DETECTION OF CHROMATIC AND LUMINANCE CONTRAST MODULATION BY THE VISUAL-SYSTEM

The data presented in this paper examine the ability of observers to d etect a modulation in the contrast of chromatic and luminance gratings as a function of the carrier contrast, duration, and spatial frequenc y. The nature of the signal underlying this ability is investigated by examining both the paradigm used to make the measurement and the effe ct of grating masks on performance in the tasks. The results show that observers' ability to discriminate amplitude modulation hom an unmodu lated carrier is dependent on carrier contrast but only up to similar to 5-8 times carrier-detection threshold. Discrimination is, however, independent of spatial frequency [10-1 cycles per degree (cpd) compone nt-frequency range], carrier color, and, most surprisingly, stimulus d uration (1000-30 ms). This set of experiments compliments data from pr evious papers and assimilates many of the conclusions drawn from this previous data. There is absolutely no evidence for the existence of a distortion product mediating performance under any of the current cond itions, and the data seriously question whether the visual system migh t use such a signal even if it does exist under more extreme condition s than those used here. The evidence suggests that the visual system d etects variations in both chromatic and luminance contrast by means of a mechanism operating locally upon the spatial structure of the carri er. (C) 1998 Optical Society of America.