Orientation specific impairment in contrast sensitivity following long-term neural adaptation to optical blur in keratoconus

Abstract

Purpose : Long term adaptation to optical blur in keratoconus (KC) alters neural spatial frequency (SF) processing, however whether this change is specific to certain orientation remains unknown. Given the predominance of vertical coma in KC eyes’ habitual optical quality, we hypothesize that fully-corrected KC eyes are less sensitive to horizontal gratings than vertical gratings relative to normal eyes.

Methods : Normal (n=3) and KC (n=3) eyes (6 mm pupil; cyclopleged using 1% tropicamide) were tested monocularly under full adaptive optic (AO) correction. Observers performed a 2-interval-forced-choice task in which they reported which one of two intervals contained the signal (oriented Gabor). Contrast sensitivity functions (CSF) (SF range: 0.5-30 cycles/degree) were measured using the quick CSF method for different Gabor orientations (0, 45, 90, and 135^o; one subject in each group was not measured at the 135^o orientation).

Results : The optics for both groups were corrected to diffraction limited optical quality. A similar oblique effect was found for normal and KC subjects, with a cardinal-to-oblique sensitivity ratio of 1.30±.16 and 1.38±.27, respectively. KC eyes are impaired across all SFs compared to normals, but are more severely impaired in the horizontal direction than the vertical–as depicted in Fig.1. KC subjects performed 20% worse than normals in the horizontal direction and 11% percent worse in the vertical. Normal eyes, on the other hand, show similar sensitivity for horizontal and vertical orientations (horizontal-to-vertical sensitivity ratio of 1.21±.28). KC eyes had a sensitivity ratio of horizontal-to-vertical of 0.83±.27.

Conclusions : Neural CSFs for different grating orientations were obtained by bypassing the eye’s optics using an AO. KC subjects, however, showed overall residual impairment in contrast sensitivity that was more dominant in the horizontal direction. This finding can be explained by KC eyes’ habitual aberrations–characterized by large amounts of vertical coma–which produces vertical optical blur more severely. Our findings suggest that long-term neural adaptation to optical defects is orientation-specific.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Fig. 1: CSF for control (n=3; blue) and KC (n=3; red) eyes tested using horizontal gratings (dashed lines) and vertical gratings (solid lines). Error bars represent ±1 standard errors of the mean.