Evaluación de patogenicidad en plantas de papa, tomate y chile

This section examines the development of the visual system using insights from the newborn, children and adults. Developmental landmarks are important because functional testing and imaging methodologies must be adapted and the results interpreted in terms of the stage of development. Interpersonal variations are important because they limit the usefulness of atlases for surgery, which makes the development of preoperative and postoperative imaging techniques so important to achieve good surgical outcomes.

2.3.1

Development of the visual system in the newborn

At birth, the brain is approximately 25% of the size of an adult brain, measuring around 16.5 cm in diameter (Boothe et al., 1985). The immaturity of the human brain is apparent not only in motor and verbal systems but in the visual system as well. The visual system lacks many abilities such as detailed vision and restricted size vision, with the newborn unable to appreciate small objects but able to respond to larger structures such as the mother’s face (Teller, 1997).

The size of the eye relative to the body is larger in babies than in adults. During the first year, the eye diameter increases and reaches the size of the adult eye when the child is in adolescence. The anterior-posterior length of the eye ball is thought to greatly increase during the first year of life (Larsen, 1971). This change is one of the factors involved in the improvement of the initially-poor visual acuity.

The anterior-posterior change of the diameter of the eye and visual acuity are directly and proportionally related in the first year. The reduced size of the retina implies a reduced size of the perceived image, as well as a less detailed image, in comparison with an adult. It has been estimated that in the newborn detailed vision is approximately one third of normal adult vision (Levin et al., 2011). The reduced size of the eye globe at birth means that a more potent lens is needed to compensate the small anterior-posterior axis. The lens at birth is spherical and has greater refractive power than that of healthy adults. It grows predominantly in the periphery over time, which turns it into a much flatter shape.

The pupillary distance also changes during the first weeks of life, reaching values in the range 48–63 mm. It is thought that the axial length and scleral broadening are regulated by the retina (Levin et al., 2011). The mechanism is still not well understood but the retina has the capability to sense the acuity of the processed image and to then regulate the axial length by scleral broadening.

The corneal diameter increases from around 6.2 mm in the 34th week of gestation to 9.0 mm at birth and 16 mm at adulthood (Tucker et al., 1992). The cornea of the newborn has a larger curvature in the periphery than in the centre, a characteristic which reverses in adulthood. The adult size of the cornea is reached at approximately 2 years of age.

Changes in the human eye after birth do not only take place anatomically, as described above, but also neurophysiologically. The optic radiations in children might be different in its relation with the pole of the temporal lobe compared to adults (van Buren and Baldwin, 1958). In very young children, they may surround the temporal tip, and once the child develops during its first months of life, due to postnatal development of the brain hemispheres, Meyer’s loop reaches the anatomical distances and relations observed in adults and older children (Pfeifer, 1920). The number of dendrites and synaptic connections in the striate cortex increases from birth to reach a

peak between 4 months (Klaver et al., 2011) and 2 years (Wong and Sharpe, 1999). This means that the grey and white matter density also increases during the first two 2 years (Huttenlocher and de Courten, 1987).

The accommodation at birth is fixed to a short distance. Normal neonates are hypermetropic by 2–3 dioptres, with a concomitant high prevalence of astigmatism (Wong and Sharpe, 1999). Children with normal development show some target distance accommodation after 3 months (Green et al., 1980), and the accommodation approaches adult norms after 4 months (Haynes et al., 1965). Although pupil reaction appears at 29–31 weeks of gestational age, blinking towards stimuli only appears at 3–4 months after birth.

2.3.2

Development of the visual system through childhood

The adult brain size is reached at 7–8 years of age.

The full length of the optic radiations has not been measured in a single study spanning the period from birth to the teenager years, and the lengths of the optic radiations in the occipital lobe and Meyer’s loop are particularly poorly understood. In an MRI study, Taki et al. (2011a) found a linear increase in white matter volume between 4 to 20 years of age and a nonlinear decrease in grey matter in the same age period. The cortical grey matter increased until age 20.

The white matter tracts volume showed a lower increase in myelination in females than males in the temporal and occipital lobe tracts. The total increase was 12.4% between ages 4 and 22 (Taki et al., 2011b). The increase in grey matter in males was also higher than in females; the female cohort had an early peak with no decrease in the post-adolescent years, which is different to other tracts such as the parietal and frontal tracts.

2.3.3

Inter-subject anatomical variations in the visual system

Significant variations in the optic radiations have been observed between adult subjects. In healthy subjects, Doyon et al. (2004) measured distances between the most anterior part of Meyer's loop and the temporal pole in the range 34–51 mm, with a mean of 44 mm. Meyer's loop did not reach the tip of the temporal horn in all cases.

Early studies of the optic radiations (e.g. Meyer, 1907) were based on gross dissection and concluded that the fibres go anteriorly almost 20 mm, with a few small fibres going even more anteriorly (Huttenlocher et al., 1982). The distance from the tip of Meyer’s loop to the tip of the temporal lobe was found to be 22–30 mm in adults (Ramon y Cajal, 1898, Traquair, 1922). The tip of Meyer’s loop was located around 5–10 mm lateral to the tip of the temporal horn and amygdala (Probst, 1906: cited in Poliak 1957). The breath of the optic radiation linking the occipital lobe was found to be 17 mm and the average adult length was 105 mm (Ramon y Cajal, 1898).

Over the whole brain, females are thought to present a higher grade of myelination along the white matter tracts than males as a result of earlier development, but no significant differences have been identified within the occipital lobe in adulthood (Watson et al., 2010). One theory for the difference in children is the protective effect of oestrogens, which delays the loss of grey matter in different areas of the brain. Inter-hemispheric differences in the inferior temporal gyrus were found in adults older than 19 years.

Meyer’s loop cannot be defined anatomically by micro-dissection techniques alone as it is located in an area of intense and dense white matter tracts. Systems located in the area include fibres from the uncinate fasciculus, occipito-frontal fasciculus, anterior commissure, inferior thalamic peduncle, posterior thalamic peduncle, as well as temporopontine and occipitopontine fibres (Yasargil et al., 2004).