Having an overactive checklist system could lead to some of the symptoms displayed by people who suffer from damage to the frontal cortex. If the brain thinks actions are done and doesn’t have the urge to do them, apathy can result. Such lack of drive can be a major symptom of frontal lobe dysfunction. At the other extreme, if the brain lacks its checkmark system, behavior may get repeated over and over again. Perseverative behaviors are a classic symptom of frontal lobe damage. It is as though the “it’s done” signal is missing. Repetitive behaviors and thoughts are also features of obsessive compulsive disorder and a range of related disorders that affect both children and adults.

This work also suggests that the prefrontal cortex-basal ganglia brain regions from which the researchers recorded help “package” the individual parts of sequential behaviors into larger chunks. The same nerve cells that make the checkmark also accentuate their activity at the beginning of a movement sequence. A defect in this system may underlie some of the symptoms of Parkinson’s disease, in which the patient has to think out each part of a sequence, even a simple movement sequence like standing up from a chair.