of accounting or correcting for their influence is a difficult one. Moreover, if these contaminating effects of outside factors are strong enough, even the most straightforward interpretation of a basic research comparison may be called into question.

The corpus callosum (CC), shown in Figure 3.7, is one brain structure that has been the source of both considerable interest and debate in the area of individual differences in human and animal research. Yet investigation of this structure's anatomy has been clouded by multiple methodological weaknesses, including small sample sizes, disagreement on which units of measurement to use, and analytic approaches that failed to account for the influence of factors such as age when examining sex differences (for further discussion, see Denenberg et al., 1989, 1991; Cowell et al., 1992, 1993, 1994a; Bishop and Wahlsten, 1997). Also, researchers

Patricia e. cowell Department of Human Communication Sciences, University of Sheffield, Sheffield, United Kingdom.

have studied the CC to great extent without having a complete understanding of its relationship to the whole brain. Clearly, the overall size of an organism's head or brain has some influence on the size of the structures contained within, but brain size, in its many possible forms of measurement, has a complex and not well-understood relationship with regional anatomical measures such as the callosum. In research in which the effects of brain size are not considered, it is possible that group differences simply reflect variation in the size of the whole brain. Alternatively, when the effects of brain size are removed from analyses of the CC in an inappropriate fashion, the result may be data that are over-corrected or undercorrected.

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