The Nature of the Putative PI4 Mechanism of W. S. Stiles

The candidacy of Stiles's (PI)(,4) field sensitivity spectrum 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 to the ((PI)(,4)) 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 (GREATERTHEQ).5 log units threshold elevation. The possibility that these failures were due to intrusion of the short-wave mechanism ((pi)(,1)) was rejected. Analysis indicates that the failures of field additivity and shape invariance are due to intrusion of the long-wave mechanism ((pi)(,5)).