Conjunction or co-activation? A multi-level MVPA approach to task set representations

While representing and maintaining rules in order to govern behavior is a critical function of the brain, it remains an open question as to how collections of rules - task sets - are represented in cortex. One possibility is that task sets are represented as the co-activation of representations of the simple rules from which a task set is composed. Alternatively, task sets could be encoded in a conjunctive manner as the unique combination of rules that belong to a task set. Using a novel multi-level MVPA approach in combination with fMRI, we attempted to answer both “where” and “how” task sets are represented in the brain. Subjects performed a delayed match-to-sample task using task sets composed of multiple, partially overlapping rules that governed which feature dimensions subjects should attend to, and MVPA was used to identify regions that encoded task set information. We identified voxels most relevant for classifying task sets, and, using these voxels as input to a second MVPA analysis, were able to identify regions in prefrontal cortex with activity consistent with co-active representation, while activity in visual cortex was consistent with conjunctive representation. These results highlight the utility of feature selection methods in neuroimaging analyses.