Drs. Sheree Fetkin and Charles Boulet continue their discussion of Milestones in Visual Development with Part 2, Clinical Applications.
It is not possible to know everything about a child’s development, but it is important to consider certain components. Although there is some degree of leeway within these expectations, it is quite important to recognize developmental delays and the role these may have on the developing visual system and vice versa. It is valuable, then, to inquire about a child’s behavior at home. One can appreciate a more complete picture of the development of gross motor function, fine motor function, speech and social development when observing certain behaviors at scheduled intervals.
We should also expect that a child is receiving sufficient visual stimulation for what the body allows the eyes to perceive, under normal circumstances, that is. For example, if a child is able to roll over and reach for objects in a normal and age-appropriate way, it is more likely that they are receiving age-appropriate visual stimulation. Also, vision cannot become versatile without suitably varied visual input and experiences. Objects of interest will encourage the baby to interact with the environment. As an infant grows, his visual world is altered and as a result, his visual demands are changed congruently. This will create new and growing opportunities to interact with the environment and develop new skills (and not just visual skills). Over the course of many years, the visual system will undergo interminable organization and re-organization, consistently adapting to the demands and the needs of the person at hand, displaying the harmonious relationship between overall child development and visual development.
In this second part of the three-part series, we consider core elements of visual development and behavior in children, as well as the impact of variations in visual development on behavior.
Early Visual Development
At five months, an infant should regard an interesting object in front of it, as well as regard him or herself within the mirror. The infant should attempt to grab objects of interest, likely bringing them to the mouth next, displaying early developmental organization of visual-prehensile behavior. If something is dropped, the infant should pursue the target visually in an effort to locate it, displaying anticipatory visualization and evolving motor planning. This behavior can give insight to the parents and the examiner as to whether an infant can locate objects, or make necessary pursuit movements to follow the objects.
By six months of age, an infant’s regard is quite versatile, greatly expanding on developments made at five months of age. The infant should be able to shift attention to different components of his or her mother’s face such as her nose, eyes, and mouth. This fine shifting of focus reveals an ability to make fine saccadic eye movements. A transfer of the infant’s fixation and attention should also be observed when the infant is looking at different objects in space, such as with blocks or cubes. As visual acuity improves and stereopsis (depth perception) develops, detailed or contrasting objects should receive more attention than before. At around 3 years of age, children undergo an organizational period of their life reflected in their personal-social attitudes and their purposeful motor behavior. Eye-hand coordination and activities are more developed and the children display a greater sense of orientation in their space world.
Refractive Status and Visual Acuity
It is not uncommon for babies to have significant refractive errors—hyperopia, myopia, and/or astigmatism. Over the first few years of life, these refractive errors have a tendency to decrease and stabilize through a developmental process called emmetropization. Emmetropia is the absence of a refractive error….. but emmetropization is a misnomer because most children do not become emmetropic. A low amount of hyperopia is typical and considered normal. This does not cause blurred vision and glasses are not required. But of course, not all children are typical and some have very high refractive errors even at a very young age. Remember Piper? She is very hyperopic and corrective lenses are having a huge impact on her visual and overall development.
Instead of showing the baby letters at the far end of the room, the doctor will often use Teller Acuity Cards held relatively close to the baby. The cards have stripes on one side of the card. Instead of asking, which is better, stripes or no stripes, the doctor watches where the baby is looking. Babies love to look at stripes if they can see them. By showing the baby skinnier and skinnier stripes, you can determine a threshold; a point where the baby no longer prefers to look at the stripes. The frequency of the stripes can then be converted to a visual acuity measurement.
To determine if the baby has a significant refractive error, the doctor uses a retinoscope. During retinoscopy, light is bounced off the retina and can be seen in the patient’s pupil. Lenses are used to change the movement of the light. The power of the lenses is an objective measurement of the patient’s refractive status. It can be challenging to perform retinoscopy on a young child, but most of the time, it can be done!
Determining whether or not to prescribe for an infant or toddler before emmetropization is complete is a decision that the clinician must make based on the overall status of the visual system, including accommodation and binocularity, the direction and the amount of the refractive error, as well as the visual demands of the individual, and whether or not the refractive error has implications for overall development.
At an infant’s first recommended examination, approximately 6 months old, visual acuity should range from 20/50 OU to 20/200 OU with preferential looking techniques such as Teller Acuity Cards and Cardiff cards. At one year, an infant will display a similar measurement of acuity with preferential looking. At preschool age, around 3 years old, it is expected that a child should have correctly undergone emmetropization and manifest a visual acuity close to 20/20 OU using methods that more closely resemble the standard Snellen chart at 20 ft.
Binocularity and Ocular Motility
Before 6 months of age, an infant’s eye may wander inward or outward while they learn to control the posture of their eyes. However, by 6 months of age and throughout the rest of the individual’s life, there should not be any signs of strabismus (an eye turn). Babies should be able to converge their eyes at 6 months old and remain through childhood. If a child is able to demonstrate convergence at a very early age, it is likely that their cortical binocular cells are being stimulated and they are experiencing stereopsis (3-dimentsional depth perception), which emerges in the fourth month and should be well developed within the 6 month and at the time of the first examination. However, stereopsis can be very difficult to assess in an infant.
Ocular motility should be developed and observed at six months of age by having the infant follow a moving target and assessing his or her ability to fixate on the target and follow it into the appropriate fields of gaze. If the infant is not fixating or following the target well, the parents can hold the child and move him or her in such a way to initiate eye movements in the appropriate gaze. Moving the head and initiating the vestibular ocular reflex to assess range of motion does not evaluate the accuracy of voluntary motility, but it does confirm that there are no restrictions. Pursuits, saccades, both horizontal and vertical, and fixation to one’s face should also be observed on the first examination.
Color vision is well developed only 1 month after birth. Adult levels of color vision should be reached by 3-5 years old.2 For younger children and infants, assessing color vision can be very difficult especially when the patient cannot subjectively participate in the examination.
Variations in Visual Development
For nine months of embryological development, the eye proves to be a resilient and persistent end organ, differentiating into specific structures. Occasionally differentiation does not go as expected and the result can range from benign anatomical anomalies to devastating vision loss.
A good example of such a deviation and the ripple effect on vision and child development is congenital cataracts. If the developing lens fails to induce the lens fibers to elongate and become organized in an appropriate way, the misaligned lens fibers will not display the necessary optical clarity and consequently a cataract will form. If the cataract obstructs the visual axis, deprivation of visual input can inhibit development and the result can be a decrease in vision, termed deprivation amblyopia.
Any disruption during early development that affects the visual input of one or both eyes can lead to complications in the organization of the brain’s cortex. Neural anomalies secondary to binocular vision anomalies were greatly explored by the animal studies of Hubel and Wiesel and published in a series of papers beginning in 1959. They concluded that monocular form deprivation led to marked physical and anatomical changes in the brain. Drawing from these studies, it was deduced that amblyopia is the result of an active inhibition, or suppression, of visual input during the critical period of vision development.
The results of monocular deprivation were then compared to binocular deprivation. These results suggested that binocular deprivation had reduced the overall functioning capability of the visual cortex, but that it had not specifically retarded the development of the visual system. More recent animal studies revealed that inducing monocular deprivation, binocular deprivation, and strabismic situations do in fact lead to diminished cortical representation. Our clinical interpretation of this diminished cortical representation is often seen as a reduction in visual acuity, which is not attributable to any other pathological or anatomical anomaly.
Monocular and or binocular deprivation frequently arises as a result of refractive error. Refractive amblyopia, can result from either high but equal refractive error called isoametropic amblyopia, or from clinically significant unequal refractive error between the two eyes relating to spherical or cylindrical power called anisoametropic amblyopia. Refractive error may result from malfunction during the emmetropization process during development, and the result can range from functionally benign to devastatingly debilitating.
Von Noorden and colleagues looked at the effects of short periods of occlusion during specific time periods early in development. They determined that there was a significant relevance of early visual stimulation conditions that ultimately affected the animal in terms of its perceptual processing capabilities. As stated by Cool, “Without a doubt, functional inputs to the visual system of the young organism do determine the way that organism’s visual system will operate in adulthood.” The implications for human development are extremely important to consider. Any deviations from the natural laws of development that deprives a child of expected sensory experiences can result in deficient visual stimulation, which, in turn, can cause long-term visual processing difficulties. The interplay between visual and overall development can spill into many facets of the individual’s life, including academic performance.
There are many other varied aspect of anomalous visual development, such as with strabismus, where fusion is either not possible or only infrequently so. Strabismus and lack of stereopsis will impact on scores of visual spatial processing. Elsewhere, high and unmanaged refractive states, in particular hyperopia and astigmatism beyond 1.50D are known to impact upon reading, visuomotor skills and cognition. Very often, children with moderate to high hyperopia (where the visual system is in a state of constant strain) will report or show varied signs and symptoms, and these can be misinterpreted as learning disabilities or developmental delays.
This second in a three-part series takes a more clinical view of vision, and provides an overview of some more specific visual developmental milestones in function and behaviour. The discussion ends by considering some of the ramifications of significant impediments to development. The third and final article in this series will shift focus to consider more generally the role of visual impediments in learning.
Don’t miss Part 1 on Clinical Principles.
Cavallini A, Fazzi E, Viviani V, Astori MG, Zaverio S, Bianchi PB, Lanzi G. Visual acuity in the first two years of life in healthy term newborns: an experience with the teller acuity cards. Func Neurol 2002; 17(2):87-92.
Calloway SL1, Lloyd IC, Henson DB. A clinical evaluation of random dot stereoacuity cards in infants. Eye (Lond). 2001 Oct;15:629-34.
Brown A, 1, Lindsey DT. Infant color vision and color preferences: a tribute to Davida Teller. Vis Neurosci. 2013; 30:243-50.