Ongoing task delays affect prospective memory more powerfully than filler task delays

The effect of delay on prospective memory (PM) is mixed. Research has typically shown that PM either decreases or remains unchanged as the time increases between intention formation and encounter with a PM cue. However, the results of one study demonstrated that PM sometimes increases with increasing delays (Hicks, Marsh, & Russell, 2000). Hicks et al. hypothesised that increasing the delay may afford an opportunity for people to spontaneously rehearse the intention, or to be reminded of the intention. In the present work, we tested delays of 6 minutes, 21 minutes, and 36 minutes. Two factors were orthogonally manipulated between-subjects. One was the duration of the filler task that came between intention formation and the beginning of the ongoing task in which PM cues were embedded. The second was the duration of the ongoing task prior to the presentation of the first PM cue. Lengthening the ongoing task delay decreased PM. However, lengthening the filler task nominally increased PM. These results suggest that delays within the ongoing task replicate the effects traditionally found in retrospective memory work. In contrast, delays between intention formation and the beginning of the ongoing task may not have straightforward effects on PM retrieval.