Visual acuity a multifactorial metric

Although image quality has a clear influence on visual acuity, it does not account for all the individual variability of other factors that must be involved including, measurement errors, or wavefront, optical or neural factors, Raymond Applegate said.

Wavefront factors could include visual performance, while optical factors include issues like scatter or chromatic aberration. Neural factors include cone sampling, internal noise and cognitive issues, he remarked.

"In eyes with normal to excellent photopic high contrast (P HC) acuity, optical quality metrics increase in predictive value with increasing difficulty of the visual task," he said.

The more difficult the task, the more detailed and refined the ultimate visual acuity measurement will be. In the case of normal healthy eyes, high photopic stimuli makes P HC acuity insensitive to minor variations in wavefront error. As such, it's a less accurate metric.

For example, higher order aberrations (HOA) account for only 4% of the variance in photopic high contrast acuity. That means, only 4% of HOAs are detected. "In comparison, mesopic low contrast (M LC) acuity is eight times more sensitive than photopic high contrast acuity to the same variation in optical quality accounting for 37% of the variance in acuity."

Mesopic low contrast stimuli contain less redundant information and are therefore more sensitive to minor variations in image quality.

In a clinical environment the effect of minor variations on optical quality can be assessed with mesopic low contrast acuity, but not with photopic high contrast acuity, Dr Applegate noted.

Ophthalmology Times Europe reporting from the XXIV Congress of the ESCRS, London, 9-13 September, 2006.