A gamma wave or gamma rhythm is a pattern of neural oscillation in humans with a frequency between 25 and 140 Hz, the 40 Hz point being of particular interest. Gamma rhythms are correlated with large scale brain network activity and cognitive phenomena such as working memory, attention, and perceptual grouping, and can be increased in amplitude via meditation or neurostimulation. Altered gamma activity has been observed in many mood and cognitive disorders such as Alzheimer's disease, epilepsy, and schizophrenia. Discovery Gamma waves can be detected by electroencephalography or magnetoencephalography. One of the earliest reports of gamma wave activity was recorded from the visual cortex of awake monkeys. Subsequently, significant research activity has concentrated on gamma activity in visual cortex. Gamma activity has also been detected and studied across premotor, parietal, temporal, and frontal cortical regions Gamma waves constitute a common class of oscillatory activity in neurons belonging to the cortico-basal ganglia-thalamo-cortical loop. Typically, this activity is understood to reflect feedforward connections between distinct brain regions, in contrast to alpha wave feedback across the same regions. Gamma oscillations have also been shown to correlate with the firing of single neurons, mostly inhibitory neurons, during all states of the wake-sleep cycle. Gamma wave activity is most prominent during alert, attentive wakefulness. However, the mechanisms and substrates by which gamma activity may help to generate different states of consciousness remain unknown. Controversy Some researchers contest the validity or meaningfulness of gamma wave activity detected by scalp EEG, because the frequency band of gamma waves overlaps with the electromyographic frequency band. Thus, gamma signal recordings could be contaminated by muscle activity. Studies utilizing local muscle paralysis techniques have confirmed that EEG recordings do contain EMG signal, and these signals can be traced to local motor dynamics such as saccade rate or other motor actions involving the head. Advances in signal processing and separation, such as the application of independent component analysis or other techniques based on spatial filtering, have been proposed to reduce the presence of EMG artifacts. In at least some EEG textbooks, we are instructed to put an electrode on an eyelid to catch these, as well as 1 on the heart, & a pair on the sides of the neck, to catch muscle-signal from the body below the neck. Clinical EEG may not do these things. Function Conscious perception Gamma waves may participate in the formation of coherent, unified perception,