Abstract
Purpose: :
Adaptation to defocus is known to influence the subjective sensitivity to blur in both emmetropes and myopes. Blur is a major contributing factor in the closed-loop dynamic accommodation response. Previous investigations have examined the static accommodation response following blur adaptation. Therefore, we investigated whether a period of blur adaptation influences the dynamic accommodation response to step and sinusoidal changes in target vergence.
Methods: :
Eighteen subjects (6 emmetropes, 6 early onset myopes and 6 late onset myopes) underwent 30 minutes of adaptation to plano (control), +1 D or +3 D myopic defocus whilst viewing a television picture. Following this adaptation period, accommodation responses to a 2 D step change and 2 D sinusoidal change (0.2 Hz) in target vergence were recorded continuously using a Shin Nippon SRW-5000 at a sampling frequency of 22 Hz.
Results: :
Adaptation to defocus failed to influence accommodation reaction times, but did influence response times to an inward step change in target vergence. Adaptation to both +1 and +3 D induced significant increases in accommodation response time. Overall mean response times increased from 0.66 ± 0.16 s at 0 D adaptation to 0.86 ± 0.21 and 0.80 ± 0.16 s for +1 and +3 D adaptation respectively. Post hoc analysis indicated that blur adaptation significantly increased the accommodation response time in both the +1 DS and +3 DS adaptation conditions, although the difference between these two adapting levels was not significant (Bonferroni; 0 DS vs +1 DS p = 0.002, 0 DS vs +3 DS p = 0.012, +1 DS vs +3 DS p = 0.850). Blur adaptation also significantly increased the peak-to-peak phase lag for accommodation responses to a sinusoidally oscillating target. Overall mean phase lags increased from 27.94 ± 9.14 deg for 0 D adaptation up to 40.25 ± 16.42 deg and 46.87 ± 16.99 deg for +1 and +3 D adaptation respectively. These changes in accommodative function were equivalent across all refractive groups. Blur adaptation failed to have a significant effect on accommodation microfluctuations.
Conclusions: :
Adaptation to a degraded stimulus causes an increased level of accommodation for dynamic targets moving towards an observer and increases response times and phase lags. An equivalent effect was found in both emmetropes and myopes. It is suggested that the contrast constancy theory may explain these changes in dynamic behaviour.
Keywords: accomodation • myopia • visual acuity