Parameters of visual simultaneity and of metacontrast and paracontrast of successively presented letters

Two stimuli of three consonants each were successively presented through a tachistoscope. Results show that the shorter the totr 1 duration of S₁, + ISI + S₂, the more often the two stimuli are perceived as simultaneous. For a given total duration, the perception of simultaneity depends on the ratio of durations S₁/S₂, according to a nonmonotonic function. The optimum is reached for a ratio higher than one. When the S₁/S₂ ratio increases from 0.25 to 4, metacontrast decreases and paracontrast increases. These two phenomena are balanced for a value of the S₁/S₂ ratio which corresponds to the optimum of the perception of simultaneity.