The loop has three inspectable roles

The source in context.

Eugene Pauly’s severe memory loss creates the chapter’s central contradiction. He repeats questions and cannot explain parts of his surroundings, yet he can find the kitchen and complete a familiar walk. Duhigg pauses that human story to examine laboratory work and introduce cue, routine and reward. He then returns to Eugene with the model in hand. Learning a correct choice between paired objects suggests one capability; presenting the objects differently reveals its limits. Everyday driving and repeated commercial cues extend the idea beyond the unusual patient. The closing family scenes preserve Eugene as a person whose experience matters beyond the explanatory use the chapter makes of it.

What this example supports

Cue, routine and reward become repeated questions readers can ask of a case. The model earns its name by helping them locate something, compare something and eventually change something.

Its limit

These are the book’s explanatory mappings. Identical labels do not establish identical mechanisms across animal learning, consumer behavior and clinical cases. The book itself warns that drawing a loop is not sufficient treatment.