Measuring context in human prefrontal cortex

Human behavior is dependent on flexibility whenever response alternatives are available. For instance, finishing a work project to meet a deadline may require a delay of more pleasant activities such as reading a favorite book. The flexibility to choose working on the project over reading a book is determined by context: the circumstances in which a response is selected (e.g., a project deadline). The aim of the present research was to identify how context is represented as neural activity, a question that is relatively undefined in the human brain. The approach I adopted to study this question was based on the Guided Activation Theory of cognitive control, which posits that activation in the prefrontal cortex (PFC) represents abstractions of context, serving to bias activity in posterior and sub-cortical regions responsible for executing a behavior. I conducted three studies, guided by this framework. In the first study I used electroencephalography to measure the onset of context updating. Voltage recorded at the surface of the brain showed a positive deflection at 180--200 ms, related to context and localized to PFC, confirming that this region is involved in context processing. In the second study I developed a behavioral index to assess context retrieval when anticipating response selection. This index was designed to maximize task specificity, the ability to differentiate context among similar tasks and to dissociate it from non-contextual cognitive processes. Finally, in the third study I used this behavioral index and functional magnetic resonance imaging to identify neural representations corresponding to each of four tasks. These representations included task specific regions in both posterior cortex and PFC. The posterior activations were consistent with what is known about the neural substrates involved in performing each task whereas the PFC regions were consistent with anatomical connections between these regions and PFC. The results of this research provide comprehensive evidence consistent with a role of PFC in representation of context, thus consistent with the Guided Activation Theory, and highlight that an exciting avenue for future research will be to address how interactions between regions implement vs. represent context.