Visual gamma oscillations and excitatory/inhibitory processes in the brain
Altered balance between neural excitation and inhibition characterizes many brain disorders, including such neurodevelopmental disorders as autism. It is therefore important to be able to measure this balance noninvasively in the human brain. The fast neuro-magnetic oscillations of the brain – ‘gamma oscillations’ (30 to 100 Hz) are generated by neural circuits that include excitatory and inhibitory neurons and are sensitive to changes in interactions between these neurons. Magnetoencephalography (MEG) is essentially more sensitive to these fast oscillations than all other noninvasive neuroimaging methods are. Therefore gamma oscillations detected by MEG can potentially provide useful information about the excitation/inhibition balance in the brain.
In a series of studies we investigate how visual gamma oscillations recorded by MEG are modulated by the strength of excitatory drive and how these modulations are related to behavioral indexes of neural inhibition. Our results show that nonlinear modulations of gamma power by the strength of excitatory drive reflect capacity of the brain to regulate the balance between neural excitation and inhibition.
These findings help understand the functional role of gamma – one of the major classes of brain rhythms. Moreover, they may help to develop non-invasive measures of homeostatic regulation of the excitation/inhibition balance in brain disorders.
Figure 1. Modulation of gamma oscillations by velocity of visual motion.
The excitatory drive to the visual cortex increases with increasing velocity of visual motion.
This increase first leads to the increase in gamma power (from 0°/s to 1.2°/s).
However yet stronger increase in motion velocity/ stimulation intensity (velocity >1.2°/s)
results in gamma suppression. The intensity-related gamma suppression may reflect down-regulation
of growing excitation in visual networks (Orekhova et al. Sci Rep 2018).
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