Behavior in complex environments requires decisions that flexibly combine stimulus representations with context, goals, and memory. Two key aspects of such cognitive flexibility are the retrieval of information from memory when needed, and the ability to selectively utilize relevant information depending on task demands. These aspects, it is believed, depend on the medial frontal cortex (MFC) and the medial temporal lobe (MTL), but it is unclear how these structures interact during memory retrieval. To study this, we simultaneously recorded single neurons in MFC and MTL while human subjects alternated between making memory and categorization-based decisions. In our population analysis, we compared the neural representations of choice, familiarity, and image category across tasks using cross-condition generalization performance. We found a striking difference between brain areas: In the MTL, representations of stimulus familiarity and stimulus category were task-demand independent, whereas in the MFC, representations of stimulus familiarity, stimulus category, and choices were all highly sensitive to task demands. Choice representations in the MFC were also task-specific but not effector specific. Lastly, choices requiring memory retrieval selectively engaged phase-locking between theta-frequency band oscillations in MTL and MFC neurons. This work reveals a mechanism for selectively engaging memory retrieval and shows that, unlike perceptual decision-making, memory-related information is only represented in the medial frontal cortex when choices require it.
1. Demonstrate the importance of studying neural representations across multiple tasks.
2. Introduce cross-task decoding and multi-dimensional scaling as quantitative and qualitative (respectively) measures of generalization performance.
3. Demonstrate task-specific interareal communication between the medial temporal lobe and medial frontal cortex.