Abstract

The candidacy of π₄ for the middle-wave cone fundamental was evaluated with the tests of shape invariance and field additivity for two observers. Field additivity was rejected: addend field sensitivities obtained in the presence of 649-nm fields displayed superadditivities near 460 nm when compared with the (π₄) field sensitivities obtained without an auxiliary background. Similar findings were made when a brief duration (10-msec) test flash was used. Shape invariance was also rejected for one of the two observers for 461- and 649-nm fields. The failure of field additivity was further investigated with bichromatic mixture curves for 461- and 649-nm fields. Superadditivities were consistently found for mixture fields causing ≥ 0.5 log units threshold elevation. The possibility that these failures were due to intrusion of the shortwave mechanism (π₁) was rejected. Analysis indicates that the failures of field additivity and shape invariance are due to intrusion of the longwave mechanism (π₅).

© 1985 Optical Society of America