Structural and functional brain asymmetries in the early phases of life: a scoping review

Abstract

Asymmetry characterizes the brain in both structure and function. Anatomical asymmetries explain only a fraction of functional variability in lateralization, with structural and functional asymmetries developing at different periods of life and in different ways. In this work, we perform a scoping review of the cerebral asymmetries in the first brain development phases. We included all English-written studies providing direct evidence of hemispheric asymmetries in full-term neonates, foetuses, and premature infants, both at term post-conception and before. The final analysis included 57 studies. The reviewed literature shows large variability in the used techniques and methodological procedures. Most structural studies investigated the temporal lobe, showing a temporal planum more pronounced on the left than on the right (although not all data agree), a morphological asymmetry already present from the 29th week of gestation. Other brain structures have been poorly investigated, and the results are even more discordant. Unlike data on structural asymmetries, functional data agree with each other, identifying a leftward dominance for speech stimuli and an overall dominance of the right hemisphere in all other functional conditions. This generalized dominance of the right hemisphere for all conditions (except linguistic stimuli) is in line with theories stating that the right hemisphere develops earlier and that its development is less subject to external influences because it sustains functions necessary to survive.

Keywords: Foetus; Hemispheric specialization; Lateralization; Neonate; Newborn; Premature.

Fig. 1

Workflow of the selection of sources of evidence

Fig. 2

The number of studies and structural methodologies used in neonates (red represents studies on full-term neonates; blue represents studies on both full-term and preterm neonates at term PCA), foetuses, and preterms (Panel a). The number of studies and functional methodologies used in neonates, foetuses, and preterms (without sensory modality [resting state], auditory [linguistic and non-linguistic] visual, tactile, and taste stimuli) (Panel b). DTI diffusion tensor imaging, EP evoked potential, EEG electroencephalogram, fMRI functional magnetic resonance imaging, HARDI high-angular resolution diffusion imaging, MEG magnetoencephalography, MRI magnetic resonance imaging, NIRS near-infrared spectroscopy, OT optical topography, PCA post-conception age, TBM tensor-based morphometry, US ultrasound

Fig. 3

Overview of the findings on structural and functional brain asymmetries in neonates, foetuses, and preterms