Foot and hand area mu rhythms
Introduction
The brain is able to generate oscillations at different frequencies, of which one type is rhythms within the alpha band. These rhythms can be recorded over different brain areas, with the largest amplitudes found during relaxed wakefulness. Adrian and Matthews (1934)considered the alpha band activity as resting rhythms of the brain, with different areas having their own characteristic resting or `idling' rhythms. The sensorimotor system is in an idling state when no somatosensory input is processed and no motor output is generated, and the visual system is in such a state when no visual information processing is performed. Therefore, the occipital alpha rhythm can be considered as an idling rhythm of visual areas and the central mu rhythm as an idling rhythm of sensorimotor areas (Kuhlman, 1978).
There is, however, not only one occipital alpha rhythm but a great variety of rhythms within the same frequency band. In this context it is important to refer to the following statement of Grey Walter in Mulholland (1969):
"We've managed to check the alpha band rhythm with intracerebral electrodes in the occipital–parietal cortex; in regions which are practically adjacent and almost congruent one finds a variety of alpha rhythms, some of which are blocked by opening and closing the eyes, some are not, some are driven by flicker, some are not, some respond in some way to mental activity, some do not. What one sees on the scalp is a spatial average of a large number of components, and whether you see an alpha rhythm of a particular type or not depends upon which component happens to be the most highly synchronized process over the largest superficial area; there are complex rhythms in everybody".
In addition to visual and sensorimotor rhythms, rhythmic activities of auditory cortical areas appear to exist. Niedermeyer (1990)described a `third rhythm' within the alpha band arising from the neocortical portion of the mid-temporal region and Papakostopoulos et al. (1980)reported on a temporal 11-Hz rhythm that reacts differently to fist clenching than do the sensorimotor rhythms. A magnetoencephalic 10-Hz rhythm from the supertemporal human auditory cortex was reported by Tiihonen et al. (1991).
A well-known phenomenon is the blocking or desynchronization of rhythms within the alpha band reported by Berger (1930), Jasper and Penfield (1949), Chatrian et al. (1959)and others. The event-related desynchronization (ERD) is a specific form of EEG desynchronization. The ERD, first quantified by Pfurtscheller and Aranibar (1977), is phasic, circumscribed and very often focused over specific cortical areas. In contrast to this desynchronization, alpha band activity can also be enhanced, i.e. the EEG activity within the alpha frequency range may become more synchronized close to an event. This phenomenon is also phasic and localized and was named event-related synchronization (ERS, Pfurtscheller, 1992). Both phenomena, ERS and ERD, can be displayed in the form of time courses (Pfurtscheller and Aranibar, 1979; Van Winsum et al., 1984; Dujardin et al., 1993), maps (Pfurtscheller and Berghold, 1989; Pfurtscheller, 1991) or chronospectrograms (Defebvre et al., 1993; Derambure et al., 1993).
Alpha desynchronization is characteristic of an activated brain state and can be seen as an electrophysiological correlate of activated or excited cortical neurons (Steriade and Llinas, 1988). The cortical modules or neuronal assemblies tend to work independently and the EEG displays low amplitudes.
A synchronized activity within the alpha band can be interpreted as a neurophysiological correlate of decreased cortical excitability or even of inhibition of neuronal populations. In this case, cortical modules display synchronized behavior, i.e. they do not work independently. Accordingly, large alpha waves can be measured on the scalp due to the cooperative behavior of cortical neurons. Taking the spatial averaging effect into consideration, it is estimated that the area of cooperative activity has to be in the order of some cm2 (Cooper et al., 1965; Lopes da Silva, 1991).
Alpha spindles are a different phenomenon and are characteristic of barbiturate anesthesia (Andersen and Andersson, 1968) and of light sleep. During the occurrence of alpha spindles, the information transmission from the thalamus to the cortex is cut off (Steriade and Llinas, 1988).
Section snippets
Data collection and processing
Similar to the computation of ERPs, the quantification of ERD is based on the averaging technique (Fig. 1). Generally, an event-related paradigm with a trial length of some seconds is used. In our experiments the subjects performed a simple finger or foot movement in response to visual stimulation. For recording of the EEG, electrodes were placed over central and pre-central areas, with an interelectrode distance of 2.5 cm. The EEG signals were recorded with a common nose reference. To overcome
Finger movement
In the past it was generally believed that the central mu rhythm was present only in a relatively small population of normal subjects (Chatrian and Lairy, 1976). When computerized methods were used, a desynchronization of mu rhythms during hand movement was found in nearly every subject (Pfurtscheller and Aranibar, 1979; Pfurtscheller and Berghold, 1989; Derambure et al., 1993; Toro et al., 1994). Examples of single EEG trials, ERD maps computed before and during right finger movement, and band
Foot movement
In contrast to hand or finger movement, a foot movement-related desynchronization can occasionally be found close to electrode Cz overlying the primary foot representation area. Such an example from one subject is shown in Fig. 5 along with some selected single EEG trials, the average power as a function of time and two ERD maps. As can be seen, the ERD is focused to the foot area and found not only within the alpha, but also the lower beta band around 20 Hz.
When the same subject performed
Enhancement of hand area mu rhythms
It was shown that different types of visual input can enhance central mu rhythms. Brechet and Lecasble (1965)reported an enhanced mu rhythm during flicker stimulation, Koshino and Niedermeyer (1975)reported on enhanced (synchronized) Rolandic rhythms during pattern vision in 33 out of 61 subjects. Recently it was demonstrated that foot or tongue movement can also enhance the hand area mu rhythm (Pfurtscheller and Neuper, 1994). Common to all these observations is that in the case of visual
Coupling between primary hand area and SMA
Along with the primary sensorimotor (MI/SI) and premotor areas, the supplementary motor area (SMA) also plays an important role in preparation or planning of movement (Roland, 1984; Goldberg, 1985). In addition to a localized ERD over the contralateral primary sensorimotor area, a localised ERD of EEG signals recorded close to the SMA has also been found prior to voluntary self-paced finger movement (Pfurtscheller and Berghold, 1989), and suggests the possibility that the SMA also has its own
Conclusion
Topographical analysis of scalp EEG data with closely spaced electrodes during discrete finger and foot movements leads us to formulate the following conclusions:
- 1.
Primary hand and foot areas have their own intrinsic rhythmic activity (mu rhythms, central beta rhythms) which desynchronize when the corresponding area becomes activated. Hand movement results in a desynchronization of mu and central beta rhythms close to the hand area in nearly every subject. A circumscribed desychronization of mu
Acknowledgements
The authors would like to thank S. Gölly, J. Kalcher and G. Florian for assistance in data recording and for software support. The project was supported by the `Fonds zur Förderung der wissenschaftlichen Forschung' project P10000.
References (41)
- Adrian, E.D. and Matthews, B.H. (1934) The Berger rhythm: potential changes from the occipital lobes in man. Brain, 57:...
- Andersen, P. and Andersson, S.A. (1968) In: A. Towe (Ed.), Physiological Basis of the Alpha Rhythm. Appleton-Century...
- Andrew, C. and Pfurtscheller, G. (1996) Event-related coherence as a tool for studying dynamic interaction of brain...
- Arroyo, S., Lesser, R.P., Gordon, B., Uematsu, S., Jackson, D. and Webber, R. (1993) Functional significance of the mu...
- Berger, H. (1930) Über das Elektroencephalogramm des Menschen II. J. Psychol. Neurol., 40:...
- Brechet, R. and Lecasble, R. (1965) Reactivity of mu-rhythm to flicker. Electroencephalogr. Clin. Neurophysiol., 18:...
- Chatrian, G.E. and Lairy, G.C. (1976) The mu rhythm. In: A. Remond (Ed.), Handbook of EEG and Clinical Neurophysiology....
- Chatrian, G.E., Petersen, M.C. and Lazarete, J.A. (1959) The blocking of the rolandic wicket rhythm and some central...
- Cooper, R., Winter, A.L., Crow, H.J. and Grey Walter, W. (1965) Comparison of subcortical, cortical and scalp activity...
- Defebvre, L., Derambure, P., Bourriez, J.L., Jacquesson, J.M., Dujardin, K., Destee, A. and Guieu, J.D. (1993)...
Cited by (351)
Acetaminophen changes Mu rhythm power related to pain empathy
2023, NeuropsychologiaNeural signature of mobility-related everyday function in older adults at-risk of cognitive impairment
2023, Neurobiology of AgingCitation Excerpt :Several electrophysiological signatures associated with movement have been identified and two of central interest to this study are presented here. First, neuronal oscillations in the mu (8–12Hz) and beta (13–28Hz) frequency range represent activation of sensorimotor cortex (Miller et al., 2010; Pfurtscheller and Aranibar, 1979; Pfurtscheller and Neuper, 1994; Pfurtscheller and Neuper, 1997; Pfurtscheller et al., 1997; Pfurtscheller and Lopes da Silva, 1999; Pfurtscheller et al., 1999;). Mu and beta desynchronization is observed during preparation and execution of movement, with effector-specific (e.g. foot, finger, and tongue) distributions in line with the somatotopic arrangement of pre and post central gyri.