Mesoscopic brain dynamics
From Scholarpedia
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Author: Dr. Hans Liljenström, Div. Biometry and Systems Analysis, Energy & Technology, SLU, Uppsala, Sweden
Dr. Hans Liljenström accepted the invitation on 19 August 2008 (self-imposed deadline: 19 January 2009).
Mesoscopic brain dynamics refers to the neural activity or dynamics at intermediate scales of the nervous system, at levels between neurons and the entire brain. It is commonly considered to relate to the dynamics of cortical neural networks, typically on the spatial order of a few cm, and temporally on the order of ms to seconds. It is usually the type of dynamics that can be measured by methods such as ECoG (electrocorticography), EEG (electroencephalography) or MEG (magnetoencephalography). Indeed, the terminology can be used in relative terms, where “meso” just indicates that the scale of interest is in between the “micro” and the “macro”. What is microscopic could be considered processes and systems studied with a microscope or microelectrodes. It could refer to ion channels or single neurons. The macroscopic scale, on the other hand, can be considered corresponding to the largest scales possible to measure with regard to brain activity. This could be the dynamics related to maps, or systems, such as cortico-thalamic, or cortico-cortical interactions, usually measured with PET or fMRI or other brain imaging techniques, capturing the dynamics associated with blood flows and metabolism.
Mesoscopic brain dynamics is characterized by its high complexity, often involving oscillations of different frequencies and amplitudes, perhaps interrupted by chaotic or pseudo-chaotic irregular behaviour. The mesoscopic brain dynamics is affected by the activity at other scales. For example, it is often mixed with noise, generated at a microscopic level by spontaneous activity of neurons and ion channels. It is also affected by macroscopic activity, such as slow rhythms generated by cortico-thalamic circuits or neuromodulatory influx from different brain regions.
Mesoscopic brain dynamics, including transitions between different states, are partly a result of thresholds and the summed activity of a large number of elements interconnected with positive and negative feedback. It is also a result of the dynamic balance between opposing processes, influx and efflux of ions, inhibition and excitation etc. Such interplay between opposing processes often results in (transient or continuous) oscillatory and/or chaotic-like behaviour. With a proper regulation, this dynamics can keep the system at a desired balance between flexibility and stability.
The mammalian olfactory system, primarily the olfactory bulb and cortex, has often been used as a model system for mesoscopic brain dynamics, due to the pioneering work of Walter Freeman and his co-workers since the 1960s. The structure of this system is also well characterized, and Freeman and others have successfully studied and described how structure, dynamics and function is related in this system. Computational models have contributed to elucidating these relationships, where simulation results have been able to closely mimic the dynamics, as captured by LFP (local field potentials), EEG or ECoG. Below is an example of real (top) and simulated (bottom) EEG from a rat olfactory cortex.
| Suggested by: | Dr. Walter J. Freeman, University of California, Berkeley, California |
| Invited by: | Dr. Eugene M. Izhikevich, Editor-in-Chief of Scholarpedia, the peer-reviewed open-access encyclopedia |
