What is Clayton Cameron referring to with the pun "a-rhythm-etic?

Limb was on a quest to understand exactly what was happening inthe brain while he was using the instinctual thought process in hisown brain. So after studying several musicians, he wasshow that the area of the brain that is connected to observation andself-monitoring is the activated when musicians are using theirbrain to improvise. His study also showed that the area of the brain

connected with self-expression became extremely active when able to being creative in instrument play.