Chromaticity coordinates of monochromatic lights were obtained 8 deg extrafoveally at a retinal illumination of 50 photopic td during the cone-plateau period and also with the subject in a dark-adapted state, while size and exposure time of the test field were varied. Unexpectedly, we found that for the dark-adapted condition the points representing the chromaticity matches of the spectral lights generally moved away from the achromatic point in the chromaticity diagram when size or exposure time of the test stimulus was increased. Furthermore, the chromaticity shift toward the achromatic point, obtained between the cone-plateau period and the dark-adapted state (i.e., with rod intrusion), tended to decrease with these two test parameters. In fact, when size and exposure time both were at the maximum level investigated (7 deg, 500 ms), there was no measurable shift in chromaticity with rod intrusion. Our results suggest that the cone system may become progressively more effective in suppressing the rod system as an effect of both size and exposure time.
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