Cognitive sex differences are consistently observed in cognitive development, cognitive performance and cognitive degeneration; however, the exact factor(s) that lead to these differences is yet to be discovered. The organizational-activational hypothesis suggests that gonadal hormones may be the underlying factors that lead to cognitive sex differences; however, this hypothesis does not suggest any direct links between gonadal hormones or specific brain structures and cognition. This thesis aimed to explore potential theoretical and experimental links between gonadal hormones, cognition and specific brain structure / function. Thus, the association between cognitive performance as well as cognitive brain type with individual masculinization / feminization levels were initially explored. Finally, the association between individual masculinization / feminization levels and brain activity was explored in order to locate a link between masculinization / feminization levels and specific brain function. In order to achieve this, three experiments were designed. The first experiment explored sex differences in a multi-trial verbal free recall test and male / female allocation on a cognitive brain type scale according to an individual’s cognitive brain type. Cognitive brain type was measured by the difference between a female-biased task (free recall) and a non-female biased task (productive vocabulary). For exploring sex differences in free recall, a two-way mixed ANCOVA was used, with the dependent variable being free recall and the independent being ‘sex’, which was used as a general index of individual masculinization / feminization levels. The covariates were factors that previous studies have indicated as significant determinants of free recall, engaging frontal lobe function such as executive function and working memory. Male / female allocation on the brain type scale was explored via a Fisher’s exact test of independence, which was used in order to explore the existence of an association between male / female group and brain type group membership. The second experiment explored the association between cognitive performance as well as cognitive brain type with individual masculinization / feminization levels. The utilized variables as well as the study design was the same as of the first experiment, while an indirect measurement of pre-natal hormone effects (2D:4D), a direct (saliva) measurement of circulating testosterone and estradiol and a measure of brain type via systemizing-empathizing (S-E) were added. Thus, individual masculinization / feminization levels were indicated by sex (as a general index), and pre-natal and post-natal indices / levels of gonadal hormones. The S-E brain type measurement was used as way to confirm the effectiveness of the cognitive brain type measure that was used in this experiment. In the third study the association between individual masculinization / feminization levels with Theta reactivity was explored. The results confirmed previous studies indicating a female advantage in verbal free recall. Moreover, masculinization / feminization levels appeared to be associated with cognitive brain type classification. S-E brain type classification was associated with individual masculinization / feminization levels only when sex was included as a factor; while S-E brain type showed a positive trend across, and an association with, cognitive brain types. Finally, Theta reactivity appeared to be sexually differentiated in the left frontal lobe; while individual masculinization / feminization levels were associated with Theta reactivity. These results were interpreted to suggest that measures of masculinization / feminization can replace ‘sex’ as a dissociating factor in verbal free recall and potentially other sex-biased cognitive tasks. That is, cognitive brain type appears to be linked to individual masculinization / feminization levels and both measures of brain type possibly rely on the same underlying biological factors. Sex differences as well as individual masculinization / feminization level differences may exist in hippocampal function.
|Date of Award||2016|
|Supervisor||Clare Wood (Supervisor) & Hayley Wright (Supervisor)|