The dorso-laterally located eyes of the southern hemisphere lamprey Mordacia mordax (Agnatha) contain a single morphological type of retinal photoreceptor, which possesses ultrastructural characteristics of both rods and cones. This photoreceptor has a large refractile ellipsosome in the inner segment and a long cylindrical outer segment surrounded by a retinal pigment epithelium that contains two types of tapetal reflectors. The photoreceptors form a hexagonal array and attain their peak density (33,200 receptors0mm 2 ) in the ventro-temporal retina. Using the size and spacing of the photoreceptors and direct measures of aperture size and eye dimensions, the peak spatial resolving power and optical sensitivity are estimated to be 1.7 cycles deg Ϫ1 (minimum separable angle of 34 ' 7 '' ) and 0.64 mm 2 steradian (white light) and 1.38 mm 2 steradian (preferred wavelength or l max ), respectively. Microspectrophotometry reveals that the visual pigment located within the outer segment is a rhodopsin with a wavelength of maximum absorbance (l max ) at 514 nm. The ellipsosome has very low absorptance (Ͻ0.05) across the measured spectrum (350-750 nm) and probably does not act as a spectral filter. In contrast to all other lampreys studied, the optimized receptor packing, the large width of the ellipsosome-bearing inner segment, together with the presence of a retinal tapetum in the photophobic Mordacia, all represent adaptations for low light vision and optimizing photon capture.

Emmetropization is dependent on visual feedback and presumably some measure of the optical and image quality of the eye. We investigated the effect of simple alterations to image contrast on eye growth and refractive development. A 1.6 cyc/deg square-wave-grating target was located at the end of a 3.3 cm cone, imaged by a +30 D lens and applied monocularly to the eyes of 8-day-old chicks. Eleven different contrast targets were tested: 95, 67, 47.5, 33.5, 24, 17, 12, 8.5, 4.2, 2.1, and 0%. Refractive error (RE), vitreous chamber depth (VC) and axial length (AL) varied with the contrast of the image (RE diff: F(10,86)=12.420, p<0.0005; VC diff: F(10,86)=8.756, p<0.0005; AL diff: F(10,86)=9.240, p<0.0005). Target contrasts 4.2% and lower produced relative myopia (4.2%: RE diff=-7.48+/-2.26 D, p=0.987; 2.1%: RE diff=-7.22+/-2.77 D, p=0.951) of similar amount to that observed in response to a featureless 0% contrast target (RE diff=-9.11+/-4.68 D). For target contrast levels 47.5% and greater isometropia was maintained (95%: RE diff =1.83+/-2.78 D; 67%: RE diff=0.14+/-1.84 D; 47.5%: RE diff=0.25+/-1.82 D). Contrasts in between produced an intermediate amount of myopia (33.5%: RE diff=-2.81+/-1.80 D; 24%: RE diff=-3.45+/-1.64 D; 17%: RE diff=-3.19+/-1.54 D; 12%: RE diff=-4.08+/-3.56 D; 8.5%: RE diff=-4.09+/-3.60 D). We conclude that image contrast provides important visual information for the eye growth control system or that contrast must reach a threshold value for some other emmetropization signal to function.

When visual information is provided at two distances, the target with the lesser incident defocus has the greater influence on the resultant refractive error. Emmetropisation responses are more accurate when information is presented at many distances.