G. Mohn

The development of visual acuity in normal fullterm and preterm infants

The development of visual acuity during the first year of life was assessed in 91 normal fullterm infants and 36 preterm infants with minimal perinatal complications, using the forced-choice preferential looking technique. Acuity in the preterm infants lagged behind that of the fullterm infants up to the age of 6-8 months if age was calculated from birth, and then reached equal levels. When age was corrected for prematurity, acuities in the two groups were very similar at all ages, but mean preterm acuity was consistently slightly higher than in the fullterm infants. The results suggest that early visual experience of preterm infants in the period up to the expected date of term may lead to a slight acceleration of the development of behavioural visual acuity. This is discussed in relation to electrophysiological studies which report a greater effect of prematurity on acuity development.

ACUITY ASSESSMENT OF NON‐VERBAL INFANTS AND CHILDREN: CLINICAL EXPERIENCE WITH THE ACUITY CARD PROCEDURE

The acuity card procedure was used to assess the visual acuity of 510 neurologically normal and abnormal infants and children. Acuity estimates were obtained for 93 per cent of 842 binocular and 279 monocular tests. The observed development of binocular acuity of normal fullterm and preterm infants agreed well with previous reports using the traditional forced‐choice preferential looking technique. Monocular tests seemed to support earlier suggestions that grating acuity may be relatively insensitive to strabismic amblyopia. Infants at risk of later neurological deficits but developing normally had only a slight delay in development of acuity, but there was a high incidence of acuity deficits (54 per cent) among those with severe neurological defects. The great majority of a group of multiply handicapped children had low acuity for age. Repeat tests showed a high degree of test‐retest consistency. The acuity card procedure was a successful and useful method for assessing the acuity of infants and children who cannot be tested with standard ophthalmological methods.

The development of vernier acuity in human infants

Vernier acuity, i.e. the detection of a small misalignment between lines, is about one order of magnitude finer than the resolution of periodic gratings in adult humans. This hyperacuity is generally attributed to cortical mechanisms, and the time-course of its development seems to differ from the development of grating resolution that probably is limited by retinal factors. We investigated 271 human infants and children between 2 months and 8 yr of age with essentially identical stimuli and experimental procedures. Vernier thresholds for Vernier targets were compared to grating resolution. The preferential looking experiments led to the following results: (i) Vernier acuity starts below grating resolution. (ii) Like grating resolution, Vernier acuity develops gradually, but more rapidly and longer; at the age of 5 yr performance becomes comparable to that of adults. (iii) Flanking borders without offset, added to the Vernier targets at various distances, did not affect thresholds consistently across distances and age groups.

Visual defects in children after cerebral hypoxia

Visual functions were examined in 18 survivors of perinatal hypoxia/ischemia with mild to severe neurological sequelae, aged between 3 months and 17 years, and in two patients, aged 8 and 13 years, who had suffered postnatal hypoxic events. All but two patients showed clear visual deficits ranging from mild defects in visual acuity, visual field size, and/or optokinetic nystagmus to blindness. In 5 patients, the visual field was restricted to tunnel vision, a finding which appeared to be specifically related to the hypoxic/ischemic nature of the brain damage. The severity of the visual defects after perinatal hypoxia was related to the occurrence of neonatal seizures, later neurological outcome, and gestational age at birth. This is discussed in relation to previous studies of the effects of perinatal hypoxia/ischemia.

Effects of very low birth weight (VLBW) on visual development during the first year after term

Behavioural visual functions were assessed in 155 very low birth weight (VLBW) infants during the first 12 months after expected term. Visual development was examined (mainly cross-sectionally) at 6 weeks, 3, 6, 9, and 12 months of corrected age by assessment of visual acuity, visual fields, optokinetic nystagmus and visual threat response. Many VLBW infants showed visual impairments (54.2%). No single visual function appeared to be specifically susceptible to impairments, deficits were often apparent across a range of functions. Visual impairments were observed at all test ages, and could already be assessed at 6 weeks of corrected age. The highest incidence of visual impairments was scored at 6 months corrected age. Beyond 6 months, less deficits were observed, suggesting in many infants a delayed rather than a permanently impaired visual development. In some infants deficits became evident at a later stage, after an apparently normal initial development. The results suggest that VLBW infants are at risk for impaired visual development.

Visual Field Measurements, Optokinetic Nystagmus and the Visual Threatening Response: Normal and Abnormal Development

Experimental studies in animals have shown that visual deprivation early during development may lead to electrophysiological and anatomical neuronal changes accompanied by permanent defects of several behavioral visual functions which include asymmetrical monocular optokinetic nystagmus (OKN), absence of the visual component of the threatening response, and visual field defects.

Optokinetic and spontaneous nystagmus in children with neurological disorders.

Binocular and monocular optokinetic nystagmus (OKN) was examined with EOG recordings in 26 visually impaired children with neurological disorders, aged 2.5 months to 15 years. Spontaneous and/or latent nystagmus, complicating the assessment of OKN, was seen in 73% of the children. Binocular OKN in 21 patients with positive visual functions was symmetrical in 5 cases, asymmetrical in 12 cases and could not be elicited in 4 patients. Monocular OKN was nearly always asymmetrical, with usually a superiority of the temporal-to-nasal (TN) components. Five blind children, 4 of them cortically blind, showed positive binocular and monocular OKN, suggesting that neural control of OKN in humans may be at least partly independent of the cortex. This, together with results from some of the sighted patients, indicates that in humans, cortical and subcortical contributions to OKN, and particularly to monocular nasal-to-temporal (NT) OKN, may be more complicated than had been thought.

Rapid Assessment of Visual Acuity in Infants and Children in a Clinical Setting, Using Acuity Cards

The preferential looking technique for assessing visual acuity in infants has over the last ten years produced very consistent results in normal infants across different laboratories, different apparatus and different methods (Teller et al 1974; Dobson and Teller 1978; Mohn and Van Hof-van Duin, this issue). The potential clinical usefulness of the technique for detecting impairments of visual development has been demonstrated by studies of infants with ophthalmological disorders (Jacobson et al 1983; Mohindra et al 1983; Mayer 1985), and infants and also older children with neurological disorders and developmental retardation (Mayer et al 1983; Lennerstrand et al 1983; Mohn and Van Hof-van Duin 1983). Up till now, however, it has not been widely adopted for clinical use. One of the main reasons for this has been the large number of trials and thus time required for a statistically reliable acuity estimate with the standard two-alternative forced-choice (FPL) method employed.

Preferential looking acuity in preterm infants

Acuity was measured in 48 prematurely born infants using the preferential looking technique. These quantitative results show that acuity development in these infants is related to conceptional rather than postnatal age, in agreement with the qualitative findings of previous studies. Severe intraventricular haemorrhage was accompanied by low visual acuity in two infants.

On the relation of stereoacuity to interocular transfer of the motion and the tilt aftereffects

Interocular transfer of both the motion and the tilt aftereffect were tested in 43 subjects with varying degrees of stereopsis, ranging from normal to stereoblind. Although there was an overall tendency for transfer to diminish with decreasing stereovision, stereoblind subjects always showed some transfer of the tilt aftereffect, and sometimes also of the motion aftereffect, while some subjects with normal stereothresholds had greatly reduced or no interocular transfer. No quantitative correlation between stereothresholds and amount of transfer could be found. The results indicate that there is no simple relationship between interocular transfer, stereopsis and cortical binocularity, as had been suggested previously.