The retina of the newborn human infant

Development of the primate area of high acuity, 3: Temporal relationships between pit formation, retinal elongation and cone packing

By establishing an avascular, highly elastic, region within the fetal area of high acuity (AHA), the developing primate eye has created a unique substrate on which the mechanical forces of intraocular pressure (IOP) and growth-induced retinal stretch (stretch) can act. We proposed (Springer & Hendrickson, 2004b) that these forces generate both the pit and high cone density found in the adult AHA. In this paper, we use quantitative measures to determine the temporal relationships between nasal and temporal retinal elongation, changes in pit depth, cone packing, and cone morphology overM. nemestrinaretinal development. Retinal length increased rapidly to about 105 days postconception (dpc; Phase 1) and then elongation virtually ceased (Phase 2) until just after birth (180 dpc). Retinal elongation due to stretch resumed during Phase 3 until approximately 315 dpc (4–5 months), after which time the retina appeared mature (Phase 4). The pit appeared during the quiescent Phase 2, suggesting that IOP acts, in conjunction with molecular changes in the inner retina, on the highly elastic, avascular, AHA to generate a deep, narrow pit and causes inner retinal cellular displacements. Subsequently (Phase 3), the pit widened, became 50% shallower and central inner retinal lamina thinned slightly due to a small amount of retinal stretch occurring in the AHA. Centripetal movement of cones was minimal until just after birth when the pit reached 88% of its maximal depth. Accelerated cone packing during Phase 3 was temporally correlated with increased stretch. A slight stretching of the central inner retina generates “lift” forces that cause the pit to become shallower and wider. In turn, these “lift” forces draw cones toward the center of the AHA (Springer, 1999). Localized changes in cone morphology associated with packing, included smaller cell body size, a change from a monolayer to a multilayered mound of cell bodies, elongation of inner segments and tilting of the apical portion toward the AHA. These changes began in cones overlying the edges of the pit, not its center. Henle cone axons formed initially in association with centrifugal displacement of the inner retina during pit formation, with an additional subsequent elongation due to cones moving centripetally. An integrated, two-factor model of AHA formation is presented. Initially, during the second half of gestation (Phase 2), IOP acts on the hyperelastic avascular zone of the AHA to generate a deep pit in the inner retina. In the first 4 months after birth (Phase 3), central retinal stretch generates tensile “lift” forces that remodel the pit and pack cones by drawing them toward the AHA center.

Development of the primate area of high acuity. 2. Quantitative morphological changes associated with retinal and pars plana growth

Mechanisms underlying the development of the primate area of high acuity (AHA) remain poorly understood. Finite-element models have identified retinal stretch and intraocular pressure (IOP) as possible mechanical forces that can form a pit (Springer & Hendrickson, 2004). A series of Macaca nemestrina monkey retinas between 68 days postconception (dpc) and adult were used to quantify growth and morphological changes. Retinal and pars plana length, optic disc diameter, disc-pit distance, and inner and outer retinal laminar thickness were measured over development to identify when and where IOP or stretch might operate. Horizontal optic disc diameter increased 500 mum between 115 dpc and 2 months after birth when it reached adult diameter. Disc growth mainly influences the immediate surrounding retina, presumably displacing retinal tissue centrifugally. Pars plana elongation also began at 115 dpc and continued steadily to 3-4 years postnatal, so its influence would be relatively constant over retinal development. Unexpectedly, horizontal retinal length showed nonlinear growth, divided into distinct phases. Retinal length increased rapidly until 115 dpc and then remained unchanged (quiescent phase) between 115-180 dpc. After birth, the retina grew rapidly for 3 months and then very slowly into adulthood. The onset of pit development overlapped the late fetal quiescent phase, suggesting that the major mechanical factor initiating pit formation is IOP, not retinal growth-induced stretch. Developmental changes in the thickness of retinal layers were different for inner and outer retina at many, but not all, of the ten eccentricities examined.

Magnocellular and parvocellular developmental course in infants during the first year of life

The visual system undergoes major modifications during the first year of life. We wanted to examine whether the magnocellular (M) and parvocellular (P) pathways mature at the same rate or if they follow a different developmental course. A previous study carried out in our laboratory had shown that the N1 and P1 components of pattern visual evoked potentials (PVEPs) were preferentially related to the activity of P and M pathways, respectively. In the present study, PVEPs were recorded at Oz in 33 infants aged between 0 and 52 weeks, in response to two spatial frequencies (0.5 and 2.5 c deg(-1)) presented at four contrast levels (4, 12, 28 and 95%). Results indicate that the P1 component appeared before the N1 component in the periods tested and was unambiguously present at birth. The P1 component showed a rapid gain in amplitude in the following months, to reach a ceiling around 4-6 months. Conversely, the N1 component always appeared later and then gained in amplitude until the end of the first year without reaching a plateau. Latencies were also computed but no developmental dissociation was revealed. Results obtained on amplitude are interpreted as demonstrating a developmental dissociation between the underlying M and P pathways, suggesting that the former is functional earlier and matures faster than the latter during the first year of life.

Paramacular coloboma

BACKGROUND

Paramacular coloboma (plural: colobomata) is a solitary oval football or torpedo-shaped chorioretinal lesion located temporal to the fovea in one or both eyes. Previous case reports have speculated varying etiology, but few have justified its pathognomonic shape and location. We believe it to be congenital in nature and caused by incomplete differentiation of the arcuate bundles along the horizontal raphe in development of the macular architecture. Associated ocular findings may include blepharophimosis, situs inversus, or other anomalous retinal vascular patterns.

CASE REPORTS

Three cases of asymptomatic unilateral paramacular colobomata are presented. In each case, a single oval chorioretinal lesion temporal to the macula was found during routine examination. Visual acuity and Humphrey threshold visual field testing were normal with no other associated congenital, systemic, or ocular abnormalities. Because the lesion is nonprogressive, these patients can be followed on an annual basis.

CONCLUSION

Due to their anatomical origin, paramacular colobomata are always located temporal to the macula and have an oval football-shaped appearance. Visual acuity and visual field testing are usually normal, although highly observant patients may be aware of a mild scotoma. Differential diagnosis is important because the clinical appearance can be similar to acquired conditions, most notably age-related macular degeneration and presumed ocular histoplasmosis syndrome.

Newborns' recognition of changing and unchanging aspects of schematic faces

The present study investigated newborns' ability to discriminate, recognize, and learn visual information embedded in the schematic face-like patterns preferred at birth. Four experiments were carried out using the visual-paired comparison paradigm. Results indicated that newborns discriminated face-like stimuli relying on their internal features (Experiments 1 and 4) and recognized a perceptual invariance between face-like configurations in conditions of low (Experiment 2) and high-perceptual variability (Experiment 3) of their inner elements. Altogether, data show that the presence of the preferred structure that schematically defines a face, displaying a triplet of elements in the correct locations for eyes and mouth, does not constitute a limit that constrains newborns' face learning processes.

Infant light adaptation shows Weber's law at photopic illuminances

In the present study we investigate the dependence of photopic contrast thresholds on retinal illuminance in infants and adults. Contrast thresholds were measured at five retinal illuminances between about 6 and about 20,000 Td in subjects in both age groups. The forced-choice preferential looking technique was used in 3-month-old infants, and standard forced-choice techniques were used in adults. The stimulus was a 0.25 cy/deg squarewave grating phase alternated at 6 Hz. Infants' contrast thresholds were more than two log units higher than those of adults at all retinal illuminances. Contrast thresholds had a similar dependence on retinal illuminance in both infants and adults. For both age groups, contrast thresholds initially decreased with increasing retinal illuminance. However, at both ages, above a critical illuminance of about 200 Td, contrast thresholds remained constant, following Weber's law. Thus a vertical shift was sufficient to bring the two data sets into correspondence. In the context of a two-site model of light adaptation, our results imply that infants' elevated contrast thresholds cannot be explained solely on the basis of photoreceptoral immaturities. Later physiological immaturities must also limit infants' photopic contrast thresholds.

Variation of chromatic sensitivity across the life span

Thresholds were measured along three directions in color space for detecting an equiluminant color change of a set of bars embedded in a larger field of spatio-temporal achromatic noise for observers ranging in age from 3 months to 86 years. Pre-verbal observers were assessed with a forced-choice preferential-looking technique while older observers responded orally or manually. Over the life span, thresholds could be described along each color axis tested by a curve with two trends. Thresholds decreased with each doubling of age by nearly a factor of two until adolescence. Thereafter, thresholds increased by a factor of 1.4-2 with each doubling of age. Sensitivity to chromatic differences varied similarly along all three axes tested, suggesting uniformity in the sensitivity of chromatic mechanisms across the life span.

Does visual sensitivity improve between 5 and 8 years? A study of automated visual field examination

In 74 normal subjects (62 children aged 5-8 years and 12 adults), we tested the widely-held belief that visual sensitivity improves substantially during childhood. Maturation of the retino-striate pathways is generally invoked to account for age-related changes in visual sensitivity. We evaluated the extent to which attentional factors unduly emphasized the effect of age on the purely physiological mechanisms. After a specially-designed familiarization procedure, sensitivity was fully evaluated at two locations in the superior temporal field using a bracketing technique (Octopus 2000R). False-positive (FP) and false-negative (FN) catch-trials were interspersed with the sequence of stimuli. Analyses demonstrated that: (1) age affected sensitivity; and (2) the general level of attentiveness varied not only with age, but also among subjects in the same age group. We then estimated the extent to which improved visual sensitivity may reflect a concomitant evolution of vigilance. Firstly, controlled variance analyses indicated that factors for evaluating attentiveness (rate of FN responses, slope of the psychometric function at the median, and goodness of fit) were indeed much better predictors than age of the sensitivity measured. Secondly and more significantly, the grouping of subjects into homogeneous subgroups, on the basis of their attentional performance, showed that children as young as 5 years may have a visual sensitivity that is only marginally lower than that of adults.

Optical, receptoral, and retinal constraints on foveal and peripheral vision in the human neonate

We examined the properties of the foveal, parafoveal, and near peripheral cone lattice in human neonates. To estimate the ability of these lattices to transmit the information used in contrast sensitivity and visual acuity tasks, we constructed ideal-observer models with the optics and photoreceptors of the neonatal eye at retinal eccentricities of 0, 5, and 10 degrees. For ideal-observer models limited by photon noise, the eye's optics, and cone properties, contrast sensitivity was higher in the parafovea and near periphery than in the fovea. However, receptor pooling probably occurs in the neonate's parafovea and near periphery as it does in mature eyes. When we add a receptor-pooling stage to the models of the parafovea and near periphery, ideal acuity is similar in the fovea, parafovea, and near periphery. Comparisons of ideal and real sensitivity indicate that optical and receptoral immaturities impose a significant constraint on neonatal contrast sensitivity and acuity, but that immaturities in later processing stages must also limit visual performance.

Oscillatory potentials in the retina: what do they reveal

This chapter is an overview of current knowledge on the oscillatory potentials (OPs) of the retina. The first section describes the characteristics of the OPs. The basic, adaptational, pharmacological and developmental characteristics of the OPs are different from the a- and b-waves, the major components of the electroretinogram (ERG). The OPs are most easily recorded in mesopic adaptational conditions and reflect rapid changes of adaptation. They represent photopic and scotopic processes, probably an interaction between cone and rod activity in the retina. The OPs are sensitive to disruption of inhibitory (dopamine, GABA-, and glycine-mediated) neuronal pathways and are not selectively affected by excitatory amino acids. The earlier OPs are associated with the on-components and the late OPs with the off-components in response to a brief stimulus of light. The postnatal appearance of the first oscillatory activity is preceded by the a- and b-waves. The earlier OPs appear postnatally prior to, and mature differently from, the later ones. The second section deals with present views on the origin of the OPs. These views are developed from experimental studies with the vertebrate retina including the primate retina and clinical studies. Findings favor the conclusion that the OPs reflect neuronal synaptic activity in inhibitory feedback pathways initiated by the amacrines in the inner retina. The bipolar (or the interplexiform) cells are the probable generators of the OPs. Dopaminergic neurons, probably amacrines (or interplexiform cells), are involved in the generation of the OPs. The earlier OPs are generated in neurons related to the on-pathway of the retina and the later ones to the off-channel system. Peptidergic neurons may be indirectly involved as modulators. The individual OPs seem to represent the activation of several retinal generators. The earlier OPs are more dependent on an intact rod function and the later ones on an intact cone system. Thus, the OPs are good indicators of neuronal adaptive mechanisms in the retina and are probably the only post-synaptic neuronal components that can be recorded in the ERG except when structured stimuli are used. The last section describes the usefulness of the oscillatory response as an instrument to study the postnatal development of neuronal adaptation of the retina. In this section clinical examples of of the sensitivity of the OPs for revealing early disturbance in neuronal function in different retinal diseases such as pediatric, vascular and degenerative retinopathies are also given.

M- and L-cones in early infancy: II. Action spectra at 8 weeks of age

Field sensitivities were measured under conditions of M- and L-cone isolation for seven infants (8-12 weeks-old) and two adults, using silent-substitution and the visually evoked potential (VEP). The efficacy of the receptor-isolation conditions were first verified by measuring psychophysical and VEP-derived action spectra from two color-normal adults under conditions of M- and L-cone isolation. M- and L-cone action spectra obtained from the two methods were found to be similar to the Smith and Pokorny M- and L-cone fundamentals, respectively. The VEP-derived action spectra obtained from infants and adults were well fit by the Smith and Pokorny M- and L-cone fundamentals. These data, in conjunction with our previous study, confirm that M- and L-cones are operating by 8 weeks and possibly as early as 4 weeks of age.

M- and L-cones in early infancy: I. VEP responses to receptor-isolating stimuli at 4- and 8-weeks of age

A silent-substitution technique combined with measures of the visually-evoked potential (VEP) was used to determine whether M- and L-cones are functional in early infancy. Data were successfully collected from twenty six infants in response to three receptor-isolation conditions (rod, M- and L-cone isolation) and a luminance-modulation condition. The efficacy of the receptor-isolation conditions was first verified by measuring VEP responses from both dichromatic and color-normal adults to each of the receptor-isolation conditions. Both 4- and 8-week-old infants demonstrated VEP responses to the M- and L-cone isolating stimuli, though the amplitude of the the responses at 4-weeks were reduced compared to those at 8-weeks. These data suggest that the functioning of M- and L-cones can be differentiated as early as 4-weeks of age.

Visual half-field development in children: detection of colour-contrast-defined forms

Recently we have shown that children's ability to perceive motion-defined forms shows an asymmetry in the left and right visual half-fields which disappears in adulthood. The present study is focused on the visual half-field development of colour contrast vision in the same group of subjects. It was found that colour contrast thresholds for the detection of colour-contrast-defined forms decrease with age to reach adult values around puberty. This improvement of colour contrast vision with age is attributed to the maturation of cortical mechanisms. However, in contrast to a visual half-field asymmetry for motion detection during childhood, no visual half-field differences were observed for colour contrast detection in children.

Effects of prenatal substance exposure: altered maturation of visual evoked potentials

We investigated the effects of prenatal substance use on visual evoked potentials (VEPs). Seventy-four children were tested at birth and 1 month of age with binocular flash VEPs and at 4, 8, and 18 months of age with binocular pattern VEPs. Regressions were run by trimester to assess the independent effects of substance exposure. Variables included in the regression model were alcohol, marijuana, tobacco, other drug use for each trimester, maternal age, education, income, race, marital status, infant sex, birthweight, and Dubowitz score. Changes in specific components of the binocular VEP were both substance- and trimester-specific. First trimester alcohol use was associated with prolonged P1 wave latencies at 1 month of age. Prolonged P1 wave latencies at birth and 18 months were associated with tobacco use during each of the three trimesters, at 1 and 18 months with third trimester marijuana use, and at 1 and 18 months with first trimester other illicit drug use. Although these women were moderate substance users during pregnancy, their offspring exhibited maturational changes in components of the VEP in the absence of neonatal behavioral disturbances.

Development of grating acuity and contrast sensitivity in the central and peripheral visual field of the human infant

Central and peripheral visual functions were measured simultaneously in 39 infants from 10 to 39 weeks old using a dual-frequency VEP technique. Central acuity and contrast sensitivity over a 4 deg circular field were measured at 6 or 8 Hz. Peripheral acuity and contrast sensitivity were measured simultaneously at the other rate with a semi-circular stimulus extending from 8 to 16 deg. The EEG was analyzed at 12 and 16 Hz to determine the separate responses for the central and peripheral fields. Both central and peripheral VEP acuity developed over the age range tested. Central acuity improved by about a factor of 2.6 over the age range tested, while peripheral acuity improved by about a factor of 2.2. Central acuity was always higher by an average factor of about 2.3. Contrast sensitivity showed similar development for the central and peripheral fields with an early rapid rise in sensitivity.

Infant peripheral vision: the development of monocular visual acuity in the first 3 months of postnatal life

Quantitative data on the early morphological development of the human retina show that the peripheral region is relatively more mature than the central region. These results have stimulated researchers to compare the development of visual functions in the central and peripheral regions of the visual field. Here, we used preferential looking to evaluate 1-, 2- and 3-month-old infants' central and peripheral (10 degrees and 30 degrees) monocular visual acuity. There were three findings: (i) both central and peripheral acuities were poor at 1 month, improved over the age range tested, but were still about 3 octaves worse than adults' acuity; (ii) at all ages monocular acuity decreased with increasing eccentricity; (iii) 2- and 3-month-olds showed higher acuity for gratings in the temporal than in the nasal visual field at 30 degrees. The implications of these results for issues in visual development are discussed.

Retinal illuminance and contrast sensitivity in human infants

Several investigators have related infants' low contrast sensitivity to immaturities in the optics and receptor lattice of the immature eye. A critical element in the modeling is how much the lower photon catch of the immature retina reduces sensitivity; the assumptions vary from square-root to Weber's law and lead to very different modeling outcomes. We measured the relationship between retinal illuminance and contrast sensitivity at different spatial frequencies. The sweep visual-evoked potential was used to measure thresholds in 2- and 3-month olds and adults over a 2.5-log-unit range of illuminances. The contrast threshold vs illuminance functions were fit by power functions. The best-fitting exponents for adults were about -0.5 at higher spatial frequencies (consistent with square-root law) and lower at lower frequencies. The best-fitting exponents for 2- and 3-month olds were -0.2 to -0.35 which indicates that threshold is less affected by changes in illuminance than is the case in adults. These results suggest that none of the models relating optical and receptoral immaturities to infants' spatial vision has assumed an appropriate relationship between lower photon catch and contrast sensitivity. Once the models are modified to incorporate the relationship obtained in the present experiment, the predictions fall well short of explaining 2-month olds' low contrast sensitivity.

Development of contrast sensitivity across the visual field in macaque monkeys (Macaca nemestrina)

Interpretation of measurements of visual performance in infants must be based on knowledge of the locus of highest sensitivity in the infant retina. While we know that adult contrast sensitivity and spatial resolution is highest at the fovea, recent anatomical data show that the infant fovea is relatively immature. We have studied that variation of contrast sensitivity across the visual field during development in infant monkeys in order to investigate the behavioral consequences of this immaturity. The results show that, unlike adults, the sensitivity of the infant foveal region is similar to that of the near periphery. Central contrast sensitivity and spatial resolution improve substantially relative to the periphery over the first 20-40 postnatal weeks. Thus, contrast sensitivity in the periphery is relatively mature in infants with respect to more central regions of the visual field. The maturation pattern seen behaviorally is consistent with physiological and anatomical maturation patterns in macaque monkey.

Maturation of electroretinograms and visual evoked potentials in preterm infants

Electroretinograms (ERGs) and visual evoked potentials (VEPs) to flash stimulation were recorded from 51 infants (gestational age 26 to 42 weeks; post-conceptional age (PCA) 31 to 47 weeks) to give cross-sectional data on the maturation of these responses. Sequential recordings were taken from a separate group of 24 preterm infants (gestational age 28 to 33 weeks) to give longitudinal data. There was a significant decrease in ERG a-wave latency and increase in a-b amplitude with increasing PCA in both groups. For the VEPs there was a significant decrease in latency of the early negative component (N1) and the major positive component (P2). Comparison between recordings made on preterm infants with those from term infants at an equivalent PCA suggested faster maturation of VEPs in the extra-uterine environment, but no difference in maturation of the ERG.

Extraocular muscles: basic and clinical aspects of structure and function

Although extraocular muscle is perhaps the least understood component of the oculomotor system, these muscles represent the most common site of surgical intervention in the treatment of strabismus and other ocular motility disorders. This review synthesizes information derived from both basic and clinical studies in order to develop a better understanding of how these muscles may respond to surgical or pharmacological interventions and in disease states. In addition, a detailed knowledge of the structural and functional properties of extraocular muscle, that would allow some degree of prediction of the adaptive responses of these muscles, is vital as a basis to guide the development of new treatments for eye movement disorders.

Spectral efficiency measured by heterochromatic flicker photometry is similar in human infants and adults

Spectral efficiency functions based on heterochromatic flicker photometry (HFP) were measured for three adults and 42 infants using a rapid visually-evoked potential (VEP) method. A 5 degrees-diameter, broadband standard (0.6 cd/m2) was presented in square-wave counterphase (15 Hz) with one of 13 monochromatic lights (420-660 nm; 20 nm steps). The intensity of the monochromatic light was continuously varied while extracting the phase-locked VEP amplitude of the fundamental component. HFP functions measured psychophysically by the method of adjustment were also obtained for the adults. Adult HFP functions from the two methods were found to be essentially the same. Both of these functions were compared to Vos'-modified 2 degrees V(lambda) function and the 10 degrees CIEV(lambda) function. The mean adult data were slightly better fit to the 2 degrees V(lambda) function than to the 10 degrees CIEV(lambda) function, although there was an elevation in sensitivity at 420 and 440 nm. Infant HFP functions were similar to Vos' modified V(lambda) except for an elevation in efficiency at short wavelengths. The mean infant HFP function agreed better with the 10 degrees CIEV(lambda) function than Vos'-modified V(lambda) function, but infant sensitivity was elevated by 0.4 log units at 420 nm compared to the 10 degrees CIE observer. The elevation found at short wavelengths for both adults and infants is attributed to individual and age-related variation in the density of the ocular media, and to reduced macular pigment screening resulting from use of a 5 degrees field size.(ABSTRACT TRUNCATED AT 250 WORDS)

Spatial vision in adults and infants: a tribute to Russell Harter

To be recognized, the retinal image of an object must first be segregated from the retinal image of its surroundings. Luminance contrast is only one way by which an object can be rendered visible. Motion contrast alone, texture contrast alone, colour contrast alone and disparity contrast alone can also render an object visible. This paper notes that psychophysical studies of patients with brain lesions show that motion-defined form and luminance-defined form are processed separately and describes how magnetic (MEG) or electrical (VEP) recording can be used to compare brain responses to luminance-defined form, motion-defined form, colour-defined form, texture-defined form and disparity-defined form. At an early stage in visual processing, the retinal images of luminance-defined objects pass through neural filters, each of which is selectively sensitive to a limited range of orientations, spatial frequencies and temporal frequencies. The properties of individual filters can be isolated and quantified by means of the two-sinewave VEP approach. The two-sinewave method also allows binocular function to be tested objectively and its normality assessed quantitatively even when one or both eyes have low acuity. Evoked potential studies of visual development in human infants and studies of labile phenomena in adults are both facilitated by the speed of our VEP sweep method.

Contrast-dependent responses in the human visual system: childhood through adulthood

Visual evoked potentials (VEPs) to temporal modulation of spatial patterns, recorded from humans ranging in age from 4-42 years, demonstrated that contrast-dependent responses exist in early childhood and change dramatically throughout childhood. Bright or dark isolated-check stimuli were used to emphasize contributions from ON or OFF pathways to the VEP. (ON and OFF pathways constitute one major pair of parallel subsystems, which process brightness [positive-contrast] and darkness [negative-contrast] information, respectively.) The developmental effects observed for each pathway were similar in magnitude and time course, suggesting maturation of a common physiological mechanism dependent on spatial contrast. Children's responses were more variable and larger than those of adults, and exhibited a relative phase lag. In addition, we recorded transient VEPs to a conventional contrast-reversing checkerboard pattern. The latency of the major positive wave (P100) was found to decrease, while the latency of the initial positive wave (P60) was found to increase, with increasing age. We propose a vector-summation model, which posits a relative decrease in cortical excitation with increasing age, to explain our major findings.

Systematic measurement of human neonatal color vision

We used a new time-efficient method to evaluate chromatic-achromatic discrimination in newborn (n = 36) and 1-month-old (n = 34) human infants. Results showed that 74% of newborns discriminated a 10.5 x 17.5 deg broadband red patch from all relative luminances of an achromatic background, but only 14% of newborns did so with a blue, 36% with a green, and 25% with a yellow patch. Most infants who "failed" did so at relative luminances very close to the respective photopic luminance match. At 1 month, performance improved somewhat although infants still show clear evidence of discriminating only the red patch. These results, the first to be obtained from individual newborns with a method incorporating a systematic variation of luminance, imply that early color vision is very limited. Possible photoreceptoral and neural bases for these immaturities are discussed.

The development of visual acuity in the peripheral visual field of human infants: binocular and monocular measurements

We measured binocular and monocular grating acuity in the peripheral visual field of infants, using a modified preferential looking procedure. Both binocular and monocular peripheral acuity increased between 2 and 11 months of age, but had not reached adult levels at the end of the first year of life. Binocular acuity was always higher than monocular acuity. At all ages tested, acuity was higher in the temporal than in the nasal visual field. We conclude that, in spite of the relative morphological maturity of the peripheral retina, visual acuity develops in the peripheral visual field.

Retinal ganglion cells within the foveola of New World (Saimiri sciureus) and Old World (Macaca fascicularis) monkeys

The morphology and distribution of retinal ganglion cells within the foveola of New World (Saimiri sciureus) and Old World (Macaca fascicularis) monkeys labeled as a result of horseradish peroxidase injections into the dorsal lateral geniculate nucleus were studied. The results indicate that monkey's foveola normally contains significant numbers of retinal ganglion cells. Most of these project ipsilaterally. Cells within the foveola are larger than other cells in monkey central retina; their dendritic fields are up to 50 times larger in area than those of cells in the foveal slope. The dendritic fields of the ganglion cells within the foveola cover at least 70-100% of its area. Among ganglion cells within the foveola (as in most ganglion cells), there was a strong tendency for the axon and dendritic tree to arise from opposite poles of the soma. The axon-dendrite axes of ganglion cells within the foveola did not show a consistent pattern. In contrast, the axes of ganglion cells in the transition zone between the foveola and the foveal slope were directed tangentially to the circumference of the fovea. The dendritic coverage of the foveola by retinal ganglion cells suggests functional significance and provides a possible neural basis for 2-3 degrees of bilateral representation of the fovea within the central visual pathways. Alternatively, or in addition, these cells may be "remnants of foveation" and provide insight into the developmental processes that mediate the development of the fovea.

Morphogenesis of retinal ganglion cells during formation of the fovea in the Rhesus macaque

The morphology of retinal ganglion cells within the central retina during formation of the fovea was examined in retinal explants with horseradish-peroxidase histochemistry. A foveal depression was first apparent in retinal wholemounts at embryonic day 112 (E112; gestational term is approximately 165 days). At earlier fetal ages, the site of the future fovea was identified by several criteria that included peak density of ganglion cells, lack of blood vessels in the inner retinal layers, arcuate fiber bundles, and the absence of rod outer segments in the photoreceptor layer. Prior to E112, the terminal dendritic arbor of retinal ganglion cells within the central retina extended into the inner plexiform layer and were located directly beneath their somas of origin or at most were slightly displaced from it. For example, at E90 the mean horizontal displacement of the geometric center of the dendritic arbor from the somas of cells within 600 microns of the estimated center of the future fovea was 4.1 microns (S.D. 2.7, range 1.0-10.0, n = 97). Following formation of the foveal depression the dendritic arbors of cells were significantly displaced from their somas. For example, at E138 the mean displacement was 41.2 microns (S.D. 12.2, range 12.0-56.0, n = 97). The displacement of the dendritic arbor which occurred during this period was not accounted for by areal growth of the dendritic arbor, the somas, or the retina, but was produced by the lengthening of the primary dendritic trunk. Moreover, no significant displacement was observed within the remaining 1.5-6.5 mm of the central retina. These observations provide evidence supporting early speculations that the formation of the foveal pit occurs, in part, by the radial migration of ganglion cells from the center of the fovea during its formation. Our analyses suggest that this migration occurs by the lengthening of the primary dendrite presumably by the addition of membrane. This migration is in a direction opposite to the inward movement of photoreceptors that occurs during late fetal and early postnatal periods (Packer et al., 1990, Journal of Comparative Neurology 298, 472-493).

The effects of luminance on FPL and VEP acuity in human infants

Grating acuity was measured in 16-week-old human infants. Three measurement techniques were used: forced-choice preferential-looking (FPL), and two visual-evoked-potential (VEP) techniques. The stimuli were counterphase flickering sinewave gratings with a space-average luminance of -1.0 or 2.0 log cd/m2. Slightly different luminance-dependent changes occur between FPL and VEP acuities, suggesting that some factor influences the two methods differently as stimulus luminance varies. A comparison between FPL acuities and VEP acuities within infants suggests a quantitative relationship between techniques. Infant's acuity for sinewave gratings with a space-average luminance of -2.0, -1.0, 0.0, 1.0 and 2.0 log cd/m2 was also measured using a single VEP paradigm. The results are compared to the same measurements in adults and to infant and adult ideal observers. VEP acuity in this group of infants improves by about 0.5 log units between -2.0 and 0.0 log cd/m2 and remains asymptotic between 0.0 and 2.0 log cd/m2. This result suggests that luminance-dependent changes in infant acuity cannot be fully accounted for by immaturities in the optics and photoreceptor spacing and efficiency.

Orientation selectivity of 3-month-old infants

A modification of the visual evoked potential (VEP) technique, first employed by Braddick, Wattam-Bell and Atkinson [(1986) Nature, London, 320, 617-619] was used to estimate the orientation selectivity of 3-month-old infants. The orientation-selective VEP was recorded in response to various changes in the orientation of a square-wave grating. The magnitude (the square root of the power of the FFT) at the frequency of orientation change was assumed to represent the response to the change in grating orientation. Orientation sensitivity was then estimated by plotting the magnitude of the FFT at the frequency of orientation change as a function of the log of the orientation change in degrees. For each individual, the data were fit by nonlinear regression and threshold was defined as the largest orientation angle for which the magnitude of the FFT was zero. The results suggest that the orientation selectivity of 3-month-old infants (1.33 deg) is similar to that of adults (1.13 deg) tested with the same stimulus parameters (1 c/deg, 9 Hz). However, when adults are tested with stimulus parameters selected to optimize their VEP response (4 c/deg, 18 Hz) instead of those which optimize the infant's response, the orientation discrimination of adults improves by a factor of 2 (0.53 deg). The results obtained from adults under optimum stimulus conditions (4 c/deg, 18 Hz) approach the estimates reported in the literature for static stimuli.

Analysis of striate activity underlying the pattern onset EP of children

The checkerboard onset Evoked Potential (EP) does not obtain its adult form before puberty. To determine the site of origin of these processes we studied the origin of the checkerboard onset EP in a group of 10 children between the ages of 6 and 16 years. Since the development of the waveform of the pattern onset EP varies with check size we also studied the dependence of these EPs on check size. The child checkerboard onset EPs described in this paper are dominated by a single source. Following an equivalent dipole source localization approach, the position, orientation and variation in strength of the equivalent dipole is estimated. The position and orientation of this dipole indicates an origin in the primary visual cortex (area 17). The variation in strength of the dipole changes from a single positive deflection, specific for children of 8 years and younger, into a negative-positive complex for the children studied between the age of 9 and 16 years. These changes in waveform must be due to changes in the activity pattern of the striate cortex.

Development of temporal properties of pattern electroretinogram and visual evoked potentials in infants

The postnatal development of the temporal properties of the responses to pattern contrast reversal has been studied by recording simultaneously the pattern electroretinogram (PERG) and visual evoked potentials (PVEP) in infants 3-22 weeks old. The stimulus grating (0.5 c/deg) was either reversed in contrast sinusoidally at frequencies 4-10.5 Hz to study the temporal frequency function of steady-state responses, or square-wave reversed at 1 Hz to evaluate the peak latency of transient responses. Developmental changes of the shape and bandwidth of the temporal frequency function of both PERG and PVEP occur post-natally and are particularly pronounced between 13 and 20 weeks from birth, possibly indicating deferred maturation of classes of retinal and central neurons with higher temporal resolution. The peak latency of the PERG decreases during the age period tested to approach adult values towards the end of the fifth month. The rate of decrease of the peak latency of the PERG differs from that of the PVEP, indicating that post-retinal factors contribute largely to the maturation of the latter, especially in the earliest life period.

The development of parafoveal and mid-peripheral human retina

The morphological development of parafoveal retina (1-1.5 mm from the foveal center) and the mid-peripheral (4 mm from the foveal center) human retina has been studied from fetal (F) 26 weeks to adulthood. At both retinal points, all layers and neuronal types are present at F26 weeks. In parafovea at F26 weeks photoreceptors have only a rudimentary inner segment and no outer segments. Short outer segments are present on both rods and cones at F36 weeks. By postnatal (P) 5-8 days the inner retina is relatively mature. Photoreceptors have elongated basal axons which cause the photoreceptor layer to become much thicker than in prenatal retina. At birth cone inner segments are untapered, but rod inner segments have already reached their adult width of 2 microns. Both rod and cone inner and outer segments are 30-50% of adult length. By 13 months both inner and outer retina are mature appearing, with the photoreceptors accounting for half the retinal thickness due to the elongation of the fibers of Henle. Cone outer segments elongate up to P5 years and rod outer segments to P13 years. At mid-peripheral or rod-ring retina outer segments are present on rods at F26 weeks and on cones at F36 weeks. At birth the inner retina is adultlike. The outer plexiform layer becomes thicker up to P45 months due to the elongation of fibers of Henle. At birth both rod and cone mid-peripheral inner segments are slightly longer and outer segments are 50% longer than in parafoveal retina. By P5 years mid-peripheral rod outer segments are slightly longer than in parafoveal retina, and this changes little thereafter. This anatomical study has found that the photoreceptors in peripheral rod-ring retina develop earlier than those in more central retina, and in turn parafoveal photoreceptors develop well in advance of foveal cones. This suggests that human neonates may utilize more peripheral retinal regions for some aspects of visual function before foveal cone vision becomes dominant.

Electrophysiological assessment of visual pathway function in infants

The flash ERG and VEP have conspicuous immature features during the first 4 months following birth. The most marked maturational changes occur in ERG amplitude and VEP latency. Concurrent recording of the skin ERG and VEP provides information which is very useful in helping to arrive at a diagnosis in the young infant with nystagmus who appears to be blind and has a fundus of normal appearance. ERG and VEP features associated with Leber's Amaurosis, congenital cone dysfunction, albinism, optic nerve hypoplasia and unilateral hemisphere dysfunction are described.

Visual development of infants with severe ocular disorders

Among 11 patients who presented as blind in early infancy, with Leber's congenital amaurosis (5 patients), optic nerve hypoplasia (4 patients), or macular colobomata (2 patients), 8 developed visually guided behavior and measurable grating acuity by age 5 to 46 months. All children with measurable grating acuity demonstrated visually guided mobility. Grating acuity was predictive of later visual performance in 10 of 11 patients by age 12 to 16 months. The best grating acuity attained by 7 months was 1.3 to 3.0 cycles/degrees (20/460 to 20/200) and 0.13 cycles/degrees (20/4700) by month 8. Two patients with Leber's congenital amaurosis and one with optic nerve hypoplasia remained blind. No clinical features existed to differentiate these three patients from the eight whose visual status improved. Posterior visual pathway maturation may underlie the improvement.

Cytogenesis in the monkey retina

Time of cell origin in the retina of the rhesus monkey (Macaca mulatta) was studied by plotting the number of heavily radiolabeled nuclei in autoradiograms prepared from 2- to 6-month-old animals, each of which was exposed to a pulse of 3H-thymidine (3H-TdR) on a single embryonic (E) or postnatal (P) day. Cell birth in the monkey retina begins just after E27, and approximately 96% of cells are generated by E120. The remaining cells are produced during the last (approximately 45) prenatal days and into the first several weeks after birth. Cell genesis begins near the fovea, and proceeds towards the periphery. Cell division largely ceases in the foveal and perifoveal regions by E56. Despite extensive overlap, a class-specific sequence of cell birth was observed. Ganglion and horizontal cells, which are born first, have largely congruent periods of cell genesis with the peak between E38 and E43, and termination around E70. The first labeled cones were apparent by E33, and their highest density was achieved between E43 and E56, tapering to low values at E70, although some cones are generated in the far periphery as late as E110. Amacrine cells are next in the cell birth sequence and begin genesis at E43, reach a peak production between E56 and E85, and cease by E110. Bipolar cell birth begins at the same time as amacrines, but appears to be separate from them temporally since their production reaches a peak between E56 and E102, and persists beyond the day of birth. Müller cells and rod photoreceptors, which begin to be generated at E45, achieve a peak, and decrease in density at the same time as bipolar cells, but continue genesis at low density on the day of birth. Thus, bipolar, Müller, and rod cells have a similar time of origin. The maximal temporal separation of cell birth is between cones and amacrine cells so that cell generation exhibits two relatively distinct phases: the first phase gives rise to ganglion, horizontal, and cone cells, and the second phase to amacrine, bipolar, rod, and Müller cells. In addition, cells of the first phase are generated faster than the second phase cells, and there are differences in the topography of spread of labeled cells between the two phases. Each cell class displays a central-to-peripheral gradient in genesis, although the spatiotemporal characteristics of the gradients differ between the classes.(ABSTRACT TRUNCATED AT 400 WORDS)

Motion thresholds in infants to sinusoidal gratings

Motion thresholds were determined at 9 degrees eccentricity in infants (mean = 14 weeks old). The stimuli used were computer-generated sinusoidal gratings presented through a 7.45 degrees aperture at a contrast ratio of .83. The range of velocities (.5, 1, 2, 4, and 6 degrees per s) was examined at only one spatial frequency (1 cycle per degree). At low velocities (less than 2 degrees per s), the infants showed no clear preference for the moving stimulus over the stationary stimulus. At faster velocities (2-6 degrees per s), the infants exhibited a clear preference for the moving stimulus. The results were interpreted as indicating that infants at 3 months of age are relatively insensitive to slow motions for low spatial frequency stimuli.

Tonic and phasic orientation in full-term and preterm infants

Thirty-three full-term infants and thirty-eight preterm infants (on average born at 30 weeks gestation) were tested for their latency to turn toward checkered stimulus patterns (phasic orienting or "attention-getting") and for the duration of their initial fixation (tonic orienting or "attention-holding"). Plotted against the logarithm of the subjects' postconceptional age, turning latency fell linearly between 36 and 120 weeks, while fixation time fell abruptly at 53 weeks. Preterm and full-term infants showed the same developmental trends, implying that both of these attentional behaviors are biologically timetabled and that neither is greatly affected by premature extrauterine experience. Unexpectedly, phasic orientation in the first 30 postnatal days was significantly faster in preterm than in full-term infants, and fixation times failed to differ. Despite the necessary functional integration of phasic and tonic orienting in mature visual scanning and attention, the present results suggest an independence in their early postnatal development and that neither is mature at birth.

Deficiencies in human neonates' color vision: photoreceptoral and neural explanations

Several photoreceptoral and neural models have been proposed to account for the development of human vision. To further evaluate those applicable to color vision, the present study examined 40 neonates' (mean age = 3.2 days) chromatic-achromatic discriminations in the mid-spectral region by using an habituation procedure and measures to minimize achromatic cues. Results indicated that newborns showed evidence of discriminating broad-band orange (lambda peak = 595 nm) but not yellow-green (lambda peak = 565 nm) lights from achromatic lights of varying luminance. Collectively, these and previous results imply that although newborns have at least dichromatic color vision, they possess relatively poor chromatic-achromatic discrimination in two spectral regions - in the short-wavelengths (including stimuli of 470-480 nm) and in the mid-wavelengths (including 565 nm). Although several hypotheses were considered, newborns' chromatic 'neutral zones' are best accounted for by models proposing that early color vision is limited by a general inefficiency of preneural (photoreceptoral and optical) mechanisms and/or by a selective immaturity of the SWS cones or the B/Y opponent channel.

Promotion of visual development of severely visually impaired babies: evaluation of a developmentally based programme

A developmentally based programme (PVD) to promote the visual development of babies with severe visual impairment was evaluated in 58 infants aged between one month and 13 months of age. Progress in nine aspects of visual development was significantly greater for babies receiving the programme. This benefit extended to those with severe learning disability, or with minimal or apparently no vision at entry. The PVD favourably influenced visual outcome when introduced at any time during the first 13 months of life, although earlier introduction enabled greater benefit to be derived from the improved vision. Therefore, it is recommended that infants are referred to a developmental centre as soon as severe visual impairment is suspected.

Sensory visual loss and cognitive deficits in the selective attentional system of normal infants and neurologically impaired children

The ability of infants to shift their gaze laterally from a central target (fixation shift) was investigated in normal one- and three-month-old infants in two visual tasks; competition (central fixating stimulus remained visible while peripheral target was presented) and non-competition (central fixating stimulus replaced by peripheral target). The younger infants were significantly more disrupted by the competition condition, in terms of latency to refixate and direction of first eye movement. An immature attention system is proposed to explain this affect. In addition, visually evoked potentials in response to comparable stimuli were easier to elicit in older infants, suggesting that the one-month-olds possessed more immature sensory and perceptual visual systems, as well as poorer neural systems for controlling selective attention. Both techniques have been applied to a group of neurologically impaired children, and the results indicate that the tests may be useful in distinguishing sensory loss from attentional impairments in these patients.

A perspective on psychophysical testing in children

In this paper, we show that the development of acuity in infancy is not due solely to foveal maturation, since there is a clear development of acuity in the peripheral visual field. The development of peripheral acuity, and the naso-temporal asymmetry during early development reinforce the idea that the visual loss in adult strabismic amblyopes is not due to an arrest of development, but rather to the chronic interocular suppression of the deviated eye. The Teller Acuity Card Test does not reliably detect strabismic amblyopia, and therefore should not be recommended as a screening test. However, the test is adequate for diagnosing visual losses in children with organic eye disorders and possibly anisometropia.

Development of looking with head and eyes

Research into stabilization of gaze has concentrated on how the eyes counterrotate to compensate for head rotation. There is little information on how head movements function as an integral part of gaze stabilization. The head and eye coordination of six adults and six infants was tested under two conditions: tracking a moving target when the body was stationary; fixating a stationary target when the body was turning. Under each condition, both infants and adults turned their heads more than their eyes in stabilizing gaze. The infants were tested at 3-week intervals between the ages of 11 and 28 weeks. During this period, the precision with which head turning was coupled to their own or the target movement developed to near adult level, showing rapid growth between 11 and 16 weeks. However, the infants' ability to couple the eyes onto the target did not change over the tested period, remaining much less precise than the adults'. These findings have important implications for the assessment of abnormal gaze stabilization, which could facilitate the early diagnosis of perceptuomotor dysfunction.