title: Why is the retina capable of resolving finer detail than the eye's optical and neural systems?
abstract: 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.
authors.affiliation: Defence Research Agency, W4 Division, Fort Halstead, Sevenoaks, Kent, UK.
authors.forename: N D
authors.initials: ND
authors.lastname: Haig
mesh_terms: 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