The default mode network (DMN) is a brain circuit which dominates brain activity when a person is least engaged with the outside world (such as being a passive observer).  Previous studies have found that a loss of coordination in the DMN is a common symptom of aging and in extreme cases can be a marker of disease, and data has suggested that older adults who are more fit tend to have better connectivity in specific regions of the DMN than their sedentary peers. Arthur Kramer, from the University of Illinois (Illinois, USA), and colleagues followed 65 adults, ages 59 to 80 years, who were sedentary (two or fewer episodes of physical activity lasting 30 minutes or more in the previous six months), who joined a walking group or stretching and toning group for a year. The researchers measured participants’ brain connectivity and performance on cognitive tasks at the beginning of the study, at six months and after a year of either walking or toning and stretching. The team employed functional magnetic resonance imaging (fMRI) to determine whether aerobic activity increased connectivity in the DMN or other brain networks.  At the end of the year, DMN connectivity was significantly improved in the brains of the older walkers, but not in the stretching and toning group. Additionally, the walkers also had increased connectivity in parts of the fronto-executive network, which aids in the performance of complex tasks; and they performed significantly better on cognitive tests than their toning and stretching peers. The researchers write that: “The study provides the first evidence for exercise-induced functional plasticity in large-scale brain systems in the aging brain …  and offers new insight into the role of aerobic fitness in attenuating age-related brain dysfunction.”