1. Introduction: A Historical Perspective
The degree to which sex differences extend to human brain anatomy has been investigated in many studies. The first postmortem studies on brain sex differences have reported larger and more asymmetrical male than female brains (Swaab DF and Horfman MA, 1984). During that period, brain sexual differences in neuroscience were exclusively restricted to the hypothalamus, sex hormones, and sexual behavior (Siva NL and Boulant JA, 1986; Levine S, 1966). Later, a surge of research focused on the influence of sex hormones on different brain regions such as the amygdala, hypothalamus, and hippocampus (Siva NL and Boulant JA, 1986), as well as brain sensory motor and cognitive functions including memory, emotion, navigation, and pain perception in healthy and diseased condition (Kimura D et al., 1996). By the 20thcentury, the development of neuroimaging techniques (MRI, PET, TMS, EEG) has extensively advanced our understanding of the differences between male and female brains during normal development, from childhood, through adolescence, to adulthood. Neuroimaging highlighted the differences in behaviors, cognitive and emotional functions, and pointed out the risk and consequences of various neurological and psychological disorders (Arnold AP et al., 2017). Analyzing such differences has a wide range of implications, from studying fundamental neuroscience to practical guidelines in the diagnosis, prognosis, and treatment of brain disorders. As research continues to deepen our knowledge of biological sex differences, a newly emerging field is intended to address the diagnostic and clinical needs of both sexes: sex-based biology and medicine. The overall purpose of this review paper is to combine the findings from neuroscience, neuroimaging, brain research, neurobehavioral, neonatology, and neurodevelopment studies covering all aspects of neonate brain development from volumetric, to microstructure and functional connectivity. We aim to provide the investigators with a global and precise overview on this topic to help the exploration of sex differences in normal brain development. This review research allows for a better understanding of the pathophysiology of brain abnormalities observed in various sex-linked diseases such as neural tube defects, neonatal seizures, autism spectrum disorder (ASD), and brain tumor, that develop early in life.