- 9. The Normal EEG of the Waking Adult Niedermeyer, E. The Normal EEG of the Ernst Niedermeyer Waking Mult. In: E. Niedermeyer & F. Lopes da Silv4 eds Electroencephal- ography: Basic Principles, Clinical Applications and Related Fields. Lippincott Williams & Wilkins, Baltimore MD, pp. 149-173(1999). The presentation of the normal mature human EEG and The sequence of these Greek letters is not logical and can "alpha" its basic features has a dual purpose. These are to promote be understoodonly in the historical view. The terms "beta" knowledge of the boundaries befween normality and abnor- and rhyttm or waves were introduced by Berger "gamma'' mality in clinical EEG work and to enhancethe understanding (1929); the term rhythm was subsequentlyused by of the EEG as a phenomenonwith important psychophysiol- Jasperand Andrews (1938) in order to designatefrequencies ogical implications. above 30 or 35/sec; these were essentially 35-45/sec and Both purposes are evident in Berger's pioneering work, superimposed on the occipital alpha rhythm (see Dutertre, "gamma" from the first report with its strong emphasison psychophy- 1977). This term was abandonedand frequencies siologyand the discovery of the alpha rhythm (Berger, 1929) became a part of the beta range. "gamma to his fourteenthreport (Berger, 1938) of predominantly clini- The use of the term rh;rthm" or "gamma fre- cal interest.The questionof how the EEG works remainsto be quency range" has made an impressive comebackduring the "fast" answeredby the experimental neurophysiologist. The nature 1990s. The use of a beta range and a "very fast" of thenorrnalEEG, how itreflects humanbehaviorandmental gamma range might have been convenient for those who uti- functions, and the boundaries of normality are most legitimate lize frequency analysis with power spectra. Furthermore, guidelinesfor a chapteron the normal adult EEG. modern EEG rhythm research in the 1990s has unearthed the "gamma all but forgotten term rhythm" (Eckhorn et al., EEGFrequencies 1992; Bullock,1992; Gray et al.,1992; Basar, 1992). In this "noise" "new The EEG, as the continuous "roar" or of the wave" of EEG rhythm research,rythmic activities of brain,contains a fairly wide frequency spectrum,but it is not the brain are conceived mainly as induced rather than as spon- simply a hodgepodgeof frequencies.Rhythmicity seemsto taneousrhythms. "delta" createsome law and order among waves of various lengths The term rhythm was introduced by Walter andamplitudes. The impression of prevailing rhythmicity and (1936) to designate all frequencies below the alpha range. organization, however, is not a yardstick for the normality Walter himself, however, found a need to introduce a special l of an EEG. honounced rhythmicity may be a sign of abnor- designation for the 4-7 .Slsecrange and used the letter theta. mality, and a prima vista anarchicappearance does not neces- He thus bypassedthe Greek letters epsilon, zeta, and eta; he sarily imply abnormality. Reactivity may be the magic word chose theta to stand for thalamus because he presumed a in such cases; an EEG of mixed frequencies may be quite thalamic origin of these waves (also see Knon, 1976b). responsiveto certain stimuli. The term "pi rhythm" has been used for the designation Tbefrequency range of the EEG has afuzzy lower and ofposteriorslowrhythms(3-4isec)withoutharmoniousrela- upper limit. There are ultra-slow and ultra-fast frequency tionship to the posterior alpha rhythm according to Dutertre componentsthat play no significant role in the clinical EEG, (1977) who recommended the preferable (although certainly "posterior with the exception of ultra-slow activity in profound coma less precise) term slow rhythms." Hardly anyone "pi ano near-terminal states.For these reasons, the frequency- uses the term rhythm" in the 1990s. response curve of an EEG apparatusconcentrates on the clini- The term "phi rhythm" was suggestedby D. Daly (ac- cally relevantfrequency range, which is also the most impor- cording to Silbert et al., 1995) for the designation of mono- tant from the psychophysiologicalviewpoint. This range lies rhythmic posterior delta waves (lessthan 4/sec),distinct from between 0.1/sec (or cps or Hz) and 100/secand, in a more the background and occurring within 2 sec of eye closure. resbicted sense,befween 0.3/sec and 70/sec. In the normal This rhythm was also described by Belsh et al. (1983) as adult, the (0.3-7lsec) slow ranges and the very fast range "posterior rhythmic slow activity after eye closure." tabove30/sec) (8-13/sec) are sparselyrepresented; medium The alpha-like anterior temporal kappa rhythm (Laugier andfast (14-30/sec) predominate. ranges and Liberson, 1937) is a controversial pattern and will be These frequencies are broken down into the following , discussedlater in this chapter. Kugler (1981) has been using oandsor ranges: "sleep the term "sigma activity" instead of spindles" and, "sigma Deltabelow 3.5/sec(usually 0.1-3.5/sec) furthermore, the term rhythm" for activity in the "rho rneta4_7.5/sec 11-15/sec range. The term waves" has been used for Alpha8-13/sec the activity known as POSTS (positive occipital shaqptran- Eeh above l3lsec (usually 14-40/sec but unlimited in the sients of sleep) (Kugler and Laub, 1973). uPPer ^ range) or more recently: Other Greek letters have been proposed for the designation leta l4-30/sec and of distinct EEG activities. Mu rhythm and lambda waves "tau Liammaabove 30/sec (unlimited in upper range) will be discussedin this chapter, The term rhythm" is -+lJW-r* Fields 150 Electroencephalography:Basic Principles, Clinical Applications, and Related "Magnetoencephalography mentioned in Chapter 60, as a Frequency ' tool of Clinical Neurophysiology,' and denotesa physiolog- The chapteron EEG maturationshows the gradualfr9- ical alpha rhythm of the temporal region (in the author's quency incriase of a posterior basic rhythm that is detectable "third opinion identical with the rhythm" discussedin this aroond ttte age of 4 months with a frequency of approximatgly chapter). 4/sec. This posterior basic rhythm shows a progressive fre- Thus, 12 ofthe 23 letters ofthe Greek alphabetare being quency increase with average values of aroun! 6isec at age used in the EEG terminology and this number could be even iZ montt t and S/sec at age 3 years' At that time, the alpha justification higher. In this author's opinion, it might be better to limit the frequency band is reached, and there is for the Gieek terms to the classical EEG frequency ranges retaining use of the term alpha rhythm. The frequency reaches a mean mean solely the lefters alpha, beta, gamma, and theta. of about 10/secat age 10 years' This is essentially the alpha frequency of adulthood;in other words, the progressive EEGAmplitudes alpha rhyttrm acceleration usually ends around the age of 10 decade of life (and to some degree also The EEG denotes voltage plotted against time. The voltage years, but the second decade) features a constant decline of intermixed of the EEG signal determines its amplitude. The passage of lhe third posterior slow activity that is usually presentin considerable the cortical EEb signal through leptomeninges,cerebrospinal quantity at age 10. fluid, dura mater, bone, galea, and scalp has a strongly attenu- - The-frequency of the alpha rhythm tends to decline in ating effect on the original signal (Cooper et al., 1965); this elderly individuals. This decline apparently reflects some de- is di-scussedin the section on the depth EEG. Corticographic gree of cerebral pathology, which is vascular or fibrillary dischargesshow amplitudesof 500-1.500 pV (0'5-1'5 mV) degenerative,in most instances'Healthy and vigorous elderly and several millivolts in prominent spiking. The amplitudes people may show little or no alpha frequency decline, even of the scalp EEG are markedly reduced and lie between 10 in the ninth decade.An alpha rhythm with a consistent S/sec and 100 pV (in adults, more cornmonly between 10 and 50 pv). frequency ought to be regarded as a mild abno:mality. ' The figure of 10.2 + 0.9/sec has been indicated as the The EEG amplitudes are measured from peak to peak' mean adult alpha frequency (Peters6n and Eeg-Olofsson, Precise determination of the voltage of each wave is unneces- 1971). An element of instability of the alpha frequency must sary and shouldbe discouragedas pseudoaccuracy;too many be taken into consideration; according to Townsend et al' variables are involved (above all, tle interelectrode distance (1975), the alpha rhythm frequency can be stabilized by si- and the type of montage, whether bipolar or referential re- nusoidally modulated light. Extreme upward gaze tends to cording). Electroencephalographers may indicate in their re- "alpha facilitate the posterior alpha rhythm (Mulholland and Evans, ports fcertain amplitude range, such as rhythm from 1965; Mulhoiland, 1969). Lateral eye deviations may have 20-30 ptY," or, even better, limit themselvesto statements "of "of similar effects (Fenwick and Walker, 1969). such as medium voltage" or low to medium An alpha rhythm frequency shift to the faster portion of voltage." the bandis not uncornrnon and is essentially within normal A given frequency can be renderedabnormal by excessive limits, as will be discussedlater. The similarities between voltafe. This is true for all frequencies,and it is particularly the frequenciesof alpha rhythm and the physiological finger important for the fast band. The problem of low voltage Oeta) tremor have been &scussed by Isokawa and Komisaruk discussed,because low amplitudes can wiil Ue thoroughly (1983). Immediately after eye closure, the alpha frequency a life-threatening decline of cerebral voltage output indicate "desyn- may be accelerated for a moment ("squeak effect," after the vast majority of low voltage recordsare whereas Storm van lreuwen and Bekkering' 1958)' chronized" (discussedlater) and a variant of normalcy. Amplitude Alpha Rhythm Alpha rhythm amplitudesvary considerablyfrom individ- moment to Definition ual to individual and, in a given person, from moment.