Why is the retina capable of resolving finer detail than the eye's optical and neural systems?
It is established that the resolving power of the eye accords with that predicted by diffraction theory. Campbell and Green (J. Physiol. 181, 576-593, 1965) and Campbell and Gubisch (J. Physiol. 186, 558-578, 1966) demonstrated that the eye exhibits nearly diffraction-limited performance at 2 mm pupil diameter, resolving up to approximately 60 cyc deg-1. However, Williams (Vision Res. 25, 195-205, 1985) has demonstrated that foveal receptors can respond at up to 200 cyc deg-1, in agreement with measured receptor separation (Hirsch and Curcio, Vision Res. 29, 1095-1101, 1989). The obvious question is: why is there such an apparent mismatch between the eye's optics and receptors? It is shown that the change in optical energy distribution corresponding to 1 just noticeable difference of defocus has the form of a difference of Gaussians, for which the contrast sensitivity function peaks at 16 cyc deg-1, with a bandwidth from 0 to approximately 70 cyc deg-1. The reason for the even higher-density packing of receptors, in the fovea, appears to be the need to allow the different colour-response cones to be spatially inhomogeneous without excessive aliasing. Prompted by this analysis, some data of Williams (1985) are re-examined, leading to a mapping of the averaged aperture-sensitivity of foveal cones, and demonstrating the reason for the diffuse cutoff of the postulated defocus channel.
Haig
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ND
Defence Research Agency, W4 Division, Fort Halstead, Sevenoaks, Kent, UK.
D015350:Contrast Sensitivity / Q000502:physiology*; D006801:Humans; D009424:Nervous System Physiological Phenomena; D016011:Normal Distribution; D009799:Ocular Physiological Phenomena; D012160:Retina / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology