The most dynamic postnatal brain development takes place during human infancy. Decades of histological studies have identified strong spatial and functional maturation gradients in human brain gray and white matter. The improvements in noninvasive imaging techniques, especially magnetic resonance imaging, magnetic resonance spectroscopy, electroencephalography, magnetoencephalography, positron emission tomography, and near-infrared spectroscopy, have provided unprecedented opportunities to quantify and map the early developmental changes at whole brain and regional levels. Unique to infant brain imaging, tailored infant image acquisition and analysis methods—such as motion correction, high-resolution imaging, optimization of imaging parameters for smaller and immature brain, age-specific brain atlas and parcellation scheme, age-specific white matter tractography, functional connectivity analysis given incomplete brain networks, and advanced gray and white matter segmentation for infant brains should be taken into consideration. Delineating functional, physiological, and structural changes of the infant brain through imaging provides insights into the complicated processes of both typical development and the neuropathological mechanisms underlying various brain disorders with early onset in infancy, such as autistic spectrum disorder. Identification of imaging biomarkers of neurodevelopmental disorders during infancy by leveraging techniques such as machine learning may offer a valuable time window for early intervention.
Human visual development is a complex dynamic psychological/neurobiological process, being part of the developing systems for cognition, action, and attention. This article reviews current knowledge and methods of study of human visual development in infancy and childhood, in relation to typical early visual brain development, and how it can change in developmental disorders, both acquired (e.g., related to at-risk births) and genetic disorders. The newborn infant starts life with a functioning subcortical visual system which controls newborn orienting to nearby high contrast objects and faces. Although visual cortex may be active from birth, its characteristic stimulus selectivity and control of visual responses is generally seen to emerge around six to twelve weeks after birth. By age six months the infant has adequate acuity and contrast sensitivity in nearby space, and operating cortical mechanisms for discriminating colors, shapes, faces, movement, stereo depth, and distance of objects, as well as the ability to focus and shift attention between objects of interest. This may include both feedforward and feedback pathways between cortical areas and between cortical and subcortical areas. Two cortical streams start to develop and become interlinked, the dorsal stream underpinning motion, spatial perception and actions, and the ventral stream for recognition of objects and faces. The neural systems developing control and planning of actions include those for directed eye movements, reaching and grasping, and the beginnings of locomotion, with these action systems being integrated into the other developing subcortical and cortical visual networks by one year of age. Analysis of global static form (pattern) and global motion processing allows the development of dorsal and ventral streams to be monitored from infancy through childhood. The development of attention, visuomotor control and spatial cognition in the first years show aspects of function related to the developing dorsal stream, and their integration with the ventral stream. The milestones of typical visual development can be used to characterize visual and visuo-cognitive disorders early in life, such as in infants with perinatal brain injuries and those born very prematurely. The concept of “dorsal stream vulnerability” is outlined. It was initially based on deficits in global motion sensitivity relative to static form sensitivity, but can be extended to the planning and execution of visuomotor actions and problems of attention, together with visuospatial and numerical cognition. These problems are found in the phenotype of children with both genetic developmental disorders (e.g., Williams syndrome, autism, fragile-X, and dyslexia), and in acquired developmental disorders related to very preterm birth, or in children with abnormal visual input such as congenital cataract, refractive errors, or amblyopia. However, there are subtle differences in the manifestation of these disorders which may also vary considerably across individuals. Development in these clinical conditions illustrates the early, but limited, plasticity of visual brain mechanisms, and provides a challenge for the future in designing successful intervention and treatment.
The role of experience in brain organization and function can be studied by systematically manipulating developmental experiences. The most common protocols use extremes in experiential manipulation, such as environmental deprivation and/or enrichment. Studies of the effects of deprivation range from laboratory studies in which animals are raised in the absence of sensory or social experiences from infancy to children raised in orphanages with limited caregiver interaction. In both cases there are chronic perceptual, cognitive, and social dsyfunctions that are associated with chronic changes in neuronal structure and connectivity. Deprivation can be more subtle too, such as being raised in a low socioeconomic environment, which is often associated with poverty. Such experience is especially detrimental to language development, which in turn, limits educational opportunities. Unfortunately, the effects of some forms of socioemotional deprivation are often difficult, if not impossible, to ameliorate. In contrast, adding sensory or social experiences can enhance behavioral functions. For example, placing animals in environments that are cognitively, motorically, and/or socially more complex than standard laboratory housing is associated with neuronal changes that are correlated with superior functions. Enhanced sensory experiences can be relatively subtle, however. For example, tactile stimulation with a soft brush for 15 minutes, three times daily for just two weeks in infant rats leads to permanent improvement in a wide range of psychological functions, including motoric, mnemonic, and other cognitive functions. Both complex environments and sensory stimulation can also reverse the negative effects of many other experiences. Thus, tactile stimulation accelerates discharge from hospital for premature human infants and stimulates recovery from stroke in both infant and adult rats. In sum, brain and behavioral functions are exquisitely influenced by manipulation of sensory experiences, especially in development.