Image Quality Shapes Eye Growth, but Limited Criteria Apply
The growth of the eye in childhood is not predetermined - it is dependent on a local feedback system within the eye itself that takes into account the quality of images formed on the retina. In general, the eye grows in an effort to keep the retinal image in good focus. In findings reported this week, researchers investigated which aspects of retinal images are used by this feedback system. They found that among the numerous image qualities that contribute to our sense of blur, only a subset of these qualities are used in the feedback system that controls eye growth. The findings suggest that the activity of only certain retinal cells may be necessary to drive the feedback system.
The findings are reported in the April 4th issue of Current Biology by a team including Robert Hess of McGill University.
Images of everyday scenes are composed of different grades of spatial details--different "spatial frequencies" --that occur at different absolute and relative contrasts. In the study, the researchers asked whether the feedback system that regulates eye growth took into account all this information, or just used a part of it. By presenting young chickens with synthetic images that had similar properties to everyday natural images, the researchers were able to independently manipulate each of these image properties and evaluate the effect on eye growth.
The researchers' first finding was that the different spatial frequencies that occur in images and define localized details, such as edges and lines, are not used by the feedback system to regulate eye growth, despite the fact that these qualities are very important for image recognition. The second finding was that the feedback system does not take into account all grades of spatial detail that are present in a scene, but only the finest detail present, to regulate eye growth. What makes these findings interesting is that our everyday perception of what is blurred is computationally more elaborate than the feedback system, since our sense of blur depends on both local spatial alignments and information across a range of different spatial scales in images. Therefore, the feedback system, which is basically restricted to the eye itself, uses much simpler information than that required by the brain to detect and discriminate image blur. The fact that the eye-growth feedback system uses more primitive information means that images of low contrast are just as effective as blurred images at initiating growth, a conclusion that may have implications for the development of myopia (i.e., short sightedness).