In this study we investigated the time course of the axial length response to 60 minute episodes of continuous myopic and hyperopic defocus, testing the hypothesis that the human eye, similar to those of other animals 17,18 , would be able to detect and respond to defocus within minutes of exposure to blur. We further assessed the persistence of axial length changes following the cessation of myopic and hyperopic defocus, during a period of clear vision, hypothesizing that the response to myopic defocus would be more enduring than the response to hyperopic defocus. Given that the visual system is also known to compensate for optical defocus through a gradual improvement in defocused visual acuity (VA) over time (blur adaptation) 24-28 , we also examined the association between the time course of changes in defocused VA and axial length during exposure to myopic defocus.

Purpose To investigate the change in axial length (AxL) and choroidal thickness (ChT) in response to continuous and alternating episodes of monocular myopic and hyperopic defocus. Methods The right eye of sixteen young adults was exposed to 60 minute episodes of either continuous or alternating myopic and hyperopic defocus (+3 DS & -3 DS) over six separate days, with the left eye optimally corrected for distance. During alternating defocus conditions, the eye was exposed to either 30 or 15 minute cycles of myopic and hyperopic defocus, with the order of defocus reversed in separate sessions. The AxL and ChT of the right eye were measured before, during and after each defocus condition. Results Significant changes in AxL were observed over time, dependent upon the defocus condition (p < 0.0001). In general, AxL exhibited a greater magnitude of change during continuous than alternating defocus conditions. The maximum AxL elongation was +7 ± 7 μm (p = 0.010) in response to continuous hyperopic defocus and the maximum AxL reduction was -8 ± 10 μm of (p = 0.046) in response to continuous myopic defocus. During both 30 and 15 minute cycles of alternating myopic and hyperopic defocus of equal duration, the effect of opposing blur sessions cancelled each other and the AxL was near baseline levels following the final defocus session (mean change from baseline across all alternating defocus conditions was +2 ± 10 μm, p > 0.05). Similar, but smaller magnitude, changes were observed for ChT. Conclusions The human eye appears capable of temporal averaging of visual cues from alternating myopic and hyperopic defocus. In the short term, this integration appears to be a cancellation of the effects of the preceding defocus condition of opposite sign.

Citation information: Delshad S, Collins MJ, Read SA, & Vincent SJ. Effects of brief periods of clear vision on the defocus-mediated changes in axial length and choroidal thickness of human eyes.

No abstract