Introduction

One of the most remarkable achievements of the human visual system is the capacity to resolve fine detail in the retinal image and efficiently to detect contrast between neighbouring regions of the image. In the central visual field these perceptual abilities appear to be limited by the physical properties of the photoreceptors themselves. The development of spatial vision provides a fine example of the way in which the efficiency of coding in the visual system emerges through an interplay between innate (presumably genetically determined) organization and plasticity of synaptic organization at the level of the visual cortex. As Barlow (1972) pointed out, developmental plasticity might allow the visual cortex to discover, in the pattern of stimulation it receives, important associations and coincidences in the retinal image that relate to the nature of the visual world.

Efficiency of spatial vision in the adult

Factors that might limit spatial vision

The resolution of spatial detail and the detection of contrast in the retinal image might, in principle, be limited by one of a number of factors. Obviously, the optical quality of the image could determine spatial performance and certainly does so in states of refractive error. Even when the eye is accurately focused, chromatic and spherical aberration degrade the image, as does the effect of diffraction, which is dependent on the size of the pupil. Interestingly, under photopic conditions, the pupil of the human eye tends to adopt a diameter that optimizes visual acuity: a larger pupil size would augment the effects of aberrations and a smaller one would increase diffraction, as well as decreasing retinal illumination (Campbell & Gregory, 1960).