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S. A. McFadden, B. N. Shields, K. J. Rounsley; Local accommodation in a lateral eyed bird facilitates seeing simultaneously in two different places in space. Journal of Vision 2001;1(3):229. doi: 10.1167/1.3.229.
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Many lateral eyed birds are bifoveate or have dual retinal specialisations corresponding with their lateral and frontal viewing modes. For example, the eye design in the pigeon, which is typical of many granivorous birds, incorporates an optic axis for lateral distant viewing and a frontal area in the temporal retina associated with close binocular viewing. In this paper we ask how these two separate visual axis are used to focus on objects, not only at different eccentricities, but also at different absolute distances. Using simultaneous infrared (IR) photoretinoscopy and IR keratometry, we find that the pigeons optical system can indeed simultaneously support differential refractive states on the lateral and frontal axis. Maps of the refractive error (RE) and corneal power in cyclopleged and anaesthetized pigeons, reveal small but antagonistic gradients in different sectors of the visual field. In particular, we find that at rest, the frontal field is relatively myopic and yet the cornea has less power in this sector. We studied how these RE and corneal power maps actively vary during a reaching behaviour. We found that during the saccadic like peck response, the optic axis is not focussed on the grain and has only 30% of the required accommodation power. In contrast, we calculate that the frontal axis does have adequate power to focus on the grain. Furthermore, at the last stationary head fixation position (which occurs at an eye-grain distance of 67mm, SD=3.9mm) before the final descent when the eyes begin to close, accommodation on the frontal axis can be supported wholly by the cornea, which allows dramatic RE changes (−13D, SD=7.7) without unduly effecting the refractive error on the optic axis (−2D, SD=1.7). We propose that the shape of the cornea facilitates this local accommodation mechanism and that this dual visual system can be simultaneously focussed at disparate distances in space.
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