Visual psychophysical methods

Imagine staring at a blank computer screen and hearing two beeps, one after the other. Each beep designates a time. A very faint picture (the stimulus) was displayed during one of the two beeps, randomly. You have to decide at which time the picture was shown. All of this constitutes one trial of a two-alternative forced-choice (2AFC) detection experiment. It is called 2AFC because you have been presented with 2 intervals and you have to choose one. This task is called “detection” because the other interval was blank. If you had your eyes closed, your expected response accuracy, given the two possible choices, would be 1/2 or 0.5. Response accuracy could be as high as 1, but you might still occasionally press the wrong button, perhaps because you were distracted or you blinked, so, in practice, maximum accuracy is typically closer to 0.99.