Sign-dependent sensitivity to peripheral defocus for myopes due to aberrations.

PURPOSE

Animal studies suggest that the periphery of the eye plays a major role in emmetropization. It is also known that human myopes tend to have relative peripheral hyperopia compared to the foveal refraction. This study investigated peripheral sensitivity to defocus in human subjects, specifically whether myopes are less sensitive to negative than to positive defocus.

METHODS

Sensitivity to defocus (logMAR/D) in the 20° nasal visual field was determined in 16 emmetropes (6 males and 10 females, mean spherical equivalent -0.03 ± 0.13 D, age 30 ± 10 years) and 16 myopes (3 males and 13 females, mean spherical equivalent -3.25 ± 2 D, age 25 ± 6 years) using the slope of through-focus low-contrast resolution (10%) acuity measurements. Peripheral wavefront measurements at the same angle were obtained from 13 of the myopes and 9 of the emmetropes, from which the objective depth of field was calculated by assessing the area under the modulation transfer function (MTF) with added defocus. The difference in depth of field between negative and positive defocus was taken as the asymmetry in depth of field.

RESULTS

Myopes were significantly less sensitive to negative than to positive defocus (median difference in sensitivity 0.06 logMAR/D, P = 0.023). This was not the case for emmetropes (median difference -0.01 logMAR/D, P = 0.382). The difference in sensitivity between positive and negative defocus was significantly larger for myopes compared to emmetropes (P = 0.031). The correlation between this difference in sensitivity and objective asymmetry in depth of field due to aberrations was significant for the whole group (R(2) = 0.18, P = 0.02) and stronger for myopes (R(2) = 0.8, P < 0.01).

CONCLUSIONS

We have shown that myopes, in general, are less sensitive to negative than to positive defocus, which can be linked to their aberrations. This finding is consistent with a previously proposed model of eye growth that is driven by the difference between tangential and radial peripheral blur.