Gamma rhythms (>30 Hz) have low energy and are difficult to record. Special measures must be taken to avoid 50- and 60-Hz artifacts as well as muscle artifacts. In animal experiments the synchrony between neuronal elements at 40 Hz has been proposed as a special mechanism of neural cooperation, called temporal binding. This temporal coordination in spiking spatially distributed neurons is needed to glue together spatially distributed representations of the image into a single percept.

In humans, scalp EEG recordings consistently reveal the existence of synchronized oscillatory activity in the gamma range when subjects experience a coherent visual percept. Naive subjects perceived such stimuli as meaningless blobs. However, when the subjects were trained to detect the Dalmatian dog that was hidden in the pictures those now meaningful dotted pictures induced gamma activity at 280 ms after stimulus onset at occipital EEG recordings. A plausible interpretation of these findings is that objects giving rise to a coherent percept recruit visual areas that are synchronized in the gamma range.

The alert brain–mind is characterized by a low-voltage fast EEG; a gamma rhythm of 40–60 c s⁻¹ can be seen. This rhythm is generated by the activated and synchronized neuronal firing in widespread cortical circuits. Drowsiness is characterized by EEG slowing in the 4–8 c s⁻¹ theta range. When thalamocortical circuits escape from brainstem control, stereotyped clusters of 12–15 c s⁻¹ spindles dominate the cortical EEG and sleep proper (stage II) commences. Gamma rhythms are not the only important oscillations in the brain; for purely historical reasons, gamma has come to mean a wide, high band of oscillations from perhaps 30 to 120 Hz and sometimes higher. This is an accident of scientific history, simply because fast oscillations were more difficult to record at one time, in part because they show up better with implanted electrodes. It is clear, however, that slower rhythms play fundamental roles.

Theta seems to ‘carry’ gamma oscillations in much the way an AM radio frequency carries a voice signal. Theta is believed to play a key role in episodic memory and frontal lobe activities. Theta is one of the basic carrier frequencies of the hippocampus, observable during both episodic memory encoding and recall. Theta is believed to enable the coding and decoding of hippocampal learning in the neocortex, especially the frontal lobes. In the surface EEG, it can be observed along the midline of the scalp when episodic memories are recalled and during executive tasks that engage both the lateral and medial frontal lobes.

There are many ways for brain waves to interact: like water waves, they can add on to each other or cancel each other. Frequency modulation is also possible, allowing for speeding and slowing of waves. What's more, there is evidence for phase modulation, so that the degree of phase locking between two sources of gamma might shift in order to pass some signal. Phase modulation has been proposed as a mechanism for hippocampal coding of place memory in rats.

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