Emmetropisation responses when visual information is presented at only one or two near target planes in chick

When visual information is confined to one object plane and zero or hyperopic defocus is present, emmetropisation is directed towards this plane, when myopic defocus is present, emmetropisation processes fail. We investigated the effect of introducing information at a second nearer plane on emmetropisation responses under these conditions.

The visual environment was controlled using lenses (+30 D, +40 D and +50 D) and cones with a Maltese cross target (MX) at one or two distances. Four different target configurations were used: 1. a single target was located at 3.3 cm, 2. a vertical hemi-field target was added at 2.5 cm, 3. a transparent target was added at 2.5 cm or 4. a single target was located at 2.5 cm. An additional cone length of 4.0 cm and nearer target distance of 3.3 cm was used with the +50 D lens. The imaging devices were applied monocularly to eight-day-old chicks and worn for four days. At the end of the treatment, refractive errors and eye growth were measured. Potential regional differences in growth were also assessed.

The configuration of the target and the interaction between target configuration and lens power had significant refractive and axial growth effects. With a single target plane, myopic and hyperopic defocus resulted in myopia. When defocus was experienced at two planes, refractive errors shifted towards the plane with the lower defocus and emmetropisation responses, although still not normal, were more consistent.

When visual information is provided at two distances, the target with the lesser incident defocus has the greater influence on the resultant refractive error. Emmetropisation responses are more accurate when information is presented at many distances.