Trends in Neurosciences
Human cortical oscillations: a neuromagnetic view through the skull
Section snippets
Source modelling
The invasive microelectrode recordings have the advantage that the neuronal structure generating the signals can be identified easily. By contrast, MEG (and EEG) sensors pick up signals from extensive brain regions, which might be even several centimetres away from the sensor. Therefore the sites of active neuronal populations have to be deduced from the measured signal distribution. Although this ‘inverse problem’ does not have a unique solution in the general case6,9, modelling the generators
Sources and reactivity of the alpha rhythm
Sources of the posterior 10 Hz alpha rhythm concentrate predominantly in the parieto–occipital region and, to a smaller extent, in the occipital areas. The two subclusters differ both by their sites and current orientations, and suggest that the strongest activity occurs in the parieto–occipital sulcus with less activity in the calcarine sulcus22,26. Even within these subclusters, several single sources with independent time behaviour might be active simultaneously11, thereby supporting
Segregation of the 10 Hz and 20 Hz somatomotor rhythms
The well-known ‘comb-like’ shape of the somatomotor mu rhythm implies that the rhythm consists of two or three frequency components with a nearly harmonic relationship. The dominant magnetic components near 10 Hz and 20 Hz (cf. Fig. 1) can occur either separately or simultaneously14, suggesting that the 20 Hz activity can be phase-locked to the 10 Hz rhythm at certain moments (and thus probably arise from the same neural generator) but stay totally independent at some other moments of time.
Hemispheric balance of somatomotor reactivity
Cortical signals associated with unilateral movements imply bilateral involvement of the somatomotor cortex. The bilaterality is particularly evident in the modulation of cortical rhythms, illustrated in Fig. 5 for unimanual finger movements46. Movement-related changes in the level of spontaneous activity occurred in both hemispheres but were strongest contralaterally, particularly at frequencies above 14 Hz. The 10 Hz activity had already started to dampen 2 s before the movement. In the
Functional significance of cortical rhythms
The available hypotheses for the role of cortical macroscopic oscillations include epiphenomena, with no functional significance, and idling, which would allow the system to start more rapidly than by cold start47. In agreement with the latter interpretation, the parieto–occipital and rolandic rhythms are strongest when the modality-specific sensory input (or motor output) is minimal or monotonous. In the visual system this happens when the eyes are closed or when one looks at a homogeneous
Concluding remarks
A surprisingly small number of synchronized cells might determine the recorded macroscopic signal that reflects the gross activity of a cell population26. Macroscopic cortical rhythms therefore probably arise in areas with the most synchronous signal transfer. It is worth emphasizing that changes in the synchronization of a neuronal population can occur without significant changes in the mean neuronal firing rates, and thus without changes of blood flow and metabolism. This means that
Acknowledgements
This work was supported by the Academy of Finland and by the Sigrid Jusélius Foundation. The magnetic resonance images were obtained at the Dept of Radiology, University of Helsinki. We thank N. Forss, J. Mäkelä, and S. Salenius for comments on the manuscript.
References (59)
Electroencephalogr Clin Neurophysiol
(1990)- et al.
Electroencephalogr Clin Neurophysiol
(1994) Electroencephalogr Clin Neurophysiol
(1984)- et al.
Neuroscience
(1989) - et al.
Electroencephalogr Clin Neurophysiol
(1959) Neurosci Lett
(1991)Electroencephalogr Clin Neurophysiol
(1995)Electroencephalogr Clin Neurophysiol
(1992)- et al.
Electroencephalogr Clin Neurophysiol
(1993) Electroencephalogr Clin Neurophysiol
(1962)
Neurosci Lett
Neuroscience
NeuroImage
Electroencephalogr Clin Neurophysiol
NeuroImage
Electroencephalogr Clin Neurophysiol
Neurosci Lett
Science
Physiol Rev
Physiological Basis of the Alpha Rhythm
Appleton–Century–Crofts
Science
J Magn Magn Mat
Science
Rev Mod Physics
Physica Scripta T49
Med Biol Eng Comput
Arch Psychiatr
Brain
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