Developmental Differences in Adolescents

Review: by Janet Spring
Schmithorst, V.J., Holland, S.K., Dardzinski, B.J. (2008). Developmental differences in
white matter architecture between boys and girls, Human Brain Mapping, 29: 696 - 710

In the research study of Schmithorst, Holland, Dardzinski (2008), they propose that the adolescent brain must be investigated and analyzed in relation to gender, where “sex differences must be taken into account” to retrieve data that explains the differences in structure of the brain of boys versus girls. The study begins with a thorough literature review presenting research findings to date, on the developmental differences in brain structure between boys and girls as well as in adult males and females. During adolescence, from ages eight to eighteen years of age, boys and girls undergo a great change in brain development as they reach their adult years. It is therefore important for researchers to investigate these differences and map these changes, for many questions in future may be answered that deal with this significant stage in child development.

Since the development of the MRI, researchers have been able to examine the human brain in much greater detail. The investigation of these changes has demonstrated that the brain of the adolescent boy undergoes an increased development in white matter than girls, and that during a male’s lifespan, they have “greater total cerebral volume as well as total gray and total white matter volumes than girls” (p. 696). However, in terms of total relative volume, the brain of the adult female contains greater gray matter area, where the adult male has a greater volume of white matter. Adolescence is therefore a time in which the brain functions and areas make marked changes. The adolescent brain is a work in progress where it is maturing to its adult state. Hence marked changes must occur in their attitudes, learning abilities and behaviour in general.

To examine the obvious changes in the adolescent brain, the MRI has proven to be a very useful tool, as well as the DTI, or the diffusion tensor imaging technique. This measures the FA, or functional anisotropy, and the MD; the mean diffusivity. Anisotropy describes a physical function that shows a variance in different areas, where the diffusivity measures the characteristics of diffusion in a region. The sexual dimorphic measurements, or the variance of different forms in the brains of boys versus girls, demonstrate marked changes and variances in boys which do not occur in girls. Subcortical gray matter in boys is larger than girls, and there is also an increase in the central white matter in boys. As the adolescent boy matures, the study of De Bellis et al. ( 2001) has found that there is a greater increase of “normalized white matter volume, and greater rates of decrease with age of normalized gray matter volume, relative to girls between the ages of 7 – 17” (p. 697). Also, in the left inferior frontal gyrus, “absolute white matter volume was shown to increase in boys, but not in girls, and the relative gray matter volume of the left inferior frontal gyrus remained larger in boys” (p. 697). This evidence was the result of another study of Blanton et al, (2004).

With the information gathered to this point, the authors predict that there is a marked difference in brain anisotropy of boys versus girls, and these differences can be pinpointed with further DTI tests. Previous studies have demonstrated that boys “possess a greater number of neurons and fewer neuronal processes with greater absolute and relative white matter volume available for the inter-neuronal connections” (p. 697). The authors therefore hypothesize that the male brain may contain “fewer, thicker, more organized and possibly more myelinated fibers, with females possessing more crossing fiber tracts” (p. 697). They also predict that the male adolescent brain is reorganizing the neural connections as it reduces the gray matter area, while it contains a larger white matter area. There will perhaps be a significant difference displayed in boys as compared to girls. Can these variations or anisotropy, be responsible for the differences in cognitive functions between boys and girls?

For the study, 106 children were recruited through advertisements in local clinics, radio and television ads. All children received a neurological examination as well as a Wechsler Intelligence test and were not accepted for the study if they did not maintain a C average in school or were under medical treatment for any neurological or psychiatric conditions. The MRI and DTI data was collected for approximately three years, measuring the FA or fractional anisotropy between boys and girls in six areas of the brain and the MD or mean diffusivity between boys and girls in five areas of the brain. These were measured from the early to the later adolescent years. Results were shown in table form, demonstrating the significant differences in age, sex and areas of the brain. The data determines that there are “developmental differences in white matter microstructure between girls and boys” and this difference “may be sexually dimorphic” (p. 707). The researchers therefore conclude that the study of these developmental differences must be examined in boys and girls separately, for the gender of the participant must be taken into account in “all DTI developmental studies” (p. 707).

The study resulted in interesting evidence that points to the fact that the differences in fractional anisotropy could be the “result of differences in fiber organization, but could also be related to myelination, fiber density, axonal diameter, and ratio of intracellular/extracellular space” (p. 704). Differences in the mean diffusivity “relate to fiber density, but are also affected by differences in axonal diameter and myelination” (p. 704). The female data identifies that the splenium of the corpus callosum experienced more FA than boys and that boys are “catching up to girls later in development” (p. 704) where the splenium area is developing later than in girls. White matter regions in boys show greater FA than girls across all age ranges and the white matter in girls more constrained. Therefore they hypothesize that “the more constrained white matter space, lower degree of fiber density, and greater dependence on intra- and inter-hemispheric connectivity in females necessitates increased crossing of white matter fiber tracts, resulting in lower FA values” (p. 705).

Other conclusions can be drawn that demonstrate a difference between boys and girls where the left inferior frontal gyrus contains a higher gray matter volume in boys, and where the relative gray matter decreases slightly in boys as they approach adulthood. To correlate with the fact that language development in boys is delayed as compared to girls, the results of the study demonstrate “continued maturation of left frontal white matter in boys, which may relate to continuing myelination” (p. 706). All data therefore “point to the importance of taking sex differences into account in developmental DTI studies”.

Reflection:
This study as well as previous studies on childhood sex differences in the brain has far reaching implications for educators who work with this interesting group of students. As boys mature physically at a different rate from girls, so do their brain functions and specific areas of the brain that control their abilities to learn and understand concepts in varying subjects. While the adolescent is often treated the same in the learning environment regardless of gender, mistakes may be made in what should be expected of them. If brain functioning is at different levels in boys versus girls, can they be evaluated the same? Should we therefore teach adolescent girls differently from adolescent boys and develop different curricula to evoke better results? Should they be segregated and taught in girls’ only and boys’ only schools? Regardless, educators need to be educated and trained accordingly so that they possess the expertise to teach the adolescent whose brain is most certainly developing at a different rate and is diverse in its physical structure.