Illusory reversal of temporal order: the bias to report a dimmer stimulus as the first.

When two objects are presented in rapid succession, observers find it difficult to discriminate their temporal order. Below certain limit (e.g., 20-70 ms), the rate of correct temporal order judgement is reported to be about 50% (i.e., close to chance level). However we have found stimulus conditions where order discrimination drops significantly below chance level: the stimulus that is presented as the second is reported as the first. It is necessary that the stimuli are very brief, spatially overlapping, clear-cut backward and forward masking is absent, stimulus onset asynchronies are very short, and luminance contrast of the following stimulus is considerably lower than luminance contrast of the first stimulus. The higher the contrast ratio, the stronger the order reversal effect. However, because also in the conditions where the two stimuli are presented synchronously, the dimmer target is perceived as the first, the effect should be attributed to some implicit bias which enforces subjects to regard a more contrasted stimulus as the one that appears subsequent to the less contrasted stimulus.