Table 1.

Cone absolute threshold, i.e. the threshold number of photons at the cornea when using the most lenient response criterion (middle column). Thresholds were interpolated at 50% probability of detection after adjusting for the non-zero false positive rate with Equation 1. The average threshold for Marriott's (1963) nine subjects, who used strict response criteria, is also shown for comparison. Values for subject 1 reflect measurements on three different days. Data for subject 4 are not included because only relative stimulus intensity information was available. To facilitate comparison across different subjects, who may have employed different internal criteria, we also report predicted 2AFC thresholds derived from empirical rating scale data using the method described by Sakitt (1972). The probability of correctly detecting stimuli in the 2AFC task is equivalent to the area under the receiver operating characteristic curve (Green & Swets, 1966), here we predict 2AFC threshold at 75% correct, roughly equivalent to the stimulus intensity with d' of 1 (Gescheider, 1997).

	Cone Thresholds (photons at the cornea)	Predicted 2AFC Thresholds (photons at the cornea)
Subject 1	212 ± 21	220 ± 15
Subject 2	169	168
Subject 3	252	250
Subject 5	178	213
Average	203 ± 38	213 ± 34
Marriott	606 ± 126	n/a