Local Ocular Compensation of Locally Imposed Refractive Errors Despite Accommodation?
Assuming that shifts of the plane of focus produced by accommodation occur equally for the entire visual field, it is difficult to explain how accommodation-derived signals could account for local compensation of refractive error. We tested whether chicks can compensate refractive errors imposed by hemifield lens segments despite accommodation being available to clear the defocus in either the lens-defocused or the untreated part of the visual field. We found that both positive (+7 D) and negative (-8D) lenses were compensated to exactly the same extent for full-field or hemifield application, although compensation was generally better for positive lenses than for negative lenses. The results rule out that accommodation provided a cue for these refraction changes. Moreover, they indicate that accommodation was apparently ignored by the retinal mechanisms controlling refractive development. We found that the chickens tried to refocus their retinal images with positive lenses 87% of the time. A local retinal error signal on defocus was therefore present but only for short periods. Nevertheless, it seemed to be sufficient to induce local changes in refraction. Although one “deprivation”-sensitive retinal mechanism could account for the results, additional experiments suggest that two different retinal mechanisms with different spatiotemporal response properties must have been involved: Positive and negative lens compensations were differently suppressed by flicker light of varying duty cycles.