A temporal, two-alternative, forced-choice (2-AFC) technique, with feedback, was used to minimize subjective test bias: the interactive staircase procedure was driven by the subject’s responses and controlled by computer. Each trial consisted of two presentations (denoted by auditory tones). In experiment 1, for the detection task, the stimulus was a Gabor oriented at 70° or 110° (chosen at random) paired with a screen of the same average luminance. The subject had to choose which interval contained the Gabor (the contrast detection threshold). For the discrimination task, a similar procedure was used, but this time the Gabor oriented at 70° was paired with the Gabor oriented at 110°. The subject had to choose which interval contained the Gabor oriented at 70° (the orientation threshold). In experiment 2, a similar technique was used to measure the detection and discrimination of Gabor stimuli of opposite contrast polarity (180° phase difference). Presentation time was limited (S t = 250 ms), in an attempt to minimize the influence on thresholds of any saccadic eye movements. Immediately after a response had been made, the next two-interval trial started, stimulus contrast being the variable between trials. The average of six reversals of the staircase constituted one mean. Each data point displayed in the figures comprises the arithmetical average of at least three means. The standard deviation was equal to the symbol size and never greater than twice the symbol size.