Luminous Impulse Response of Positive- and Negative-Contrast Stimuli in Different Spatial Frequencies

An interesting question is whether positive- and negative-contrast stimulations have temporally different visual responses. Luminous impulse response functions (IRF) were measured to investigate this question. The IRF represents a theoretical response to a flash of infinitely short duration. It can be estimated from detection thresholds of double-pulses divided by various inter-stimuli-intervals (ISI). Each threshold was measured independently 5 times included 20∼30 trials each, controlled by a psi method combined with a spatial 4-alternative-forced-choice (4AFC) method. The stimulus, which has a water-wave-like structure in terms of luminance with one of various spatial frequency configurations (0 cpd, 1 cpd, 2 cpd, 4 cpd, 8 cpd, and 16 cpd), was presented on a 10 cd/m2 background (equal-energy-white). About 135,000 trials were measured for five observers. The results show that; (1) thresholds of positive contrast detection were significantly higher than that of negative contrast detection except a part of spatial frequencies on two observers; (2) there was no significant difference between positive and negative contrast IRFs. These results suggest that the responses for positive- and negative-contrast stimulations are the same in terms of IRF.