Performing a nonfocal prospective memory (PM) task results in a cost to ongoing task processing, but the precise nature of the monitoring processes involved remains unclear. We investigated whether target context specification (i.e., explicitly associating the PM target with a subset of ongoing stimuli) can trigger trial-by-trial changes in task interference according to stimulus relevance for the nonfocal PM task. Participants performed a lexical decision task in which a PM task (press F6 when a target syllable appeared) was embedded. The target syllable always occurred in word trials, but we manipulated participants' expectations regarding the target context by instructing them that targets would occur in words only (specific condition) or in both words and nonwords (nonspecific condition). A control condition with no PM demands was also included. Although having a PM task led to noticeable slowing on the ongoing task, specifying the PM target context reduced cost to items irrelevant to the intention (nonwords) while leaving PM performance intact. Moreover, higher cost for nonwords in the nonspecific than specific condition was persistent across the ongoing task even though the target syllable was repeatedly presented in words. These results suggest that stimulus processing can be modulated according to participants' expectations about the lexical properties of the target, with trial-by-trial changes in task interference as a function of stimulus relevance to a nonfocal intention observed as a consequence.

Prospective memory (PM) research has often investigated if having an intention interferes with ongoing activities, but rarely by linking the intention to a particular context. We examined effects of trial-by-trial changes in whether the context (defined by colour) was relevant for the nonfocal PM task. The ongoing task involved speeded decisions about the position (left/right) of the upper-case letter in a pair, and the PM task consisted of pressing an additional key if the upper-case and lower-case letters were in a specified colour and the same letter. Trials switched between two colours either randomly or predictably in eight-trial blocks. We also manipulated the presence/absence of occasional same-letter pairs in the irrelevant context. Results showed higher cost of having a nonfocal PM task when ongoing stimuli matched than when they mismatched the target's colour. Moreover, cost for intention-irrelevant stimuli was minimized, though never eliminated, by blocking match/mismatch trials. These findings highlight the role that local changes in intention-related context play in task interference and support a view of monitoring as a flexible mechanism. Additionally, the study introduced a novel way of embedding intention-related events in the irrelevant context shortly before the occurrence of PM targets, with results tentatively suggesting that such events might impair target detection.

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