NO EFFECT OF AGE ON SPATIAL INTERVAL DISCRIMINATION AS A FUNCTION OF ECCENTRICITY OR SEPARATION

Purpose. Performance on positional acuity tasks exhibits marked resist ance to the effects of optical image degradation. This places position al acuity tasks in a unique position for psychophysical examination of the effects of age upon visual performance because any observed age-r elated changes reflect losses in retinal/neural function. One position al acuity task with important consequences for ''real-world'' vision i s spatial interval discrimination, a task in which the subject is requ ired to detect changes in the size of the gap between two objects. In the present study we examine spatial interval discrimination in young and elderly observers as a function of separation and eccentricity. Me thods, Stimuli were two Gaussian-modulated luminance patches placed si de by side around an imaginary iso-eccentric are in the upper visual f ield, allowing eccentricity and separation to be varied independently. Changes in separation were achieved by moving the stimuli around the are, while eccentricity was varied by changing the radius of the are. Thresholds were obtained for healthy young and elderly observers using a forced-choice method of constants at two eccentricities (1.25 degre es and 10 degrees) and five separations. Results. When thresholds, exp ressed as Weber fractions, are plotted as a function of the geometric ratio of the stimuli (separation/eccentricity) the data from the young and elderly groups collapse to a single function. Performance is inde pendent of age and eccentricity, and depends only on the geometric rat io of the stimuli. Conclusions. No effect of age was found for spatial interval discrimination. Our results suggest that spatial interval di scrimination belongs to the wider group of positional acuities whose n eural substrates are unaffected by ageing.