Not all aberrations are equal: reading impairment depends on aberration type and magnitude

The eye's optical components are imperfect and cause distortions in the retinal image that cannot be corrected completely by conventional spectacles. It is important to understand how these uncorrected aberrations (those excluding defocus and primary astigmatism) affect visual performance. We assessed reading performance using text with a simulated monochromatic aberration (defocus, coma, or secondary astigmatism), all of which typically occur in the normal population. We found that the rate of decline in reading performance with increasing aberration amplitude was smaller for coma than for secondary astigmatism or defocus. Defocus and secondary astigmatism clearly had an impact on word identification, as revealed by an analysis of a lexical frequency effect. The spatial form changes caused by these aberrations are particularly disruptive to letter identification, which in turn impacts word recognition and has consequences for further linguistic processing. Coma did not have a significant effect on word identification. We attribute reading impairment caused by coma to effects on saccade targeting, possibly due to changes in the spacings between letters. Effects on performance were not accompanied by a loss of comprehension confirming that even if an aberration is not severe enough to make text illegible it may still have a significant impact on reading.