Abstract

Observers’ ability to combine intensity information over time was measured by testing accuracy in time-average intensity judgments of periodic time-varying intensity stimuli (frame sequences). The subjects’ task was to choose the highest time-average intensity stimulus (target) in a six-alternative forced-choice experiment. The intensities for each frame for each of the non-targets were assigned by randomly sampling values from a Gaussian sampling distribution (non-target sampling distribution). The same procedure was used to generate the target intensity values but by using a Gaussian sampling distribution with a higher mean than the non-target sampling distribution. Performance was measured for sequences of 4, 16, 32, and 50 frames, all displayed at 35 frames per second. For an ideal observer who perfectly integrates the intensity information, d' increases as the square root of the number of frames in the frame sequence. Our results show that the human visual system is able to integrate intensity information for a critical period, t_c, of 700–900 ms. Such findings cannot be accounted for by the well studied Bloch’s Law, which has a critical period about 200–300 ms. This study suggests the existence of another temporal intensity integration mechanism distinct from the mechanism described by Bloch’s Law.

© 1992 Optical Society of America