This essay will explore the development of visual acuity and depth perception within the first year of an infant’s life. It explains the reason for the fast development of visual acuity within the first 6 months and the development of visual acuity based on how cone photoreceptors and the fovea mature to provide good resolution. OKR and VOR are also looked at for stabilising images. The development of depth perception using motion parallax and stereopsis is explained and along with the reasons for the age of onset of stereopsis.
Visual Acuity Infantile Development
Researchers Daphne Maurer and Telle L Lewis (Maurer and Lewis 2001) explained the rapid change in acuity within the first 6 months is due to the cone shape change. The cone changes to become a funnel shape preventing extra light from falling thorough and therefore allowing fine details to pass on to the visual cortex. Simons ( Simons 1993) using research from (Bach and Seefelder, 1914; Abramov et al, 1982; Hendrickson and Youdelis, 1984; Youdelis and Hendicson, 1986) that the spacing for cones in an adult is more densely packed than that of an infant, so as infants cones spacing decreases, less information is lost from the gaps, increasing fine detail and therefore increasing visual acuity.
In the parafovea, the rod cells have already matured, but the cone cells are still developing, Simons (Simons 1993) explains that initially cones are untapered so they are thinner and smaller at one end.
Foveal Maturation
Changes in the fovea aid visual acuity as it affects spatial vision. As the fovea matures, more light and information from the world is caught, more information is processed, so less of the world is blurry therefore improving the acuity for an infant and allowing them to gradually recognise faces and important detail.
The fovea at birth is immature, after which it shows rapid development which aids with visual acuity. Hendrickson and Youdellis (Anita E Hendrickson Christine Youdelis 1984) explain the fovea shows thinning in the inner nuclear and ganglion cell layers which leads to a central depression by centripetal migration forming the foveal pit. They also include that there is shallow depression one week after birth and the rods are still thick without outer segments. Researchers (Hendrickson and Drucker 1992) state that the inner retina within 5-8 days is nearly mature. Within these days, the basal layers of the axons of the photoreceptors elongate too, leading to a thicker photoreceptive layer.
Simons says (Simons 1993) as the axial length of Rhodopsin increases in the rod outer segment it leads to an increase in quantum catch therefore it leads to higher resolution. The chart shows maximum rhodopsin being produced post 6 months and rapid development within first 6 months disregarding preterm babies. As rhodopsin is important for transduction, more light is transduced into energy signals therefore more information reaches the visual cortex, and objects are seen with a greater acuity.
Neurons at birth are immature at birth. The lateral geniculate nucleus responsible for relay between the retina and the visual cortex neurons are visibly different, they have spiky textures and more dendrites in comparison to adult neurons, greatest spines are at 4 months, and reach adult spine level at 9 months. The maturation of neurons is important for transmitting information to the visual cortex and the continued development of visual post the early post- natal months could be attributed to the maturation of neurons, as greater fine detail is achieved.
In his book (Simons 1993) explained the importance of the development of vestibuloocular and optokinetic reflexes to maintain images on the retina, if they are moved, blurry images are seen lowering visual acuity. Head Movement causes this response. During head rotation, the visual field moves in the opposite direction, to maintain the image the eyes move in opposite direction to the head movement these are controlled by two reflexes:
Vestibulooculular reflex: the movement of the eyes
VOR gain (ratio eye velocity: Head velocity) determines the efficiency of VOR, a study (finocchio Preston and Fuschs 1991) cited in Simons book shows infants have higher gain than adults therefore allowing for short periods of head rotation.
Optokinetic reflex: Determine velocity for movement of eyes to ensure image is stabilised. It is dependent on development of visual acuity, contrast sensitivity and other such factors.
The importance of VOR and OKR for infants is that at birth, smooth pursuit and tracking has not yet developed and so to maintain image, the infant relies on these reflexes. Smooth pursuit only develops at about 3 months of age when both eyes can co-ordinate together.
Depth Perception in Infants
Depth perception is the ability to view objects in 3d using depth cues. Infants use monocular and binocular depth perception.
Motion Parallax is important for the development of monocular depth perception, by making head movements as explained earlier in OKR the closer objects move in the opposite direction. It is a monocular depth cue that relies on motion as proven ( Nawrot et al 2009) when infants discerned depth when using smooth pursuit. They also develop it close to the age of stereopsis
Nawrot (Nawrot and Nawrot) in their journal claim that motion parallax develops slightly before the onset of stereopsis and therefore prepares the cortical developments required for depth processing.
Stereopsis is a means of depth perception using binocular vision. It requires retinal disparity, this allows information about depth to be made. Due to the different geometrical positions of the eyes, different images are seen. The greater the difference between images, the closer the object is and vice versa.
(Simons 1993) in his book had defined onset stereopsis as the youngest age in which an infant is able to differentiate between images with and without binocular disparity. Using the diagram, it is concluded that onset stereopsis is shown as 3-5 months of age, however, some infants display stereopsis at as young as two months. Simons surmised the age for the sudden onset can be determined by various factors:
Retinal maturation: cited from (Aslin and Dumais, 1980) but, there is no sudden increase in acuity from the retina, and therefore it cannot be the only factor. The maturation increases the retinal resolution, however Simon states at 2 months old, the spatial resolution is enough for stereopsis to occur.
Vergence: Simons also cites research carried out by scientists (Aslin and Durmais 1980, Braddrick and Atkinson1983) in that Vergence Control is a factor for development of stereopsis, considering that postnatal vergence control improves greatly.
Cortical Maturation: it is important for the coding of original visual information to be accurately depicted using depth disparity (Birch et al 1982, Atkinson 1984, Held 1988, Wilson 1988). Simons cites that a study (Huttonlocher et al 1982) that the development of synapses peaks between 2-8 months, this coincides with the onset of stereopsis. Due to the development of synapses, visual information is passed on the cortex, the disparity differences would be more visible and a more accurate depth perception can be carried out.
The fusion of the data received from the right and left eyes that causes depth disparity need to be analysed and combined, this process is carried out by binocular neurons in the visual cortex which need to be developed to work.
Simons also uses research carried out on a monkey that depicts during the early stages of infancy, the neurons of both eyes at the entry of the striate cortex are overlapped and share synapses with the same cells, this causes the disparities to be wrongly processed and therefore stereopsis is unable to occur. Research carried out with cats (Timney 1981) suggests that the separation of these columns and the age of onset stereopsis are similar and therefore neural development too could be a factor for onset stereopsis.
The experiment (Birch et al 1982) carried out to investigate stereoacuity for crossed and uncrossed disparities indicates stereoacuity develops very early on between 3-6 months of an infants life, which is on par with stereopsis development, they also concluded that the difference between crossed and uncrossed disparities aren’t present post 6 months this shows that convergence isn’t a main factor for stereopsis. Birch et al also conclude that the data support the hypothesis that convergence is important for development of onset stereopsis: inaccuracy of convergence leads to a decrease in acuity for stereopsis both crossed and uncrossed disparities. In addition to this, it supports the idea that the visual cortex is important for the development of stereopsis as fixation and fusion are necessary. This further supports Simons conclusions.