The brain sends signals via neural connections called synapses. In young children, synapses develop rapidly as they are constantly learning and remembering new information. Important connections are nurtured and reinforced, whilst unused connections are deemed unnecessary and pruned away. Brain plasticity is dictated by the number of neurons the brain is actively maintaining.
Adult brains continue pruning and form synapse connections at a slower rate, but it is unclear how or why synapses in the adult brain get eliminated. Psychological disorders are genuinely destructive, with Parkinson’s disease, Alzheimer’s disease, schizophrenia, depression, bipolar disorder, and alcoholism all limiting or actively destroying grey matter.
One major activity that promotes synapse growth is meditation and relaxation. There are two types of meditation that studies have shown increase brain plasticity: FA (focused attention), and OM (open monitoring). For more information on the psycho-restorative abilities of these meditation forms, see the attached PDF.