To determine the linear, unadapted responses of the cone pathways, we have measured the critical fusion frequency (CFF) for green (555-nm) and red (642-nm) flicker as a function of retinal illuminance. Both functions obeyed the Ferry–Porter law (CFF proportional to log illuminance) to high accuracy over a ≥5-log-unit range. In both foveola and periphery the CFF/illuminance functions were significantly steeper for green light than for red light. The peripheral 555-nm function had an average slope 1.26 times the average slope of the 642-nm function. An additive model of flicker detection could not account for the observed differences in slope. A threshold independence model, in which detection is based on the most sensitive mechanism, accurately fits the data. Whichever model is assumed, the presence of different slopes for the two wavelength flicker conditions strongly implies that the R- and G-cone pathways have different temporal properties. The occurrence of steeper CFF/illuminance slopes in response to green light implies that the linear (near-CFF) response of the G-cone pathways is inherently faster than that of the R-cone pathways at both retinal loci. These differences in R- and G-cone-mediated temporal properties complicate the fundamental concept of luminance and invalidate it for precise application over the full illuminance range.
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