Moser, E. I. et al
In:- Dynamic Coordination in the Brain: From Neurons to Mind – Eds. Christoph von der Malsburg, William Phillips & Wolf Singer
Neuroimaging of the human brain indicates coordination of activity between different brain regions. Large scale coordination is shown to span the entire brain. Rhythmic modulation of electrical activity is seen as a possible mechanism to change the couplings amongst neurons. Networks undergoing electrical oscillation facilitate the establishment of synchrony through entrainment and resonance. The precision of synchronisation increases with the frequency of the oscillation. Synchronisation increases the influence of the output of cell assemblies on target neurons.
In synchronised cell populations response to strong excitatory inputs will occur earlier than weak inputs on the rising phase of the oscillation, and this acts as a code to indicate the relative strength of signals. Studies of the retina show that this process indicates that relative strength of visual stimuli. Moser’s group considered that oscillation-based synchronisation has a role in cognitive processing. Studies have shown that visual attention correlates with increases in coherence between the parietal and the frontal cortex. They also show an increase in coherence between two different frequency bands, 35-55 Hz (gamma) for bottom-up attention and a lower frequency band for top-down attention.
Coupling between neuron populations depends on the phase relationship between the different groups. Oscillations in different frequency bands such as theta, beta and gamma can coexist. The point in the phase of an input can code for whether it is processed or suppressed. Brain activity has been shown to be organised in spatiotemporal patterns corresponding to gamma fluctuations. These appear to be related to learnt activity, and may represent an attractor that recruits particular brain networks. The role of neuromodulators is suggested to be one of providing the necessary conditions for oscillations rather than directing the oscillations.
This chapter also discusses the question of the zero-phase lag between oscillations in different populations of neurons. This zero-phase lag is ubiquitous in the brain, manifesting over large spatial separations and even between the hemispheres. This is despite significant conduction times between the separated populations. However, it is admitted that the mechanism for the synchronous gamma firing is not well understood.