Phase Alignment of Neocortical Gamma Oscillations by Hippocampal Theta Waves

An empty brain is the devil’s workshop, goes the proverb. Actually, the brain is never empty. Even in our deepest slumber, the brain continues to weave waves of electrical rhythms that can be seen with the aid of electroencephalogram or EEG. When we place electrodes on the scalp or on the cortex (inside the skull), and amplify the faint signals via bioinstrumentation amplifier, we can lay our hands on these fluctuating rhythms. (More on the electronics of EEG may be found at the OpenEEG project site).

We have as many as 100 billion neurons in the brain. In the superficial layers of the cortex, the neurons have numerous dendrites branching out from the soma or cell body (shown in grey oval in this picture).These neurons have been compared to a forest of trees where the branches are the dendrites and the trunk the axon. These dendrites make extensive connections among each other. They also get connections from the axon collaterals of neighboring axons (i.e. the 'trunks' of other trees connect to these 'twigs' by offshoot from the trunks). Since there are a lot of axons converging on the dendrites of each neuron, and given the fact that these axons can be excitatory (red) or inhibitory (green) depending on the neurotransmitter, the sum of input may be either negative or positive (with respect to the cell body). Thus an alternating current (cortical dipole) will flow between the shifting dendrites and the soma. This along with thalamocortical oscillations produces the EEG waves.

The brain doesn’t churn out the rhythm just like that. Had the neurons fired randomly the oscillations would have cancelled out.EEG waves occur due to synchronous discharge of neurons producing the alpha, beta, theta, gamma and other telltale waves. Like all other electrical waves, they too have a frequency and amplitude. Alpha waves, for example, have a frequency of 8-12 Hz (cycles per second) and an amplitude ranging from 50-100 microvolt when recorded from the scalp, and it is found when a person is resting comfortably with eyes closed and the mind wandering. On the other hand, gamma rhythm has a frequency of 30-80 Hz, and it is found when a person is deeply engrossed on some work.

It was known for a long time that the hippocampus exerted a role in learning by fostering long term potentiation (LTP) by aligning the neocortex, where memories are stored. The mechanisms behind this are now emerging. Sirota et al and Siapas et al have analyzed rat brains and found out that there were many localized gamma oscillators within the brain that gave rise to neocortical gamma bursts. These oscillators had varying frequencies but they phase aligned themselves with the arrival of hippocampal theta waves. A large fraction of pyramidal cells and interneurons too were phase aligned to the hippocampal theta rhythm.This is similar to a bar magnet aligning iron dust or other ferromagnetic materials by virtue of its magnetic field. Apart from the cerebral cortex, the cerebellar cortex and the hippocampus too can generate brain waves. Such a mechanism may explain the orchestration of many parts of the cortex (and hence the memory engrams they contain); and data synchronization and downloading to the hippocampus for memory retrieval. It also shows how hippocampus does the ‘indexing’ of cortical contents. These experiments throw light on neuronal plasticity and information flow, and may be someday they could help clinicians in fighting memory loss as it occurs in neurodegenerative diseases like Alzheimer’s disease.

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References:

Prefrontal Phase Locking to Hippocampal Theta Oscillations
Athanassios G. Siapas, Evgueniy V. Lubenov and Matthew A. Wilson. doi:10.1016/j.neuron.2005.02.028
A SIROTA, S MONTGOMERY, S FUJISAWA, Y ISOMURA, M ZUGARO, G BUZSAKI (2008). Entrainment of Neocortical Neurons and Gamma Oscillations by the Hippocampal Theta Rhythm Neuron, 60 (4), 683-697 DOI: 10.1016/j.neuron.2008.09.014