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Changes in direct current activity during experimental focal seizures
ELECTROENCEPHALOGRAPHYAND CLINICALNEUROPHYSIOLOGY CHANGES
IN DIRECT
CURRENT
EXPERIMENTAL
ACTIVITY
FOCAL
63
DURING
SEIZURES x
ROBERT J. GUMNrr, M.D. 2 AND TAKEOTAKAHASHI,M.D. 3 Department of Neurology and Division of Electroencephalography and Neurophysiology (Department of Psychiatry), State University of lowa College of Medicine, lowa City, lowa and the Division of Neurology, University of Minnesota School of Medicine, Minneapolis, Minn. (U.S.A.)
(Accepted for publication: December 14, 1964)
INTRODUCTION
The physiological mechanisms whereby a small portion of brain tissue gives rise to a seizure are poorly understood. In the case of one widely investigated type of experimental epileptic focus (produced by the topical application of penicillin to the cerebral cortex) there is even disagreement as to whether the organized seizure discharges are the result of an increased frequency of sharp wave discharge (Walker et ai. 1945) or whether they arise from an afterdischarge following the sharp waves (Ralston 1958). We have reinvestigated the development and activity of the penicillin-induced focus with the expectation that the situation would be clarified by an analysis of the D.C. changes, especially in (I) the transition from interictal to ictal episodes and (2) the structure of the electrical field about the focus. METHOD Experiments were carried out in 66 cats. Under general anesthesia (i.v. Pentothal in the first 25 animals, ether in the others), a tracheotomy was performed and trephine holes were placed symmetrically over both hemispheres. Local anesthesia (1% lidocaine) served to reduce the amount of the general agent required. A nylon ring and lucite insert assembly, cemented to the skull, permitted an area of either 10 or 16 1 This investigation was supported in part by Research Grants NB 4362 and NB 5466 of the National Institute of Neurological Diseases and Blindness. Present address: Department of Neurology, Ancker Hospital, Saint Paul, Minn., U.S.A. a Fuibright Scholar, on leave from the Department of Neuropsychiatry, College of Medicine, Hirosaki University, Hirosaki, Japan.
mm in diameter to be surveyed (Grossman and Gumnit 1960). Although this device allowed only a portion of the field to be sampled at any one time, it kept the cortex warm and moist and fixed the salt bridges securely. A smaller ring cemented to the bone over the frontal sinus anchored the reference electrode. At the conclusion of the operation the local anesthesia was reinforced and additional local anesthesia was used throughout the experiment. The animal was loosely held in a modified (non-painful) Czermack head holder, paralyzed with gallamine triethiodide and maintained on artificial respiration under conditions designed to keep discomfort to a minhnum. General anesthesia is a potent anticonvulsant, therefore it could not be maintained throughout the experiment. Calomel half-cell electrodes (Beckman #270) with saline-agar salt bridges (0.6 mm inner diameter) were used in all experiments. Records were obtained with a Grass Model 5 polygraph and 5Pl chopper-modulated pre-amplifiers (Gumnit 1960) and (with a Grass C4 camera) from a Tektronix 502 CRO, at times using Grass P5 or P6 pre-amplifiers. Stin,.alation was carried out with a Grass $4 stimulator through a Grass SIU4 isolation unit or the circuit described by Landau (1956). The stimulating electrodes consisted of a tripolar arrangement of stainless steel wires (0.25 mm in diameter) 0.5 mm apart at the apices of an equilateral triangle. The unfiltered light of a Grass PS-2 photo stimulator was the source of light flashes. lr~ several experiments, arterial blood pressure was monitored with a Statham P23AC transducer and cortical temperature with a thermistor bridge circuit. Cortical temperatures as low as 33°C did not affect the seizure activity, and arElectroenceph. olin. NeurophysioL, 1965, 19:63-74
64
R.J. GUMNIT AND T. TAKAtI~,3HI
cil!i~_ G were topically appalled in a variety of ways. The most satisE~gor,:, m e t h o d was to m a k e a tkick paste o f pe.,,.icill/n a n d methylene blue and apply a piece of '-~o~,dng .... " p a p e r 3 m r n ;q d i a m e t e r , s o a k e d ~n tkds paste, to t h e C[~:Le,X.
terial b l o o d pressure d i d n o t vary w i t h a p a r o x y s m or at other ,[mes d u r i n g the experiment. S u r f a c e electrode locations were m a r k e d with an e'¢ctroc o a g u l a t i n g unit. Both s o d i u m a n d p o t a s s i u m crysm!iMe peni-
6rain
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.
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Fig. { Cha::,~,-:,~ shape of the sharp wave with time. Simultaneous D.C. and R.-C coupled recording (T.C. is the value of the time constant of R-C syst-,m). Note the increase in amplitude and duration. The after-discharge riding the peak of the sharp wave is accompanied by an additie~al negative swing. The R-C coupled record distorts the true wave form, especially fl~e Fosition of the after-discharge. In this and all suboacquent, figures an upward deflection indicates that the active (cortical) electrode is negative relative to a refe~',,snceelectrode on the frontal periosteum Calibration: 2 :¢~V tor the D.C. traces, 1 mV for the R-C traces. Time marker: I scc.
A-O.15 see
A--DC
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P~' ,,c*.alspiking, iarvai p~.-o~:-~--. and ~bc ~ginning of a sustained ~iox.ysm ¢,.vhich lasted an additional 2 rain). Tho two up~.:r ~.'- and " a D.C. a a:Fiitier • tra¢;~ ~re f"om an R-~ ~ou#,~;; ' 7 ~ n.'...; connee*~J to the same pr~Jr ,,~ ~lectr.".?.c,s. f n e ~.ter-di~cbo., ~e r;dcs t.'~ peal: of a scco~,~exy ne:~ative deflation. Note 'that ~!*.hous~: tae sha~, .~':vo~ar~ p i:6do.,z,iea~,~iy pos:tive 4 mm from th6 foc;,.g (¢race. B) the after-discharge .-',tiffis ~:,. at th= peak ,~f the g~i:ondary negat.h.,o w,e.ve.
Elec:roenc¢,)£ clin. Neu.~pi,ysiol., ~96~, 19:63-'/4
D. C. ACTiViT'f iN FOCAL SEIZURES
65
DC
.I 2mV TC 0.12
Isec
Fig. 3 "Larval" seizures consisting of rapidly repeated sharp waves summating to form a "staircase" shift (absent from the R-C tracing). Note the marked periodicity in the absence of an identifiable stimulus. The two traces are from the same pair of electrodes.
Injecting the cortex with amounts as small as 0.01 ml of a solution of penicillin (I million units/ml), or applying the penicillin as a drop, while suitably epileptogenic, led to a spread of the solution over a wide area and the creation of a complex, highly irregular focus. The electrica~ structure of the field was determined bv ~imultaneous recording from 4-6 point~ arranged in a straight line. Lesions wer," :,ado in tb~ middle suprasylvian gyrus .('., ex~,eriments) and the lateral gyru~, (:~ J e.~peri.~ents). The penicillinsoaked r,.,Jtting paper was removed w hen seizure ....tray became established (usually 30-60 min after application) in e",der t,o map the fie~d. Rf!SUL'rS Analysis of the various phenomena was simpl~.q.ed by t~'Jng ,.heir relative time c.f occurfence into accou,~.o .'n general, an e x ~ r i m e a t could be divi,ded into 3 phases'O) the establishment of ~harp wave discharges ~,:~d "larval" i¢~al episodes; (II) orgaoized r~pJd seizure di,:charges of limited d~r~tiort s~ith a clear establishment c,f the f : | d l, and 01D a period of intense. The termjbc~z is used to de~ri~: the central region cf the electric~/ield fr~.:n which local (i.e.. not projected) sharp w~¢6 and seizure discharge~ were recor,led. The focus was located just inside the edge of the lesion produce.d by the topical application of penicillin. Thus the lesion was 3-4 mm in diameter, the field approximately 2 cm in diameter and the focus was a small, roughly circular area in the center. Out technique did not permit a more detailed analysis of the electrical .structure_cf t~:,
focus.
prolonged paroxysms spreadir.g widely throughout the cortex and the depths of the brain.
1. Immediate effects o f the penicillin wf, after recording from the normal cortex ~o obtain a baseline, a drop of penicillin was applied next to one of the agar-agar salt bridges, a negative D.C. shift of 2-10 mV (reference electrode on the frontal periosteum) was seen (el Esberard t % t ) . w~, large part this el tang0 is an artifact generated at the tip of the salt bridge. A similar change was found in control experiments in which the salt bridges touched a piece of salinesoaked b'otting paper resting on a piece of clean wood. Although it would be of significance for certain theorie.z of epileptoger, esis if a genuine ch~,,nge in the standing potential of the cortex occurs when an epileptogenic agent is applied, filrther experiments will be necessary before it can be determined whether or not such a change takes place. The application of penicillin to the cortex initially produced hyperemia followed in some experiments by blanching.
2. Pre-ictal activity Sharp wave discharges appeared 30 see.-5 min after the application of the penicillin. At first of fairly low voltage (0.2-0.5 mV), they soon become larger and of longer duration (Fig. 1). Thirty to 60 min later, one or more low voltage fast waves appeared at, or shortly after, the peak of the sharp wave. This after-discharge (as Ralston-1958-named it) was associated with Electroeneeph. olin. NeurophysioL, 1965, 19:63-74
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R. J. GUMNIT AND T. TAICAHASHI
temporal prolongation and an abrupt increase of the negative phase (Fig. 2). Prior to the appearance of well developed paroxysmal activity, "larval" forms were noted. Earliest seen was a series of high voltage sharp waves which occurred so close together in time that one built upon the other, result~.ng in a deflection from the baseline (Fig. 3). These often appeared with a remarkable periodicity in the absence of any indentifiable trigger (cf Prince !964). After the appearance of the after-discharge a short (about I scc) paroxysm arising from it with an abrupt shift in the D. C. level was more commonly seen (Fig. 2).
3. The transition from interictal to paroxysmal activity Repetitive, well formed paroxysms appeared 30-120 min after the appli¢'~tion of the penicillin. A prolonged series of sharp wave discharges with an increase in frequency to approximately 2-3/sec, associated with a staircase shift from the baseline, was always seen. In less than 10% of the cats it was the only type of paroxysm to occur and would last 5-50 sec. In the other - 100
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F7
Fig. 5 Field about the focus. In this and subsequent figures, the numbers indicate the distance in mm from the focus (F). Positive and negative numbers indicate distance in opposite direction. At 5 mm from the focus a low voltage paroxysm is recorded without significant shift. However, at 7 mm from the focus a positive shift is seen. Note that the shift is negative at 3 mm from the focus despite the predominantly positive sharp waves. The large interictal sharp wave at F7 is not common (compare with Fig. 6).
animals, once ictal activity was well established, the predominant pattern was one of an abrupt shift of the baseline accompanying a 10/sec or faster discharge growing out ofthe after-discharge (Fig. 2). Although our technique did not permit a detailed analysis, we have the impression that the discharge rate during the paroxysm varied directly with the size of the D. C. shift (e.g., Fig. 11, trace F). Such paroxysms lasted from 5 sec to well over 2 rain. If the penicillin had been applied to the post. lateral gyrus, photic stimulation readily elicited typical sustained discharges, even before they had appeared spontaneously.
Fig. 4
Scattergram showing the distribution and relative size of the D.C. shifts accompanying paroxysms front one plotting of the field from each of : 1 experiments. To make the different experiments comparable, shin amplitudes are expressed as percent of the amplitudq: at the focus (always the l~.rgest). The data were grouped in blocks representing 2 mm intervals. All the data were plotted to one ,.fide of the fc~us, resulting in more than 11 points in certain intervals.
4. Direction and size of shift At the focus a baseline shift invariably accompanied the paroxysm and was always negative with respect to the frontal periosteum, the depth of a distant portion of the hemisphere or the spinal fluid in the cisterna magna. Such shifts averaged 3 mV (range 1-6 mV). The location of Electroenceph. clin. Neurophysiol., 1965, 19:63-74
67
D. C. ACTIVITY IN FOCAL SEIZURES
2sec
F 16
F I0
F8
F6
Fig. 6 D.C. and sharp wave fields about the focus. The field disappears between 10 and 16 mm from the center. The contrast between the abrupt shift at the focus and the more ~adual change in the periphery is particul~.rlyclear, ,as is the change in the shape of the sharp wave. Also seen is a spread of D.C. change and fast wave discharge from center to periphery. the reference electrode is important. The paroxysmal activity spread through the depth of the cortex and subcortically, especially late in an experiment, and a careful selection of a reference point was necessary. Therefore the results of transcortical recording were often difficult to evaluate.
5. Electrical structure of the field The electrical field was not uniform in time or in space. In all experiments in which the penicillin lesion was restricted to an area of 4 mm in diameter or less (as indicated by the spread of the methylene blue) a characteristic distribution of potential change during the paroxysm was obtained (Fig. 4). The center of the field was marked by the highest voltage and the most abrupt surface negative shifts. Wi~h increasing distance from the center, the shifts became more gradual and smaller in amplitude
until a narrow zone was reached in which no shift in the baseline (Fig. 5) was seen and in which the rapid rhythmic seizure activity was hardly appreciable. Still further from the center, positive shifts occurred with the rapid seizure discharges. The gradient of change from negative to positive shifts was often quite steep. OrL one occasion there was a 2 mV negative shift at the focus and a 0.6 mV positive shift at a point only 1.5 mm distant. The positive shifts also decreased in amplitude with distance from the center until the field disappeared and neither conventional EEG nor D.C. changes were observed (Fig. 6). This pattern of central negativity and peripheral positivity was reliably seen although it was not always symmetrical. The interictal sharp wave activity followed a similar distribution. The center of the field was marked by high voltage, monophasic, surface negative sharp waves with a prolonged decay.
Electroenceph. clin NeurophysioL, 1965, 19:63-74
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R . J . GUMNIT AND T. TAKAHASHI
J Irr;/ F2
F6
Fig. 7 Field about the focus. The sharp waves are largest and longest and the shift most a~rupt over the focus, in this experiment, the field was quite symmetrical along the sagittal .v~ane.
|o~
FI
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Fig. 8 March of a D.C. front along the length of the suprasylvian gyrus at a speed of approximately 10 mm/min. Although in this example the seizure appears to cease at the focus (FI) I se¢ before ending in the periphery, this is not the usual situation.
Electroenceph. clin. Neurophysiol., 1965, 19:63-74
D. C. ACTIVITYIN FOCAL SEIZURES
~
With increasing distance from the center the sharp waves at first became briefer, smaller in amplitude (Fig. 7) and polyphasic with an initial positive spike. They were often predominantly positive while still in the region of negative shifts (Fig. 5). The negative shift at the focus was always larger than any positive shift in the periphery. However, in the center of the area to which the penicillin had been applied, activity similar to that seen at the edge of the lesion, but lower in voltage, was occasionally encountered. This was probably electrotonic spread of activity from the edge of the lesion in a situation where the strong penicillin solution had severely damaged the underlying cortex. Late in the experiment, when the organized rapid discharges lasted for several minutes at a time (phase HI), the seizure activity spread
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Fig. 9 Scattergram showing the distribution and -elative size of the D.C. shifts accompanying paroxysms from one plotting of the mirror field from each of 12 experiments. Data plotted as in Fig. 4 except that the largest shift, positive or negative, was assigned a value of 100~. Note that the positive shifts in the peripheral portions of the field were more prominent here than about the primary focus.
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Fig. 10 Distribution of D.C. shifts within a mirror field. F: activity at the focus. MO: center of the mirror field, note small negative shift. M2-MS: Numbers indicate distance in mm from center of field in a chain rostrally from the center of the mirror focus along the supraaylvian gyms. Electroencepk. elin. Neurophyslol., 19(~, 19:63-74
70
R. .I. G U M N I T A N D T. TAKAHASHI F t |
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...J 0.2mV
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Fig. I I Distribution of D.C. shifts within a mirror field. F: activity at the focus. Note that no shift is seen at the center of the field (MO) during paroxysm, but that positive shifts are encountered on either side. beyond the bounds of the field seen during phase [1 to invade hitherto uninvolved areas of surrounding cortex (Fig. 6, 8). In the initial few seconds of the seizure the typical configuration and behavior was observed. The peak of the negative D.C. shift (preceded in several experiments by a smaller positive swing) was then observed to "march" across the cortex at a speed of 5-20 mm/min. As the peak of the wave arrived at a point distant from the center the rapid discharge in this region would intensify and, if the front invaded a previously silent area, appear for the first time. In this late phase of the experiment the entire brain became involved in the seizure activity which appeared to be dependent on triggering by the focus. In contrast to the complex relations among various regions of the cortex during the paroxysms, the cessation of each seizure was essen-
tially simultaneous throughout. After the paroxysms the D.C. level slowly decayed to the previous baseline over a period of 3-30 sec, during which time the conventional EEG recorded electrical silence. Late in the experiment, after very long seizures, this return was eollowed by a low voltage slow (up to 60 sec) swing in the opposite direction.
6. Mirror activity In an attempt to identify epileptiform activity projected to the homologous contralateral cortex (mirror area), the primary cortex was stimulated prior to the application of the penicillin and the point of maximal cailosal response located. This point did not always coincide with that of maximal projected seizure activity and differences of 2-6 mm were frequently seen. The latter was defined as the site of the earliest and largest Llectroence~h. clin. Neurophysiol., 1965, 19:63-74
D. C. ACTIVITYIN FOCALSEIZURES amplitude projected activity. Sharp wave discharges appeared in the mirror area 5-30 rain later than in the primary area. Still later, after organized rapid seizure discharges developed, the temporal relations between sharp wave dfscharges in the primary focus and the mirror area became complex and variable. Even with CRO recording the projected sharp waves followed, appeared simultaneously or even preceded the sharp wave discharges at the focus. Organized seizure activity in the mirror area invariably was of lower voltage and appeared later than in the primary area. It was initially clearly dependent upon activity at the focus. The electrical structure of the mirror field was similar to that of the primary field (Fig. 9). However, unlike the latter, the center of the mirror field occasionally displayed only small negative shifts (Fig. 10) or~ rarely, was isopotential (Fig. ! I) during the rapid seizure discharge. Positive shifts were relatively more prominent in the periphery of the mirror field than in that of the primary area and could even be larger than the central negative shift. In the mirror field, the largest shifts of either polarity were smaller than the primary activity, ordinarily by a factor of 10 or more (average 0.2 mV). In only one experiment were mirror shifts larger than 1 mV seen. Mirror sharp waves were also smaller, but only by a factor of 2-5 to 1. In the late s:ages of a few experiments rapid seizures discharges appeared in the mirror field independently from those of the primary focus. DISCUSSION 1. lnterictal sharp wave discharges There was a clear alteration in shape of these discharges in the primary field proceeding from the center to the edge. Not only were the sharp waves at the most active portion of the field larger, they were also longer, with a prominent negative slow component upon the crest of which an after-discharge might appear. Ralston (1958) had clearly established the importance of the after-discharge in an understanding of fully developed seizure activity. Our data suggest that the abrupt D.C. change accompanying the afterdischarge might have a causal relationship with the associated paroxysmal discharge.
71
Goldring and O'Leary (1954) and O'Leary and Goldring (1960), ie their study of seiz,lre activity produced by the topical application of veratrine and strychnine, found similar slow phenomena with some differences in polarity. Since they routinely used a transcortical derivation, one cannot readily compare the wave forms recorded in their experiments with the ones recorded here. 2. D.C. changes with rapid seizure discharges Previous studies of D.C. changes in association with seizure discharge have yielded confusing results. The complex baseline swings reported before, during and after the organized rhythmical paroxysms produced by intravenous injection of convulsants (Vanasupa et al. 1959) may have been the results of the systemic effects of the drug, with the widespread changes affecting the reference electrode. The use of a carefully controlled method of topical application and the analysis of the field about the lesion in our experiments revealed a consistent and clear-cut picture. Both the actual seizure and the afterdischarge, which Ralston (1958) identified as an embryo or larval seizure, were associated with an abrupt change in the D.C. level occupying the same time period. No such abrupt changes were ever seen in the absence of paroxysmal activity in our experiments. Although a causal relation cannot be asserted with certainty, the results suggest that the fully developed seizure discharge occurs when a critical change in the D.C. level has taken place. 3. The structure of the field Aithough seizure activity may be widely projected from the focus, we agree with Ralston and Papatheodorou (1960) that the after-discharge tends to be more localized. In our experiments it was generally restricted to a radius of 1 cm from the center of the penicillin lesion. However, an even more striking degree of localization was revealed by D.C. recording. The large prolonged negativity following the interictal sharp wave was seen only in the immediate vicinity of the penicillin-produced lesion. Activity recorded only a few millimeters away, while quite striking, was often polyphasic and briefer than that seen at tile center of the field. This might be of clinical Electroenceph. clin. NeurophysioL, 1965, 19:63-74
72
Z . J . GUMNIT AND T. TAKAHASHI
significance in the localization of naturally occurring foci. The higher voltage and prolonged afternegativity of the sharp wave at the focus are of theoretical significance in that they suggest that the D.C. field in the vicinity of the penicillin lesion is much stronger than in the periphery. This concept is supported by the morphology and size of the D.C. shift associated with the fully developed seizure discharges. It is only in the immediate neighborhood of the penicillin lesion that high voltage and abrupt negative D.C. shifts were seen at the onset of the paroxysms. We conclude that this is the locus of the greatest field strength since not only was the amplitude greatest but also the potential gradient was steepest. Mahnke and Ward (1961) reported a gradient of I mV/mm in the standing field about a chronic epileptic focus in a monkey. Although we did not routinely make precise measurements of the gradients for the shifts associated with rapid seizure discharges, a few were clearly of the same order of magnitude. The scattergram shown in Fig. 4 suggests a median slope of about 0.5 mV/mm. However, we do not feel our data should serve as the basis for quantitative speculations about field strength or intensity of current flow. The scatter above and below the zero line at the 3-5 mm interval reflects the variations in field strength and extent among the cats. In most experiments phase reversal occurred between 3 and 6 mm from the center of the field. The distribution about the focus resembles that which is obtained as an active electrode passes through the isopotential boundary to the other pole of a dipole. The dipole in this case would be in the form of a circle (negative in the center) horizontally oriented along the plane of the cortex. Supporting evidence for this concept comes from the results of depth electrode studies in our laboratory (to be published elsewhere) and by Matsumoto and Ajmone Marsan (1964c). No reversal of polarity was seen within the depth of the cortex in the region of the penicillin lesion. There are alternative explanations, however, including the possibility that the active area is deep within the cortex underlying the region from which surface positive shifts were obtained. It would be of great theoretical significance if
the presence of a strong horizontal dipole be confirmed. Most theories dealing with the role of field changes in the function of the cortex have emphasized the vertical orientation of neuronal elements and have interpreted dipoles in terms of relative differences in activity between apical and basal dendrites. A horizontal dipole 1-2 mm in extent could be interpreted in terms of relative depolarizations of one portion or another of dendrites extending horizontally in the cortex. However, the large distances involved in the experiments reported here (5 mm or more)might imply that the cortex under these conditions is functioning as neuropil (for an alternative view and a more extensive discussion of the lateral spread or cortical activity, see Brooks and Enger 1959). The existence of such a dipole might explain why the edge of a clinical lesion is the site of maximum epileptiform activity. The locus of steepest gradient would be the site of greatest current flow and, presumably, neurons in tills region would be most strongly affected. The spread of the D.C. wave front over the field (at 5-20 mm/sec) should not be identified with spreading depression. Although spreading depression may be accompanied by seizure activity (Van Harreveld and Stamm 1953), its rate of spread is about 2-3 ram/rain and not more than 5 mm/min (Marshall 1959). In addition, the very large (over l0 mV) swings frequently seen in spreading depression were absent in these experiments. Adrian (1936) described the spread of activity in seizures induced by electric shock at 25 cm/ sec. Rosenblueth and Cannon (1942) observed similar changes. Hughes and Mazurowski (1964) reported seizure spread in the cingulate cortex at a rate of 20-25 mm/min. No D.C. observations were made in any of these studies.
4. Mirror activity The structure of the field on the mirror area has many similarities to that of the primary. However, the relative prominence of peripheral positive shifts as compared to the central negative shifts may indicate that the primary focus projects to a large degree to elements deep within the cortex of the opposite hemisphere. The lack of agreement between the point of maximal callosal response to a shock applied to Electroenceph. clin.NeurophysioL, 1965, 19:63-74
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D. C. ACTIVITY IN FOCAL SEIZURES
the center of the area of penicillin application and the center of t!~e mirror field may be a reflection of the more intense sharp wave discharges at the edge of the penicillin lesion. However, *.he conplex relations between the activities at the two loci suggest that subcortical structures may also be involved (cf Gastaut and Fisher-Williams 1959). Rutledge and Kennedy (1960) have presented evidence for such a mechanism in the case of the transcallosal response.
changes from the depth of the cortex in the neighborhood of the penicillin focus. A negative shift was an invariant accompaniment to each paroxysm recorded by the conventional EEG. However, they did not find a precise temporal correlation with either the surface EEG changes or unit depolarization~ These authors, however, report no attempt to analyze the temporal relations as a function of the location of the electrodes within the field. Such an analysis might clarify the situation. The paroxysm and the D.C. shift need not be considered as separate events; one does not exist without the other. The initiation of the process depends upon the local sharp wave discharge, which can arise in part from the effect of incoming impulses from other portions of the brain (Smith and Pupura 1960; Matsumoto 1964; Matsumoto and Ajmone Marsan 1964a). This might explain the clinical phenomena of photogenic and audiogenic epilepsy, as well as the ability of photic stimulation to trigger paroxysms in our (and many other) experiments.
5. Theoretical formulation The data now at hand suggest the following heuristic t~ypothesis. Each sharp wave discharge at the focu,.; is accompanied by a surface negative swing which represents a relative depolarization of many cortical elements (Li 1963; Matsumogo and Ajmone Marsan 1964b), resulting not only in a localized increase in excitability but also in a flow of current from surrounding cortex. This induced current flow affects the excitability of other elements in the neighborhood of the focus (cf GIoor et ai. 1961). When a critical level of depolarization is reached, perhaps as a result of SUMMARY temporal summation, the conditions are suffiExperimental seizure foci were produced by cient for a localized rapid seizure dischargem the after-discharge. The notion of a critical level the topical application of penicillin to the cortex of depolarization is not entirely speculative. of cats. D.C. recording revealed that an electrical Matsumoto and Ajmonc Marsan (1964c) found field was produced which surrounded the lesion (cortical intracellular D.C. recording) a prolonga- by a radius of approximately l cm. Sustaine~ tion and an oscillation of paroxysmal mem- paroxysms were associated with D.C. shifts; brane depolarization of single units closely strongly negative with an abrupt rise at the cencorrelated with the after-discharge in an indenti- ter o r the field, more gradual and positive in the cal experimental situation. If a very high level of periphery. The interictal sharp wave discharges excitability is attained, a sustained paroxysm had a similar distribution. They were high voltoccurs, accompanied by a more intense flow age, monophasic, negative and with a long duraof current from surrounding structures. As each tion at the center; briefer, polyphasic and usually surrounding portion of cortex becomes involved predominantly positive at the periphery. When in the seizure, the current flow spreads outward, an after-discharge was present, it rode the crest resulting in the "march" of both D.C. shift and of the prolonged negative phase of the sharp paroxysmal discharge across the field. As the wave discharge. The activity projected to the diameter of the circle increases, the density of homologous area of the opposite hemisphere was current ltow decreases until a point is reached also surveyed and a similar distribution was where neighboring elements are not sufficiently found. However, the activity was of lower voltage and the peripheral positive shifts were relaexcited and the process ends. Implicit in this scheme is the notion that not tively more prominent. It is concluded that D.C. all units are active simultaneously. Results from changes play a prominent role in the activity of single unit recording in the experiments of the penicillin focus and suggested that a horiMatsumoto and Ajmone Marsan (1964c) are in zontal dipole is generated which may play a agreement. They also recorded extracellular D.C. causal role in the genesis of the paroxysms. Electroenceph. olin. Neurophysiol.,
19~5,
19:63-74
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R.J. GUMNIT AND T. TAKAHASHI
The authors are indebted to Drs. J. R. Knott, D. W. McAdam, W. R. Ingram, A. L. Sahs and M. W. Van Al!¢n for their generosity in providing laboratory space, equipment and advice. REFERENCES
ADRIAN,E. D. The spread of activity in the cerebral cortex..L Physio,. (Lend.), 1936, 88: 127-161. BROOKS,V. B. and ENGER, P. S. Spread of directly evoked responses in the cat's cerebral cortex. J. gen. Ph~,sioi., 1959, 42: 761-777. ESBERARD, C. A. The influence of penicillin on steady cortical potentials. J. Physiol. (Lend.), 1961, 1.59: 28P-29P. GASTAUT, H. and FISCHER.WiLLIAMS, M. The physiopathology of epileptic seizures. In J. FIELDet al. (Eds.), Handbook of physiology, Sect. I, Amer. Physiol. See., Washington, 1959, 1: 329-363. GLOOR, P., VERA, C. L., SPERTI, L. and RAY, S. N. Investigations on the mechanism of epileptic discharge in +he hippocampus. Epilepsia (Amst.), 1961, 2: 42-62. GOLDRING, S. and O'LE,~.~v, J. L. Correlation between steady transcorfical potential and evoked response. II. Effect of veratrine and strychnine upon the responsiveness of visual cortex. Electroenceph. clin. Neurophysiol., 1954, 6: 201-212. GROSSMAN,R. G. and GUMNIT,R. J. A method of recording from the cortex while maintaining physiological conditions. Electroenceph. clin. NeurophysioL, 1960, 12: 920-921. GUMNIT, R. J. D.C. potential changes from auditory cortex of cat. J. NeurophysioL, 1960, 23: 667-675. HUOHt~S, J. R. and MAZUROWSgI, J. A. Studies on the supracallosal mesial cortex of unanesthetized, conscious mammals, ll. Monkey, D. Vertex sharp waves and epileptiform activity. Electroenceph. clin. Neurophysiol., 1964, 16: 561-574. LANDAU, W. M. ,~,n anal>sis of tP" cortical response to antidromic pyramidal tract stimulation in the cat. Electroenceph. clin. Neurophysiol., 19$6, 8: 445-456. Lt, C.-L. Cortical intracellular sy~aptic potentials in response to thalamic stimulation. J. cell. comp. PhysioL, 1963, 6,: 165-179. MAHNKE, J. H. and WAnD JR., A. A. Standing potential characteristics of epileptogenic focus. Epilepsia
(Amst.), 1961, 2:161 -169. MARSHALL,W. H. Spreading cortical depression of Le~o. Physiol. Rev., 1959, 39: 239-277. MATSUMOTO,H. Intracellular events during the activation of cortical epileptiform diseharges. Electroenceph. clin. NeurophysioL, 1964,17: 294-307. MATSUMOTO,H. and AJMoh,~ M,~SAN, C. Cellular mechanisms in experimental epileptic seizures. Science, 1964a, 144: 193-194. MATSUMOTO, H. and AJMONE MARSAN, C. Cortical cellular phenomena in experimental epilepsy: Interictal manifestations. Exp. Neurol., 1964b, 9: 286-304. MATSUMOTO, H. and AJMONE MARSAN, C. Cortical eel lular phenomena in experimental epilepsy: Ictal manifestations. Exp. NeuroL, 1964c, 9: 305-326. O'LEARY, J. L. and GOLDRINO, S. Slow cortical potentials. Their origin and contribution to seizure discharge. Epilepsia (Amst.), 1960, 1: 561-574. PRINCE, D. Long duration cyclical changes in excitability of spike loci. Eiectroenceph. din. NeurophysioL, 1964, 17: 453-454. RALSTON,B. L. The mechanism of transition of interictal spiking foci into ictal seizure discharges. Electroenceph, clin. Neurophysiol., 1958, 10: 217-232. RALSTON, B. L. and PAPATHEODOROU,C. A. The mechanism of transition of interictal spiking loci into ictal seizure discharges. II. Observations in ,-nan. Electroenceph, clin. NeurophysioL, 1.o50, 12: 297-304. ROSENnLUETH, A. and CANNON,W. B Cortical responses to electrical stimulation. Amer. J. Physiol., 1942, 135: 690-741. RUTLED~E, L. T. and KENNEDY, T. T. Extracallosal delayed responses to cortical stimulation in ch|oralosed cat. J. Neurophysiol., 1960, 23: 188-196. SMITH JR., T. G. and PURPURA, D. P. Electrophysiological studies on epileptogenic lesions of cat cortex. Electroenceph. clin. Neurophysiol., 1960, 12: 59-82. VANASUPA,P., GOLDRINO, S. and O'LEARY, J. L. Seizure discharges effected by intravennusly administered convulsant drugs. Eiectroenceph. clin. Neurophysiol., 1959, li: 93-106. VAN HARREV~LD,A. and STAMM,J. S. Spreading cortical convulsions and depressions. Y. Neuropl~ysicL, 1953, 16: 352--366. WALKER, A. E., JOHNSON, H. C. and KOLLROS, J. J. Penicillin convulsions. Surg. Gynec. Obstet., 1945, 81: 029-701.
Reference: GUMNIT, R. J. and TAKAHASHI,T. Changes in direct current activity during experimental focal seizures. Electroenceph. clin. Neurophysiol., 1965, 19: 63-74.
IN DIRECT
CURRENT
EXPERIMENTAL
ACTIVITY
FOCAL
63
DURING
SEIZURES x
ROBERT J. GUMNrr, M.D. 2 AND TAKEOTAKAHASHI,M.D. 3 Department of Neurology and Division of Electroencephalography and Neurophysiology (Department of Psychiatry), State University of lowa College of Medicine, lowa City, lowa and the Division of Neurology, University of Minnesota School of Medicine, Minneapolis, Minn. (U.S.A.)
(Accepted for publication: December 14, 1964)
INTRODUCTION
The physiological mechanisms whereby a small portion of brain tissue gives rise to a seizure are poorly understood. In the case of one widely investigated type of experimental epileptic focus (produced by the topical application of penicillin to the cerebral cortex) there is even disagreement as to whether the organized seizure discharges are the result of an increased frequency of sharp wave discharge (Walker et ai. 1945) or whether they arise from an afterdischarge following the sharp waves (Ralston 1958). We have reinvestigated the development and activity of the penicillin-induced focus with the expectation that the situation would be clarified by an analysis of the D.C. changes, especially in (I) the transition from interictal to ictal episodes and (2) the structure of the electrical field about the focus. METHOD Experiments were carried out in 66 cats. Under general anesthesia (i.v. Pentothal in the first 25 animals, ether in the others), a tracheotomy was performed and trephine holes were placed symmetrically over both hemispheres. Local anesthesia (1% lidocaine) served to reduce the amount of the general agent required. A nylon ring and lucite insert assembly, cemented to the skull, permitted an area of either 10 or 16 1 This investigation was supported in part by Research Grants NB 4362 and NB 5466 of the National Institute of Neurological Diseases and Blindness. Present address: Department of Neurology, Ancker Hospital, Saint Paul, Minn., U.S.A. a Fuibright Scholar, on leave from the Department of Neuropsychiatry, College of Medicine, Hirosaki University, Hirosaki, Japan.
mm in diameter to be surveyed (Grossman and Gumnit 1960). Although this device allowed only a portion of the field to be sampled at any one time, it kept the cortex warm and moist and fixed the salt bridges securely. A smaller ring cemented to the bone over the frontal sinus anchored the reference electrode. At the conclusion of the operation the local anesthesia was reinforced and additional local anesthesia was used throughout the experiment. The animal was loosely held in a modified (non-painful) Czermack head holder, paralyzed with gallamine triethiodide and maintained on artificial respiration under conditions designed to keep discomfort to a minhnum. General anesthesia is a potent anticonvulsant, therefore it could not be maintained throughout the experiment. Calomel half-cell electrodes (Beckman #270) with saline-agar salt bridges (0.6 mm inner diameter) were used in all experiments. Records were obtained with a Grass Model 5 polygraph and 5Pl chopper-modulated pre-amplifiers (Gumnit 1960) and (with a Grass C4 camera) from a Tektronix 502 CRO, at times using Grass P5 or P6 pre-amplifiers. Stin,.alation was carried out with a Grass $4 stimulator through a Grass SIU4 isolation unit or the circuit described by Landau (1956). The stimulating electrodes consisted of a tripolar arrangement of stainless steel wires (0.25 mm in diameter) 0.5 mm apart at the apices of an equilateral triangle. The unfiltered light of a Grass PS-2 photo stimulator was the source of light flashes. lr~ several experiments, arterial blood pressure was monitored with a Statham P23AC transducer and cortical temperature with a thermistor bridge circuit. Cortical temperatures as low as 33°C did not affect the seizure activity, and arElectroenceph. olin. NeurophysioL, 1965, 19:63-74
64
R.J. GUMNIT AND T. TAKAtI~,3HI
cil!i~_ G were topically appalled in a variety of ways. The most satisE~gor,:, m e t h o d was to m a k e a tkick paste o f pe.,,.icill/n a n d methylene blue and apply a piece of '-~o~,dng .... " p a p e r 3 m r n ;q d i a m e t e r , s o a k e d ~n tkds paste, to t h e C[~:Le,X.
terial b l o o d pressure d i d n o t vary w i t h a p a r o x y s m or at other ,[mes d u r i n g the experiment. S u r f a c e electrode locations were m a r k e d with an e'¢ctroc o a g u l a t i n g unit. Both s o d i u m a n d p o t a s s i u m crysm!iMe peni-
6rain
~min
ZOmin
i
&
|
i
/!
,\ !
.
|
Fig. { Cha::,~,-:,~ shape of the sharp wave with time. Simultaneous D.C. and R.-C coupled recording (T.C. is the value of the time constant of R-C syst-,m). Note the increase in amplitude and duration. The after-discharge riding the peak of the sharp wave is accompanied by an additie~al negative swing. The R-C coupled record distorts the true wave form, especially fl~e Fosition of the after-discharge. In this and all suboacquent, figures an upward deflection indicates that the active (cortical) electrode is negative relative to a refe~',,snceelectrode on the frontal periosteum Calibration: 2 :¢~V tor the D.C. traces, 1 mV for the R-C traces. Time marker: I scc.
A-O.15 see
A--DC
o°
d.
P~' ,,c*.alspiking, iarvai p~.-o~:-~--. and ~bc ~ginning of a sustained ~iox.ysm ¢,.vhich lasted an additional 2 rain). Tho two up~.:r ~.'- and " a D.C. a a:Fiitier • tra¢;~ ~re f"om an R-~ ~ou#,~;; ' 7 ~ n.'...; connee*~J to the same pr~Jr ,,~ ~lectr.".?.c,s. f n e ~.ter-di~cbo., ~e r;dcs t.'~ peal: of a scco~,~exy ne:~ative deflation. Note 'that ~!*.hous~: tae sha~, .~':vo~ar~ p i:6do.,z,iea~,~iy pos:tive 4 mm from th6 foc;,.g (¢race. B) the after-discharge .-',tiffis ~:,. at th= peak ,~f the g~i:ondary negat.h.,o w,e.ve.
Elec:roenc¢,)£ clin. Neu.~pi,ysiol., ~96~, 19:63-'/4
D. C. ACTiViT'f iN FOCAL SEIZURES
65
DC
.I 2mV TC 0.12
Isec
Fig. 3 "Larval" seizures consisting of rapidly repeated sharp waves summating to form a "staircase" shift (absent from the R-C tracing). Note the marked periodicity in the absence of an identifiable stimulus. The two traces are from the same pair of electrodes.
Injecting the cortex with amounts as small as 0.01 ml of a solution of penicillin (I million units/ml), or applying the penicillin as a drop, while suitably epileptogenic, led to a spread of the solution over a wide area and the creation of a complex, highly irregular focus. The electrica~ structure of the field was determined bv ~imultaneous recording from 4-6 point~ arranged in a straight line. Lesions wer," :,ado in tb~ middle suprasylvian gyrus .('., ex~,eriments) and the lateral gyru~, (:~ J e.~peri.~ents). The penicillinsoaked r,.,Jtting paper was removed w hen seizure ....tray became established (usually 30-60 min after application) in e",der t,o map the fie~d. Rf!SUL'rS Analysis of the various phenomena was simpl~.q.ed by t~'Jng ,.heir relative time c.f occurfence into accou,~.o .'n general, an e x ~ r i m e a t could be divi,ded into 3 phases'O) the establishment of ~harp wave discharges ~,:~d "larval" i¢~al episodes; (II) orgaoized r~pJd seizure di,:charges of limited d~r~tiort s~ith a clear establishment c,f the f : | d l, and 01D a period of intense. The termjbc~z is used to de~ri~: the central region cf the electric~/ield fr~.:n which local (i.e.. not projected) sharp w~¢6 and seizure discharge~ were recor,led. The focus was located just inside the edge of the lesion produce.d by the topical application of penicillin. Thus the lesion was 3-4 mm in diameter, the field approximately 2 cm in diameter and the focus was a small, roughly circular area in the center. Out technique did not permit a more detailed analysis of the electrical .structure_cf t~:,
focus.
prolonged paroxysms spreadir.g widely throughout the cortex and the depths of the brain.
1. Immediate effects o f the penicillin wf, after recording from the normal cortex ~o obtain a baseline, a drop of penicillin was applied next to one of the agar-agar salt bridges, a negative D.C. shift of 2-10 mV (reference electrode on the frontal periosteum) was seen (el Esberard t % t ) . w~, large part this el tang0 is an artifact generated at the tip of the salt bridge. A similar change was found in control experiments in which the salt bridges touched a piece of salinesoaked b'otting paper resting on a piece of clean wood. Although it would be of significance for certain theorie.z of epileptoger, esis if a genuine ch~,,nge in the standing potential of the cortex occurs when an epileptogenic agent is applied, filrther experiments will be necessary before it can be determined whether or not such a change takes place. The application of penicillin to the cortex initially produced hyperemia followed in some experiments by blanching.
2. Pre-ictal activity Sharp wave discharges appeared 30 see.-5 min after the application of the penicillin. At first of fairly low voltage (0.2-0.5 mV), they soon become larger and of longer duration (Fig. 1). Thirty to 60 min later, one or more low voltage fast waves appeared at, or shortly after, the peak of the sharp wave. This after-discharge (as Ralston-1958-named it) was associated with Electroeneeph. olin. NeurophysioL, 1965, 19:63-74
66
R. J. GUMNIT AND T. TAICAHASHI
temporal prolongation and an abrupt increase of the negative phase (Fig. 2). Prior to the appearance of well developed paroxysmal activity, "larval" forms were noted. Earliest seen was a series of high voltage sharp waves which occurred so close together in time that one built upon the other, result~.ng in a deflection from the baseline (Fig. 3). These often appeared with a remarkable periodicity in the absence of any indentifiable trigger (cf Prince !964). After the appearance of the after-discharge a short (about I scc) paroxysm arising from it with an abrupt shift in the D. C. level was more commonly seen (Fig. 2).
3. The transition from interictal to paroxysmal activity Repetitive, well formed paroxysms appeared 30-120 min after the appli¢'~tion of the penicillin. A prolonged series of sharp wave discharges with an increase in frequency to approximately 2-3/sec, associated with a staircase shift from the baseline, was always seen. In less than 10% of the cats it was the only type of paroxysm to occur and would last 5-50 sec. In the other - 100
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Fig. 5 Field about the focus. In this and subsequent figures, the numbers indicate the distance in mm from the focus (F). Positive and negative numbers indicate distance in opposite direction. At 5 mm from the focus a low voltage paroxysm is recorded without significant shift. However, at 7 mm from the focus a positive shift is seen. Note that the shift is negative at 3 mm from the focus despite the predominantly positive sharp waves. The large interictal sharp wave at F7 is not common (compare with Fig. 6).
animals, once ictal activity was well established, the predominant pattern was one of an abrupt shift of the baseline accompanying a 10/sec or faster discharge growing out ofthe after-discharge (Fig. 2). Although our technique did not permit a detailed analysis, we have the impression that the discharge rate during the paroxysm varied directly with the size of the D. C. shift (e.g., Fig. 11, trace F). Such paroxysms lasted from 5 sec to well over 2 rain. If the penicillin had been applied to the post. lateral gyrus, photic stimulation readily elicited typical sustained discharges, even before they had appeared spontaneously.
Fig. 4
Scattergram showing the distribution and relative size of the D.C. shifts accompanying paroxysms front one plotting of the field from each of : 1 experiments. To make the different experiments comparable, shin amplitudes are expressed as percent of the amplitudq: at the focus (always the l~.rgest). The data were grouped in blocks representing 2 mm intervals. All the data were plotted to one ,.fide of the fc~us, resulting in more than 11 points in certain intervals.
4. Direction and size of shift At the focus a baseline shift invariably accompanied the paroxysm and was always negative with respect to the frontal periosteum, the depth of a distant portion of the hemisphere or the spinal fluid in the cisterna magna. Such shifts averaged 3 mV (range 1-6 mV). The location of Electroenceph. clin. Neurophysiol., 1965, 19:63-74
67
D. C. ACTIVITY IN FOCAL SEIZURES
2sec
F 16
F I0
F8
F6
Fig. 6 D.C. and sharp wave fields about the focus. The field disappears between 10 and 16 mm from the center. The contrast between the abrupt shift at the focus and the more ~adual change in the periphery is particul~.rlyclear, ,as is the change in the shape of the sharp wave. Also seen is a spread of D.C. change and fast wave discharge from center to periphery. the reference electrode is important. The paroxysmal activity spread through the depth of the cortex and subcortically, especially late in an experiment, and a careful selection of a reference point was necessary. Therefore the results of transcortical recording were often difficult to evaluate.
5. Electrical structure of the field The electrical field was not uniform in time or in space. In all experiments in which the penicillin lesion was restricted to an area of 4 mm in diameter or less (as indicated by the spread of the methylene blue) a characteristic distribution of potential change during the paroxysm was obtained (Fig. 4). The center of the field was marked by the highest voltage and the most abrupt surface negative shifts. Wi~h increasing distance from the center, the shifts became more gradual and smaller in amplitude
until a narrow zone was reached in which no shift in the baseline (Fig. 5) was seen and in which the rapid rhythmic seizure activity was hardly appreciable. Still further from the center, positive shifts occurred with the rapid seizure discharges. The gradient of change from negative to positive shifts was often quite steep. OrL one occasion there was a 2 mV negative shift at the focus and a 0.6 mV positive shift at a point only 1.5 mm distant. The positive shifts also decreased in amplitude with distance from the center until the field disappeared and neither conventional EEG nor D.C. changes were observed (Fig. 6). This pattern of central negativity and peripheral positivity was reliably seen although it was not always symmetrical. The interictal sharp wave activity followed a similar distribution. The center of the field was marked by high voltage, monophasic, surface negative sharp waves with a prolonged decay.
Electroenceph. clin NeurophysioL, 1965, 19:63-74
68
R . J . GUMNIT AND T. TAKAHASHI
J Irr;/ F2
F6
Fig. 7 Field about the focus. The sharp waves are largest and longest and the shift most a~rupt over the focus, in this experiment, the field was quite symmetrical along the sagittal .v~ane.
|o~
FI
-
' . . . . . . . . . . . . . .
- . . . . . . . . . . . .
~
~3
Fig. 8 March of a D.C. front along the length of the suprasylvian gyrus at a speed of approximately 10 mm/min. Although in this example the seizure appears to cease at the focus (FI) I se¢ before ending in the periphery, this is not the usual situation.
Electroenceph. clin. Neurophysiol., 1965, 19:63-74
D. C. ACTIVITYIN FOCAL SEIZURES
~
With increasing distance from the center the sharp waves at first became briefer, smaller in amplitude (Fig. 7) and polyphasic with an initial positive spike. They were often predominantly positive while still in the region of negative shifts (Fig. 5). The negative shift at the focus was always larger than any positive shift in the periphery. However, in the center of the area to which the penicillin had been applied, activity similar to that seen at the edge of the lesion, but lower in voltage, was occasionally encountered. This was probably electrotonic spread of activity from the edge of the lesion in a situation where the strong penicillin solution had severely damaged the underlying cortex. Late in the experiment, when the organized rapid discharges lasted for several minutes at a time (phase HI), the seizure activity spread
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Fig. 9 Scattergram showing the distribution and -elative size of the D.C. shifts accompanying paroxysms from one plotting of the mirror field from each of 12 experiments. Data plotted as in Fig. 4 except that the largest shift, positive or negative, was assigned a value of 100~. Note that the positive shifts in the peripheral portions of the field were more prominent here than about the primary focus.
.
.
.
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Fig. 10 Distribution of D.C. shifts within a mirror field. F: activity at the focus. MO: center of the mirror field, note small negative shift. M2-MS: Numbers indicate distance in mm from center of field in a chain rostrally from the center of the mirror focus along the supraaylvian gyms. Electroencepk. elin. Neurophyslol., 19(~, 19:63-74
70
R. .I. G U M N I T A N D T. TAKAHASHI F t |
.
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= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
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Fig. I I Distribution of D.C. shifts within a mirror field. F: activity at the focus. Note that no shift is seen at the center of the field (MO) during paroxysm, but that positive shifts are encountered on either side. beyond the bounds of the field seen during phase [1 to invade hitherto uninvolved areas of surrounding cortex (Fig. 6, 8). In the initial few seconds of the seizure the typical configuration and behavior was observed. The peak of the negative D.C. shift (preceded in several experiments by a smaller positive swing) was then observed to "march" across the cortex at a speed of 5-20 mm/min. As the peak of the wave arrived at a point distant from the center the rapid discharge in this region would intensify and, if the front invaded a previously silent area, appear for the first time. In this late phase of the experiment the entire brain became involved in the seizure activity which appeared to be dependent on triggering by the focus. In contrast to the complex relations among various regions of the cortex during the paroxysms, the cessation of each seizure was essen-
tially simultaneous throughout. After the paroxysms the D.C. level slowly decayed to the previous baseline over a period of 3-30 sec, during which time the conventional EEG recorded electrical silence. Late in the experiment, after very long seizures, this return was eollowed by a low voltage slow (up to 60 sec) swing in the opposite direction.
6. Mirror activity In an attempt to identify epileptiform activity projected to the homologous contralateral cortex (mirror area), the primary cortex was stimulated prior to the application of the penicillin and the point of maximal cailosal response located. This point did not always coincide with that of maximal projected seizure activity and differences of 2-6 mm were frequently seen. The latter was defined as the site of the earliest and largest Llectroence~h. clin. Neurophysiol., 1965, 19:63-74
D. C. ACTIVITYIN FOCALSEIZURES amplitude projected activity. Sharp wave discharges appeared in the mirror area 5-30 rain later than in the primary area. Still later, after organized rapid seizure discharges developed, the temporal relations between sharp wave dfscharges in the primary focus and the mirror area became complex and variable. Even with CRO recording the projected sharp waves followed, appeared simultaneously or even preceded the sharp wave discharges at the focus. Organized seizure activity in the mirror area invariably was of lower voltage and appeared later than in the primary area. It was initially clearly dependent upon activity at the focus. The electrical structure of the mirror field was similar to that of the primary field (Fig. 9). However, unlike the latter, the center of the mirror field occasionally displayed only small negative shifts (Fig. 10) or~ rarely, was isopotential (Fig. ! I) during the rapid seizure discharge. Positive shifts were relatively more prominent in the periphery of the mirror field than in that of the primary area and could even be larger than the central negative shift. In the mirror field, the largest shifts of either polarity were smaller than the primary activity, ordinarily by a factor of 10 or more (average 0.2 mV). In only one experiment were mirror shifts larger than 1 mV seen. Mirror sharp waves were also smaller, but only by a factor of 2-5 to 1. In the late s:ages of a few experiments rapid seizures discharges appeared in the mirror field independently from those of the primary focus. DISCUSSION 1. lnterictal sharp wave discharges There was a clear alteration in shape of these discharges in the primary field proceeding from the center to the edge. Not only were the sharp waves at the most active portion of the field larger, they were also longer, with a prominent negative slow component upon the crest of which an after-discharge might appear. Ralston (1958) had clearly established the importance of the after-discharge in an understanding of fully developed seizure activity. Our data suggest that the abrupt D.C. change accompanying the afterdischarge might have a causal relationship with the associated paroxysmal discharge.
71
Goldring and O'Leary (1954) and O'Leary and Goldring (1960), ie their study of seiz,lre activity produced by the topical application of veratrine and strychnine, found similar slow phenomena with some differences in polarity. Since they routinely used a transcortical derivation, one cannot readily compare the wave forms recorded in their experiments with the ones recorded here. 2. D.C. changes with rapid seizure discharges Previous studies of D.C. changes in association with seizure discharge have yielded confusing results. The complex baseline swings reported before, during and after the organized rhythmical paroxysms produced by intravenous injection of convulsants (Vanasupa et al. 1959) may have been the results of the systemic effects of the drug, with the widespread changes affecting the reference electrode. The use of a carefully controlled method of topical application and the analysis of the field about the lesion in our experiments revealed a consistent and clear-cut picture. Both the actual seizure and the afterdischarge, which Ralston (1958) identified as an embryo or larval seizure, were associated with an abrupt change in the D.C. level occupying the same time period. No such abrupt changes were ever seen in the absence of paroxysmal activity in our experiments. Although a causal relation cannot be asserted with certainty, the results suggest that the fully developed seizure discharge occurs when a critical change in the D.C. level has taken place. 3. The structure of the field Aithough seizure activity may be widely projected from the focus, we agree with Ralston and Papatheodorou (1960) that the after-discharge tends to be more localized. In our experiments it was generally restricted to a radius of 1 cm from the center of the penicillin lesion. However, an even more striking degree of localization was revealed by D.C. recording. The large prolonged negativity following the interictal sharp wave was seen only in the immediate vicinity of the penicillin-produced lesion. Activity recorded only a few millimeters away, while quite striking, was often polyphasic and briefer than that seen at tile center of the field. This might be of clinical Electroenceph. clin. NeurophysioL, 1965, 19:63-74
72
Z . J . GUMNIT AND T. TAKAHASHI
significance in the localization of naturally occurring foci. The higher voltage and prolonged afternegativity of the sharp wave at the focus are of theoretical significance in that they suggest that the D.C. field in the vicinity of the penicillin lesion is much stronger than in the periphery. This concept is supported by the morphology and size of the D.C. shift associated with the fully developed seizure discharges. It is only in the immediate neighborhood of the penicillin lesion that high voltage and abrupt negative D.C. shifts were seen at the onset of the paroxysms. We conclude that this is the locus of the greatest field strength since not only was the amplitude greatest but also the potential gradient was steepest. Mahnke and Ward (1961) reported a gradient of I mV/mm in the standing field about a chronic epileptic focus in a monkey. Although we did not routinely make precise measurements of the gradients for the shifts associated with rapid seizure discharges, a few were clearly of the same order of magnitude. The scattergram shown in Fig. 4 suggests a median slope of about 0.5 mV/mm. However, we do not feel our data should serve as the basis for quantitative speculations about field strength or intensity of current flow. The scatter above and below the zero line at the 3-5 mm interval reflects the variations in field strength and extent among the cats. In most experiments phase reversal occurred between 3 and 6 mm from the center of the field. The distribution about the focus resembles that which is obtained as an active electrode passes through the isopotential boundary to the other pole of a dipole. The dipole in this case would be in the form of a circle (negative in the center) horizontally oriented along the plane of the cortex. Supporting evidence for this concept comes from the results of depth electrode studies in our laboratory (to be published elsewhere) and by Matsumoto and Ajmone Marsan (1964c). No reversal of polarity was seen within the depth of the cortex in the region of the penicillin lesion. There are alternative explanations, however, including the possibility that the active area is deep within the cortex underlying the region from which surface positive shifts were obtained. It would be of great theoretical significance if
the presence of a strong horizontal dipole be confirmed. Most theories dealing with the role of field changes in the function of the cortex have emphasized the vertical orientation of neuronal elements and have interpreted dipoles in terms of relative differences in activity between apical and basal dendrites. A horizontal dipole 1-2 mm in extent could be interpreted in terms of relative depolarizations of one portion or another of dendrites extending horizontally in the cortex. However, the large distances involved in the experiments reported here (5 mm or more)might imply that the cortex under these conditions is functioning as neuropil (for an alternative view and a more extensive discussion of the lateral spread or cortical activity, see Brooks and Enger 1959). The existence of such a dipole might explain why the edge of a clinical lesion is the site of maximum epileptiform activity. The locus of steepest gradient would be the site of greatest current flow and, presumably, neurons in tills region would be most strongly affected. The spread of the D.C. wave front over the field (at 5-20 mm/sec) should not be identified with spreading depression. Although spreading depression may be accompanied by seizure activity (Van Harreveld and Stamm 1953), its rate of spread is about 2-3 ram/rain and not more than 5 mm/min (Marshall 1959). In addition, the very large (over l0 mV) swings frequently seen in spreading depression were absent in these experiments. Adrian (1936) described the spread of activity in seizures induced by electric shock at 25 cm/ sec. Rosenblueth and Cannon (1942) observed similar changes. Hughes and Mazurowski (1964) reported seizure spread in the cingulate cortex at a rate of 20-25 mm/min. No D.C. observations were made in any of these studies.
4. Mirror activity The structure of the field on the mirror area has many similarities to that of the primary. However, the relative prominence of peripheral positive shifts as compared to the central negative shifts may indicate that the primary focus projects to a large degree to elements deep within the cortex of the opposite hemisphere. The lack of agreement between the point of maximal callosal response to a shock applied to Electroenceph. clin.NeurophysioL, 1965, 19:63-74
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D. C. ACTIVITY IN FOCAL SEIZURES
the center of the area of penicillin application and the center of t!~e mirror field may be a reflection of the more intense sharp wave discharges at the edge of the penicillin lesion. However, *.he conplex relations between the activities at the two loci suggest that subcortical structures may also be involved (cf Gastaut and Fisher-Williams 1959). Rutledge and Kennedy (1960) have presented evidence for such a mechanism in the case of the transcallosal response.
changes from the depth of the cortex in the neighborhood of the penicillin focus. A negative shift was an invariant accompaniment to each paroxysm recorded by the conventional EEG. However, they did not find a precise temporal correlation with either the surface EEG changes or unit depolarization~ These authors, however, report no attempt to analyze the temporal relations as a function of the location of the electrodes within the field. Such an analysis might clarify the situation. The paroxysm and the D.C. shift need not be considered as separate events; one does not exist without the other. The initiation of the process depends upon the local sharp wave discharge, which can arise in part from the effect of incoming impulses from other portions of the brain (Smith and Pupura 1960; Matsumoto 1964; Matsumoto and Ajmone Marsan 1964a). This might explain the clinical phenomena of photogenic and audiogenic epilepsy, as well as the ability of photic stimulation to trigger paroxysms in our (and many other) experiments.
5. Theoretical formulation The data now at hand suggest the following heuristic t~ypothesis. Each sharp wave discharge at the focu,.; is accompanied by a surface negative swing which represents a relative depolarization of many cortical elements (Li 1963; Matsumogo and Ajmone Marsan 1964b), resulting not only in a localized increase in excitability but also in a flow of current from surrounding cortex. This induced current flow affects the excitability of other elements in the neighborhood of the focus (cf GIoor et ai. 1961). When a critical level of depolarization is reached, perhaps as a result of SUMMARY temporal summation, the conditions are suffiExperimental seizure foci were produced by cient for a localized rapid seizure dischargem the after-discharge. The notion of a critical level the topical application of penicillin to the cortex of depolarization is not entirely speculative. of cats. D.C. recording revealed that an electrical Matsumoto and Ajmonc Marsan (1964c) found field was produced which surrounded the lesion (cortical intracellular D.C. recording) a prolonga- by a radius of approximately l cm. Sustaine~ tion and an oscillation of paroxysmal mem- paroxysms were associated with D.C. shifts; brane depolarization of single units closely strongly negative with an abrupt rise at the cencorrelated with the after-discharge in an indenti- ter o r the field, more gradual and positive in the cal experimental situation. If a very high level of periphery. The interictal sharp wave discharges excitability is attained, a sustained paroxysm had a similar distribution. They were high voltoccurs, accompanied by a more intense flow age, monophasic, negative and with a long duraof current from surrounding structures. As each tion at the center; briefer, polyphasic and usually surrounding portion of cortex becomes involved predominantly positive at the periphery. When in the seizure, the current flow spreads outward, an after-discharge was present, it rode the crest resulting in the "march" of both D.C. shift and of the prolonged negative phase of the sharp paroxysmal discharge across the field. As the wave discharge. The activity projected to the diameter of the circle increases, the density of homologous area of the opposite hemisphere was current ltow decreases until a point is reached also surveyed and a similar distribution was where neighboring elements are not sufficiently found. However, the activity was of lower voltage and the peripheral positive shifts were relaexcited and the process ends. Implicit in this scheme is the notion that not tively more prominent. It is concluded that D.C. all units are active simultaneously. Results from changes play a prominent role in the activity of single unit recording in the experiments of the penicillin focus and suggested that a horiMatsumoto and Ajmone Marsan (1964c) are in zontal dipole is generated which may play a agreement. They also recorded extracellular D.C. causal role in the genesis of the paroxysms. Electroenceph. olin. Neurophysiol.,
19~5,
19:63-74
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R.J. GUMNIT AND T. TAKAHASHI
The authors are indebted to Drs. J. R. Knott, D. W. McAdam, W. R. Ingram, A. L. Sahs and M. W. Van Al!¢n for their generosity in providing laboratory space, equipment and advice. REFERENCES
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Reference: GUMNIT, R. J. and TAKAHASHI,T. Changes in direct current activity during experimental focal seizures. Electroenceph. clin. Neurophysiol., 1965, 19: 63-74.