Effects of stimulus order on duration discrimination sensitivity are under attentional control.

When participants are asked to discriminate between a fixed-magnitude standard stimulus and a variable comparison stimulus, discrimination performance is typically better when the standard precedes rather than follows the comparison. To date it is unclear whether this Type B effect is the sign of an automatic process or whether it is under participants' control. In a series of three experiments, participants compared the duration of two successively presented intervals. At the beginning of each trial, a symbolic cue either indicated the temporal position of the comparison validly or it was neutral with respect to comparison position. With neutral cues, a strong Type B effect was observed in all experiments. With valid cues, however, this effect was reduced. This pattern of results suggests that the Type B effect is, at least in part, under participants' control. Recently, we have shown that the Type B effect might be due to the formation of an internal reference that is dynamically updated from trial to trial. Within this framework, the attention-based reduction of the Type B effect might be explained by a flexible weighting mechanism that regulates the level of integration of remote stimulus information.