Rostral-caudal gradients of abstraction revealed by multi-variate pattern analysis of working memory

The lateral frontal cortex (LFC) is thought to represent contextual and rule-based information that allows adaptive behavior according to circumstance. Recent progress has suggested that the representations of the LFC vary along its rostral-caudal axis with more abstract, higher level representations associated with rostral areas of the LFC and more concrete, lower level representations associated with caudal areas of the LFC. Here, we investigated this proposal. Subjects responded to stimuli based upon a nested series of contextual cues stored in working memory (WM) while being scanned with fMRI. Higher level context cues denoted an abstract rule set while lower level context cues provided more concrete information. Using multi-variate pattern analysis (MVPA), we found varying forms of representation along the rostral-caudal axis of the LFC depending on the type of information stored in WM. Rostral areas of frontal cortex in the lateral orbitofrontal cortex (OFC) represented the higher level context, but not more concrete information, and only when more concrete information was unavailable. Mid-level areas in the mid-dorsolateral prefrontal cortex (DLPFC) and inferior frontal junction (IFJ) represented more concrete rules, but only when the forthcoming response could not be anticipated. By contrast, the dorsal premotor cortex (PMd) and primary motor cortex (M1) represented contextual and response information when the forthcoming response could be anticipated on the basis of context. Collectively, these data indicate that representations dedicated to higher levels of abstraction become less discriminating when more concrete information becomes available. These patterns are consistent with rostral-caudal abstraction proposals of the LFC.