It is clear that Ricco's area has a very strong neural component because it is highly affected by factors such as stimulus duration and background adaptation. However, it is also apparent that optical factors may make a contribution, primarily because of widening of the point-spread-function (PSF) and forward light scatter. By comparing the performance of live subjects and an ideal observer model, Davila and Geisler
7 demonstrated that optical factors can account for most, if not all, of the phenomenon of Ricco's area in the fovea. However, in a recent adaptive optics study in young subjects, Dalimier and Dainty
22 showed that though the foveal Ricco's area estimate is diminished in size, it is not eliminated by the correction of optical aberrations; an area of complete spatial summation is still observed, demonstrating that Ricco's area has a neural component. The low-pass effect of young optics on the smallest stimuli would result in a steepening of the left limb of the spatial summation function. When a subsequent two-phase regression model is fitted to these data and the slope of the first line is constrained to −1, this will result in an
apparently larger Ricco's area without adaptive optics. In the peripheral retina, as tested in the experiments described here, however, in which Ricco's area and the stimuli to measure it are much larger, any low-pass effect of optics would be much smaller. Artal et al.
56 measured the change in the optical modulation transfer function (MTF) with age and found the greatest deterioration at low spatial frequencies. It would follow, therefore, that, in the present study, thresholds for our larger stimuli should be disproportionately affected by aging optics, resulting in an effect opposite to that shown by Dalimer and Dainty, with an apparent decrease in Ricco's area. As a control experiment, Schefrin et al.
8 modeled the effects of increased intraocular straylight and reduced MTF on their spatial summation data. They concluded that though these factors caused an overall elevation in stimulus detection threshold, neither had a significant effect on the Ricco's area estimate in their model. Although this kind of modeling at the
retinal level might lead one to conclude that there is no effect of reduced optical quality on Ricco's area, the same cannot be concluded from our data, for which spatial summation curves were plotted using measurements of stimulus size from the monitor. Considering an enlarged PSF resulting from increased wide-angle scatter, one might appreciate that because the spatial extent is larger, Ricco's area is “filled” sooner than if it were not affected. The amount of luminous flux within this area would govern threshold. If this is correct, one might expect an
apparent reduction in Ricco's area under such conditions. In the present study, it is reasonable to suggest therefore that an increased Ricco's area is masked by an artifactual decrease as a result of age-related optical change.