When you daydream, or ruminate on something bothersome, or ponder the past, or plan for the future, the part of your brain most engaged is the default mode network, or DMN, which includes part of the prefrontal cortex. Scientists have long hypothesized that changes to DMN dynamics play major roles in certain behaviors, such as those associated with attention deficit-hyperactivity disorder; and diseases, such as Alzheimer’s and Parkinson’s; and conditions such as depression and autism.