The Impact of Sex Steroid Hormones on the Neural Control of Movement

Abstract

Eye-hand coordination is essential to our functional independence and relies on communication between widespread brain regions for successful unimanual and bimanual motor control. Eye-hand coordination performance has been shown to differ between the sexes, as well as between young and older adults. The studies included in this dissertation were designed to gain a better understanding of these previously reported differences by investigating whether sex steroid hormone concentrations are related to the functional and behavioural correlates of movement control in working-aged adults. The first study investigated bimanual control in a younger working-aged group. Sex differences in unimanual and bimanual eye-hand coordination have been extensively documented. However, sex differences in the neural correlates of bimanual control have not yet been fully elucidated. We’ve previously shown differences in functional connectivity for uni- and bimanual task performance between men and women. Here, we aimed to examine whether sex steroid hormones underlie these differences. Task-based functional imaging data demonstrated that despite equivalent behavioural performance between males and females, levels of estradiol, progesterone, and testosterone distinctly modulated connectivity for uni- and bimanual movement in a sex-specific manner. The second and third studies investigated changes in brain function and behaviour for complex unimanual performance in adults from the working age to retirement years. Non-standard, rule-based visuomotor transformation requires intact connections between frontal, parietal, and subcortical regions. Sex and age differences in the neural control and performance of this skill have been separately reported in children, adolescents, and older adults. The second and third studies aimed to characterize age- and hormone-related visuomotor differences in the working-and middle-aged demographic. We observed that in working-aged females, testosterone influenced functional connectivity between visuomotor-related regions in a task-specific manner. Behavioural performance in a rule-based task was likewise only predicted by testosterone levels in males and females, and a small effect of age was observed. Taken together, these data provide novel insight into the neural correlates underlying one of our most fundamental human behaviours, eye-hand coordination, and the potential implications of sex steroid hormones in preserving brain function during healthy aging.