Variability in the accommodative response to a naturalistic target in adults and infants
Riddell P, Horwood A, Turner J, Houston S.. Investigative Ophthalmology & Visual Science. 2000 41 (4): S2264
- Infants can converge and accommodate well by three months of age
The visual targets for young infants differ considerably in their accommodative demands from the text or high contrast gratings used with older subjects. This study compares adult accommodation to both text and a naturalistic target (a 12x8cm coloured clown picture) and then compares adult and infant responses to the naturalistic target.
A remote haploscopic photorefractor was used to study accommodation and convergence binocularly and monocularly. Subjects were 25 children and adults (9-44 years) and 134 healthy infants (1-54 weeks). Adults were required to read detailed text or looked at a clown presented at 0.5m-2m. Infants were tested only with the clown target at the same distances. Participants were rendered monocular remotely by placing an opaque occluder between the target and the eye at the level of the upper concave mirror (See figure 3). Adult subjects reported that the occluder was invisible during testing, and were often surprised to discover that they had only been viewing with one eye. Vergence and accommodation of each individual were calculated from their photographs. Vergence was calculated from the position of the corneal reflection in relation to the pupil centre, applying the Hirschberg ratio (1mm corneal reflection change per 12.2o of vergence change; Slater & Findlay, 1975; Riddell, Hainline & Abramov, 1994). Accommodation was measured using a smooth two dimensional surface derived from a comparison of photorefraction data to dynamic retinoscopy. This gave an estimate of accommodative response from a given pupil and fundal reflex crescent size. Vergence is reported in metre angles with accommodative plane in dioptres to allow comparison to be made between these two measures.
For the text target, mean accommodation for unoccluded adults followed target demand with a small lag for targets at 4D.This did not significantly reduce on occlusion (p=0.65), although 1/12 ceased to accommodate. For the clown target adults’ mean unoccluded accommodation was lower than for text, with more lag, which significantly reduced on occlusion (p=0.02) with 25% showing flat responses. Mean infant responses for the clown target compared well to those of the adults, showing a similarly significant reduction in accommodative response on occlusion with 30% showing flat responses. In order to look more closely at individual behaviours and their developmental progress, vergence and accommodation responses were rated on a scale from 0 to 4. 0 – flat or erratic behavior,1 – 2-3 points on the line, 2 – linear response with most points within 1.5 D/MA of the target, 3 – linear response with most points within 0.75 D/MA of the target, 4 – linear response with most points within 0.25 D/MA of the target. Responses for both unoccluded vergence and accommodation improve during the first 3 months of life. After this time, the range of behaviours is similar to that of adults. Occluded accommodative performance shows little change across development. Occluded vergence performance improves slightly over the first 2 months, and then remains constant over the first year of life. There is some improvement, however, between the end of the first year, and adulthood since no adults show completely flat responses. This might result from a continuing development of sensitivity to proximal cues.
This study demonstrates that by 3 months of age the infants’ vergence and accommodative performance is the equivalent to that of uninstructed adults when using naturalistic targets. This is true for both the unoccluded and occluded conditions. In addition, the range of individual behaviours found in adults and infants is similar