Psychology

At the time of birth the human eye is almost half the size and weight of a mature eye in an adult. Although the infants eye is grossly similar to that of an adult, the only difference is that the way the parts relate to one another is quite is not the same. The different parts of the eye also develop at different rates. For example, at birth the cornea is almost completely developed as compared to iris which is still crude at birth. Retina is also well developed at birth as compared to other parts of the eye.

This does not imply that retina does not continue to develop after birth. At birth macula which is a part of retina and contains fovea, the area of clearest vision in adults is partially developed at birth. They only achieve complete differentiation at the age of four months, but macular continues to develop until early childhood. At birth, the systems which reconcile brightness and color vision are functional (Arterberry, 2008).

The optic tract which connects the eyes to the vision center in the brain is not fully myelinated at the time of birth. Myelin is the protective sheath which covers the axon of a nerve cell. It functions by insulating the axon from the structures around and improves the conductivity of the nerves. When myelin is absent, conductivity of the nerve is reduced and the electrical activity is spreads to the tissues around. Absence of myelin sheath around the optic tract would mean that there will only be diffuse flashes of light. The myelin surrounding the optic tract is fairly thin in infants as compared to adults (Allison, Gillam,  Vecellio, 2009).

The visual system of an infant is anatomically and physiologically functional at birth and it is capable of responding to stimuli except that it is still immature. This ability gratifies the condition for getting environmental information, although the condition is not adequate enough. In order to achieve adequate extraction of information from the environment, the organism should be in a position to concurrently accustom both the eyes towards the origin of stimulation, concentrating on the source, deciding on the details of the stimulus, and being able to perform compensatory eye movements should the stimulus move a bit. Mature adult eye is capable of making these adjustments suddenly (Allison, Gillam  Vecellio, 2009).

Depth perception can be described as the ability to see objects in three dimensions and to estimate the spatial distances of objects from you and from one another. Depth perception is very useful for organisms survival, since it is vital to effectively explore the surrounding and function in the world. Without depth perception, it is not possible to tell the distance between us and the objects, and what distance we need to cover so as to reach or even to avoid the object. It is also not possible to tell the difference between stepping down from a stair and from a tall building (Arterberry, 2008).

Depth perception also includes space perception which is the ability to distinguish the difference in distances between objects in space. Depth perception is a product of vision, but hearing also plays a crucial role in perception. There are two wide classes of cues which are used to assist visual depth perception, they include monocular and binocular. In monocular cue, only one eye is required while binocular cue, working of the two eyes together is required. There are cues which only require one eye for their perception. They give information that aids in estimation of spatial distances and perception of three dimensions. Interposition is a situation where objects appear to partially block or overlap each other. When an object appears as being partially blocked, the object which appears full is perceived as being closer and this is usually true. Here below are some studies which have been conducted on monocular and binocular views in infants (Arterberry, 2008).

Infants sensitivity to the depth cue of height in the picture plane
The research was aimed at assessing sensitivity to the pictorial depth cue of height in the plane in five and seven month old babies. Infants were given two objects with one being placed higher than the other. The ability of the infants to reach the objects was recorded under monocular and binocular viewing conditions (Arterberry, 2008). The results indicated that both the age groups reached the lower object more than the higher one. The lower object was under monocular view while the higher one was under binocular view. This shows that both ages are sensitive to depth information provided by height in the picture plane (Allison, Gillam,  Vecellio, 2009).

According to the study, seven months old infants are sensitive to the depth cues of shading, linear perspective, interposition, texture gradient, familiar size, relative size, and surface contour. In the study, reaching was used as the measure. Infants of this age will always reach for nearer objects even if the difference between the two objects is very small (Arterberry, 2008).

A longitudinal study was also carried on the same infants. The assessment was done after every two weeks to ascertain sensitivity for depth cues in linear perspective and texture ingredients. This made it possible to analyze the emergence of sensitivity to pictorial depth information both within and across children (Arterberry, 2008). The final results showed a variation in sensitivity to pictorial depth. It was realized that in most children, the start of pictorial depth sensitivity occurs between the ages of 22 and 32 weeks, and full development may be achieved in 2 to 8 weeks (Arterberry, 2008).

The study also included the cue of height in the picture plane also referred to as relative height in the field and relative upward location in the field. This cue is normally found together with other cues in the picture. The main part of height in the picture plain is the position of the object in relation to the horizon. In a picture, objects that appear to be close to the horizon are usually far away (Arterberry, 2008).

There is always an assumption linked to all pictorial depth cues in accordance to the regularities in the environment. The assumption states that light comes from above, texture elements are uniform in size, parallel lines converge with distance, objects that overlap others are near, and objects size do not change (Arterberry, 2008). For infants to achieve height in picture plain, they need to view objects in relation to the horizon, and getting the horizon is not a difficult task provided that the ground has been identified. This suggests that sensitivity to the height in the picture plane develops earlier as compared to other pictorial cues (Arterberry, 2008).

Sensitivity of infants to height in picture plain was done solely. In this research, infants were provided with a trapezoid shaped board where the information on surface slant was mad available by linear perspectives and texture ingredients. Both the objects were placed on the board with one being raised higher than the other. When adults viewed the objects monocularly, the lower object appeared nearer as compared to the higher object. Random texture was used in this study (Arterberry, 2008).

In assessing the infants sensitivity to pictorial depth information, within objects and between objects design can be employed. Within objects design carries out both monocular and binocular testing on the same infant. This design allows for cross comparison across viewing, although the numbers of reaches are limited and cannot exceed six since infants become bored with repeated activities (Arterberry, 2008). Between subject design is used to test different infants under both conditions, and does allow many trials per an infant per a condition. Between subjects design was used in this study because it does not limit the number of reaches per infant per viewing condition (Allison, Gillam,  Vecellio, 2009).

The study involved five and seven month old infants who were tested both monocularly and binocularly and their abilities to reach the objects were put down. In monocular viewing conditions, the relative distance between the two objects was distinguished by height in the picture plane, which showed that the object which was lower was the nearer (Arterberry, 2008). It was envisaged that infants who would reach to the lower object under monocular viewing condition are sensitive to the available depth information. It was also foreseen that all the infants would reach the two objects equally under binocular view. This was the case because stereopsis supersedes pictorial information for depth, and this information is developed in infants at the age of 4 months (Allison, Gillam,  Vecellio, 2009).
Development of infants surface contour information for spatial layout

The aim of the study was to investigate the development of sensitivity to static monocular depth cue to shape and surface contours. The study was done with about sixty infants distributed equally in three groups of 5, 512, and seven months. It showed that frontoparalell cylinder is slanted away in depth such that one end appears nearer than the other end. Better reaching was recorded in both conditions. There was more reaching to nearer end in the monocular conditions as compared to binocular conditions. Infants aged 7 months old demonstrated sensitivity to surface contour information, while infants aged 5 and 5  did not show any sensitivity (Richards,  Rader, 1981).  A control study using approximately twenty 5 months old infants, most of them reached for the closer end of cylinder which was rotated in depth. Other studies with static monocular depth information indicated that infants sensitivity to surface contour information develops at about 6 months of age. Surface contours can be described as surface markings that are understood to be inhibited by the three dimensional shapes of the surface on which the objects lies (Richards,  Rader, 1981).

The findings indicated that there was more reaching to the apparently closer ends of the cylinder in monocular viewing conditions as compared to binocular conditioning by 7 months old infants. This shows that they are sensitive to surface contour information for layout. The absence of sensitivity in 5 months old infant indicates that they are not responsive to this information. It can therefore be assumed that 5 months infants are sensitive to cue, but they reach both sides of the cylinder because of absence of motor control (Richards  Rader, 1981).

Development of human visual system monocular and binocular VEP latency
Visual evoked potentials to pattern stimuli have been employed in the study of development of the human visual system and to test visual function in young children and children who are unable to speak. It has proved very successful in testing visual functions in clinical environment, and even in the people with neurological disorders. Success has also been recorded in its use in acuity estimation in normal subjects (McCulloch,  Skarf, 1991).

P-VEPS stands for pattern visual evoked potentials. P-VEPS was recorded in 161 human infants aged between 3 weeks and 2 years of age. The latency of the first positive peak in the P-VEP was then measured monocularly and binocularly for about five sizes of phase interactions checkerboard stimuli (McCulloch,  Skarf, 1991). The study demonstrated a swift maturation in the first six months of life. Monocularly conditioning lasts longer as compared to binocular P_VEPS. The latency different of monocular P-VEPS is a bit longer and varies with age (McCulloch,  Skarf, 1991).

The results of this study showed that almost three thirds of the infants and young children managed to complete binocular and monocular P_VEPS recording series with not less than four pattern sizes. Binocular testing was successful of 95 visits. Eight of the binocular sessions could not be analyzed because they demonstrated poor reproducibility as a result of poor cooperation as a result of crying (McCulloch,  Skarf, 1991).

Crawling onset age predicts visual cliff avoidance in infants
The study was conducted to test the impacts of crawling onset age, the amount of crawling experience, and testing the age on avoidance of the deep side of the of visual cliff apparatus by human infants. The study involved two experiments. In the first experiment, the ages of infants used was ranging from 7 to 13 months. Discriminant results revealed that crawling onset age was used to differentiate between the infants who crossed and those who shied away from apparent drop off (Richards,  Rader, 1981). Those infants who crossed the deep side of the cliff had early crawling onset age. The second experiment involved testing infants on visual cliff apparatus. The first experiment was done at the age of 9 months and the second done at 12 months of age. Analysis of the experiment showed that the only way the infants could be differentiated depended on those who crossed and those who failed. Using age alone did not prove useful (Richards  Rader, 1981).

These results indicated that experience in crawling is important in eliciting guided avoidance behavior in infants. It is stated that crawling onset age effect happens because crawling during the tactile phase of infancy interrupts later visual control of locomotion. Other studies have also revealed that the locomotor experience is very significant in the development of avoidance response (Richards,  Rader, 1981).

Discussion of the studies
The first research was aimed at assessing the infants sensitivity to pictorial depth cue of height in the plane. The research involved infants aged five months and 7 months. The ability of the infants to reach the objects was then recorded under monocular and binocular viewing conditions (Arterberry, 2008). The results indicated that both the age groups reached the lower object several times than the higher object. The lower object was under monocular view while the higher one was under binocular view. It can therefore be concluded that both ages are sensitive to the depth information provided by the picture plane. It was realized that in most children, the start of pictorial depth sensitivity occurs between the ages of 22 and 32 weeks, and full development may be achieved in 2 to 8 weeks (Arterberry, 2008).

The second research was on development of infants surface contour information for spatial layout. The aim of the research was to find out development of sensitivity to static monocular depth cue to shape and surface contours. The infants used in this study were aged 5, 512, and 7 months old. From the study it was realized that the cylinder has one side which appears nearer than the other side. Reaching for the sides of the cylinder was recorded under two different views. Under monocular view, more reaching was made to the nearer side in all the ages. This is more or less the same to the first study where most infants preferred reaching to the nearer end. It implies that by the age of five months, infants have developed pictorial depth information under monocular view (Allison, Gillam,  Vecellio, 2009).

Infants aged 7 months old demonstrated sensitivity to surface contour information, while infants aged 5 and 5  did not show any sensitivity. This also can be confirmed with other studies which also indicated that static depth monocular depth information is developed at the age of six months in children. The first study also indicated that pictorial depth sensitivity starts to develop at the age of 512 to 8 months and full development is achieved in 2 to 8 weeks.

The third research was on the development of human visual system. The development was measured by monocular and binocular VEP latency.  The study involved 161 human infants aged between 3 weeks and two years and P-VEPS was recorded (McCulloch,  Skarf, 1991). The latency of the first positive peak was measured in both views (Richards,  Rader, 1981). The study showed that in the first 6 months of life, there is rapid maturation of both the views, but monocular latency period tends to last longer as compared to binocular view. This study also concurs with the previous ones which also showed that by five months of age monocular views are developed and binocular starts at the age of five and half onwards (McCulloch  Skarf, 1991).

The last study discussed was on the crawling onset age and cliff avoidance in infants. It was aimed at testing the age at which an infant starts avoiding the deeper side of the cliff with the use of visual cliff apparatus. The study showed that the infants who crossed the deeper side of the cliff started crawling at a tender age (Richards,  Rader, 1981). The second experiment was done on two different ages that 9 and 12. The results indicated that infants who had experience in crawling exhibited guided avoidance behavior. It is stated that crawling onset age effect are experienced because crawling during the tactile phase of infancy interrupts later visual control of locomotion. Other studies have also revealed that the locomotor experience is very significant in the development of avoidance response. In this study the age did not prove useful (Allison, Gillam,  Vecellio, 2009).

Future research areas
There is very scarce information on infants perception of depth from motion parallax despite the knowledge that they are both responsive to motion and depth from motion cues at a tender age. There is need to come up with a research to asses whether infants are responsive to definite depth specified by parallax motion and when the sensitivity is first developed (Nawrot,  Mayo, 2009). Although stereopsis has been found to emerge at the age of 13 to 14 weeks, there is another potent, definite depth cue that may also emerge at the same time or even earlier because it is based on motion cues that may be present to the developing visual system. Motion parallax can be defined as a monocular depth cue that develops from the relative motion of objects at various distances and is created when an observer translates plainly (Nawrot,  Mayo, 2009).