Relation of testosterone and hand preference in right-handed young adults to sex and familial sinistrality

Serum testosterone levels in male and female subjects with standard and anomalous dominance

Serum testosterone levels were determined in female and male subjects. Hand preference was assessed by the Edinburgh Handedness Inventory. Subjects with anomalous dominance (left-handers, mixed-handers, and right-handers with familial sinistrality) were compared to subjects with standard dominance (right-handers without familial sinistrality). The mean serum testosterone levels were found to be significantly higher in subjects with anomalous dominance than those with standard dominance. It was concluded that the results are in accord with the testosterone hypothesis of cerebral lateralization.

Opposite effects of androgen receptor CAG repeat length on increased risk of left-handedness in males and females

Prenatal exposure to testosterone has been hypothesised to effect lateralization by influencing cell death in the foetal brain. Testosterone binds to the X chromosome linked androgen receptor, which contains a polymorphic polyglutamine CAG repeat, the length of which is positively correlated with testosterone levels in males, and negatively correlated in females. To determine whether the length of the androgen receptor mediates the effects of testosterone on laterality, we examined the association between the number of CAG repeats in the androgen receptor gene and handedness for writing. Association was tested by adding regression terms for the length of the androgen receptor alleles to a multi-factorial-threshold model of liability to left-handedness. In females we found the risk of left-handedness was greater in those with a greater number of repeats (p=0.04), this finding was replicated in a second independent sample of female twins (p=0.014). The length of the androgen receptor explained 6% of the total variance and 24% of the genetic variance in females. In males the risk of left-handedness was greater in those with fewer repeats (p=0.02), with variation in receptor length explaining 10% of the total variance and 24% of the genetic variance. Thus, consistent with Witelson's theory of testosterone action, in all three samples the likelihood of left handedness increased in those individuals with variants of the androgen receptor associated with lower testosterone levels.

Paw preference in rats

We applied a food-reaching test to investigate paw preference in 114 Spraque-Dawley rats and to determine whether a correlation exists between paw preference and gender. We found that 70.2% of rats were right-pawed, 19.3% were left-pawed, and 11.9% were ambidextrous. No significant difference was found between right-pawed, left-pawed, and ambidextrous male and female rats. The results indicate that the distribution of paw preference in rats is similar to that of other animals and to human handedness.

Immunohistochemical detection of progesterone receptors and the correlation with Ki-67 labeling indices in paraffin-embedded sections of meningiomas

Female sex steroids may play a role in the proliferation of meningiomas, which usually have a high level of progesterone receptors (PgRs). We aimed to investigate the presence of PgRs and the Ki-67 labeling index (Ki-67 LI) in meningioma cases (N = 110) in relation to the severity of the disease. We studied PgR immunoreactivities and the Ki-67 LI in paraffin-embedded sections from meningiomas. Immunodetection of PgRs was conducted with peroxidase-antiperoxidase complex. Immunodetection of Ki-67 antigen was achieved by streptavidin-biotinperoxidase complex. Of 110 meningiomas, in 57 (52%) the immunostaining for PgRs was moderately to strongly positive (Group 1), in 23 (20%) weakly positive (Group 2), and in 30 (28%) negative (Group 3). The positive immunostaining rate for the PgR in the benign meningiomas (76%) was significantly higher than that in nonbenign tumors. The positive immunostaining rate for the PgR was significantly higher in women (81%) than men (55%). The results suggested that progesterone may play a role in the growth of meningiomas. Our results confirmed that the immunodetection of the PgR and Ki-67 antigens on the paraffin sections of meningiomas provides a valuable tool for estimating the biological behavior of meningiomas.

Testosterone and grasp-reflex differences in human neonates

According to the Geschwind-Behan-Galaburda (GBG) hypothesis, prenatal testosterone (T) causes a slowing in the development of the left brain with a consequent compensatory growth in the right brain, creating a reverse organisation of the cerebral lateralisation. That is, left- and right-handedness might be associated with high and low prenatal T levels, respectively. To test this hypothesis, the relations of T levels (umbilical cord blood) to grasp-reflex strengths were studied in human neonates. Handedness was assessed by measuring the grasp-reflex strengths from the right and left hands in 10 trials from each hand alternatively. There were two handedness groups: right-handers (R-L significantly greater than zero) and left-handers (significantly smaller than zero). Contrary to the GBG model, the mean free T concentration was found to be significantly higher in right-handers than left-handers for males and females. There was no significant difference in the total T levels between right- and left-handers. Free T concentrations positively correlated with RL grasp-reflex strengths, i.e. right-handedness increased as T increased, and left-handedness increased as T decreased. Contrary to these positive correlations, T negatively correlated with the grasp-reflex strengths from the right and left hands. These results partly supported the GBG hypothesis for this spinal-motor-asymmetry model. Total T did not significantly correlate with grasp-reflex strengths. The results suggest that prenatal T may at least play a role in prenatal determination of spinal motor lateralisation, with a possible consequent upward regulation of cerebral lateralisation.

Steroid fluctuations modify functional cerebral asymmetries: the hypothesis of progesterone-mediated interhemispheric decoupling

This study examines the modulation of functional cerebral asymmetries by gonadal hormones in three distinct groups. Young, normally cycling women performed a prototypical left (lexical decision) and two prototypical right-hemispheric tasks (figural comparison and face discrimination) during the low steroid menses and the high steroid midluteal phase. Saliva progesterone levels were measured with radioimmunoassay (RIA). Parallel to younger females, young men, and postmenopausal women were tested at matching time intervals. Results revealed significant interactions between cycle phase and visual half-field in the accuracy of all three tasks for the younger women; stronger lateralization patterns occurring during menses, while a more bilateral or at least less asymmetric cerebral organization predominated the midluteal phase, when highest levels of progesterone appear. Progesterone seemed to have a significant influence on lateralization in the figural comparison task, with high hormone levels enhancing the performance of the left hemisphere (for this task subdominant), thereby decreasing asymmetry. After menopause, when the levels of gonadal hormones are lower and more stable, the lateralization patterns for all three tasks were similar to those of men and normally cycling women during menses. These results make it likely that steroids and especially progesterone are able to reduce cerebral asymmetries. We hypothesize that progesterone attenuates the effect of glutamate on non-NMDA receptors. This could diminish cortico-cortical transmission which is mostly dependent on a glutamate-induced initial EPSP in pyramidal neurons which receive transcallosal input. The reduction in callosal transfer could then suppress the functional asymmetries.

Facial asymmetry in right- and left-handed men and women

A posteroanterior cephalometric radiographic study was performed on the right- and left-handed men and women with normal occlusion. A posteroanterior cephalometric radiography was conducted in these subjects. Method of triangulation was used to measure various face areas. The surface areas of these triangles were compared with their equivalents on the contralateral side. Sex and its interactions with handedness and side were significant factors influencing facial areas. Areas on the left were found to be significantly larger than those on the right in right-handers. Left-handers were inconsistent in facial asymmetry, but they tended to have larger facial areas on the right than the left. Sex was especially significant for left-handers. It was suggested that an asymmetric development in some brain regions may be responsible for the development of asymmetric facial regions.

Relation of serum free-testosterone level to grasp-reflex strength in human neonates with right-ear and left-ear facing out in-utero positions

The relation of the grasp-reflex strength to serum free-testosterone level was studied in human neonates with right and left ear facing out in utero positions. The grasp-reflex strengths from the right and left hands were found to be inversely correlated with testosterone in neonates with right-ear-out position. In neonates with left-ear-out in utero position, the grasp-reflex strength only from the left hand inversely correlated with testosterone. Left-dominance decreased and right dominance in grasp reflex increased linearly with testosterone only in females with left-ear-out in utero position. These results supported the theory of prenatal origins of cerebral lateralization (Previc, 1991). It was concluded that testosterone may favour the maturation of both hemispheres in neonates with right-ear-out in utero position, and only the right-hemispheric development in subjects with left-ear-out in utero position; testosterone may be a male hormone influencing the cerebral lateralization on the ground of genetically and even environmentally predetermined cerebral organization.

The grasp reflex from the right and left hand in human neonates indicates that the development of both cerebral hemispheres in males, but only the right hemisphere in females, is favoured by testosterone

The grasp reflex was studied in relation to serum free testosterone levels in human neonates. In the total and -FS (no familial sinistrality) males, the grasp reflex (especially that from the right hand) significantly increased as testosterone increased. In the total and -FS females, there was a significant positive linear correlation between testosterone and the grasp-reflex strength from the left hand, but not from the right hand. In +FS males, there was a significant negative correlation between the grasp-reflex strength from the left hand and testosterone, but not from the right hand; there were no significant correlations in the +FS females. The right minus left (R - L) grasp-reflex strength linearly increased with testosterone in females with normal testosterone levels, but linearly decreased in females with high testosterone levels. The log R - L grasp reflex increased with testosterone in males. It was concluded that testosterone may be beneficial for the development of the left brain in -FS males and the left brain in -FS females, but may have detrimental effects on the right brain in +FS males. The growth-promoting asymmetric effects of testosterone on the brain may depend upon genetic organization of the brain.

Association of serum-free-testosterone level with hand preference in right-handed young females

The relationship between the degree of right-hand preference and serum free-testosterone level was studied in right-handed female subjects, who were 17-19 years old. It was assumed that free testosterone levels in serum of young female subjects would reflect the perinatal testosterone levels. Hand preference was assessed by the Edinburgh Handedness Inventory (Geschwind score, GS) and Waterloo Handedness Questionnaire (Waterloo score, WS). The degree of the right-hand preference linearly and significantly increased as the serum free-testosterone level increased in the moderately right-handed (GS = < 70) subjects. There was only a marginally significant negative linear correlation between these parameters in the strongly right-handed subjects (GS > 70). These results only partly supported the testosterone hypothesis of cerebral lateralization that testosterone would slow the growth of the left cerebral hemisphere, reducing the degree of right hand preference. The results also suggested a trophic effect of testosterone especially on the left cerebral cortex in females. Activational effects of testosterone may also contribute to an increase in the degree of right-hand preference in right-handed young females.

Dermatoglyphic asymmetry and testosterone levels in normal males

Dermatoglyphic prints and salivary samples were taken on a sample of 39 adult males. A statistical relationship between dermatoglyphic asymmetry and adult testosterone levels as measured in saliva was examined for seven dermatoglyphic variables by means of correlation, regression, and analysis of covariance, controlling for age and stature when necessary. The first two types of analyses indicated a significant effect of testosterone level upon the asymmetry of three dermatoglyphic variables: a-b ridge count, palmar pattern intensity, and the combined pattern intensity of palm and digits. Analysis of covariance, which examined the effect of testosterone level as a categorical variable, while holding age or stature constant, demonstrated the asymmetry of five variables to be significantly affected by testosterone level: radial digital count, digital pattern intensity, palmar pattern intensity, total digital ridge count, and the combined palmar and digital intensity. Although there is as yet only associational evidence linking levels of prenatal and secondary testosterone, the results of the present study lend support to the hypothesis that prenatal testosterone levels may have a significant effect on the development of dermatoglyphics.

Relationship of serum zinc levels to hand preference, skull length and serum gonadal hormone levels in right-handed young adults

The association between serum zinc (Zn) level and the degree of hand preference was studied in right-handed young adults. Hand preference was scored by the Waterloo Handedness Questionnaire. Serum Zn level was determined by the flow injection analysis-atomic absorption technique. Zinc was found to be positively linearly and significantly correlated with the degree of right hand preference in the sample of males with familial sinistrality (FS+) and FS- females with right eye and right foot preference. There was a negative linear correlation between these variables in FS+ females. Skull length showed a significant negative linear correlation with zinc in FS+ males and females, but a positive linear correlation in FS- females. In FS- females, there was a significant positive linear correlation between serum gonadal hormone binding globulin (GBG) and serum Zn levels. In FS+ males, there was a significant negative linear correlation between serum GBG and Zn levels, and a significant positive linear correlation between serum free testosterone level and serum Zn level. It was suggested that zinc may influence the degree of the right hand preference by hormonal mechanisms according to genetically established brain organization.

Testosterone and estradiol in right-handed men but only estradiol in right-handed women is inversely correlated with the degree of right-hand preference

Relations of sex hormones to the degree of right-hand preference was studied in right-handed male and female adult subjects. Hand preference was assessed by the Edinburgh and Waterloo Handedness Questionnaires. Males consisted of weakly, moderately, and strongly right-handed subjects. Females comprised only moderately and strongly right-handed subjects. In females, only serum estradiol was found to be negatively linearly and significantly correlated with the degree of right-hand preference. In males, testosterone, as well as estradiol, showed a significant negative linear correlation with the degree of right-hand preference. Sex hormone binding globulin also showed a similar relation to hand preference in males. Thus, nearly all sex hormones caused a decrease in the degree of right-hand preference in males, whereas only estradiol had the same effect on hand preference in females. This would be the cause of stronger right-hand preference in females than males and vice versa.

Association of serum testosterone levels with latencies of somatosensory evoked potentials from right and left posterior tibial nerves in right-handed young male and female subjects

The relationships between serum testosterone levels and somatosensory evoked potential (SEPs) latencies from the right and left posterior tibial nerves (PTNs) were studied in right-handed young men and women. The corrected P1 (P39) and N1 (N49) latencies from the right PTN (left hemisphere) were found to be significantly longer in females than males. The corrected P2 and N2 latencies from both PTNs were longer in females than males. Testosterone was not associated with N1 latencies. In females, there was a negative linear correlation between testosterone and latencies from the right and left PTNs. These relationships were complex in males depending upon foot and eye preferences. The following results were obtained from males: no correlation with P1, N1, and P2, but a direct relation with P2 from left PTN in total sample; in males with right eye and right foot preference, a direct correlation only with P1 from right and left PTNs; in mixed- and left-eyed males, inverse correlations with P1, N1, and P2 latencies especially from right PTN; in right-eyed males, direct correlation with P1, inverse correlation with P2 from left PTN; in right-footed males, direct correlation with P1 and N2 waves from right and left PTNs. The interpeak latencies also showed sex-related differences. The overall results suggested that the left brain would be the main target for testosterone effects in both sexes, which may be beneficial for females but mainly disadvantageous for males.