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N. C. Strang, S. Mucke, V. Manahilov, D. Seidel, L. S. Gray; Visual Suppression During Ocular Accommodation Is Velocity Dependent. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4565.
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Previously we have shown the presence of non-optical visual suppression for high spatial frequencies during large (>2D) accommodation step responses. This supports the suggestion that large accommodation step responses are pre-programmed and do not require the negative feedback information which operates during steady-state and small step responses. As slow ramp changes in accommodation response are also thought to be dependent upon negative feedback we would predict that visual suppression would be absent for slower velocity large step responses. In this study, we assess the extent of visual suppression present during accommodation responses of varying velocity.
5 young emmetropic subjects participated with informed consent in the study. Contrast thresholds were measured using a 2-alternative forced-choice staircase procedure during static and dynamic accommodation responses. Sinusoidal (1, 4, and 9 cpd) gratings were presented on a monitor at 1m for a duration of 26ms. In the dynamic condition a 2 dioptre (1-3D) far to near accommodation stimulus was presented at velocities of 0.5, 2, 3 and 10D/s while contrast threshold measurements were obtained at pre-determined time points after stimulus onset. Contrast thresholds were also obtained during steady state accommodation responses to 5 stimulus distances ranging from 1m-33cm. Dynamic accommodation responses to the accommodation stimulus were recorded by a modified infrared, open field autorefractor (Shin-Nippon SRW-5000).
Significant elevation (mean difference = 0.33 ± 0.12 log units, p<0.05) in contrast thresholds for 9cpd were found during the fastest accommodation responses (10D/s and 3D/s) in comparison to the static condition. This contrast threshold elevation was found only at time points during the accommodation response. No elevation of contrast thresholds was found for the slowest accommodation response velocities (0.5 and 2 D/s) There were no significant elevations of contrast threshold for 1 and 4 cpd at any accommodation response velocities.
Non-optical visual suppression is present during fast accommodation responses but is not found for slow responses. The results support the suggestion that for large amplitude accommodation step responses there is an initial fast pre-programmed component followed by a small slow component which relies upon continuous negative feedback about the degree of blur in the retinal image.
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