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This chapter considers the visual capabilities of the normal newborn infant, starting with crude visual orienting behaviour towards visually conspicuous objects. The evidence for a specific ability in newborns to recognize face configurations, and its possible neural underpinnings is reviewed. Studies that measure the development of acuity and contrast sensitivity from birth are discussed, including the likely optical, retinal, and neural factors limiting these processes, and the relationship between measurements based on preferential looking and those obtained by VEP/VERP methods. It presents data from two unique intensive longitudinal studies of the author's daughters, Fleur and Ione, on development of contrast sensitivity and acuity. The results show that compared across many testing sessions, OKN (optokinetic nystagmus), VEPs, and preferential looking measures show broad agreement in the time course of development of contrast sensitivity.

This chapter describes some of the methods used for testing vision in infants and young children in research and clinical assessment, including behavioural measures from observing eye movements (forced choice preferential looking /habituation), video/photorefraction, and visual evoked potentials (VEP) or visual event related potentials (VERP). It outlines how these methods have been used and compared, including the use of forced-choice preferential looking for acuity testing and acuity measures in preschool children using the Cambridge Crowding Cards, devised in the Visual Development Unit. It describes the subtests from the Atkinson Battery of Child Development for Examining Functional Vision (ABCDEFV), a behavioural battery which assesses children's functional use of sensory, perceptual, and cognitive vision, from birth to five years.

It is generally accepted that children born very prematurely or with a very low birth weight (VLBW; 〈1500 g) are at risk for visual impairment, at least during the first few years after birth. Whether visual impairment such as low visual acuity is only a sign of delayed development, or is irreversible, is still questionable. Visual acuity development continues until puberty which possibly allows a catching up in this lag in development. The purpose is to determine whether VLBW and/or very prematurely born children are at risk. For permanent impairment of visual acuity by comparison of acuity values obtained in five-year-old children who are at risk for an impaired visual development because of a very low birth weight and/or a very short gestational duration, to those of healthy control five-year-olds. In this earlier study, it is demonstrated that with the Rotterdam C-chart acuity estimates of both five-year-old control children and adults show a normal distribution and no ceiling effect. Therefore, the Rotterdam C-chart seems suitable for acuity assessment in young and visually impaired children.

In the assessment of visual function, visual and stereo acuity measurement play important roles in diagnosing vision problems and in monitoring the effects of clinical treatments. Grating-acuity measurement determined by preferential-looking methods is the most widely available tool for assessment of visual function in infants and toddlers. Visual acuity may be measured binocularly and monocularly but requires monocular measurements to determine the presence of unilateral blur. Stereo acuity is the perception of depth as a consequence of cortical recognition of horizontal retinal disparity between the two eyes. It is a form of hyper acuity and is measured binocularly. Stereopsis is the result of activating cortical binocular cells with identical but disparate images. Disrupted or discordant binocular vision during human infancy results in amblyopia and/or suppression thereby interrupting stereopsis. The development of stereopsis has important implications for both understanding human visual development and the clinical treatment of visual anomalies.

The extent of the adult visual field gives important clues on brain functioning. Visual field deficits often accompany, and sometimes predict, the onset of neurological problems. It is therefore of utmost importance to have reliable methods of early assessment of the visual field, in addition to the assessment of visual resolution. The behavioural function that is used for visual acuity assessment is the spontaneous orienting towards a salient visual stimulus in preference to a homogeneous field. In this chapter, an overview on the achievements and limitations of the methods of visual perimetry used with infants and children is given, including correlative data from cats and monkeys.

D. B. was randomly presented with either a grating of a given spatial frequency and constant contrast, or a plain stimulus matched in flux (actually a stimulus with ultra-high spatial frequency to guarantee a flux match) and asked to respond, ‘lines’ or ‘no lines’. The spatial frequency was changed in groups of thirty trials and the cut-off spatial frequency determined for 75% performance. Two different apparatus were used for presenting sine-wave gratings: the Moiré fringes generated by light passing though a pair of diffraction gratings, or non-luminous non-projected photographic prints mounted on cardboard. In his blind field, acuity actually improved with increasing eccentricity out to about 30°. His spatial frequency acuity compared to his good field was reduced by about 2 octaves, a result similar to that reported by the Pasiks for the monkey without striate cortex. D. B. never reported any experience of the gratings and said he was ‘just guessing’.

This chapter discusses the anatomy and physiology of the retina in aquatic tetrapods. It focuses on the input and output stages of retinal processing: the photoreceptors and the ganglion cells. It describes the characteristics of rod and cone visual pigments, photoreceptor sensitivity, and the importance of temporal summation of photoreceptor signals. It also discusses ganglion cell densities and topographies, and examines how ganglion cells receive photoreceptor inputs and determine anatomical visual acuity.

Visual evoked potentials provide a quantitative measure of the visual system. The function measured includes that of the optic nerve, through the optic chiasm and tract, to the lateral geniculate bodies, and the geniculocalcarine projection to the visual cortex. The use of small-sized stimuli tests the foveal region, emanating primarily from the central fifteen degrees. The most reliable information relates to lesions of the optic nerve, and is derived by individual testing of each eye. An important application of visual evoked responses is as a screen for multiple sclerosis lesions. Abnormalities have also been shown in other conditions such as glaucoma, parkinsonism, and cortical blindness. It can also be used to measure visual acuity in infants. Since the response relies on a visual image reaching the retina, it is important to screen initially for any significant decrease in visual acuity. If possible, this should be corrected. If not, flash stimuli may yield some information, although this may or may not be as sensitive a test.

‘D. B.’ is the subject of the book who underwent intensive tests of his blindsight. His visual cortex in one hemisphere was removed surgically to excise an invasive tumour when he was 33 years old. The findings of Pöppel et al. (1973) were confirmed, in which brain damaged subjects could move their eyes to the location of an ‘unseen’ stimulus in their blind fields. The use of animal testing methods were then applied by asking D. B. to guess the location of stimuli by reaching for them. He was also asked to guess between two alternatives: whether a line was horizontal or not, and whether a stimulus was a particular shape or an alternative shape. Asking him to guess whether a sine-wave grating or a homogenous luminous-matched patch was present or not made it possible to measure his visual acuity and compare it with that of the intact visual hemifield. Following a battery of tests, his verbal commentaries were recorded in which he characteristically said he was just guessing and thought he was ‘at chance’ because he could not see anything, although in some tests he had a ‘feeling’ that something was there.

This chapter presents a review of the development of quantum-statistical theories for visual functions such as absolute thresholds for steady and moving targets, contrast thresholds, visual acuity, adaptation, and color identification. Soon after the corpuscular nature of light was discovered by Planck, the minimum perceptible amount of light for human vision was compared with the energy of the quantum of light. At about the same time as the discovery of the quantum, fairly accurate measurements were made by different investigators of this minimum perceptible. It is argued in this chapter that, for each state of adaptation, the nerve system in the retina sets an organizational pattern through which action potentials can find their way to the higher centers. Instability phenomena in color identification and dependence of visual acuity on brightness are also discussed with reference to quantum-statistical approaches.

Amblyopia is a developmental disorder of binocular vision resulting from an anomalous visual experience early in life. It is the primary cause of monocular vision loss in children and young adults, affecting more than 2% of the population. Many investigators have strongly recommended the screening and correction of amblyogenic factors—such as high refraction errors, strabismus, and dioptric media opacity—at the earliest feasible age (from six months onwards) to improve visual outcome and reduce cases of squint and the progression of myopia. Amblyopia can be screened by subjective or objective testing. From birth, an estimation of visual acuity can be obtained from preferential direction of gaze but this method is unsuitable for large-scale screening because it is time-consuming and relies on the subject's co-operation. Objective refractometry without cycloplegia may represent the ideal method for screening infants for amblyogenic factors because the test is fast, it requires no contact, and it can be performed by non-professional staff. Various improvements have been proposed in an attempt to solve problems such as controlling image quality, accurate fixation, and accommodation, to extend the range of sensitivity to refractive defects and to measure ocular deviation.

The muscles of giraffes are similar to those of other artiodactyls but differ in order to accommodate the stresses associated with an extraordinary shape. Extensor muscles of the neck are relatively small but flexor muscles of the neck that have to contract against the elasticity of the ligament nuchae are well-developed. The muscle bodies of neck muscles are located at the base of the neck and exert their effects via long tendons, which minimizes neck mass. Giraffes rarely lie down, or sleep, or run. Consequently limb muscles give support more than movement and are more tendinous that muscular. The power to run is generated by hip flexors and extensors, aided by contractions of the gastrocnemius muscles. The forelimb is the main support for body mass and acts as the fulcrum for movement. Chest and forelimb muscles keep the shoulder and elbow joints rigid, but allow drinking and the complications that arise from it. Brain mass forms 0.5% of adult body mass. Eyes are large and the retina has a unique distribution of photoreceptors that provides both clear images of close objects and allows focusing on ground level objects. Peripheral nerves contain more sensory than motor fibers. The skin of giraffes forms ~10% of body mass, and has more collagen than elastic fibers. Skin panniculus muscles are absent, an absence compensated by sebaceous gland secretion of volatile compounds that repel insects. The development of the colors of skin markings begins in the last trimester of pregnancy.