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Gamma and other rhythmsWhat are the local circuit design features concerned with coordinating rhythms? Miles Whittington, Nancy Kopell, Roger Traub Newcastle University, UK and IBM In:- Dynamic Coordination in the Brain – Eds. Christoph von der Malsburg, William Phillips & Wolf Singer INTRODUCTION: This chapter examines some of the detail of gamma and other rhythms in the brain as studied by recent neuroscience. It emphasises the relationship between cortical rhythm generation and cortical function. In particular, it indicates gap junctions between axons as a driver of brain rhythms including gamma rhythms. In terms of consciousness studies, this is important background given the correlation between the gamma synchrony and consciousness. The cortex gives rise to rhythmic activity over a broad range of frequencies. This involves rhythmic change of the neuronal membrane electrical potential between periods of activity and periods of quiescence. Rhythmic activity can rise as high as 400 Hz when glutamate is involved. Evidence suggests that the majority of rhythm generating properties amongst neurons are in local circuits. Very selective frequency filters for neuronal inputs can determine which local circuit rhythm a neuron can be involved in. This leads to resonance between particular neurons. Synaptic inhibition is an important cause of rhythm generation in local networks through the theta to gamma range (up to 80 Hz). Even very low levels of neuronal excitatory activity can cause inhibitory interneurons to fire. The frequency of rhythms is mainly set relative to inhibitory postsynaptic potentials. Theta rhythms also depend on inhibition. Local rhythms above the gamma range are known as 'high gamma' or 'VFO' and may relate to high frequency discharges in interneurons (1. Buzsaki et al, 1992). These higher frequencies may be generated via gap junctions rather than synapses. Gap junctions between axonal compartments can allow rapid transmission of action potentials from one axon to another. These very fast oscillations (VFO) can be nested within slower ones. In persisting gamma rhythms, VFOs can be related to each period and they are seen as a driving force in these rhythms. Networks of such interconnected axons can generate rhythms, and gap junction coupled axonal networks are suggested to generate rhythms in local circuits. Cell assemblies are defined by synchronisation of axon spikes with a near-zero time delay. This is thought to be possible because of inhibitory neuronal activity. With gamma rhythms there is often coordination of activity in a number of separate neuronal populations. In addition to coordination of rhythms at the same frequency, there is also coordination of rhythms at different frequencies in the same or separate brain regions. Gamma rhythms are generated by interaction between principle cells and interneurons. Slower theta rhythms derive from a different set of interneurons inhibiting the dendritic compartments of principle cells. The relationship between the two rhythms is suggested to be handled by the interaction of two types of inhibitory interneuron. Here output from at least one neuron determines both circuits. In some cases, the duration of a locally generated rhythm is an integer multiple of another locally generated rhythm. A ratio of about 1.6 between frequencies expressed at the same time may allow information channels to operate at the same time without interference. The phase relationship between different rhythms also shows a cycle, and reduction of cortical excitation can see a stabilisation of the phase relationship between cortical layers. Rhythms are seen to coordinate firing patterns of neurons. The phase relationship between rhythms at spatially separated brain sites governs the timing of local activity and also future interactions between the two sites. The amount of coherence of rhythms governs the degree of communication between structures. The multiple frequencies in the cortex suggests an overall scheme where different frequency channels are used to process different types of information. So far this has been observed mainly in the association cortex. Gamma rhythms in the visual, parietal and entorhinal cortex are different from one another. Even within the visual field different levels of detail can fall into different frequency channels, with for instance general features related to theta frequencies, but more detailed features related to beta frequencies. Information processed at different frequencies can later be combined at some further frequency. |
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