There are many causes for variation in the responses of the motor apparatus to neural commands. Fast-timescale disturbances occur when muscles fatigue. Slow-timescale disturbances occur when muscles are damaged or when limb dynamics change as a result of development. To maintain performance, motor commands need to adapt. Computing the best adaptation in response to any performance error results in a credit assignment problem: which timescale is responsible for this disturbance? Here we show that a Bayesian solution to this problem accounts for numerous behaviors of animals during both short- and long-term training. Our analysis focused on characteristics of the oculomotor system during learning, including the effects of time passage. However, we suggest that learning and memory in other paradigms, such as reach adaptation, adaptation of visual neurons and retrieval of declarative memories, largely follow similar rules.