Vision: Developmental Disorders

Outline of Normal Development

Developmental disorders must be seen in the context of the process of normal perceptual development. This has several key aspects:

• Visual acuity, studied with behavioral and electrophysiological methods, shows a development from less than 1/1 0th of adult values at birth, with rapid development over the first few months, but with adult values not attained until 4 to 6 years of age. This is limited in part by the immaturity of photoreceptors, but development of neural systems in the eye and brain is undoubtedly necessary.
• The visual abilities of the newborn are primarily mediated by subcortical systems. The characteristic selectivity of the primary visual cortex emerges over the first 3 to 6 months of life, first for contour orientation and later for direction of motion. These two forms of selectivity can be regarded as the basis of subsequent processing in the ventral cortical stream (occipital to temporal, for identification) and the dorsal stream (occipital to parietal, for spatial actions), respectively.
• Binocularity, the combination of information from the two eyes on the same cortical neurons, also develops rapidly from about 3 months, rather later than other functions of the primary visual cortex.
• Facelike stimuli are a target of infants' fixation from birth, but the development of specialized cortical systems for face processing is a long process and is not mature before middle childhood.
• Global processing in the extrastriate visual cortex is revealed by the integration of information in patterns of motion (for dorsal-stream areas) and static form (for ventral-stream areas). This continues on from initial visual cortical development. Global motion sensitivity appears in infants more rapidly than for global form; however, in later development through childhood, the dorsal stream function of global motion takes longer to reach adult levels than ventral form processing, and as discussed later it is more vulnerable.
• Apart from eye movements, visually controlled action systems (reaching, grasping, locomotion) develop from about 5 months onward. Progressive integration of visual action planning and the control of visual attention required for this also develop through infancy. These are also predominantly dorsal functions, and again are particularly vulnerable. Attention consists of multiple developmentally dissociated systems. Earlier maturity in selective attention (before 6–7 years), for example, contrasts with the continued development of sustained attention into adolescence and rapid development of executive attention between 7 and 11 years.

Refraction and Focusing

The first stage in vision is the optics of eye bringing images to a sharp focus on the retina. This depends on the size and shape of the eyeball. In young infants, the eye is on average hyperopic or longsighted—but the action of accommodation (focusing) adjusts the lens to overcome this, so that in the first few months, the infant's eyes are usually focused on near objects, although they will adjust in the appropriate direction for an object that moves toward or away from the child.

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