Stimulus requirements for the decoding of myopic and hyperopic defocus under single and competing defocus conditions in the chicken.

PURPOSE

The bidirectional nature of emmetropization, as observed in young chicks, implies that eyes are able to distinguish between myopic and hyperopic focusing errors. In the current study the spatial frequency and contrast dependence of this process were investigated in an experimental paradigm that allowed strict control over both parameters of the retinal image. Also investigated was the influence of accommodation.

METHODS

Defocusing stimuli were presented through lens-cone devices with attached targets. These devices were monocularly applied to 5-day-old chickens for 4 days. Defocus conditions included: (1) 7 D of myopic defocus, (2) 7 D of hyperopic defocus, and (3) a combination of the two. Two high contrast target designs, a spatially rich, striped Maltese cross (target 1) and a standard Maltese cross (target 2) were used, except in some experiments where target contrast or spatial frequency content was further manipulated. To test the role of accommodation, the treated eye of some chicks underwent ciliary nerve section before attachment of the device. Refractive error (RE) was measured by retinoscopy and axial ocular dimensions measured by A-scan ultrasonography, both in chicks under anesthesia.

RESULTS

With imposed myopic defocus and high contrast, target 1 elicited significantly better compensation than did target 2. With imposed hyperopic defocus, both targets elicited near normal compensatory responses. Reducing image contrast to 32% for target 2 and to 16% for target 1 precluded compensation for myopic defocus, inducing myopia instead. The low-pass-filtered target also induced myopia, irrespective of the sign of imposed defocus. With competing defocus and intact accommodation, target 1 induced a transient hyperopic growth response, whereas myopia was consistently observed with target 2. When accommodation was rendered inactive, both targets induced myopia under these competitive conditions.

CONCLUSIONS

Compensation to myopic defocus is critically dependent on the inclusion of middle to high spatial frequencies in the stimulus and has a spatial frequency-dependent threshold contrast requirement. With competing myopic and hyperopic defocus, the former transiently dominates the latter as a determinant of ocular growth, provided that the stimulus conditions include sufficient middle to high spatial frequency information and that accommodation cues are available.