For its space missions, NASA wants astronauts with excellent vision without corrective lenses or glasses. This doesn't prevent its Vision Science and Technology Group to study human vision of ordinary people like you and me. Two members of this group recently discovered that a new formula connecting optical quality with visual acuity could lead to automatic eyeglasses prescriptions. The researchers also said this 'could also enable surgeons to more accurately assess and correct the vision of patients undergoing LASIK or refractive surgery.' But read more...
Here is an example of what the NASA researchers have found. You can see on the left "how the optical transfer function (OTF) was computed for a given aberration state using standard methods: (Top) wavefront aberration image; (middle) point-spread image and OTF; (bottom) original letter and aberrated image of letter." (Credit: NASA Vision Group)
This technique has been developed by Andrew B. Watson, Director of the NASA Vision Group at NASA's Ames Research Center, Moffett Field, CA, USA, and by Albert J. Ahumada, Jr., Research Psychologist in the same group.
Their study evaluated "the performance of several simple metrics that predict visual acuity from wavefront aberrations -- the eye measurements provided by an aberrometer." And the scientists made a clear distinction between a metric and model. "'A metric is a formula that describes a quantitative relationship,' explains Watson. 'It is accurate but not necessarily adaptable to different situations. A model is a mechanistic description that explains why a relationship exists. If the parameters of a model change, one can make predictions of how an outcome will change.'"
So the NASA scientists built a model and a metric. They "developed a model that successfully predicts visual acuity using both wavefront aberrations and simulations of the complex task of identifying individual letters from the widely used Sloan letter set. They then designed a simple metric that performs as well as the more elaborate model."
This research work is available in the online Journal of Vision, a publication of Association for Research in Vision and Ophthalmology (ARVO) under the name "Predicting visual acuity from wavefront aberrations" (Volume 8, Number 4, Article 17, Pages 1-19, April 22, 2008).
Here is a link to the abstract. "It is now possible to routinely measure the aberrations of the human eye, but there is as yet no established metric that relates aberrations to visual acuity. A number of metrics have been proposed and evaluated, and some perform well on particular sets of evaluation data. But these metrics are not based on a plausible model of the letter acuity task and may not generalize to other sets of aberrations, other data sets, or to other acuity tasks. Here we provide a model of the acuity task that incorporates optical and neural filtering, neural noise, and an ideal decision rule. The model provides an excellent account of one large set of evaluation data. Several suboptimal rules perform almost as well. A simple metric derived from this model also provides a good account of the data set."
Here is an excerpt from the conclusion. "In this study, we evaluated the fit of a set of template-matching models, incorporating both optical and neural filtering, to a set of acuity versus aberration data. In general, the class of models we have considered fit very well. Relative to prior metrics, which attained a correlation of 0.85, the best model here attains a correlation of 0.913, and indeed all models tested (including our metric) attain correlations of over 0.86. [...] The success of the ID model means that we now have a successful operational account of how particular aberrations reduce visual acuity. This account may prove useful in future study of the effects of specific aberrations."
A final good news: the metric developed by NASA could be used internationally. Here is a last quote from Watson. "The same metric, because of its generality, can predict acuity measured with other symbol sets, such as Chinese characters. With this metric, the aberrometer will be able to give direct predictions of visual acuity, and could also provide an automatic optimal refractive prescription for the patient."
Sources: Association for Research in Vision and Ophthalmology (ARVO) news release, May 14, 2008; and various websites
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