Abstract

Flash-on-flash thresholds for sinusoidal targets were measured in the dark-adapted eye. The stimuli were increment-Gabor patches (the sum of a Gaussian-damped sine wave and a simple Gaussian of equal amplitude) presented for 50 ms at the onset of a 500-ms flashed background. The Gabor stimuli had a bandwidth of 0.5 octaves, with center frequencies of 1, 2, 4, 8, and 12 c/deg. For all spatial frequencies, the slopes of the increment-threshold functions increased continuously with flashed-background intensity, reaching values greater than 2.0 at 10,000 td. In the dark, the thresholds for the different spatial frequencies were spread out over a range of 1.0 log unit, with the higher spatial frequencies yielding the higher thresholds (i.e., the CSF fell off steeply with spatial frequency). However, the spread of the thresholds decreased as the flashed-background intensity increased; above 100 trolands the CSFs were nearly flat over the range of spatial frequencies tested. These results suggest that different mechanisms mediate detection at low and high flash intensities.

© 1992 Optical Society of America