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