Neural correlates of preparatory attentional control: Relationship to behavior and processing of conflict stimuli

Top-down control is required when subjects must selectively attend to one stimulus feature in the presence of conflicting information. The neural basis of attentional control is hypothesized to involve modulation of regions processing task-relevant features (sites) by frontal and parietal control regions (sources). Given that reaction time (RT) on attentional tasks is improved when advance information is provided, it is surprising that few studies have directly examined preparatory activity at the sources and sites of control in relation to RT. Using ERPs and fMRI, we examined the relationship between RT to conflicting spatial Stroop stimuli and neural activity measured prior to target presentation. Subjects received informative cues identifying whether the spatial location or meaning of upcoming words was task-relevant, allowing preparatory implementation of feature biasing. In the ERP study, when neural activity was segregated according to RT variability within each subject, pre-stimulus right frontal and bilateral central-parietal increases were associated with faster RT. On word meaning trials, those subjects exhibiting more pre-stimulus right frontal activity showed greater post-stimulus left inferior temporal activity, suggesting that preparatory biasing at the sites of verbal processing (left temporal cortex) by right frontal cortex results in an enhancement of processing once the target arrives. For the fMRI study, subjects additionally received advance cues that served to increase general anticipatory processes but were neutral regarding the task-relevant feature. Results comparing these cue conditions again revealed that preparatory right prefrontal activity was related to faster RT, this time on a between-subjects basis and when the influence of general anticipation was minimized. For word meaning trials only, semantic selection was associated with greater activity in the left inferior frontal gyrus. Additionally, we found preparatory modulation of both task-relevant and task-irrelevant information such that subjects with greater pre-stimulus temporal activity exhibited faster RT on word trials but slower RT on position trials. These studies are the first to demonstrate that increased activation of both the sources and sites of control during a preparatory period is associated with improved selection of task-relevant information.