Inter-module credit assignment in modular reinforcement learning.

Critical issues in modular or hierarchical reinforcement learning (RL) are (i) how to decompose a task into sub-tasks, (ii) how to achieve independence of learning of sub-tasks, and (iii) how to assure optimality of the composite policy for the entire task. The second and last requirements are often under trade-off. We propose a method for propagating the reward for the entire task achievement between modules. This is done in the form of a 'modular reward', which is calculated from the temporal difference of the module gating signal and the value of the succeeding module. We implement modular reward for a multiple model-based reinforcement learning (MMRL) architecture and show its effectiveness in simulations of a pursuit task with hidden states and a continuous-time non-linear control task.