Use of an early nonlinearity to measure optical and receptor resolution in the human infant

Circuit Reorganization Shapes the Developing Human Foveal Midget Connectome toward Single-Cone Resolution

The human visual pathway is specialized for the perception of fine spatial detail. The neural circuitry that determines visual acuity begins in the retinal fovea, where the resolution afforded by a dense array of cone photoreceptors is preserved in the retinal output by a remarkable non-divergent circuit: cone → midget bipolar interneuron → midget ganglion cell (the "private line"). How the private line develops is unknown; it could involve early specification of extremely precise synaptic connections or, by contrast, emerge slowly in concordance with the gradual maturation of foveal architecture and visual sensitivity. To distinguish between these hypotheses, we reconstructed the midget circuitry in the fetal human fovea by serial electron microscopy. We discovered that the midget private line is sculpted by synaptic remodeling beginning early in fetal life, with midget bipolar cells contacting a single cone by mid-gestation and bipolar cell-ganglion cell connectivity undergoing a more protracted period of refinement.

The Importance of the Interaction Between Ocular Motor Function and Vision During Human Infancy

Numerous studies have demonstrated the impact of imposed abnormal visual experience on the postnatal development of the visual system. These studies have provided fundamental insights into the mechanisms underlying neuroplasticity and its role in clinical care. However, the ocular motor responses of postnatal human infants largely define their visual experience in dynamic three-dimensional environments. Thus, the immature visual system needs to control its own visual experience. This review explores the interaction between the developing motor and sensory/perceptual visual systems, together with its importance in both typical development and the development of forms of strabismus and amblyopia.

Potential downside of high initial visual acuity

Children who are treated for congenital cataracts later exhibit impairments in configural face analysis. This has been explained in terms of a critical period for the acquisition of normal face processing. Here, we consider a more parsimonious account according to which deficits in configural analysis result from the abnormally high initial retinal acuity that children treated for cataracts experience, relative to typical newborns. According to this proposal, the initial period of low retinal acuity characteristic of normal visual development induces extended spatial processing in the cortex that is important for configural face judgments. As a computational test of this hypothesis, we examined the effects of training with high-resolution or blurred images, and staged combinations, on the receptive fields and performance of a convolutional neural network. The results show that commencing training with blurred images creates receptive fields that integrate information across larger image areas and leads to improved performance and better generalization across a range of resolutions. These findings offer an explanation for the observed face recognition impairments after late treatment of congenital blindness, suggest an adaptive function for the acuity trajectory in normal development, and provide a scheme for improving the performance of computational face recognition systems.

The gaze stability of 4- to 10-week-old human infants

The relationship between gaze stability, retinal image quality, and visual perception is complex. Gaze instability related to pathology in adults can cause a reduction in visual acuity (e.g., Chung, LaFrance, & Bedell, 2011). Conversely, poor retinal image quality and spatial vision may be a contributing factor to gaze instability (e.g., Ukwade & Bedell, 1993). Though much is known about the immaturities in spatial vision of human infants, little is currently understood about their gaze stability. To characterize the gaze stability of young infants, adult participants and 4- to 10-week-old infants were shown a dynamic random-noise stimulus for 30-s intervals while their eye positions were recorded binocularly. After removing adultlike saccades, we used 5-s epochs of stable intersaccade gaze to estimate bivariate contour ellipse area and standard deviations of vergence. The geometric means (with standard deviations) for infants' bivariate contour ellipse area were left eye = -0.697 ± 0.534 log(°2), right eye = -0.471 ± 0.367 log(°2). For binocular vergence stability, the infant geometric means (with standard deviations) were horizontal = -1.057 ± 0.743 log(°), vertical = -1.257 ± 0.573 log(°). These values were all not significantly different from those of the adult comparison sample, suggesting that gaze instability is not a significant limiting factor in retinal image quality and spatial vision during early postnatal development.

Maturation of Binocular, Monocular Grating Acuity and of the Visual Interocular Difference in the First 2 Years of Life

The sweep visual evoked potential method (sVEP) is a powerful tool for measurement of visual acuity in infants. Despite the applicability and reliability of the technique in measuring visual functions the understanding of sVEP acuity maturation and how interocular difference of acuity develops in early infancy, as well as the availability of normality ranges, are rare in the literature. We measured binocular and monocular sVEPS acuities in 481 healthy infants aged from birth to 24 months without ophthalmological diseases. Binocular sVEP acuity was significantly higher than monocular visual acuities for almost all ages. Maturation of monocular sVEP acuity showed 2 longer critical periods while binocular acuity showed three maturation periods in the same age range. We found a systematic variation of the mean interocular acuity difference (IAD) range according to age from 1.45 cpd at birth to 0.31 cpd at 24 months. An additional contribution was the determination of sVEP acuity norms for the entire age range. We conclude that binocular and monocular sVEP acuities have distinct growth curves reflecting different maturation profiles for each function. Differences in IAD range shorten according to age and they should be considered in using the sVEP acuity measurements for clinical diagnosis as amblyopia.

Three-month-old infants' sensitivity to horizontal information within faces

Horizontal information is crucial to face processing in adults. Yet the ontogeny of this preferential type of processing remains unknown. To clarify this issue, we tested 3-month-old infants' sensitivity to horizontal information within faces. Specifically, infants were exposed to the simultaneous presentation of a face and a car presented in upright or inverted orientation while their looking behavior was recorded. Face and car images were either broadband (UNF) or filtered to only reveal horizontal (H), vertical (V) or this combined information (HV). As expected, infants looked longer at upright faces than at upright cars, but critically, only when horizontal information was preserved in the stimulus (UNF, HV, H). These results first indicate that horizontal information already drives upright face processing at 3 months of age. They also recall the importance, for infants, of some facial features, arranged in a top-heavy configuration, particularly revealed by this band of information. © 2016 Wiley Periodicals, Inc. Dev Psychobiol 58: 536-542, 2016.

Retinal image quality and postnatal visual experience during infancy

Studies of animal models have demonstrated that abnormal visual experience can lead to abnormal visual development. The provision of normal optical experience for human infants and children requires an understanding of their typical retinal image quality in the natural dynamic environment. The literature related to this topic is reviewed.

Higher order monochromatic aberrations of the human infant eye

The monochromatic optical aberrations of the eye degrade retinal image quality. Any significant aberrations during postnatal development could contribute to infants' immature visual performance and provide signals for the control of eye growth. Aberrations of human infant eyes from 5 to 7 weeks old were compared with those of adult subjects using a model of an adultlike infant eye that accounted for differences in both eye and pupil size. Data were collected using the COAS Shack-Hartmann wavefront sensor. The results demonstrate that the higher order aberrations of the 5-to-7-week-old eye are less than a factor of 2 greater than predicted for an adultlike infant eye of this age. The data are discussed in the context of infants' visual performance and the signals available for controlling growth of the eye.

Multiple sensitive periods in human visual development: Evidence from visually deprived children

Psychophysical studies of children deprived of early visual experience by dense cataracts indicate that there are multiple sensitive periods during which experience can influence visual development. We note three sensitive periods within acuity, each with different developmental time courses: the period of visually-driven normal development, the sensitive period for damage, and the sensitive period for recovery. Moreover, there are different sensitive periods for different aspects of vision. Relative to the period of visually driven normal development, the sensitive period for damage is surprisingly long for acuity, peripheral vision, and asymmetry of optokinetic nystagmus, but surprisingly short for global motion. A comparison of results from unilaterally versus bilaterally deprived children provides insights into the complex nature of interactions between the eyes during normal visual development.

Does a front-end nonlinearity confound VEP acuity measures in human infants?

The visual evoked potential is commonly used to estimate visual acuity in infants. The stimulus used is temporally modulated in order to drive the cortical response. Here it is proposed that distortion products generated by a front-end nonlinearity may contaminate the acuity estimate. Specifically, the nonlinearity might convert temporal modulation of a high spatial frequency grating into apparent whole-field flicker. Thus, the VEP may reflect an artifactual response to a high spatial frequency that is not resolved at the cortical level. If this were the case, one could null or attenuate the flicker response by adding whole-field flicker to the grating stimulus. We looked for such nulling effects in 18 infants aged 6-17 weeks. No consistent evidence was found for the nulling effect, so it was concluded that infant VEP acuity estimates are not significantly contaminated by the hypothesized distortion product.