The hippocampus is a seahorse-shaped region of the brain (actually, there are two of them) that helps us form new memories from experiences. As with the amygdala, the hippocampus responds to stress. However, with stress, the hippocampaus shrinks. A study with sixteen people, who took an eight-week Mindfulness-Based Stress Response training, found that the hippocampus is sensitive to cortisol, is negatively impacted by stress hormones, and gets smaller with stress. The group of meditators increased the concentration of gray matter in the left hippocampus following the mindfulness training.
The pre-frontal cortex thins with age, contributing to cognitive decline in later years. However, regular and long-term meditators are able to avert and even reverse this change.