Effect of localized defocus on detection thresholds for different sized targets in the fovea and periphery

Purpose: Previous studies of optical blur in perimetry have measured the ef fect of foveal refractive error on peripheral perimetric detection threshol ds. Since peripheral refractive error can be significantly different from t hat of the fovea we wished to remove the ambiguity of previous results by c orrecting the actual peripheral refractive error first before adding blur. Methods: We measured detection thresholds in the fovea and at 30 degrees in the horizontal temporal field in two trained observers. Peripheral refract ive error was determined at each location and thresholds measured at the sa me locations for stimuli ranging in size from 0.2 to 6.4 degrees and refrac tive errors between +/-4.00 diopters, Results: Foveal thresholds increased immediately with increasing refractive error, particularly for smaller stimulus sizes. At 30 degrees, thresholds for smaller stimuli were less affected by defocus initially and then increa sed more sharply. Larger stimuli were relatively unaffected by defocus such that when stimulus size reached 1.6 degrees there was Little or no increas e in threshold for refractive error between +/-4.00 diopters, Conclusions: Peripheral refractive error, largely forgotten by perimetrists , has a significant effect on performance, particularly for smaller stimuli . Differences in foveal vs peripheral viewing can be explained by differenc es in ganglion cell receptive field sizes.