3.4. Gyrification
The sulcal and gyral patterns start to form during the third trimester of pregnancy (White T et al., 2010), and continue to develop in the first 2 years of life in parallel with the increase in brain size (Li G et al. 2013). As the volume of the cortical hemispheres increases by 88% during infancy (Knickmeyer RC et al., 2008), the thickness of the cerebral cortex increases by 31% (Lyall AE et al., 2015), 42% in sulcal depth (Meng Y et al., 2014), and expands by about 80% in surface area (Li G et al., 2013; Lyall AE et al., 2015),, particularly during the first postnatal year. In neonates, inter-individual variability of cortical folding was investigated, and significant sex differences were detected in the superior temporal gyrus, inferior frontal gyrus, and cingulate cortex (Duan D et al., 2019). Male infants showed greater GM volumes in the fusiform gyrus, superior, middle, and inferior temporal gyrus, Heschl’s gyrus, left medial frontal gyrus, left superior frontal gyrus, as well as left precentral and postcentral gyri (Knickmeyer RC et al., 2014). However, females demonstrated higher GM volumes in bilateral superior and middle frontal gyri, precentral gyri, inferior and middle temporal gyri, and medial frontal gyri (superior), as well as the left superior temporal gyrus. These possibly explain the enhanced social interactive capacities and visual preference, along with better self-regulation in newborn girls than boys (Knickmeyer RC et al., 2014). It is worth mentioning that some differences in gyrification present only in children and adults are primarily located in regions associated with social cognition (Mutlu AK et al., 2018). Furthermore, at birth, males’ cortical thickness of the left fusiform gyrus was remarkably greater than that of females (Lyall AE et al., 2015). Kim et al (2016) observed higher cortical shape complexity in the middle cingulate and parieto-occipital sulcal regions in male infants, but a more complex Broca’s area in females (Kim SH et al., 2016). In the other hand, the authors reported no sex differences in the right anterior superior temporal gyrus after birth until 2 years of age (Kim SH et al., 2016). Moreover, males displayed significantly faster expansion rates of the surface areas of many gyri in the left hemisphere compared to females i.e., precentral gyrus, superior frontal gyrus, middle frontal gyrus, olfactory gyrus, insula, superior parietal gyrus, and the right calcarine gyrus (Lyall AE et al., 2015). Variation in cortical gyrification emerges as a potentially useful marker for normal development, as structural abnormalities in cortical folding were evident in certain neuropsychological disorders such as ASD and schizophrenia (Yang DY et al., 2016). An investigation using T2W fetal MRI aimed to assess the timing of sulcal emergence and its temporal variability. The study identified a trend indicating that females exhibited a significantly earlier sulcal emergence than males in the right superior-temporal-sulcus. In addition, a larger temporal variability of sulcal emergence timing was found in male fetuses in the right intraparietal sulcus, while the right precentral sulcus showed lower variability in males. These findings are attributed to the influence of various cellular and molecular processes, including sex hormones and sex-biased gene expression (Yun HJ et al., 2022).