This study aims to investigate the brain mechanisms underlying trial-and-error problem solving in marmosets. To achieve this, we will develop our original behavioral tasks and measurement techniques specifically suited for marmosets, along with molecular manipulation tools. We will then simultaneously measure neural spike activity in the cerebral cortex and basal ganglia, as well as dopamine release and physiological indicators. By theoretically analyzing these experimental data, we will seek to understand the neural activity dynamics associated with the realization of higher-order brain functions in primates.