Using brain imaging to track problem solving in a complex state space

This paper describes how behavioral and imaging data can be combined with a Hidden Markov Model (HMM) to track participants' trajectories through a complex state space. Participants completed a problem-solving variant of a memory game that involved 625 distinct states, 24 operators, and an astronomical number of paths through the state space. Three sources of information were used for classification purposes. First, an Imperfect Memory Model was used to estimate transition probabilities for the HMM. Second, behavioral data provided information about the timing of different events. Third, multivoxel pattern analysis of the imaging data was used to identify features of the operators. By combining the three sources of information, an HMM algorithm was able to efficiently identify the most probable path that participants took through the state space, achieving over 80% accuracy. These results support the approach as a general methodology for tracking mental states that occur during individual problem-solving episodes.