Investigating the sub-regional organization of the prefrontal cortex

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Cookson, Savannah L.
Schumacher, Eric H.
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The prefrontal cortex (PFC) is involved in many cognitive processes important for complex, flexible human behavior (e.g., Duncan & Owen, 2000). Recent research has posited at least two axes of functional organization in PFC: a rostrocaudal axis, along which the PFC processes tasks of varying abstractness or complexity (e.g., Badre 2008); and a dorsoventral axis, along which the PFC handles various modes of task-related information (e.g., Goldman-Rakic, 1995; O’Reilly, 2010; Petrides, 1995). However, it remains unclear how these two axes may interact with one another, as well as with other known organizational principles in PFC (viz., lateralization of motor control). The present experiment aimed to address these questions using a novel “hierarchical precuing” task that combined a traditional cuing procedure with a hierarchical mapping structure in an event-related functional magnetic resonance imaging (fMRI) design. Participants made one of four possible judgments about pairs of stimuli based on simple characteristics shared by the pair. Two judgments related to spatial features of the stimuli (viz., left/right or above/below fixation), and two to nonspatial features (viz., color or shape of stimulus). One spatial judgment and one nonspatial judgment are mapped to each hand. Cues presented at the start of each trial allowed participants to prepare response sets based on whether they received information about the upcoming judgment type, response hand, both, or neither. The cues produced a stair-step effect on reaction time as a function of the amount of information presented a priori; that is, as the amount of information contained in the cue increased, reaction time decreased, regardless of the exact information contained in the cue. The fMRI data demonstrated segregation of activity in PFC at the cue time point for the main effects of each factor: a rostrocaudal distribution as a function of cue content; a dorsoventral distribution according to judgment domain; and lateralization of activity as a function of response hand. We then investigated how different combinations of cue content, processing domain, and response hand interact to influence the distribution of activity within these regions of interest (ROIs). These results demonstrate how the functional structure of the PFC integrates these different axes across the cortex and validate the hierarchical precuing task as a procedure for integrating multiple cognitive factors into a single event-related task design.
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