The significance of retinal image contrast and spatial frequency composition for eye growth modulation in young chicks

Abstract

Purpose: This study sought further insight into the stimulus dependence of form deprivation myopia, a common response to retinal image degradation in young animals.

Methods: Each of 4 Bangerter diffusing filters (0.6, 0.1, <0.1, and LP (light perception only)) combined with clear plano lenses, as well as plano lenses alone, were fitted monocularly to 4-day-old chicks. Axial ocular dimensions and refractive errors were monitored over a 14-day treatment period, using high frequency A-scan ultrasonography and an autorefractor, respectively.

Results: Only the <0.1 and LP filters induced significant form deprivation myopia; these filters induced similarly large myopic shifts in refractive error (mean interocular differences+/-SEM: -9.92+/-1.99, -7.26+/-1.60 D, respectively), coupled to significant increases in both vitreous chamber depths and optical axial lengths (p<0.001). The other 3 groups showed comparable, small changes in their ocular dimensions (p>0.05), and only small myopic shifts in refraction (<3.00 D). The myopia-inducing filters eliminated mid-and-high spatial frequency information.

Conclusions: Our results are consistent with emmetropization being tuned to mid-spatial frequencies. They also imply that form deprivation is not a graded phenomenon.

Figure 1

Image degradation resulting from the Bangerter diffusing filters; the images displayed were recorded with a Nikon Coolpix camera with either no filter (top row), or one of the diffusing filters attached to the camera lens. There is a progressive loss of high and medium frequency detail, with the 2 densest filters transmitting only low frequency detail.

Figure 2

Fourier analysis of the images shown in Figure 1; MTFs compared to those derived for the plano (no filter) treatment (MTF=1) to characterize the spatial frequency transmission properties of the diffusing filters.

Figure 3

Mean interocular differences ±SEM in A. optical axial length, B. vitreous chamber depth, C. inner axial length, D. anterior chamber depth, normalized to baseline values, plotted against days of treatment for each of the treatments. Only the groups fitted with the two densest (LP and <0.1) filters, showed significant increases in optical axial length. The induced changes in these two groups are similar to each other (p>0.05) and significantly greater than those in the other 3 groups, i.e. with no filter, 0.6, or 0.1 filter (p<0.001). The trends in vitreous chamber and inner axial length data are similar to those described for optical axial length, because increases in vitreous chamber depth account for most of the increases in optical axial length for all groups, and treatment-induced changes in retinal and choroidal thickness are minimal. There were significant increases in anterior chamber depth (p<0.001) with the two densest (<0.1 and LP) filters compared to the effects of the other 3 treatments.

Figure 4

Mean interocular differences ±SEM in A. refractive error, normalized to baseline values, plotted against days of treatment for each of the treatments. The LP and <0.1 filters induced large myopic shifts in refractive error while the other treatments induced only very small myopic shifts. B. Refractive error changes plotted against vitreous chamber depth changes. Parameters are significantly correlated (R² = 0.1144, F_{1, 38} = 4909, p = 0.0328).

Figure 5

A. Effects of plano lens-filter combinations on the visual spatial resolution of normal chicks, measured using an optokinetic nystagmus paradigm and black and white grating stimuli. B. Effects of plano lens-filter combinations on contrast sensitivity thresholds of human subjects, assessed using a custom-designed computer-based test.