Müller and his student Alfons Pilzecker are credited with the term "consolidirung" (consolidation). This terms means that learning doesn't automatically create permanent, consolidated memories, they take time to be permanently learned or consolidated. To test consolidation Müller used nonsense syllables, introduced by Hermann Ebbinghaus, to see how many trials it was needed to learn the nonsense syllables and fully reproduce them. Müller and Pilzecker quantified the amount of learning by measuring the percentage correct. Some of the participants in their experiment said that occasionally the nonsense syllables would come to mind between training sessions even when they were trying to suppress it, they called this perseveration.
