'Who is a visually impaired child? A model is needed to address this question for children with cerebral visual impairment'

The effects of visual degradation on face discrimination

PURPOSE

People with reduced visual acuity (VA) and/or contrast sensitivity have difficulty recognizing faces and facial expressions. We have quantified these difficulties, using a synthetic face discrimination task employing both normal and artificially degraded vision.

METHODS

VA and contrast thresholds were measured using an optimised staircase procedure [Freiburg acuity Test (FrACT)] in 25 young adults (aged 18-24 years) with corrected visual acuity of 0.0 logMAR or better and with four levels of vision degraded with Bangerter occlusion foils. For face discrimination, male face images were synthesised from 37 cardinal points (position of eyes, width of nose, head shape etc) derived from frontal face photographs and manipulated by altering the points as a fraction of the mean head radius. Face discrimination thresholds (% difference) were measured from a simultaneous four-alternative forced choice of 'odd one out' from three identical faces and one that differed. Psychometric functions were measured for four participants with normal and degraded vision. Subsequently, the difference between faces was fixed at twice the discrimination thresholds and the size of the faces manipulated using the FrACT threshold procedure in 25 participants. Data were converted to equivalent face discrimination distances for realistic face dimensions.

RESULTS

With normal vision, face discrimination thresholds ranged from 2.7% to 5.6%; these increased systematically and were more variable with visual degradation. When manipulating face size, face discrimination distance was highly correlated with both acuity and contrast sensitivity (r(2) = 0.77 and 0.80 respectively, p < 0.01). The mean distance with normal vision was 15.3 m (14.5-16.2 ± S.E.M.). With vision degraded to 0.6 logMAR (6/24 Snellen, contrast threshold 15%) the mean face discrimination distance was reduced to 3.9 m (3.7-4.1, ±S.E.M.).

CONCLUSIONS

Poor face discrimination has a profound impact on real-life social communication. Here we report that artificial visual degradation also adversely impacts a synthetic face recognition task. As a rule of thumb, reduction in VA of 0.3 logMAR (halving the decimal VA) reduces the face recognition distance by a factor of 0.6 times. The FrACT-based face discrimination task provides an efficient new tool to quantify and monitor face discrimination ability.

Spectrum of visual disorders in children with cerebral visual impairment

Cerebral visual impairment is a visual function deficit caused by damage to the retrogeniculate visual pathways in the absence of any major ocular disease. It is the main visual deficit in children in the developed world. Preperinatal hypoxic-ischemic damage is the most frequent cause of cerebral visual impairment, but the etiology is variable. The authors set out to evaluate the presence of visual disorders not attributable to any major ocular pathology in a sample of children with central nervous system disease and to describe the clinical picture of cerebral visual impairment in this cohort. One hundred twenty-one patients with central nervous system damage and visual impairment underwent a protocol developed at the authors' center that included neurologic, neurophthalmologic, and neuroradiologic assessments (brain magnetic resonance imaging). Reduced visual acuity was found in 105 of 121 patients, reduced contrast sensitivity in 58, abnormal optokinetic nystagmus in 88, and visual field deficit in 7. Fixation was altered in 58 patients, smooth pursuit in 95, and saccadic movements in 41. Strabismus was present in 88 patients, and abnormal ocular movements were found in 43 patients. Of the 27 patients in whom they could be assessed, visual-perceptual abilities were found to be impaired in 24. Fundus oculi abnormalities and refractive errors were frequently associated findings. This study confirms that the clinical expression of cerebral visual impairment can be variable and that, in addition to already well-documented symptoms (such as reduced visual acuity, visual field deficits, reduced contrast sensitivity), the clinical picture can also be characterized by oculomotor or visual-cognitive disorders. Cerebral visual impairment is often associated with ophthalmologic abnormalities, and these should be carefully sought. Early and careful assessment, taking into account both the neurophthalmologic and the ophthalmologic aspects, is essential for a correct diagnosis and the development of personalized rehabilitation programs.

Visual-perceptual impairment in children with periventricular leukomalacia

We set out to define visuo-perceptual impairment related to periventricular leukomalacia (PVL) using the Developmental Test of Visual Perception (DTVP). Correlations were sought between visual-perceptual deficits and DTVP profile and neuroradiological and neurophthalmological findings. The DTVP was administered to 20 children (m/f: 10/10), aged between 5 and 8 years (mean: 6.95 years), presenting with: spastic diplegia; PVL documented by brain MRI; normal or mildly impaired visual acuity; mild-moderate upper limb functional impairment. The mean General Visual-Perceptual Quotient was impaired, showing a great variability among the patients. Despite this, an uneven DTPV profile, characterised by a significant difference between the VMIQ and the Non-Motor Visual-Perceptual Quotient (P < 0.001) and a poor result on the Closure subtest (identification of whole figures from incomplete visual information) was observed in all the subjects. This profile reflects a deficit in eye-hand coordination and in praxic-constructional abilities and could be the expression of malfunctioning of the occipital-parietal pathway of visual integration, the so-called 'dorsal stream,' a hypothesis reinforced by the emergence of a statistically significant correlation between the neuroradiological data and the presence of visual-perceptual impairment.

A survey of visual impairment in children attending the Royal Blind School, Edinburgh using the WHO childhood visual impairment database

PURPOSE

To assess the aetiology and changing patterns of childhood blindness in one school for the blind in the UK and to assess the use of the World Health Organisation Prevention of Blindness (WHO/PBL) methodology and reporting form in a developed country.

METHODS

One hundred and seven children in one school for the blind and visually impaired in Edinburgh were examined using the WHO/PBL childhood blindness assessment form.

RESULTS

Of the 107 children examined, 87 (81%) were blind or severely visually impaired (corrected visual acuity of <6/60 (20/200) in the better eye). Perinatal related blindness (40%), hereditary disease (26%) and developmental factors (26%) formed the three largest aetiological categories.

CONCLUSION

The pattern of childhood blindness seen in this study was similar to reports from other developed countries. The WHO/PBL reporting form allows detailed comparisons between countries and over time. Additional fields for more detailed reporting of cerebral visual impairment and associated handicaps would increase the usefulness of the WHO/PBL form for population-based studies and for use in developed countries.