Mathematics, sex hormones, and brain function

Sex hormonal modulation of interhemispheric transfer time

It is still a matter of debate whether functional cerebral asymmetries (FCA) of many cognitive processes are more pronounced in men than in women. Some evidence suggests that the apparent reduction in women's FCA is a result of the fluctuating levels of gonadal steroid hormones over the course of the menstrual cycle, making their FCA less static than for men. The degree of lateralization has been suggested to depend on interhemispheric communication that may be modulated by gonadal steroid hormones. Here, we employed visual-evoked EEG potentials to obtain a direct measure of interhemispheric communication during different phases of the menstrual cycle. The interhemispheric transfer time (IHTT) was estimated from the interhemispheric latency difference of the N170 component of the visual-evoked potential from either left or right visual field presentation. Nineteen right-handed women with regular menstrual cycles were tested twice, once during the menstrual phase, when progesterone and estradiol levels are low, and once during the luteal phase when progesterone and estradiol levels are high. Plasma steroid levels were determined by blood-based immunoassay at each session. It was found that IHTT, in particular from right-to-left, was generally longer during the luteal phase relative to the menstrual phase. This effect occurred as a consequence of a slowed absolute N170 latency of the indirect pathway (i.e. left hemispheric response after LVF stimulation) and, in particular, a shortened latency of the direct pathway (i.e. right hemispheric response after LVF stimulation) during the luteal phase. These results show that cycle-related effects are not restricted to modulation of processes between hemispheres but also apply to cortical interactions, especially within the right hemisphere. The findings support the view that plastic changes in the female brain occur during relatively short-term periods across the menstrual cycle.

Insensitivity of the analysis of variance to heredity-environment interaction

It makes sense to attribute a definite percentage of variation in some measure of behavior to variation in heredity only if the effects of heredity and environment are truly additive. Additivity is often tested by examining the interaction effect in a two-way analysis of variance (ANOVA) or its equivalent multiple regression model. If this effect is not statistically significant at the α = 0.05 level, it is common practice in certain fields (e.g., human behavior genetics) to conclude that the two factors really are additive and then to use linear models, which assume additivity. Comparing several simple models of nonadditive, interactive relationships between heredity and environment, however, reveals that ANOVA often fails to detect nonadditivity because it has much less power in tests of interaction than in tests of main effects. Likewise, the sample sizes needed to detect real interactions are substantially greater than those needed to detect main effects. Data transformations that reduce interaction effects also change drastically the properties ofthe causal model and may conceal theoretically interesting and practically useful relationships. If the goal ofpartitioning variance among mutually exclusive causes and calculating “heritability” coefficients is abandoned, interactive relationships can be examined more seriously and can enhance our understanding of the ways living things develop.

Gender differences in relations of mental rotation, verbal fluency, and SAT scores to finger length ratios as hormonal indexes

One hundred thirty-four university students (93 women, 41 men) were administered the Vandenberg Mental Rotation Test and the Thurstone Word Fluency Test, and they were asked to report their Scholastic Achievement Test scores. Finger lengths were measured, because literature has reported gender differences in the ratio of the 2nd to 4th, 2nd to 3rd, and 2nd to 5th finger lengths, such that the ratio is larger in women than in men. The goal of this study was to evaluate the relations between finger-length ratios and cognitive skills, such as spatial skills and verbal fluency, which have shown gender differences and direct relations to hormonal effects. Gender differences were found in the expected directions, such that the men performed better than the women for mental rotation, the women performed better than the men for verbal fluency, and the finger-length ratios were in the directions reported in the literature. The finger-length ratios showed an interesting relation with the cognitive variables for the men and women. For the men, better performance on the measures, including mental rotation, verbal fluency, and verbal Scholastic Achievement Test score was associated with less of a male-typical finger-length ratio pattern, or higher ratios. For the women, better performance for mental rotation and verbal fluency was associated with less of a female-typical finger-length ratio pattern, or lower ratios. Thus, in this group of college students, better cognitive performance was associated with a less gender-typical finger-length ratio, for both men and women. These findings are discussed in the context of other similar reports and a possible curvilinear relation between hormones and cognition in normal populations.

Curvelinear correlations between total testosterone levels and fluid intelligence in men and women

The relationship between serum total testosterone (T) level and fluid intelligence (Cattell's Culture Fair Intelligence Test) was studied in young men and women. There was no significant difference between IQs of men and women. IQ tended to increase with T in men, except at very high T levels. It was concluded that (i) T may be associated with IQ, even in samples with no sex-related IQ difference; (ii) too low or too high T may be disadvantageous for fluid intelligence in women; (iii) T may be advantageous for this kind of IQ in men, except very high T levels.