He found that mice with a defect in their CREB activator gene were virtually incapable of forming long-term memories. They were amnesiac mice. But even these forgetful mice could learn a bit if they had short lessons with rest in between. Scientists theorize that we have a fixed amount of CREB activator in the brain that can limit the amount we can learn in any specific time. If we try to cram before a test, it means that we quickly exhaust the amount of CREB activators, and hence we cannot learn any more--at least until we take a break to replenish the CREB activators.