Thanks for bringing this 1972 study to my attention! It's interesting that they noticed the impact of "complicated motor planning" on stuttering. Here's another study discussing the brain activation of people who stutter compared to controls. They found that the left inferior frontal gyrus (speech production) connection to the supplementary motor area (speech planning) is under-activated compared to controls. [https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0179255](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0179255)