Functional magnetic resonance imaging was used to examine the maintenance of intentions during an ongoing task involving implicit cues. Participants were required to detect target words while engaging in the ongoing task. Cues matched to the target category and cues matched to the action for targets were presented implicitly during the ongoing task. Implicit categorical target cues were found to enhance prospective memory performance, and implicit action cues accelerated responses more than irrelevant implicit cues in the prospective memory task. We found increased caudal anterior cingulate cortex (cACC) activity during the ongoing task with both implicit target and action cues, suggesting that alertness or preparatory attentional processes were strengthened by implicit cues. Implicit target cues elicited enhanced activity in the lateral anterior prefrontal cortex (Brodmann's area 10), which may be involved in dual processing of the ongoing task and the maintained intention. These results suggest that implicit cues may boost primitive attention toward an intended action and may be accessible to off-line maintenance. The facilitation of prospective target detection and the action execution by target cues were associated with increased supplementary motor area (SMA) activity. In contrast, we found enhanced premotor cortex (PMC) activation with action cue during the prospective memory task, suggesting that the PMC may be involved in stimulus-induced and rather automatic action. These results demonstrate the effectiveness of implicit cues for examining the maintenance of intention. Overall, we found that implicit target cues can facilitate prospective memory performance, and implicit action cues can accelerate responses. The above-mentioned neural underpinnings suggest that attentional and motor control processes are involved in the enhancement of prospective memory.
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