Laminar analysis of spindles and of spikes of the spike and wave discharge of feline generalized penicillin epilepsy

Electroencephalography and Clinical Neurophysiology, 1982, 5 3 : 1 - - 1 3

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Elsevier/North-Holland Scientific Publishers, Ltd.

L A M I N A R ANALYSIS O F SPINDLES AND OF SPIKES OF THE SPIKE AND WAVE D I S C H A R G E OF FELINE G E N E R A L I Z E D PENICILLIN EPILEPSY i G. K O S T O P O U L O S

2, M. A V O L I 3, A. P E L L E G R I N I 4 and P. G L O O R

Department of Neurology and Neurosurgery, McGill University and Montreal Neurological Institute, 3801 University Street, Montreal, Que. H 3 A 2B4 (Canada)

(Accepted for publication: August 25, 1981)

Jasper and Droogleever-Fortuyn in 1946 and Gloor in 1979 proposed that the bilaterally synchronous spike and wave discharges of generalized corticoreticular ('centrencephalic') epilepsy are generated by thalamocortical inputs similar to those implicated in the genesis of spindles. Using the feline generalized penicillin epilepsy (FGPE) model, we have shown in an earlier study that after i.m. penicillin injection spindles gradually change into spike and wave discharges, probably because of an increase in the excitability of cortical neurons (Kostopoulos et al. 1981a, b). This is characterized b y an amplitude increase, especially of the positive phases of spindles which gradually evolve into the spikes of the spike and wave complexes. Simultaneously one of every two spindle waves, or, t w o of every three in animals with a transection of the midbrain reticular formation, decrease in amplitude and axe finally replaced by a more or less well developed slow wave. The result of these changes is the emergence of a spike and wave rhythm which has an intraburst frequency of a b o u t one-half or one-third of

the original spindle wave frequency (Kostopoulos et al. 1981a). The postulate that the spike and wave bursts in FGPE represent modified spindles implies (i) that the thalamocortical inputs which trigger the two phenomena are the same, and (ii) that the same cortical neurons are activated by the same synaptic contacts, that spike and waves are the result of a more vigorous response of the same cortical neurons to incoming volleys and that the latter leads to recruitment of the intracortical recurrent inhibitory pathway responsible for the appearance of the slow wave c o m p o n e n t (Gloor 1979; Kostopoulos et al. 1981b). If this explanation is correct, the intracortical laminar surface to depth profiles for spindles and for the spikes of the spike and wave complexes should be very similar, since the two, according to our hypothesis, basically represent the same phenomenon. The present study examines this question.

Methods

(a) Anesthesia and surgical procedures 1 This work was supported by Grant MT-3140 awarded by the Medical Research Council of Canada to Dr. P. Gloor. 2 Dr. G. Kostopoulos is an MRC Scholar. 3 Dr. M. Avoli was a NATO Fellow. 4 Dr. Andrea Pellegrini's present address is: Clinica Neurologica dell'UniversitY, via Giustiani 1, 35100 Padova, Italy.

Acute experiments were carried out on 34 non-anesthetized and painlessly immobilized cats (for details, see Kostopoulos et al. 1981a).

( b) Recording and stimulation Recording started at least 1 h following discontinuation of anesthesia. Concentric bipolar

0013-4649/82/0000--0000/$02.75 © 1982 Elsevier/North-Holland Scientific Publishers, Ltd.

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stimulating electrodes were inserted into the nucleus centralis medialis or reuniens of the thalamus at coordinates A: +10.5, V: +1, L: 0 of the atlas of Jasper and Ajmone Marsan (1954). The best depths for eliciting recruiting responses were selected. Single or double shocks of 1--3 mA and 0.25--0.5 msec from a Nuclear Chicago constant current stimulator were applied to this area to trigger spindles and spike and wave discharges. Spindles were facilitated by small subanesthetic doses of thiopentai (2--5 mg/kg i.v.). Following the recording of spindles a large dose of penicillin was administered i.m. (350,000 IU/kg). One to 2 h later bilaterally synchronous spike and wave discharges appeared either spontaneously or in response to single thalamic stimuli. The EEG was recorded from both hemispheres with surface electrodes using an 8 ~ h a n n e l Elema-Schoenander Mingograph. Laminar analysis of the intracortical EEG during spindles and spike and waves was conducted in the anterior part of the middle suprasylvian gyms in two different ways:

(i) Sequential recordings at different cortical depths. With this m e t h o d of laminar analysis the surface EEG was continuously recorded while a recording microelectrode, driven by a micro
G. K O S T O P O U L O S

E T AL.

microelectrode was minimized. Surface and microelectrode EEGs were recorded on a magnetic tape, every 200 /am, as the microelectrode was moved up to the zero point. For calculation of the location of the microelectrode we considered as zero depth the point at which it was flush with the annular surface electrode as viewed under the microscope prior to placing the circular plate on the cortex, as well as at the end of the recording. All recordings were monopolar with a clip on the neck muscles serving as the reference electrode.

(ii)Simultaneous recordings at multiple cortical depths. For this purpose we used the multielectrode probe described by Prohaska et al. (1977). This electrode, manufactured by thin-film technology, carried 8 Ag-AgC1 contacts of 50 × 50/am 2 each at distances of 300 lam. The electrode was inserted perpendicularly to the surface of the middle suprasylvian gyms, driven down and then partially withdrawn until its most superficial contact was visible. The exposed area of the brain was covered with Agar to minimize pulsations. Although we did not verify our recording position histologically, we monitored, by inspection through the microscope, the position of the highest contact of the multicontact electrode throughout the experiment. Cases with severe brain swelling were discarded. Recordings from the multicontact electrode were monopolar, the reference electrode being clipped to the neck muscles.

(c) Computer analysis The wave-triggered EEG averages described by Ball et al. (1977) were used with other modifications. The EEG was digitized usually at 200 Hz, occasionally at 100 or at 400 Hz (for the multicontact electrode experiments). With digitizing at 200 Hz the limit for accurate time resolution was 5 msec. A transient from the epicortical EEG which has an identifiable peak can either be picked up automaticaUy by the computer on the basis of polarity, duration and amplitude, or can be selected by the experimenter on the Tektronix

LAMINAR ANALYSIS OF SPINDLES AND SPIKE AND WAVE terminal with a pointer. Simultaneous epochs (0.5 or 1 sec long) of both epicortical and sequential intracortical EEGs (or of the simultaneous EEGs recorded with the multicontact electrode at multiple levels) were averaged. Time zero fell at mid-point of the selected epoch and was chosen to coincide with the peak of the selected surface EEG wave. The c o m p u t e r also calculated maximum and minimum amplitudes of the intracortical EEG averages, as well as the time of occurrence of these two extremes relative to time zero.

Results

The features of spindles and spike and wave discharges in these experiments were similar to those described in previous reports from our laboratory (Gloor and Testa 1974; Ball et al. 1977; Gloor et al. 1979; Kostopou-

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los et al. 1981a). The slow waves of the spike and wave complexes were often small and sometimes even difficult to identify, as is often the case in acute experiments. The temperamental nature of the slow wave component in the present series of experiments, however, made it impossible to subject it to laminar analysis.

(a) Laminar analysis obtained with sequential recordings at different depths With this method we studied the laminar surface to depth profiles of spindles and of spikes of spike and wave discharges in 22 cats. In 13 instances it was possible to compare the two phenomena in the same animals. Fig. 1 demonstrates the raw data of typical laminar profiles of spindles, spike and waves and recruiting responses obtained sequentially with a penetrating microelectrode in the same cat. The spindles and spike and waves were spontaneous, while the recruiting

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G. KOSTOPOULOS ET AL.

responses were produced by 6 Hz stimulation of nucleus centralis medialis. Two features stand out: (i) The amplitude of all 3 phenomena tended to decrease at a depth of 200 #m. A clear and stable zero isopotential level, however, could n o t be established, even though it was sometimes fortuitously demonstrable for a single wave, as, for instance, for the 3rd and 4th surface negative spindle wave in row A of Fig. 1. Maximum amplitude was usually attained at depth of 600--800 pm and this was followed by a decline in amplitude at deeper cortical levels which could, however, be very small. (ii) The polarity of individual waves reversed below a depth of 200-400 pm. This applied both to the positive and the negative transients of individual spikes: As much as the above features could be easily identified in the majority of the experiments, difficulties were sometimes encountered. In several experiments no substantial decrease in amplitude was observed at any depth. In other cases, some of the individual waves, in a burst recorded by the intracortical microelectrode, were found to be in phase with their epicortical counterparts, some in clear opposition of phase and others even missing. Sometimes individual transients of a bior triphasic spike were apparently reversing at slightly different depths. Also, a closer inspection of what appeared to be 'mirror

images' often revealed time differences in the order of 5--10 msec between peaks of 'corresponding' individual waves recorded by epiand intracortical electrodes. Such 'phase shifts' were usually, but not exclusively, confined to waves recorded near the polarity reversal level, waves at deeper levels appearing 180 ° o u t of phase with their surface counterparts (Fig. 1B at 200 ~m). These difficulties prompted us to treat the data statistically by using computer averaging. This helps to tease o u t the statistically most prevalent profiles prevailing both in time and space, b u t we remain fully cognizant of the fact that such averaging hides the true complexity of cortical electrogenesis. The most prominent peaks of spindles and spikes of spike and waves of the surface EEG were chosen as time zero, and both the epicortical and the simultaneously recorded intracortical EEGs were averaged for 0.25 or 0.5 sec before and after the peak of the chosen waves (Figs. 2 and 3). F o r 9 experiments in which a phase reversal could be demonstrated in the depth EEG by inspection of the raw data as well as by computer analysis, we found the spikes of the spike and wave discharges to reverse at the same depth, or at slightly deeper levels, as the spindles. The mean depth of the zero isopotential was at 255 ~m for the spindle waves (range 100--700 /~m) and 310 /~m for the spikes of

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LAMINAR ANALYSIS OF SPINDLES AND SPIKE AND WAVE

the spike and wave discharges (100--800/~m). For every single animal the level of phase reversal for the spikes was, however, somewhat deeper than for spindle waves, but never more than 200 #m. Negative and positive phases of spindles and spikes appeared to reverse phase at similar depths (Figs. 2 and 3). In Figs. 2A and 3B spikes were averaged from their negative peaks while in Figs. 2B and 3C they were averaged from their positive peaks. Surface recorded spindles usually consisted of symmetrical monophasic negative waves. Spikes of spike and wave complexes were usually bi- or triphasic and asymmetric. At the surface, the most commonly observed spike consisted of a succession of positive and negative phases. Surface negative spindle waves (Fig. 3A) and surface negative phases of spikes of spike and wave complexes (Figs. 2A and 3B) were coincident with intracortical positivities which attained their maximum amplitude at about 600 pm and showed very little amplitude decline throughout the remaining depth of the cortex. In contrast, surface positive phases of spikes were coin:cident with deep negativi'ties which had theiz maximum amplitude at depths between 600 and 1000 pm with a marked further amplitude decrease in deeper layers (Figs. 2B and 3C). Spindle waves rarely had positive phases

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of sufficient amplitude for the study of their depth profile. However, when present, they showed intracortical laminar profiles similar to those of the positive phases of spikes. The lack of significant change of averages of the sequentially recorded surface EEGs throughout an entire laminar analysis session suggested that the phenomenon under study changed very little over the time it took to perform a laminar analysis by the sequential method. In Fig. 4, the sequential intracortical EEG averages have been placed in two vertical columns lined up below the surface EEG averages. This facilitates the direct comparison of spindles (Fig. 4A) and spikes of the spike and wave complexes (Fig. 4B) at different depths. At least, when the latter retained their biphasic form they seemed to consist of two components: one of them, the surface negative phase, had a depth profile identical to that of the predominantly negative spindle waves. For the distribution of the potential amplitude through the thickness of the cortex, this wave seemed to result from a current sink located superficially in the first 200--400 /~m of cortex, the rest of the cortical gray matter serving as the current source. The other component, the surface positive, seemed to result from a current sink at depths between 600 and 1000 #m with the upper 770609

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depth resolution provided by the multicontact electrode, we found that recruiting responses produced by different frequencies of stimulations also had similar depth profiles (Fig. 6). Both positive and negative phases of the spikes of the spike and wave complexes (studied in 8 animals) reversed at depths between 210 and 510 /~m (Figs. 6 and 7). When the surface to depth profiles of spindles and spikes of the spike and wave complexes obtained in the same animal were compared (Figs. 5 and 7), the phase reversals were located between the same contacts for both phenomena in 5 of the 8 experiments, i.e. within a distance of less than 300 #m, while in 2 animals, the spikes of the spike and wave complexes reversed phase at a slightly deeper level than the spindle waves, but never beyond 300 pm. As had been the case with the sequential

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(b) Laminar analysis obtained with simultaneous recordings at multiple cortical depths The depth profiles of spindles were studied in 12 cats with this method and in 8 of these the profiles of spikes were also analyzed. The spindle waves recorded at the surface were predominantly negative in polarity. A zero isopotential was found at depths between 150 and 450 #m; at deeper layers the spindles were positive (Figs. 5 and 6). Spontaneous spindles and spindles induced by i.v. pentothai or by single stimuli to nucleus centralis medialis or nucleus lateralis posterior had similar depth profiles. Within the limits of the

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8 recordings of the laminar profiles, we often observed time-lags between surface waves and their corresponding intracortical mirror images in the order of 2--10 msec. This was rarely seen with recruiting responses, more often with spindles and still more so with spikes. Most c o m m o n l y , this p h e n o m e n o n was seen at a depth close to that of the zero isopotential level (Fig. 7). In the example of Fig. 7, for instance, the surface positive phases of all spikes in the spike and wave burst coincided in time with their intracortical counterparts recorded from a depth of 400 ]~m and below, b u t the positive phases of the spikes at a depth of 100 pm either preceded or followed their counterparts of the surface EEG. Sometimes consistent time differences were seen b e t w e e n waves recorded at the surface and all the reversed waves seen at deeper layers.

Discussion

G. KOSTOPOULOS ET AL. wave complexes are not time-locked to a stimulus, the averages had to be triggered from the peak of waves in the surface EEG. This method has its weaknesses, however, since it tends to obscure the presence of any EEG generator which is not conspicuously represented in the surface EEG record. The use of the multicontact electrode developed by Prohaska et al. (1977) eliminates one of the difficulties of the sequential m e t h o d of laminar analysis, namely, that a change of pattern in electrogenesis over time may distort the results of laminar analysis. It was therefore reassuring that since the two methods yielded similar results, factors due to change in activity in time might have a negligible influence on the results obtained with the sequential method. However, the multicontact electrode, because of its size, inflicts more damage to the cortex than a microelectrode (diameter twice as large at the surface and longer length of penetration into white matter}. This may distort some of the electrophysiological phenomena under study.

(a) Comparison of the two techniques employed for laminar analysis

(b) Surface to depth profiles of spindle waves

Before interpreting our findings we have to consider several possible technical limitations. With sequential recordings, using a penetrating microelectrode, the main drawback is that the recordings at different cortical depths are n o t made simultaneously. Although the similarity of the successive epicortical averages demonstrates that electrophysiological conditions have remained stable, some changes over time may still have been undetected. Computer averaging partly compensates for this problem and also facilitates analysis because it smooths o u t the momentt o - m o m e n t variability of the EEG waves. The spatiotemporal average obtained b y this method, however, hides the true complexity of cortical electrogenesis. By showing a consistent picture, these averages are nevertheless useful because they demonstrate the statistically prevailing pattern o f intracortical distribution of potentials in time and space. Since both spindle waves and spike and

Most of the spindle waves in this study were predominantly surface negative and thus conformed to t y p e II of Spencer and Brookhart (1961a, b). The surface to depth laminar profiles of these waves resembled those reported earlier by other authors (Li et al. 1956; Spencer and Brookhart 1961b; Calvet et al. 1964; Ball et al. 1977). The intracortical depth of the zero isopotential level was, however, consistently found to be more superficially located in the cortex (100--700 #m, average: 255 pm) than in the study by Ball et al. (1977) (500--1500 pro, average: 800 #m), even though both studies were carried o u t in the same cortical area of the same species using apparently identical methods. This systematic discrepancy of values between the t w o studies must reflect some undetected differences in methodology and indicates that caution should be exercised in equating measured intracortical depths with specific layers of the cerebral cortex.

LAMINAR ANALYSIS OF SPINDLES AND SPIKE AND WAVE In over 20 additional experiments we have studied the depth profiles of recruiting responses. Our results characteristically represented in Fig. 1C essentially confirm the recent study of Foster (1980).

(c) Surface to depth profiles of spikes of spike and wave discharges Laminar profiles of these or other models of spontaneous generalized spike and wave discharges have n o t been studied before, but such studies have been made for the spike and wave complexes evoked by 3/sec thalamic stimulation. These showed an apparent zero isopotential level at 600--1000 pm for the surface slow negative waves (Pollen et al. 1964; Pollen 1969). Our studies provide direct evidence for the intracortical location of the electrical generators of the spike components. We were unable to study the depth profiles of the slow wave c o m p o n e n t of the spike and wave complexes because in acute experiments of the penicillin model, in an exposed brain, the slow wave exhibits a variable amplitude and polarity (see Gloor et al. (1963) for clarification of this point). In a few instances in which well developed surface negative waves were recorded, the intracortical microelectrode recorded a polarity-reversed positive wave in the depth of the cortex (see Figs. 1 and 10A in Kostopoulos et al. 1981b).

(d) Comparison of depth profiles of spindles to those of spike and wave discharges Our study demonstrates a similarity of the depth profiles of spindles and spikes of the spike and wave complexes recorded in the same animals with both methods of laminar analysis. The surface negative phases of both p h e n o m e n a extended from the cortical surface to a depth of 100--400/~m, while their positive counterparts were recorded throughout most of the remaining deeper cortical gray matter. Surface positive phases of spindles, when t h e y could be demonstrated (type I spindles of Spencer and Brookhart 1961b), as well as those of the spikes of spike and

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wave complexes reverted to negativity between depths of 400--1200 lam which gradually decreased in amplitude to almost zero at deeper cortical levels. A vertical organization of sinks and sources could thus be demonstrated for both surface negative and positive phases of spindles and spikes. Since in an earlier study (Ball et al. 1977) no significant tangential current flow could be detected for spindles, the findings conform in general with the postulates of the dipole hypothesis of cortical electrogenesis (Li et al. 1956, Spencer and Brookhart 1961a, b; Calvet et al. 1964; Creutzfeldt and Houchin 1974; Ball et al. 1977) which assumes that most EEG waves are generated by neural elements producing sets of stationary dipoles arranged perpendicularly to the cortical surface. The most obvious candidates for this role are pyramidal neurons. In agreement with the earlier studies by Ball et al. (1977), it was found that the computer averaged data were in better agreem e n t with the dipole hypothesis than the raw EEG data. Some of our data, namely the time-lag sometimes observed between surface and intracortical waves, which do not seem to fit some of these postulates, will be discussed later. The intracortical location of sinks and sources of stationary dipole fields indicate the approximate location of the neuronal generatots of the recorded EEG potentials within the cortex. Theoretically an accurate localization of these generators would require a current density analysis (Humphrey 1968; Nicholson and Freeman 1975). However, voltage profiles provide the essential indications for such a localization as long as (i) the voltage profile is relatively simple and (ii) the medium is considered electrically homogeneous which is the case below layer I (Hoeltzell and Dykes 1979). Thus, on the basis of our data, surface negative SPindle waves as well as the negative phases of the spike and wave complexes could be attributed to a single generator organized like a dipole with its sink located approximately in the upper 250 pm of cortex. The surface positive phases of type I spindles and

10 those of the spikes of the spike and wave complexes had laminar profiles conforming to a generator with its sink located at cortical depths below 250 pm. We interpret our findings as indicating the existence of two generators for type I spindle waves and for spikes of spike and wave complexes, one of them located in the upper 200--400 gm of cortex and the other at the mid-cortical level around 600--1000 pro. The existence of two separate generators is also suggested b y the fact that the phase-inverted deep intracortical counterparts of the surface positive phases rapidly decreased in amplitude with increase in cortical depth, while those of the surface negative phases did not. That t w o generators contribute to the genesis of t y p e I spindles was also proposed by $asaki et al. (1970). They concluded that thalamocortical responses related to spindles are made up of two independent processes: a late superficial thalamocortical response which is negative on the cortical surface and inverts to positivity at a b o u t 0.25 mm and an early deep thalamocortical response which is positive at the surface and inverts to negativity at 0.25 mm. While augmenting responses consist of a deep response succeeded by a superficial one, the latter only is apparent in the recruiting response. The depth profiles of the surface negative phases of spindle waves and of the surface negative phases of spikes in the present study certainly resemble those of the superficial thalamocortical responses of Sasaki et al. (1970), and those of the surface positive phases of t y p e I spindle waves and of spikes are similar to those of their deep thalamocortical responses. A major feature of the transition between spindles and spike and wave discharges is the development of and/or increase in amplitude of the surface positive phases of spindle waves (Kostopoulos et al. 1981a). Although with our recordings we could n o t conclusively differentiate between an 'active' and a 'passive' source or sink, several studies indicate that the thalamocortical responses in question are at least partly due to the summation of EPSPs

G. KOSTOPOULOS ET AL. (Jasper and Stefanis 1965; Andersen and Andersson 1968; Creutzfeldt and Houchin 1974). Also, it is unlikely that the surface positive waves represent superficial 'active' sources. Surface negative and surface positive phases of spindles and spikes therefore probably result from depolarizations at superficial and deep lying synapses respectively. The epileptogenic action of penicillin in this model may thus shift the weight of the thalamic input to the cortex from superficial to more deeply located synapses. If this is the case, the EPSPs induced b y thalamocortical volleys after penicillin can be expected to become more efficient in triggering action potentials from cortical neurons. A recent study has indeed demonstrated that the number of action potentials related to individual spindle waves progressively increases after penicillin as spindles gradually develop increasingly prominent surface positive phases and are ultimately transformed into the spikes of the spike and wave complexes (Kostopoulos et al. 1981b). The similarity between the intracortical laminar surface to depth profiles of spindle waves and of the spikes of the spike and wave complexes add further support to the hypothesis that the two phenomena indeed represent the response of the same population of cortical neurons mediated by the same synaptic contacts formed by thalamocortical fibers. The development of the slow wave c o m p o n e n t of the spike and wave complex is most likely due to secondary activation of the recurrent intracortical inhibitory pathway resulting from the increased penicillin-induced enhanced responsiveness of cortical neurons to thalamocortical impulses (Kostopoulos et al. 1981b).

(e) Data in apparent conflict with the dipole hypothesis of cortical electrogenesis According to the dipole hypothesis of cortical electrogenesis in a laminar analysis conducted perpendicularly to the cortical surface, all waves above the zero isopotential should show a 0 ° phase shift with respect

LAMINAR ANALYSIS OF SPINDLES AND SPIKE AND WAVE to those recorded at the surface, while all waves below it should show 180 ° phase shift. Intermediate values should not occur. Grossly our results fit with this prediction. However, in some experiments, there were phase-lags of more than 0 ° or less than 180 ° between the surface cortical waves and those in the depths of the cortex. These corresponded to time differences measuring 2--10 msec between 'corresponding' peaks. Such time-lags were usually confined to waves recorded close to the zero isopotential, while waves recorded deeper in the cortex were 180 ° o u t of phase with the surface records, i.e. there was no time-lag for these deeper waves, only a reversal o f electrical signs with respect to the surface EEG. Since these deviations from the theoretically expected pattern were inconsistent and often confined to a relatively narrow zone of cortical depths, we do n o t feel that they, b y themselves, invalidate the dipole hypothesis. It would indeed be difficult, in the light of our present knowledge of cortical neuroanatomy and neurophysiology, to conceive of another theoretical framework into which our findings could be fitted. It seems probable, however, that the deviations from what is theoretically predicted is due to the anatomical complexity of mammalian neocortex. The experimental data obtained in the laminar analysis of more simply organized cortex, such as that of the hippocampus, fit the dipole hypothesis very well (Green et al. 1960; Gloor et al. 1963). The 6-layered cerebral neocortex is much more complexly organized (Globus and Scheibel 1967; Szent~gothai 1969) and this probably accounts for some of the apparent discrepancies from the pattern predicted by the dipole hypothesis. We cannot give an exact explanation of h o w the deviations from the strict postulates of this hypothesis come about. However, it is conceivable that the deep potentials near the zero isopotential level which, in our experiments, appeared phase-shifted with regard to the waves recorded at the surface (and also with regard to those at deeper levels) are produced by a differently timed set of synaptic

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inputs which is separate from that creating the large dipole spanning the entire thickness of the cortex and which is the predominant feature of the laminar profiles, especially in the computed averages. This different set of synaptic inputs may create a dipole of lesser amplitude and perhaps smaller vertical extent and thus at the surface and in deeper layers becomes masked b y the principal, larger dipole underlying the surface EEG waves, except in the area near the zero isopotential of the main transcortical dipole where the 'signal to noise ratio' becomes favorable to the smaller dipole. We must also not forget that the starting point in all our laminar analyses (as well as in those reported by others) was an identifiable surface EEG wave. Averaging from this obviously biases the procedure in favor of detecting those waves in the depth of the cortex which directly correspond to those seen on the surface. The assumption that differently timed separate synapses may account for time differences between the principal cortical dipole and that confined to some layers in the depth, is in no conflict with neuroanatomical data. Thus, to take one example, it is known that some thalamocortical fibers contact the apical dendrites of cortical pyramidal cells directly, while others do so only through the intermediary of stellate interneurons which, in their turn, give origin to climbing axons which make multiple sequential contacts on apical dendrites along their way (Globus and Scheibel 1967; Szent~gothai 1969). The timing of the postsynaptic responses induced by thalamocortical volleys employing these two sets of synapses would obviously be different.

Summary Intracortical laminar profiles of spindles and spikes of spike and wave complexes in feline generalized penicillin epilepsy were studied using two methods: (i) sequential

12

microelectrode recordings at various cortical depths, and ,.., simultaneous recordings at multiple cortical depths using a fine multicontact electrode. Raw EEG data and EEG epochs averaged with respect to peaks of surface EEG waves were analyzed. Spindles and the spikes of the spike and wave complexes showed similar laminar profiles. This supports the hypothesis that the two are basically the same cortical electrophysiological phenomenon, the spike being a spindle wave enhanced and slightly altered because of the penicillin-induced increased cortical excitability. The latter causes the weight of the thalamic input to shift from superficial to more deep lying synapses. Both surface negative and surface positive phases of spindles and of spikes of spike and wave complexes show similar laminar profiles, those of the former suggesting activation of excitatory synapses in the superficial cortical layers, those of the latter suggesting activation of more deeply located excitatory synapses. The profiles generally conform to the dipole hypothesis of cortical electrogenesis and suggest that spindles and spikes of spike and wave complexes are generated by the same pyramidal neurons, probably through activation of the same sets of synapses. Some inconstant and relatively minor deviations of the laminar profiles from the pattern predicted by the dipole theory of cortical electrogenesis were encountered and are tentatively explained in the light of some of the complexities of the microanatomical organization of mammalian neocortex.

Rdsumd

Analyse laminaire des fuseaux et des pointes des d~charges de pointe-ondes dans l'dpilepsie gdndralisde d la pdnicilline chez le chat Les auteurs 6tudient les profils intra-corticaux laminaires des fuseaux et des pointes des complexes pointe-ondes dans l'~pilepsie g6n6ralis6e ~ la p~nicilline chez le chat ~ l'aide de deux m~thodes: (i)enregistrement s6quentiel

G. KOSTOPOULOS ET AL.

par micro~lectrodes ~ diffdrentes profondeurs du cortex, et (ii) enregistrement simultan~ de multiples profondeurs corticales au moyen d'une dlectrode fine ~ contacts multiples. Les donn6es EEG brutes et les ~poques EEG moyennes sont analys~es par rapport aux pics des ondes EEG de surface. Les fuseaux et les pointes des complexes pointe-ondes montrent des profils laminaires semblables. Ceci est en faveur de l'nypoth~se suivant laquelle ces deux phdnom~nes constituent fondamentalement un m~me ph6nom6ne 61ectro-physiologique cortical, la pointe ~tant une onde d'un fuseau agrandie et 16g~rement d~form~e du fait de l'accroissement de l'excitabilit~ corticale induite par la pdnicilline. Ceci fait que le poids de l'aff~rence thalamique varie des synapses superficielles aux synapses plus profondes. Les phases surface n6gatives et surface positives des fuseaux et des pointes des complexes pointe-ondes montrent des profils laminaires semblables, celles des phases n~gatives soulevant l'hypoth~se d'une activation des synapses excitatrices dans les couches corticales superncmlles, celles des phases positives soulevant l'hypoth~se d'une activation des synapses excitatrices localis~es plus profonddment. Ces profils sont dans l'ensemble conformes ~ l'hypoth~se d'un dipole ~ l'origine de l'~lectrog6n~se corticale et sugg~rent que les fuseaux et les pointes des complexes pointe-ondes prennent leur origine dans les m~mes neurones pyramidaux, probablement au travers d'une activation des m~mes r~seaux de synapses. Quelques d~viations inconstantes et relativement mineures par rapport aux profils laminaires du pattern pr~dit par la th6orie du dipole de l'~lectrog~n~se corticale se rencontrent et sont provisoirement expliqu~es ~ la lumi~re de certaines des complexitds de l'organisation microanatomique de n~o-cortex des mammif~res. We thank Mrs. S. Schiller and Mr. E. Puodziunas for technical assistance, Miss G. Robillard and Mrs. K. Douglas for secretarial assistance and Dr. R. Dykes for his helpful comments in preparing this paper.

LAMINAR ANALYSIS OF SPINDLES AND SPIKE AND WAVE

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