A gamma wave is a pattern of neural oscillation in humans with a frequency between 25 to 100 Hz though 40 Hz is typical.
Experiments on Tibetan Buddhist monks have shown a correlation between transcendental mental states and gamma waves.
A 2004 study took eight long-term Tibetan Buddhist practitioners of meditation and, using electrodes, monitored the patterns of electrical activity produced by their brains as they meditated. The researchers compared the brain activity of the monks to a group of novice meditators (the study had these subjects meditate an hour a day for one week prior to empirical observation). In a normal meditation state, both groups were shown to have similar brain activity. However, when the monks were told to generate an objective feeling of compassion during meditation, their brain activity began to fire in a rhythmic, coherent manner, suggesting neuronal structures where firing in harmony.
This was observed at a frequency of 25-40 Hz, the rhythm of gamma waves. These gamma-band oscillations in the monks' brain signals were the largest seen in humans. Conversely, these gamma-band oscillations were scant in novice meditators. Though, a number of rhythmic signals did appear to strengthen in beginner meditators with further experience in the exercise, implying that the aptitude for one to produce gamma-band rhythm is trainable.