3.5. Hemispheric asymmetry
The human brain exhibits hemispheric asymmetry as early as 20 weeks of gestation (Bisiacchi P and Cainelli E, 2022). In line with studies across the infancy period, leftward asymmetry was found in the volume of the hemispheric brain, the occipital lobe, prefrontal lobe, parietal lobe volumes, and lateral ventricles (Gilmore JH et al., 2007; Choe MS, et al., 2013; Hering-Hanit R et al., 2001). On the other hand, rightward asymmetry was documented in the volume of the temporal lobe (Lehtola SJ et al., 2019) indicating the formation of specialized neuronal circuits during the earliest period of brain development (Dean DC et al., 2018).
Significant sex differences in asymmetry were found in the occipital and prefrontal lobes where female infants showed a more pronounced leftward asymmetry in both regions (Gilmore DH et al., 2007). Hemispheric asymmetries of the mean depth of the sulcal pits also exhibited sexual differences. Males, but not females, showed considerably greater mean sulcal pit depth in the right hemisphere than that of the left hemisphere at 1 and 2 years of age (Meng Y et al., 2015). In their 2022 study, Alexopoulos et al. employed functional Near Infrared Spectroscopy –(fNIRS) to examine sex differences in the neural processing of speech and non-speech (reversed) stimuli in infants during the first days after birth. They found that girls exhibited significant oxyhemoglobin changes exclusively within the left hemisphere in response to speech stimuli. In contrast, boys demonstrated simultaneous neural activations in both hemispheres, with a larger and more significant fronto-temporal cluster observed in the right hemisphere (Alexopoulos J et al., 2022). In a separate investigation utilizing T2W MRI, Lu et al. (2019) explored the lateral ventricle volume (LVV) of normally developed infants. The results indicated a significantly larger total LVV for males (13,093.25 ± 5972.09 mm3, 95 % confidence interval) compared to females (11,092.70 ± 4647.55 mm3). The authors observed a left-ward asymmetry of LVV (Left > Right) in both male and female infants. These findings align with prior research on neonates, demonstrating larger total brain volume, hippocampus and thalamus in males compared to females (Lu Z et al., 2019).
4. Brain Structures