Lens-Compensation in Chicks Depends on Spatial Factors Other Than the Power Spectrum

Abstract

Purpose:: Chick eyes compensate for defocus imposed by spectacle lenses by changing eye length and choroidal thickness. Because some psychophysical theories of blur make use of the slope of the power spectrum as a metric of defocus, one could imagine the same to be true of the emmetropization mechanism, and hence of the compensation for defocusing spectacle lenses. Because a recent paper (Hess et al., 2006) offered evidence that only the spatial frequencies were important for lens-compensation, we tested whether chicks would compensate better when viewing natural images than noise-like patterns with the same spectral distribution of spatial frequencies.

Methods:: Three images were compared for their effects on lens-compensation: a photograph with a natural spatial frequency profile (power spectrum of 1/f²), the same photo with its phase information randomized, and a blank piece of paper. Chicks, wearing a +7 D lens on one eye with the other eye covered, viewed one of these images on the inside wall of a 60 cm cylinder under monochromatic light for 20 minutes. The lens and patch were then switched to the other eye, and the chick was placed in another drum with a different image for 20 minutes. Chicks underwent three exposures per day for three days, and were in the dark for the remainder of the time. Refraction and ultrasound measurements were taken before and after the experiment.

Results:: Eyes that viewed the phase-scrambled image compensated less well than those that viewed the photograph but better than those that viewed the blank paper. Compensation was significantly (p<0.005) better for the photograph than for the phase-scrambled image with respect to refractive error (+3.9 D more hyperopic), choroidal thickness (67 µm thicker), vitreous chamber depth (142 µm shallower) and axial length (78 µm shorter). Similarly, chicks compensated better for the phase-randomized image than for the blank wall. Results were significant (p<0.05) for the refractive error, vitreous, and axial length, but not the choroid.

Conclusions:: Spatial frequency contrast amplitudes do not appear to be the only visual information in the image that the chick emmetropization system uses to guide lens compensation.

Keywords: emmetropization • myopia • refractive error development