The analysis of the electroencephalogram

Abstract

The interest of analytical studies of electroencephalographic record which considerably increase the information which they give to the clinic, stands out. When the time comes to choose the method to be used in the analysis of the E. E. G. it is to be remembered that this is not the real expression o£ the pheno- mena which are difined in inside of the brain, níƒor even of the variations of the field which take place on its surface. We must take into account the figure ofTO” neurones which act as gene- rators of electric energy which rarely discharge syncronically, and also the thousands of metres and the complexity of the interneuronal circuits, together with potentials originated by the glia bringing on threedimensional fields interrupted by planíƒe waves which go partly in relation to the rhythm of those which are whithin the circuits of regulation and partly in- terrupt those fields in an anarchial form corresponding to signáis originated by the sensory stimuli. Potentials which rhythmically or anarchially create a field of forcé on the cranial surface, of which the electroencephalograph selects and re- gisters only one special type of phenomenon. The whole group and each sepárate one of the variable which it registers has to be estimated for its real valué as it is not very scientific to break up the graphic expressions of a phenomenon whose real form has been deformed by the technical characteristics which condition the recording ; on the other hand, the introduction in certain cases of pace to integration is very useful.

The method of analysis is not only considercd advisable but also indispensable, the histogram of the frequencies and the other variably which occur in the organization of the E. E. G. being the simplest.

The analytical solution, like the graph, is far too laborious to be done in a routine way. These and the niechanical method only determine the coeficients of the integral of Fourier which must correspond to múltiple frequencies of fundamental fre- cuency, therefore these methods can only be applied to perio- dical functions but it is not advisable for the analysis of the E. E. G. Since the electrocerebral phenomena are not the same as those which origínate with the use of A/C generators, the simple inspection of the curves obtained by the making of the analysis of the same segment of the records with the analyzer, nathematically and by means of the histogram of frequencies show, with clarity, how much the relative valúes obtained in the computation of the slow or rapid extreme frequencies se­ párate. This particular increase in the extreme potentials obtained with the analyzer, fluctuate according to the number of terms of the series, while in the histogram there is no posible fiuctuation since it is organized by a simple count of frequency. The result of the histographic analysis keeps a cióse relation with the direct expression of the electroencephalogram so that it is easier to understand that the curve obtained by the analysis of the Fourier series.

The method chosen in the analysis of the frequencies of the E. E. G. enables us tq deduct the following :

In the fetus, whose earliest age, in our explorations, corre- sponds to a premature one of four and a half months, and two hours after its life out of the maternal claustrum, the graph, being almost a electrical silence is very difficult to analyze. Up to six months of life in the womb the inactive records persists. Between seven and eight months it takes on characteristics very similar to those of newborn, even if occasionally during the inactive period and even towards six fetal months, outbursts of regular elements of theta frequency — and low voltage are observed—, always preceding spasmodic movements of the pre­ mature. The bioelectric characteristics of the newborn always correspond to the state of being awake, a state which is difficult to recognise as even with the eyes open, graphs similar to those of sleep are obtained. With these real bases it con be understood why the data which we give differ from a large part included in the literature referring to this subject and even from the opinion we held in 1955. Fundamentally the E. E. G. record of the newborn is characterized by the great dispersión of the class of frequencies which make it up, oscillating from delta to beta band, inter-mixing anarchially without reaching a rhythmical organization. The curve of distribution of frequencies is asymmetric, the niode falling back in relation with the average towards the slowest band in whose direction it becomes sharper and extends

with a more gradual dip lowards the more rapid rhythms. The result of this is that on the curve becoming normal an average of 6.8 c/s, is obtained and a standard deviation o- = 2 which marks a valué of the frequencies at the distance — 2 <r — 2, fre- quencies which we did not find in the normal E. E. G. of the vigil of the newborn. Attention is drawn to the coincidence of these results with the ones obtained on analyzing the first re- gister of Gibbs Atlas.

In the first week of life the average becomes slightljr slower (average 6,3 c/s) but the dispersión is better, with a standard deviation o- = 1.8. From the first week on, and up to ten years of age, the evolution of the frequency could appear, with a very slight error, as a lineal positions with a dip of approximately 0.3. This permits to determine with great approximation the average frequency of this period which is the most variable, by having only the initial average, approximately 6 c/s, and the coefficient by which the age must be multiplied, in order to ad this product to the said average. That is to say, ex- pressed in mathematical law :

Fm=6+0.3x E

where Fm is the average frequency and E the age.
On the other hand, the dispersión does not follow the same law, decreasing more rapidly in the early ages : in the firstweek 1.8 : at five years of age 1,4 and at ten yars of age 1,1. In the interval between the age of ten and twenty years,only one cicle per second is gained on an average but the limits of dispersión are conisderabl}r reduced, reaching the máximum stability at thirty years of age, with an average for the nor- motypes within our ambicnt of 10 c/s and a standard deflection 0.7. Fig. 13 of the text gives a clear idea of the modification of the perccntages of frequencies which have operated from the period of the newborn until maturity.

The relation which exists between the so much per cent alfa and the so much per cent theta through the years is very expressive, while in the period of the newborn the percentage theta is higher than double that of alfa, and at six years of age the quotient is inverted, coming closes ti infinity from twelve years of age on. The curve corresponds to an exponentíal equation with the asymptote parallel to the axis OY for the valué of X ncar to the age of fifteen.

The indices of cerebral maturation of deflection from the norniality which has becn put forward up to now are considered

of little discriminative valué and even deceptive, including these o£ the author, but on the other hand the quotient between the frequencies above F1 and below Fr the average M is eonsidered very useful, this latter is expressed by means of the function

F1 < M

o = ----------- Fr > M

The valué of this Iíndex for the control of a possible slow- ness or evolutive recuperation of an E-E.G. record of cerebral suffering is evident from the marked deflection in relation to the unity which throws the function to the least grade of mo- dification of the frequencies. It is assigned that the first plan

of rhythmical organization corresponds approximately to three months oíd, the sporadic presence of sliort regular bursts of frequency somewhat below the average being observed. On coming up to the age of one year, the pace of the bioelectrical activity becomes organized to a greater extent and with more persistence. At that age there still exists a total asymmetry between the periodical discharges picked up over homological areas. At about five years of age symmetrical bursts begin to be picked up, becoming nearer and nearer and more and more sepárate over extensive irregular segments, gravitating the waves above and below the base-line. Complete syinmetry is níƒor reached untill total maturity, corresponding to thirty years of age : then it is possible to observe in the E-E.G. recording the presence of spindly synchronous and continous burts.

The characteristic which is reached in the end is the re- gularity of the cerebral activity all along, being the periodicity in the spindly bursts the datum which speaks under cover of a greater cerebral bioelectrical stability. This rhythmic orga­ nization of the E.E.G. of the time is impossible to get in the healthy subject while awake. At the age of psychic somatic maturity the E-E.G. recording is organized with constant va- riation of the conditions in which the E-E-G. record is obtened, never free of stimuli.

In the newborn the amplitude of E.E.G. is as variable as the frequencies but the limits between which it oscillates are very cióse (5-25/x V). As time passes the voltage increases in posterior areas, this increment of the amplitude making itself little manifest untill the first two months are completed, althouhg at the age of three months numerous waves which double the highest amplitude of the E-E-G. record of newborn are observed.

The potentials increase little by little bringing up the máximum at about five years of age (up to 120/x V) and decrease more slowly from the same age. The máximum amplitude of E.E.G. record of mature normotypes does not surpass 70p V. The amplitude is the least uniform of all variables of the E.E.G within those considered as normal subjects.

The topographic specific organization of E.E.G. has a Pro­ gressive differentiation bringing up the meximum specification with maturation. In the newborn there is no appreciable diffe- rence in the topographic cerebral activity. It is not possible to be sure that the most precocius rhythmic organization corresponds to the motor areas. The occipital areas are dif- ferentiated primarily this differentiation up to increasing little by little up to the adult age. This differentiation consist fundamentaly, within the first stage, in the modifications of the amplitude and regularity and does not consist in the inc’edence of the; determined frequencies.

The data which provide the máximum, differential electro- encephalographic expression within what are considered normal records, are determined by the increments of the deviation on amplitude and frequency of the electric activity in the different cerebral areas with regard to the activity recording occipital areas. Occipital areas are the cerebral region where it is pos­ sible to obtain the máximum functional repose while awakes. It is also very interesting to determine the above-mentioned increments with respect to the study of the encircling of the spindless which we cali macro-waves.

In another work, given the great iconographic volume, the results of the study of the analytical particularities of the E.B.G. during the evolutionary period and each of their va­ riables, will be published.