CHAPTER Normal and Abnormal Visual 2 Development

Transcription

1 ch.02.qxd 29/3/04 05:49 AM Page 9 CHAPTER Normal and Abnormal Visual 2 Development Daniel L Adams THE DEVELOPING VISUAL PATHWAYS Human vision requires precise collaboration of diverse structures. From the eye to the cerebral cortex, the components mature in parallel, each influencing the development of the whole. Some developmental processes follow an innate plan that is programmed using molecular cues forming hard-wired neural circuits. Others are controlled by the neuronal activity within the system itself that arises spontaneously, or from visual stimulation. Thus, the anatomical configuration of the visual system is sculpted by both nature and nurture. In a developing individual, visual experience adjusts the neural structures such that they best represent the world they are exposed to. Combined with the innate, hard-wired, plan, this produces an efficient visual system because only elements that function appropriately are maintained: use it or loose it. Reliance on visual experience makes the system vulnerable: a fault during development may be detrimental. With anomalous visual experience, the system develops abnormally. Thus, the processes that normally generate an efficient visual system can cause abnormal development. An example is monocular deprivation. Here, a problem that obscures vision in one eye, like a congenital cataract, causes abnormal visual development. The visual system allots less cortical tissue to the deprived eye, generating a permanent visual loss that persists after removal of the cataract. The interdependent elements of the visual system must all develop appropriately. A fault at any point from eye to brain can have effects on the whole system. The normal development of each component will be discussed individually. The eye The eyes differentiate early from the neural plate. They first appear as the optic pits by the fifth week of gestation, and then extend from the neural tube to form the optic vesicles. These spherical pouches invaginate to form the optic cups, attached to the prosencephalon by stalks that become the optic nerves. Further differentiation of the optic cup gives rise to each of the components of the eye. The lens and cornea arise from the surface ectoderm; the retina, pigment epithelia, and optic nerve from neural ectoderm; and the vasculature and sclera from paraxial mesoderm. By three months gestation, each of the major anatomical structures is in place. At birth, the axial length of the human eye is about 7 mm (about 74% of adult), and increasing by about 0.6 mm/week. The eye grows nonuniformly; most of its increase in volume is from posterior segment growth. The neonatal corneal surface area is 3/4, and the scleral surface area /3 of the emmetropic adult s. 2 The lens continues to grow after birth, increasing in diameter more than in thickness, resulting in a less spherical and more disk-like adult shape. By 3 years of age, the eye has reached an average axial length of 23 mm, its developmental endpoint. 3 The retina The fovea develops before the peripheral retina; 4 6 yet it is immature at birth. 7,8 It first appears as a bump formed by ganglion cells. Over about the next 25 weeks, foveal ganglion cells and inner nuclear layer cells migrate peripherally, creating the familiar foveal depression at about 5 months. 8,9 Among the many specializations that endow the primate fovea with supreme vision is its peak density of photoreceptors. At birth, the density of foveal photoreceptor cells is a tiny fraction of the adult s. Peripheral photoreceptor cells migrate toward the fovea from before birth to at least 45 months (longer than the centripetal migration of ganglion cells). As the cones pack together there is a reduction in their diameter; their short, squat inner segments elongate, and the rudimentary stumps of outer segments lengthen into the long, thin appendages of the adult (Fig. 2.). Since ganglion cells and photoreceptors migrate in opposite directions, extended connecting processes form between the cone pedicles and their cell bodies. Reaching radially as far as 0.4 mm, these specialized axons are the Henle fiber layer, which surrounds the fovea by 2.5 mm in the adult. 0 The human fovea remains immature, even at six to eight months postpartum. Cell morphology and cell density take 5 months to approach maturity, and it may be four years before the retina is largely adult-like. This time course is consistent with some aspects of vision development measured experimentally. The retina contains seven principal cell types, each with its own circuitry, organized into layers. The different cell types derive from progenitor cells in the inner layer of the optic cup. Progenitor cells generate different retinal cell types, right up to its final division, raising the following question What influences the fate of retinal progenitor cells? The type of cell a progenitor may become follows a temporal order during development that is preserved between species. 2 4 Ganglion cells develop first, followed in overlapping phases by horizontal cells, cones, amacrine cells, rods, bipolar cells, and Müller cells (Fig. 2.2). 5 The acquisition and loss of their ability to differentiate into particular cell types suggests that extrinsic factors serially bias progenitor cells to a particular fate. However, in vitro experiments have not confirmed this. Environmental factors can change the proportions of different cells types generated at a particular stage, but they cannot induce the production of cell types inappropriate for that stage. 6,7 Thus, progenitor cells pass through a number of states during which they are only competent at producing a subset of cell types, and the proportions of city types that they produce at each stage is 9

2 ch.02.qxd 29/3/04 05:49 AM Page 0 SECTION EPIDEMIOLOGY, GROWTH AND DEVELOPMENT Cone pedicle Fiber of Henle Stage one brings cells imprecisely into bands at roughly the appropriate depths and is moderated by the attractive/repulsive adhesion properties of the cells. Mutations in genes encoding adhesion molecules or integrins disrupt retinal lamination Stage two more precisely organizes cells into uniform mosaics, distributed tangentially, and with perfect laminar distribution. In the rat, at birth the horizontal cells have migrated to within a ~50- m-deep sheet, but by day six, they form a regularly spaced monolayer. 23,24 The mechanisms of the second stage may work by maintaining constant distances between cells, perhaps by minimizing dendritic overlap. If one cell is removed from, or cells are added to, a developing retinal mosaic, the others shuffle over to regularize the mozaic weeks gestation 5 µm Newborn Outer segment Inner segment 45 month postpartum Fig. 2. A schematic drawing showing the stages of development of a human foveal cone (left to right) at 22, 24 to 26, and 34 to 36 weeks gestation, newborn, and 5 and 45 months postpartum. The inner segment is present before birth, while the outer segment develops mainly postnatally, being little more than a stump at birth. The cone pedicle and the fiber of Henle are present before birth. All four structures undergo extreme postnatal thinning and elongation. Adapted from Hendrickson and Yuodelis. Ganglion Horizontal Cones Amacrine Rods Bipolar Müller glial 3 6 Birth Embryonic Retinogenesis days Postnatal Fig. 2.2 Retinal neurogenesis proceeds in a characteristic sequence. Ganglion cells and horizontal cells differentiate first, followed in overlapping phases by cones, amacrine cells, rods, bipolar cells, and Müller glial cells. Curves represent the relative proportions of cells differentiating at each stage, not their absolute numbers. The time scale refers to mouse development. Adapted from Marquardt and Gruss. 9 controlled by environmental factors. 8 This is the competence model of retinal development. 9 Once differentiated, retinal neurons must migrate to their adult positions before they form synapses and to generate the laminar structure of the retina. This occurs in two stages: The chiasm In primates, ganglion cell axions enter the optic stalk at about six weeks of gestation. When they reach the optic chiasm they either cross or remain ipsilateral. Their decision is influenced by (among other factors 26 ) adhesions molecules and pathway markersl, 27 differential gene expression; 28,29 and chiasmal template neurons. 30,3 These mechanisms guide axons using attractive and repulsive molecules. 32 One such family of molecules is the slits. These are thought to govern where the chiasm forms by defining a restricting corridor. Disrupting slit expression produces a large, more anterior, secondary chiasm and prevents retinal ganglion cell (RGC) axons from finding their way into the appropriate optic tract. 33 The zinc finger transcription factor, Zic2, is expressed in ipsilaterally projecting RGCs during their growth from the ventrotemporal retina to the chiasm. 34 Zic2 regulates RGC axon repulsion by cues at the chiasmal midline. The proportion of RGC axons that cross is related to the size of the animal s binocular visual field. Retinal Zic2 levels correlate with the animal s degree of binocular vision, suggesting Zic2 is an evolutionarily conserved determinant of ipsilateral projection. In primates, the RGCs in nasal and temporal retina are directed to the contralateral and ipsilateral hemispheres respectively, except for a 5 wedge along the vertical meridian (with the overlap increasing with vertical distance from the fovea) where ganglion cells project to either hemisphere. 35 The normal decussation pattern is disrupted in albinos 36 and rarely in otherwise normal primates 37 (see Chapter 45). Myelination of the optic nerve begins only after all RGC axons have reached the geniculate body (fifth month in humans) and continues into early childhood in a brain-to-eye direction, stopping at the lamina cribrosa. 38 Occasionally, it proceeds into the retina, where it appears as white streaks in the nerve fiber layer. 39 Such errant myelination is normally benign, but rarely it can be associated with a visual deficit. 40,4 Retinogeniculate projections About 90% of primate RGCs project to the lateral geniculate body (LGB), the remainder mostly go to the pretectum and the superior colliculus. 42 The LGB contains six principal layers specified by their eye of input and by their cell type (Fig. 2.3). Four of the principal layers (two for each eye) are made up of small, parvocellular (P) cells, and two (one for each eye) contain large, magnocellular (M) cells. Tiny koniocellular (K) cells constitute a third class that occupy the leaflets between the six principal layers. 43 The layers of the LGB are present at birth. Their development exemplifies the interaction between activity-dependent and hard-

3 ch.02.qxd 29/3/04 05:49 AM Page Normal and Abnormal Visual Development CHAPTER 2 geniculate first and innervate the medial segment that will later develop into the four P layers. The M-type retinal afferents arrive later and innervate the lateral segment that will become the two layers of the M division. Thus, retinal afferents innervate their appropriate presumptive M or P divisions exclusively, suggesting selective targeting rather than corrective selective elimination. 50 Thus, the segregation of the geniculate into magnocellular and parvocellular layers is less dependent on visual experience than its segregation by eye. Fig. 2.3 Macaque LGN following injection of a radioactive tracer ([ 3 H] proline) into the right (contralateral) eye. Layers containing the tracer appear bright. The six principal layers are monocular in macaque (and human) and follows the sequence: ipsi-contra-contra-ipsi-contra-ipsi. Layers and 2 are magnocellular, and layers 3 6 are parvocellular. Koniocellular layers are situated between the six principal layers (not see in this section). M, magnocellular; P, parvocellular; c, contralateral; i, ipsilateral. Geniculocortical connections Most geniculate cells project to layer 4 of the striate cortex, where inputs from the two eyes differentially activate single cells ocular dominance. 5 As an electrode is advanced parallel to the cortical layers, the ocular dominance of cells alternates between the left and right eyes. Small lesions of single (monocular) layers of the LGB cause degeneration of terminals in layer 4 of the striate cortex in a stripy pattern. 52 These 300- to 400- mwide stripes of left- and right-eye inputs form a mosaic of discreet columns. The complete pattern of ocular dominance columns can be visualized by radioactive tracer injected into one eye. 53 The tracer is taken up by RGCs and transported to layer 4 of the striate cortex (Fig 2.4). Formation of the ocular dominance column pattern cannot be dependent on visual experience because (at least in the macaque) it is adult-like at birth. 54,55 However, this does not necessarily mean that it forms independently of neuronal activity. It has long been held that the geniculocortical afferents are initially intermingled and are segregated into ocular dominance columns under the influence of retinal activity Thus, pharmacological blockage of retinal activity abolishes column formation in the cat. 57 The spontaneous waves of neuronal activity that roll across each retina in utero could play a role in column segregation by generating firing patterns in the RGCs that are spatially correlated within, but not between, each eye. 59 This suggests that cells with synchronous activity are segregated into a single ocular dominance column, i.e., cells that fire together, wire together. 60 However, ferrets binocularly enucleated before their geniculocortical afferents arrived at the striate cortex form normal wired mechanisms in the formation of the visual pathways. In the human, optic tract fibers begin to reach the LGB by about the th week. Initially, left and right eye afferents are intermingled over the prospective left and right eye layers. 44 Between weeks 4 and 30, a time corresponding to the formation of eye-specific layers in the geniculate body, the population of RGC axons reduces from 3.5 million to about million. 45,46 Perhaps the purpose of this cell loss is to generate eye-specific layers by eliminating inappropriately connected axons: selective elimination. 45 Selective trimming of axon terminal arbors also segregates binocular inputs to the LGB. In the cat, axons innervating the LGB grow promiscuous side-branches that contact cells in both left and right eye columns. By birth, the side-branches contacting the inappropriate layer for their eye are withdrawn, leaving a precise segregation of inputs. 46,47 However, selective elimination of whole retinal afferents (rather than single branches) can account for the segregation of binocular afferents in the primate LGB. 48 In contrast to the laminar segregation by eye, development of the M and P layers of the LGB does not employ selective elimination of afferent arbors. RGCs become M and P types soon after their final mitosis. 49 The P-type retinal afferents reach the Fig. 2.4 Macaque monkey left striate cortex following injection of a radioactive tracer ([ 3 H] proline) into the left eye. The tracer appears bright. The tissue has been dissected from the rest of the brain, unfolded, and flattened to show the entire striate cortex. The mosaic of ocular dominance columns is visible because the tracer was transported to only those columns belonging to the injected eye. The oval in the center is the representation of the blind spot a monocular region of the visual field.

4 ch.02.qxd 29/3/04 05:49 AM Page 2 SECTION EPIDEMIOLOGY, GROWTH AND DEVELOPMENT 2 ocular dominance columns. 6 Thus, retinal activity cannot be a prerequisite for columnar segregation in this species. The formation of the ocular dominance column pattern may rely on intrinsic signals, e.g., molecular cues on thalamic axons, on cortical cells, or on both. 62 Extrastriate cortical areas Cells in the striate cortex project to multiple extrastriate visual areas, which form an interconnected hierarchy reaching into the parietal and temporal lobes. Many regions in this network are defined as single visual areas by their retinotopic organization, cell selectivity, and/or unique pattern of connections with other regions. 63 It is thought that different areas process different visual modalities, e.g., motion, color, and form. 64 The development of extrastriate cortical areas has been investigated by the early removal of cortical tissue, before any inputs have arrived at the neocortex: 65 If cortical areas form in predetermined regions, subsequent mapping of visual areas in these animals in adulthood would show a reduced number of areas. However, they show a full compliment of cortical areas, squeezed onto a reduced area of cortex. Thus the primitive neocortex is an unspecialized substrate whose subdivision into areas occurs in unison. NORMAL VISUAL DEVELOPMENT To study visual development, it is important to know about the vision of the newborn. Newborn humans can see, they prefer to look at faces, 66,67 they can discriminate between mouth opening and tongue protrusion, and rapidly imitate either. 68 They initially fixate simple high-contrast patterns (like their mother s hairline), and are later attracted to more subtle features (like their mother s eyes). Measurement techniques Since infants are unable to report what they see, adult vision measurement techniques must be adapted and other indicators of seeing used. 69,70 The three most commonly used techniques will be described: Preferential looking Given a choice of looking at a striped grating or a uniform field, an infant will prefer to fix the grating A hidden observer guesses which of the stimuli contains a grating based on the infant s fixation pattern. 74 As less visible gratings are presented, the observer makes more incorrect inferences. The visual threshold is defined by the grating stimulus that generates only 75% correct inferences from the observer. While this technique has been successfully used to asses infant visual development, its reliability is dependent on many trials and it is not always possible to hold the infant s attention for the necessary time. 77 Visually evoked potentials The visually evoked potentials (VEP) technique measures electrical activity directly from the scalp using surface electrodes. 78,79 Visual stimulation produces a stereotypical wave whose amplitude and timing can be measured. Repeated responses are recorded and averaged to improve signal-to-noise ratio. A transient VEP is recorded in response to a single event, e.g., a flashed stimulus, 80 and a steady-state VEP is a continuous standing wave pattern produced by a rapidly repeating stimulus. 8 The raw VEP signal must be analyzed to estimate acuity. A simple method is to compile a set of transient VEPs using a single high-contrast stimulus, and measure the Snellen acuity in an emmetropic adult with various degrees of optical blur. The infant s acuity can then be estimated by comparing their VEP to the blurred adult set. The acuity of the infant is presumed to be equal to the acuity of the adult whose vision was blurred such that their VEP signals were most similar. 80,82 This method relies on the unlikely assumption that infant and adult VEPs are equivalent. An alternative method is to present finer and finer grating stimuli until a transient VEP is no longer measurable above noise, 83 or to extrapolate to zero response amplitude, using either the transient 84 or the steady-state VEP. 85 Extrapolation to zero is used because the VEP signal is inherently noisy, making it more difficult to define the exact point where a small signal disappears. Optokinetic nystagmus A wide field-drifting stimulus generates optokinetic nystagmus that can be used to estimate infant visual acuity because it is only generated by resolvable stimuli and it is easily observed. The earliest investigation used a stimulus attached to a metronome wand; 86 later experiments employed scrolls of paper, printed with gratings and streamed over the infants visual field by a hand crank. 73,87 Eye movements were observed or measured with electro-oculograms. 88 Visual acuity Visual acuity ( grating acuity, resolution acuity ) is a measure of the finest feature detectable by an observer. It can be described as the visual angle subtended by a single stripe element (minutes per stripe), or more formally as its reciprocal (cycles per degree), i.e., the threshold spatial frequency of a 00% contrast square-wave grating. 89 In normal adults, resolution acuity is equal to about min/stripe, or 30 cycles/degree. By setting this value to be equivalent to the standard Snellen acuity of 20/20, it is possible to roughly convert between the two scales. In classic studies using optokinetic nystagmus (OKN), 93 of 00 infants aged 80 minutes to 5 days responded to a 0.56 cycles/degree grating moving at 8.5 /sec, but none responded to a 0.9 cycles/ degree grating moving at the same speed. 87 Using a greater range of grating sizes, a large percentage of another 00 newborns responded to a 0.25 cycles/degree grating. 90 Nearly all the infants tested had at least 20/400, and some 20/300, Snellen equivalent vision. Others found lower values for newborn acuity, 9 but this range is consistent with most investigations of zero- to threeweek-old infant acuity, using OKN 73,88 and PL. 75 Natural variation, nonstandardized testing techniques, different viewing distances, and illuminations produced differences in measured acuity between studies. To overcome this, the standardized Teller Acuity Card system was devised. 92,93 Using these cards, 40 infants showed a mean acuity at one week of 0.9 (± ~0.5 SD) cycles/degree. 94 This data, along with other measures of the development of acuity using PL, is shown in Fig A comparison of PL with VEP data shows that the VEP technique gives acuity values about to 2 octaves better than PL measures at all ages (an octave is a doubling of acuity). This could be due to the stationary stimuli used in PL studies, whereas VEP studies use temporally modulated gratings that may produce a lower acuity threshold. If checkerboard stimuli are used in VEP studies, the amplitude of the signal shows a peak at a particular check size. If the location of this peak is used instead of the VEP amplitude, the two acuity measures agree. 95 When VEPs and FPL

5 ch.02.qxd 29/3/04 05:49 AM Page 3 Normal and Abnormal Visual Development CHAPTER Acuity (cycles/deg) Contrast sensitivity were measured in the same infants, VEP signals could be detected for spatial patterns that were below threshold for behavioral measures. This could be due to the signal averaging used in the VEP technique. 96 It seems that the increased signal-to-noise ratio generated by averaging in VEP studies is of benefit to the experimenter but not the visual system! If VEP latency was used instead of amplitude, comparable scores could be generated with the two techniques. Contrast sensitivity Age (months) Gwiada et al Birch & Hale Van Hof-van Duin and Mohn Allen Mayer and Dobson McDonald et al Courage and Adams Fig. 2.5 Representative examples of data showing the development of binocular grating acuity in normal infants tested with the PL procedure. 70,94, From Dobson. 240 A more complete evaluation of the spatial performance of the visual system can be gained by measuring many contrast thresholds over a range of spatial frequency. This produces a graph of threshold contrast versus spatial frequency contrast sensitivity function. 97,98 The conventionally measured grating acuity is then represented by the abscissa of the x axis (00% contrast). The adult curve has a peak value at 3 5 cycles/degree and a decline in sensitivity at both lower and higher spatial frequencies. Overall contrast sensitivity of the infant is about 0 times higher than the adult but infants are relatively more sensitive in the low-frequency region. This reduced low-frequency cut was demonstrated in a PL study of infants from age 5 to 2 weeks. 99 The low-frequency cut was smallest in the 5-week group, more pronounced by 2 weeks. The absence of a low-frequency cut in the 5-week group, and its relatively smaller size in the older groups, suggests that the undeveloped visual pathway might be relatively well suited to transmit coarse features that do not require the high-resolution components of the retina, or it could be central in origin. Inhibitory cortical connections may tune cortical cells to higher spatial frequencies; 00,0 and perhaps these are underdeveloped in infants. It could be artifactual, due to the reduced number of cycles visible in the low-frequency stimuli, or perhaps infants just prefer to fixate lower spatial frequencies. The effect has been verified by Spatial frequency (c/deg) Adult 3 months 2 months month Fig. 2.6 Average contrast sensitivity functions for -, 2-, and 3-montholds and an adult obtained using an FLP procedure. The solid line represents data obtained with the infant apparatus; the dashed portion represents typical high-frequency data for an adult under similar conditions. Data from Banks and Salapatek. 76 further PL investigations (Fig. 2.6) but the origins of changes in the contrast sensitivity function over the first 3 months of infancy would have to be determined with other techniques. The maturation of contrast sensitivity has been studied in the infant using the steady-state sweep VEP, where the spatial frequency of the stimulus is swept over a range of values during recording, while holding contrast constant. These findings substantiated the main PL findings and narrowed the search for anatomical correlates. The absence of low-frequency attenuation in young infants was reproduced, and its time course clarified. Up to 9 weeks of age, the contrast sensitivity function shows an increase in sensitivity at all spatial frequencies. Thence, sensitivity increases are restricted to the higher frequency domain, indicating an improvement in spatial resolution, rather than sensitivity per se. Thus, the development of low-spatial-frequency vision follows a time-course shorter than that of high spatial frequencies. Given that high spatial frequencies are detected by the fovea, which develops more slowly than the periphery, 7 it is likely that the slow increase in high-spatial-frequency sensitivity is a result of the prolonged development of the fovea. Likewise, since low-spatialfrequency sensitivity is unaffected by exclusion of the fovea, 08 it follows that the early relative sensitivity to low spatial frequencies is due to the relatively advanced maturation of peripheral photoreceptors. An infant monkey, operant trained to indicate with a lever push the screen that contains a grating, shows that the macaque visual system is very similar to the human s, but it develops much faster. 09 Infant humans and monkeys show the same depression of sensitivity compared to adults. The higher relative sensitivity 3

6 ch.02.qxd 29/3/04 05:49 AM Page 4 SECTION EPIDEMIOLOGY, GROWTH AND DEVELOPMENT 4 to low spatial frequencies of human infants is also apparent, as is the differential developmental time-course for low and high spatial frequencies. 0 Young monkeys foveal contrast sensitivity is similar to the near periphery but develops to a greater degree during maturation. Thus, the developmental time-courses of high- and low-spatial-frequency sensitivity matched those of the fovea and peripheral retina, respectively. To compare the development of grating acuity and contrast sensitivity in the central and peripheral visual field of the human infant, sweep VEPs were measured in infants ranging from 0 to 39 weeks of age. 2 By testing the central and peripheral fields simultaneously, each at a different temporal frequency, it was found that peripheral acuity reached adult values by 26 weeks while central acuity did not reach asymptosy up to 33 weeks. The human infant s fixation sensitivity was better than its peripheral vision at all ages. However, this could be due to the infant using an eccentric fixation strategy, rather like the one an adult might use to view a distant star at night. Photoreceptor development is probably not the only limiter of maturation of contrast sensitivity. 3 Optical influences are modest because the neonatal media are clear, and accommodation does not affect acuity over distances between 30 and 50 cm. 4 To gauge any influence the LGB and striate cortex may have on contrast sensitivity is not straightforward because they receive signals already filtered by the immature retina. Binocular vision Primates and carnivores perceive depth from their binocular view of the world: stereopsis. For stereopsis to develop, eye movements must bring both foveas at the same point in 3-D space and the eyes must move conjugately to maintain binocular fusion during vision. When the images from the eyes are continuously fused, the disparities are analyzed by cortical cells for the perception of depth. Stereopsis is robust: stereograms can be perceived when one eye s image is blurred; 5 likewise, it can develop in infants despite significant anisometropia. 6 Stereopsis is the result of a neuronal calculation, so it requires specialized stimuli to isolate it from other visual cues. Clinically, stereopsis is often tested with the Titmus test. The patient wears polarized spectacles with the plane of polarization at right angles for each eye. Test images consist of superimposed stereo-pairs, presented differently to each eye by a polarized filter layer. One example is of a large housefly, which stands out from the page to an observer with stereopsis. Others are circles and animal pictures at different depth planes. While these tests are adequate for assessing the presence or absence of stereopsis in children, random dot stereograms 7 contain a form that can only be seen with stereopsis. Viewed monocularly or by the stereoblind, a random dot stereogram appears as a flat field of noise. When viewed stereoscopically, pictures appear in front of, or behind the plane of the page. By about two months of age, some infants, tested with random dot stereograms, apparently discriminated disparity when tested with PL and a habituation recovery test. 8 Computers made dynamic random dot stereograms more amenable. Using FPL, infants were presented with a square stimulus, defined by stereo alone, that drifted to the right or left. The hidden observer guessed the direction of the stimulus by the infant s behavior and eye movements. 9 No significant difference from chance was measured in the observer s direction guesses for infants up to 3.5 months of age, and even later (up to 6 months) in a longitudinal experiment. 20 Random dot stereograms are suited for use with VEPs because they are perceived only if stereopsis is present. Any modulation of the VEP signal at the same frequency as the stimulus is indicative of stereopsis. Random dot stimuli that oscillate in depth (stereograms) or that counterphase in one eye (correlograms) evoke large potentials, enabling reliable determination of stereopsis in infants Stereo stimuli evoked responses in infants at 8 to 20 weeks. 24,25 Stereoacuity improves until about 24 months, when it approaches adult levels 26,27 ; some improvement is from increased interocular distance but most is due to development of the central visual pathways. Disparity-tuned binocular neurons have been found in the striate cortex in both the cat and monkey. 28,29 Although these cells signal the disparity of stimuli, their responses do not necessarily correlate with depth perception. 30,3 Cells in the monkey striate cortex can be tuned to disparity by the sixth postnatal day, 32 several weeks before the onset of stereopsis. Furthermore, infant monkeys had an adult proportion of disparity-tuned cells and their ocular dominance histograms are adult-like. 33 The development of stereopsis is not due simply to a proportional increase in the numbers of disparity-tuned cells in the striate cortex, but to a refinement of their spatial response properties and overall responsiveness. The onset of stereopsis may correlate with the refinement of extrastriate visual connections and increasing populations of disparity-tuned cells in higher visual areas. Orientation selectivity Cells activated selectively by bars or gratings presented over a small range of orientations are implicated in form vision. 34 Recordings from striate cortex cells of newborn kittens showed that no neurons were adult-like in their responses to oriented contours. 35 Orientation-selective cells have been found in visually inexperienced macaque striate cortex at three weeks. 36 Even at this early stage, the cells are organized into adult-like columns that are in register with ocular dominance columns. 37 The relationship between orientation selectivity and visual experience has been investigated by raising animals in an artificial environment that exposed them to a restricted range of contour orientations: stripe rearing. 38,39 Stripe-reared cats were first shown to have a larger than normal proportion of cells tuned to the orientation they were exposed to most. Despite the dramatic changes in the distributions of orientation preference of cortical cells, only modest specific behavioral deficits could be measured, incommensurate with the magnitude of the physiological effects. 40 This inconsistency led others to re-examine the phenomenon. At first, no effect was found; 4 later, a small effect was described. 42 The inconsistencies in stripe rearing experiments were probably due to different techniques and sampling methods. The overall effect, though probably real, was certainly not as striking as it was first described. Motion perception Motion information is crucial to many visual and motor functions, for example, the encoding of depth through parallax, estimating trajectories, segmenting figures from backgrounds, and controlling posture and eye movements. 43 OKN studies show that motion vision develops early in humans. 44 However, OKN is a crude and reflexive measure of motion sensitivity that does not require fine direction discrimination.

7 ch.02.qxd 29/3/04 05:49 AM Page 5 Normal and Abnormal Visual Development CHAPTER 2 The ability to discriminate opposite directions of motion develops at about 0 to 3 weeks. 45,46 VEP studies show it within the first two months of life. 47,48 Finer discriminations of motion direction have been tested using FPL where infants were presented with windows of dots moving in a different direction to the background dots. 48 The angle between target and background directions was reduced until preferential looking by the infant was no longer detectable. By the age of 2 weeks, infants made quite fine discriminations, on the order of 20, and by 8 weeks they were down to ~5 These values are still far from those of adults, who have no trouble making discriminations of less than. Neural motion perception is thought to result from the activity of direction-selective cells. These are found in layer 4B of the striate cortex, 50 and in many extrastriate cortical areas, most notably V5 (MT). 5 Electrical microstimulation of V5 cells in the monkey has shown that their activity correlates directly with the perception of motion direction and can affects the animal s direction discrimination. 52 Little is known about the development of direction selectivity in area V5. However, single-cell recordings in the striate cortex of -week-old monkeys have shown that direction selectivity is absent or very broad. The tuning width was by 2 weeks of age, narrowing to an adult-like 30 by 4 to 8 weeks. 53 Thus, the time-course of direction selectivity in monkey striate cortex approximately matches that of the psychophysical measurement of direction discrimination in the human infant, counting the four-times slower maturation of the human visual system. 09 Color vision Early studies suggest that very young infants are able to discriminate different colors. 54 However, most natural-colored stimuli also differ in their real and perceived brightness. To test color vision exclusively it is necessary to use colors of the same perceived brightness that can only be discriminated by their wavelength composition (isoluminant colors). Different individuals can have different isoluminance points, so to eliminate individual variability, isoluminance points must be measured in every experimental subject. In early studies, adult isoluminance points were used, 55 introducing a luminance confound. 56 FPL techniques for measuring isoluminance points in infants were soon developed. 57,58 FPL-derived isoluminance points are unlikely to be perfectly accurate, so residual luminance artifacts were camouflaged either by testing over small ranges of luminance differences from trial to trial or by dividing the stimuli into a number of tiles and randomly jittering their luminance. With the luminance confound removed, the infants chromatic discrimination can be tested, usually with FPL, using patterned (preferred) stimuli, commonly two isoluminant color checks or gratings, versus uniform (nonpreferred) intermediate color stimuli. Thus it was shown that 8-week-old infants can distinguish a red and isoluminant gray square wave grating from a uniform luminance matched stimulus. 58 Female infants were used to reduce the probability of a color-blind infant being tested. A few 4-week-olds, some 8-week-olds, and all 2-week-olds demonstrated color vision Sensitivities to different wavelengths may develop at different times, making infants functionally deuteranopic for a developmental period. 60 However, a wide range of normal variation exists in development of spectral sensitivities. 6,62 Luminance and chromatic sensitivity are both dependent on the same photoreceptors: red and green cones. 63 If the time-course of changes in contrast sensitivity are the same for luminance and chromatic stimuli, it suggests that their neural correlate lies with photoreceptors development, 64 but if their time-course is different, separate (post-receptor) mechanisms may be responsible for each. 65 To differentiate, it is necessary to measure chromatic contrast sensitivity in infants over a range of ages. Using VEPs, it was found that chromatic and luminance contrast sensitivity functions at all ages were well described by curves of a common shape, with developmental changes confined to upward shifts in sensitivity and rightward shifts in spatial scale Thus, their poorer color discriminative ability (like their poorer luminance contrast sensitivity) can be explained by the smaller percentage of photons caught by their immature photoreceptors. 64 ABNORMAL VISUAL DEVELOPMENT: AMBLYOPIA Definition Amblyopia is from the Greek amblyos, blunt, and opia, vision. Albrecht von Graefe defined amblyopia as the condition in which the observer saw nothing and the patient very little. This definition remains valid because it emphasizes an important feature of amblyopia that looking into the eye reveals nothing about the disease itself. Eye examination does reveal factors that cause amblyopia, like cataract, strabismus, and anisometropia. A more formal definition of amblyopia is visual impairment without apparent organic pathology. Critical periods The term critical period was first used by Konrad Lorenz in his studies of imprinting in birds. It was adopted by Hubel and Wiesel to refer to the time when deprivation changes the ocular dominance of cells in the striate cortex. 70 It falls between 4 and 6 weeks in the cat, 7 when closure of one eye for 3 days or more leads to a visual cortex dominated by cells responsive only to the open eye. Some susceptibility to deprivation persists until about 9 months. 72 Since Lorenz coined the term, it has taken on a more general use. A critical period can be defined for any function as the time when, if deprived of normal stimulation or unused, its development may be permanently disrupted. Visual critical periods begin after the initiation of visual stimulation (eye-opening in cats, birth in primates) and last between weeks and years, depending on the species and visual faculty in question. Critical periods have been defined for strabismus, 73 for the development of direction selectivity in cat striate cortex, 73 and for orientation selectivity. 75 Neurones with higher functions, like binocularity, have critical periods that end later than those with earlier-processed properties, like ocular dominance. This is exemplified within the striate cortex, where the critical period is over in layer 4 (the input layer) before the other layers. 76 Early monocular deprivation is catastrophic to the visual system because it affects many critical periods. Critical periods can be inferred by monocular deprivation after various delays. In the monkey, monocular deprivation before 3 months affects absolute light sensitivity, between 3 and 6 months sensitivity to wavelength and brightness, up to 8 months high-spatial-frequency vision, and up to 24 months binocular vision. 77 Human critical periods are less well defined, and can be deduced by studying children with amblyopia following unilateral cataract surgery. 78 By comparing children in whom the age of cataract onset and correction of vision were known, the human critical period for visual acuity loss seems much longer than that in cats and monkeys. 5

8 ch.02.qxd 29/3/04 05:49 AM Page 6 SECTION EPIDEMIOLOGY, GROWTH AND DEVELOPMENT 6 Weeks of deprivation between 6 and 8 months and months of deprivation up to age 8 produce permanent visual deficits. Early correction is therefore imperative for visual recovery following deprivation. Amblyopia may recover in adult humans following loss of the nonamblyopic eye. This occurs well beyond the critical period, when no amount of monocular occlusion would result in the induction of amblyopia. Improvements have been measured following visual loss of the good eye in teenagers and adults 79 and a 65-year-old. 80 One year after the loss of their good eye, 20% of 254 amblyopes aged years or older had some improvement in their amblyopic eye, 8 and half improved by two or more Snellen lines. At least in a minority of patients, neural plasticity can occur past the critical period. Factors other than neural plasticity could also lead to an apparent improvement in visual acuity. Fixation may become more stable and accommodation accuracy may improve in an amblyopic eye once the individual is forced to use it alone. Adult recovery from amblyopia may help us understand the factors that normally act to restrain plasticity beyond the critical period but children should remain the focus of detection and treatment. 8 Causes Amblyopia is caused by abnormal visual experience: mostly by strabismus, anisometropia, monocular form deprivation, or a combination of these. Defining the cause in any particular patient is not always straightforward because anisometropia and strabismus can arise as a consequence of amblyopia, 82 making it difficult to distinguish cause and effect unless a patient is examined early enough. The three most recognized causes are strabismus, anisometropia, and monocular form deprivation. Strabismus Strabismus is a misalignment of the optic axes resulting from motor or sensory deficits. 83 The optic axes may be crossed (esotropic), diverged (exotropic), or vertically misaligned (hyper/hypotropic). Humans are often born with a slight exodeviation, thought to represent the anatomic positions of the divergent orbits. 82 During the first six months, binocular fusion emerges, and a normal infant attains orthotropic vision. 83 One to 2% of infants do not develop binocular fusion and acquire strabismus. 8 Esotropia is the most common form of childhood strabismus and is often associated with hyperopia. Infants are born hyperopic but generally attain emmetropia, though some remain hyperopic until 2 3 years or more. 86 Some of these infants accommodate to correct their blurred vision. Normally, accommodation is reflexively accompanied by a tendency to converge but in hyperopia this reflex can cause the eyes to cross, preventing binocular fusion and resulting in infantile esotropia. Strabismics rarely complain of double vision because they suppress perception from the deviated eye. Some are able to alternate fixation and suppression between their eyes, and they rarely develop amblyopia. Strabismics who fix constantly with one eye and suppress its deviated fellow are most at risk of developing amblyopia. In adult monkeys made exotropic surgically, the metabolic activity of one or other set of ocular dominance columns was found to be locally depressed. 87 The retinotopic locations of depressed columns corresponded to the locations of suppression scotomas in human exotropes. This demonstrates that strabismic suppression reduces the activity of cells in the striate cortex. The neural mechanism of suppression may be similar to that of normal binocular rivalry. 88 Anisometropia Anisometropia is an interocular difference in refractive power, often the result of a difference in size or shape of the globes. An inequality of greater than 2 diopters is potentially amblyogenic if it persists until the age of 3 or longer. 83,89 In humans, about onethird of cases of anisometropia are accompanied by strabismus. Blurred vision in one eye may lead to inaccurate binocular fusion, resulting in a small angle strabismus, but confounding effects of the strabismus are difficult to isolate. Anisometropic amblyopia has been produced in primates by daily uniocular administration of the cycloplegic, atropine, from birth to eight months. 88 This provides a realistic model of the effects of anisometropic amblyopia without the confounding factors that exist in humans Astigmatism occurs when one or more of the refracting surfaces of the eye contain a cylindrical component, resulting in refractive power that varies at different meridians. Astigmatism is common (30 to 70%) in the first two years of life Early astigmatism may not to have any detrimental affect on visual development but, if persistent, it is a risk factor for amblyopia. 98 Astigmatism may be responsible for a form of amblyopia that is orientation specific meridional amblyopia, 99 where subpopulations of cortical cells are selectively affected according to their visual response properties. In humans, the angle and magnitude of meridional amblyopia correlate well with that of astigmatism. 200 Adult monkeys, raised with fixed orientation cylindrical lenses to imitate binocular astigmatism, have shown orientation-specific acuity deficits. 90,20 Monocular form deprivation It is not the lack of light that causes amblyopia, but the lack of a sharp image. Monocular blur (anisometropia) is a form of monocular form deprivation in an eye with clear optics. Form deprivation can also be caused by light scattering from imperfections in the optical components of the eye. Cataracts are a common cause. Surgery for early cataracts is urgent because, at the peak of the critical period, as little as 2 weeks of deprivation can initiate amblyopia. However, the operated, aphakic eye is far from normal: accommodation is abolished, so focus is fixed at a single distance and anisometropia occurs at some fixation distances. Aggressive patching following cataract surgery may enable some recovery of vision, but it rarely prevents amblyopia entirely Suturing the lids of one eye of neonatal cats causes profound amblyopia reliably but nonspecifically, because it blocks all modalities of vision. Monkeys raised with one of three different strengths of diffuser spectacle lenses in front of one eye and a clear zero-powered lens in front of the fellow eye were found, in adulthood, to have a close correspondence between the magnitude of the amblyopia and the reduction in retinal image contrast produced by the diffuser lenses. 206 Thus, the depth of nonstrabismic amblyopia is strongly influenced by the degree of retinal image degradation early in life. Classification Patients are usually classified as strabismic or anisometropic amblyopes based on the symptoms at the time of study. However, strabismus and anisometropia can also arise as a consequence of amblyopia, making the classification of amblyopia at best difficult. Those amblyopic from an isolated cataract are an exception; it is presumed that monocular deprivation is the sole cause.

9 ch.02.qxd 29/3/04 05:49 AM Page 7 Normal and Abnormal Visual Development CHAPTER 2 A fundamental distinction exists between strabismic and other forms of amblyopia. Anisometropic and deprivation amblyopias are caused by an optical degradation of one retinal image, but in strabismic amblyopia both retinal images are initially perfect. It has been proposed that there are distinct patterns of visual deficits in strabismic and anisometropic amblyopes, 207,208 but this has been based on studies of few patients Recently, 427 amblyopes between the ages of 8 and 40 were classified by ocular deviation, surgical history, refractive errors, eccentric fixation, and deprivation history, and compared with 68 controls. 23 Measures of acuity, contrast sensitivity, and binocular and stereo vision were undertaken. Three patterns of visual loss corresponding roughly to traditional classifications based on the associated condition were found: strabismics, anisometropes, and strabismic anisometropes. Deprivational amblyopes had functional deficits distinguishable from anisometropes (Fig. 2.7). Thus, two developmental anomalies could account for the patterns of visual loss in amblyopia poor image formation in one eye (anisometropes) and a loss of binocular function (strabismics). A combination of these factors produces the third, worst affected group (strabismic amblyopes). Visual deficits Clinically, amblyopia is characterized by a significant reduction of Snellen visual acuity that does not correct with refraction. However, Snellen acuity is a general measure of visual function. If more specific characteristics of vision are examined, a more precise picture of amblyopia can be gained. Comparing grating acuity, Vernier acuity, and Snellen acuity in strabismic and anisometropic amblyopes shows that grating acuity is more reduced in anisometropic than strabismic amblyopia Conversely, contrast sensitivity is more elevated in strabismic amblyopes than those with preserved binocular vision (including controls) but reduced below normal in anisometropic amblyopes. 23 Interference effects, characteristics of normal spatial vision, manifest as a reduced discrimination of closely spaced stimuli, e.g., orientation, 29 stereoacuity, 220 and Vernier acuity. 29 Spatial interference, or crowding, is elevated in amblyopic eyes, 22,222 so amblyopes poor performance at Snellen charts organized in rows may be improved by using single optotypes. 22,223 Visual measures adversely affected by crowding all rely on hyperacuity; i.e., they are limited by cortical processing rather than the spatial resolution of foveal cones. 224,225 Since amblyopia is a cortical deficit, diminished hyperacuity and the increased effects of visual crowding are typical in amblyopic individuals. The acuity and sensitivity deficits in amblyopia could be the result of changes in striate cortex neurons, but other deficits are less easily accounted for by low-level visual neuron activity. If strabismic amblyopes are asked to count highly visible features that vary in number, or change orientation in briefly presented stimuli, they systematically undercount. This suggests a limit to the amount of information that the amblyopic visual system can attend to individually. 229 Cueing the observer to the relevant part of the display improved performance in amblyopes and normals alike, suggesting that the amblyopic deficit was not the result of reduced spatial attention. It is unlikely that this high-level visual processing is in the striate cortex; but probably reflects unreliable signals reaching higher visual areas from the striate cortex. Anatomical correlates The balance of left- and right-eye cells in the cat striate cortex can be tilted in favor of one or the other eye by manipulating early visual experience. Immediately after newborn kittens open their eyes, a few weeks of monocular eye-lid closure result in a paucity of cortical cells responding to the closed eye. 230,23 This regime produced monocular deprivation similar to a congenital cataract in humans. In macaques, the changes are accompanied by a change in the relative widths of the ocular dominance columns. 232,233 The normal pattern of ocular dominance columns, roughly equal in width, is remodeled so that the columns belonging to the sutured eye shrink and the space is taken by the expanded columns of the nondeprived eye (Fig. 2.8). This anatomical evidence of postnatal remodeling of ocular dominance columns in monocular deprivation suggests that amblyopia could be caused by a lack of striate cortex devoted to the deprived eye. However, other forms of amblyopia are not Low Acuity (cycles/deg) High Eccentric fixators Strabismics Strabismic anisometropes Deprivationals -0.5 Anisometropes Inconstant strabismics Former strabismics Inconstant strabismic anisometropes Other abnormals Refractives Normals Fig. 2.7 Eleven clinically defined categories of amblyopia in the study of McKee et al. 2 The mean position of each is plotted against acuity and sensitivity, calculated from a number of tests. The normal, strabismic, and anisometropic observers fall into different regions of the twofactor space. The strabismic amblyopes appear to represent a mixture of the strabismic and anisometropic categories. Error bars represent SEM along the principal axes of each category s elliptical distribution Low Factor ("acuity") High Strabismic-anisometropes Strabismics Anisometropes Normals 7

10 ch.02.qxd 29/3/04 05:49 AM Page 8 SECTION EPIDEMIOLOGY, GROWTH AND DEVELOPMENT Fig. 2.8 The ocular dominance column pattern of a macaque, following early monocular eye-lid suture to simulate congenital cataract. The columns belonging to the deprived eye appear shrunken and reduced to small islands, while those of the unaffected eye have expanded their territory. associated with differential column shrinkage. The ocular dominance columns of a human strabismic and an anisometropic amblyope were to shown to have the same width for both eyes 234,235 and a naturally occurring anisometropic amblyopic macaque also had normal and equal width ocular dominance columns. 236 Thus, column shrinkage does not necessarily have a causal relationship to amblyopia; nevertheless, column shrinkage without amblyopia has never been described. A fmri study showed a biased share of cortical territory in favor of the nonamblyopic eye in strabismic, anisometropic, and strabismic-anisometropic amblyopes whose visual deficit developed during infancy, but no effect if the deficit developed after two years of age. 237 This finding contradicts the histological studies in humans and animals. Perhaps the humans and animals studied histologically had late-onset amblyopia, occurring after remodeling of the columns was possible. The resolution of fmri is limited to about 0.5 mm, about the width of a single ocular dominance column, making it difficult to resolve columns, let alone measure the subtle changes that they may undergo in amblyopia. The spatial resolution of histological tissue is not limited. Anatomical tracer injections have shown that neurons in the striate cortex of strabismics have abnormal wiring. At birth, intralaminar horizontal connections exist between neighboring ocular dominance columns. This pattern of horizontal fibers normally persists into adulthood, 238 but if strabismus is induced during the critical period, there is a change in the horizontal network: projections between left- and right-eye columns are reduced, leaving only fibers that connect cells activated by the same eye. 239 Amblyopia is caused by anatomical and functional changes in the brain. They have been observed in the striate cortex, but it is unlikely that this is the only region altered; it is merely the best studied. Further investigations of the anatomical wiring and physiological properties of neurones in amblyopic and normal animals and humans may tell us more about the mechanisms that cause amblyopia. One hopes that such knowledge will facilitate new approaches for the treatment and prevention of the disease. 8 REFERENCES. Fledelius HC, Christensen AC. Reappraisal of the human ocular growth curve in fetal life, infancy, and early childhood. Br J Ophthalmol 996; 80: Swan KC, Wilkins JH. 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