Increment and decrement contrast thresholds have been measured for stimuli 15 min to 4.3° diam and repeated durations of 20 to 1000 msec. The fixation point was 7° from the fovea. The increment threshold was consistently greater than decrement threshold.
A. S. PATEL and R. W. JONES, "Increment and Decrement Visual Thresholds," J. Opt. Soc. Am. 58, 696-699 (1968)