Effects of Prematurity on the Development of Contrast Sensitivity: Testing the Visual Experience Hypothesis
Rain G. Bosworth and Karen R. Dobkins
Vision Res. Author manuscript; available in PMC 2014 Apr 19. Published in final edited form as: Vision Res. 2013 Apr 19; 82: 31–41. Published online 2013 Feb 24.
In order to investigate the effects of visual experience on early visual development, the current study compared contrast sensitivity across infants born with different levels of moderate-to-late prematurity. Here the logic is that at any given postterm age, the most premature infants will have the oldest postnatal age. Given that postnatal age is a proxy for visual experience, the visual experience hypothesis predicts that infants who are more premature, yet healthy, should have higher sensitivity. Luminance (light/dark) and chromatic (red/green) contrast sensitivities (CS) were measured in 236 healthy infants (born −10 to +2 weeks relative to due date) between 5 and 32 weeks postterm age from due date and 8 to 38 weeks postnatal from birth date. For chromatic CS, we found clear evidence that infants who were most premature within our sample had the highest sensitivity. Specifically, 4 to 10 additional weeks of visual experience, by virtue of being born early, enhanced chromatic CS. For luminance CS, similar but weaker results were seen. Here, only infants with an additional 6 to 10 weeks of visual experience, and only at later age points in development, showed enhanced sensitivity. However, CS in preterm infants was still below that of fullterm infants with equivalent postnatal age. In sum, these results suggest that chromatic CS is influenced more by prematurity (and possibly visual experience) than is luminance CS, which has implications for differential development of Parvocellular and Magnocellular pathways.
A randomised controlled study was conducted. The study sample consisted of 89 very low birth weight premature infants, admitted during a 24-month period to the neonatal care unit at Tygerberg Hospital in Cape Town, South Africa. The infants were recruited by means of certain criteria and then randomly assigned to one of two groups: 1) the intervention group was cared for according to the Sensory Developmental Care Programme for ten recorded days; and 2) the control group that received the standard care of the unit, also for ten days. The intervention group consisted of 45 infants of whom 22 completed the study, while the control group consisted of 44 infants of whom 20 completed the study. Both study groups were followed up at six, 12 and 18 months (corrected age) when the Test of Sensory Functions in Infants was used to do a sensory developmental assessment. At 18 months (corrected age) a Griffiths Developmental Scale assessment was also conducted to determine function in other areas of development. Test results were analysed using repeated measures of ANOVA, and the Bonferoni t procedure to determine the effect that the Sensory Developmental Care Programme had on the sensory development of the infant up to 18 months (corrected age).
The results of the comparison of the performance of both groups (group effect), measured by the Test of Sensory Functions in Infants are of great importance to this study. The intervention group had a significant difference on the total score (p<0.00), as well as on the following four of the five sub-tests scores: reactivity to tactile deep pressure (p<0.03); adaptive motor functions (p<0.03); visual-tactile integration (p<0.00); and reactivity to vestibular stimulation (p<0.01).
The results of this study signify that the infants in the intervention group benefited from the Sensory Developmental Care Programme concerning their sensory functions up to the age of 18 months (corrected age). The Sensory Developmental Care Programme was demonstrated to be both practical and successful in terms of its aims. The Programme could therefore be fruitfully utilised in other neonatal intensive care units.