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Effects of bilateral partial diencephalic lesions on cortical epileptic activity in generalized penicillin epilepsy in the cat
EXPERIMENTAL
NEUROLOGY
66,285-308
(1979)
Effects of Bilateral Partial Diencephalic Lesions on Cortical Epileptic Activity in Generalized Penicillin Epilepsy in the Cat ANDREA Montreul
Neurological
PELLEGRINI
AND PIERRE GLOOR’
Institute, 3801 University QlrPhec H3A 2B4, Canuda Received
April
Street,
MontrPul.
23. 1979
The effects of bilateral symmetric partial diencephalic lesions on the epileptic discharges of generalized penicillin epilepsy in the cat were studied. Bilateral cooling of the inferior thalamic peduncle (ITP) reversibly abolished recruiting responses and epileptic discharges, whereas bilateral destructive lesions of the ITP were without effect. The effect of ITP cooling on these electrocortical phenomena is most likely due to inadvertent cooling of the nearby preoptic area. Preoptic cooling releases posterior hypothalamic and mesencephalic reticular mechanisms from inhibition and induces an electrocortical arousal response which is inimical to the elaboration of recruiting responses and of the epileptic bursts of generalized feline penicillin epilepsy. Of all other bilateral partial diencephalic lesions. only those of nucleus lateralis posterior exerted a pronounced effect on the epileptic discharges in this model. Even these lesions, however. rarely produced total abolition of the discharges, but reduced or modified them considerably. This may be related to the fact that the epileptic discharges in this model predominate in the cortical projection area of this nucleus. Bilateral lesions of Forel’s fields had no effect on the epileptic discharges. We conclude that the thalamocortical volleys responsible for triggering the epileptic discharges of feline generalized penicillin epilepsy probably arise from multiple potential pacemakers which are widely distributed throughout the thalamus. It is, however, possible that the thalamocortical projection ofthe nucleus lateralis posterior, possibly through the “dorsal spindle system” of F. E. Horvath and P. Buser (1972, Brain Res. 39: 21-41), represents the principal, though not the exclusive, final common path through which the thalamocortical volleys precipitating the epileptic bursts in this model are mediated. Abbreviations: EEG-electroencephalogram, ITP-inferior thalamic peduncle. ’ This work was supported by grant MT-3140 awarded by the Medical Research Council of Canada to Dr. Gloor, to whom reprint requests should be addressed. Dr. Pellegrini’s present address is Clinica Neurologica dell’Unitersit8, Via Giustiniani 1, 35100 Padova. Italy. 285 0014-4886/79/l 10285-24$02.00/O Copyright 0 1979 by Academic Press, Inc. All rights of reproduction in any form rewrved.
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INTRODUCTION In a previous study (36), we demonstrated that the characteristic 3 to 6/s spike and wave, or multiple spike or sharp wave bursts of generalized penicillin epilepsy of the cat do not occur in cortex that has been completely or nearly completely disconnected from all thalamic inputs, although such deafferented cortex retains the capability of producing widespread, synchronous epileptiform activity of a different form, even without the influence of penicillin. Similar results were recently reported by Hablitz (20). These observations suggested that the spontaneous bursts of feline generalized penicillin epilepsy are as dependent on thalamocortical volleys as those evoked by single shock or low-frequency electrical stimulation (16,40). The thalamocortical volleys inducing these spontaneous epileptic bursts seem to be the same as those which mediate recruiting responses and spontaneous spindles in the normal cat (15, 16, 36, 40). In the light of these findings, it appeared of interest to determine whether more restricted bilateral and symmetric thalamic or other diencephalic lesions, particularly in regions which have been considered important for the initiation or the mediation of spindles and recruiting responses, would abolish the typical epileptic bursts of feline generalized penicillin epilepsy. Among these regions, the midline and intralaminar thalamic nuclei (13, 16, 25, 27, 40, 49, as well as the inferior thalamic peduncle (ITP) (43, 44), seemed to be of particular interest. METHODS Fourty-two acute experiments were carried out on cats of either sex weighing 2.5 to 4.5 kg. The surgical, anesthetic, and analgesic procedures were the same as those described by Quesney er al. (40). Generalized penicillin epilepsy was induced by the i.m. injection of 300,000 to 400,000 IU/kg sodium penicillin G. Bipolar and monopolar electroencephalograms (EEGs) were recorded from the dura or directly from cortex with an 8-channel Elema-Schonander Mingograf or an 8-channel Model T Offner electroencephalograph. The reference electrode was placed in the neck muscles. In some experiments the ITP was cooled bilaterally with two stereotaxically implanted thermodes, of the type used by Testa and Gloor (46), each having its tip placed in the region of the ITP where it emerges from the rostra1 pole of the thalamus and forms a relatively compact bundle [stereotaxic coordinates F + 13.5, L +3, H -3.5, according to the atlas of Jasper and Ajmone-Marsan (26)]. The technique of perfusion with coolant was that used by Testa and Gloor (46), except that in some animals, instead of supercooled methylbutane, water at t0.5”C was perfused through the
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thermode. With this method, the brain temperature at the tip of the thermode could not be lowered to less than + 10°C. When cooling was done with supercooled methylbutane, the temperature at the tip of the thermode was never allowed to be less than + 1°C. After generalized epileptic discharge had been induced by the i.m. injection of penicillin, diencephalic lesions were made with a Wyss Model C coagulator using a radio frequency current through an electrode with an uninsulated tip of 1 mm equipped with a small thermocouple. The temperature at the tip of the probe was kept at +75”C for 3 min; this produced a lesion about 3 mm in diameter. Larger confluent lesions were made by multiple insertions of the thermocoagulating probe in adjacent regions. Stereotaxic coordinates for the lesions were chosen from the atlas of Jasper and Ajmone-Marsan (26). In several groups of cats symmetrical destruction of the following diencephalic areas was carried out: ITP, anterior thalamus, massa intermedia, nucleus lateralis posterior of the thalamus, and Forel’s fields. In some cats in which the ITP was cooled, midbrain lesions were made bilaterally at F +2, either by electrocoagulation or by a stereotaxically inserted leucotome. At the end of the experiments, the animals were killed with an overdose of anesthetic agent, and the brain was perfused and fixed in 10% formalin solution. The location of the thermodes or the site and extent of the lesions was identified in 15pm serial histologic sections stained with cresyl violet. In some experiments electrical stimulations were carried out with stereotaxically implanted bipolar concentric 35gauge stainless-steel electrodes with an interelectrode distance of 0.5 mm and a resistance of 40 to 60 ka. RESULTS The bilaterally synchronous epileptic bursts induced by i.m. penicillin were similar to those described by earlier investigators (17, 37, 40). They consisted of 3 to 6/s spike and wave, or multiple spike or sharp wave bursts. One aspect to which little attention was paid in previous investigations, but which proved to be important for the interpretation of some of the results reported in this study, was the mode of development and the topographical distribution of these bursts. They most often appeared first in the middle suprasylvian gyrus and sometimes appeared to remain confined to that gyrus. Only later did they develop in other gyri as they gradually became generalized (Fig. 1). Even then, however, the epileptic bursts usually predominated in the suprasylvian gyrus. That the lower voltage bursts in the other gyri could not be attributed to volume conduction of larger bursts occurring in the suprasylvian gyrus was
REF-
99
min
after
Penicillin
IM
110
min
after
Penicillin
IM 135
min
after
Penicillin
IM 167
min
after
IPOX
Penicillin
IM 35b
FIG. 1. Gradual development of generalized epileptic activity after intramuscular injection of penicillin: Epileptic bursts first appear in the middle suprasylvian gyri bilaterally and still predominate after the epileptic discharges have become generalized. Abbreviations in this and subsequent figures: AES-anterior ectosylvian gyrus, AS-anterior sigmoid gyrus, ASS-anterior suprasylvian gyrus, C-cingulate gyrus, ITP-inferior thalamic peduncle, L-left, MES-middle ectosylvian gyrus, ML-midportion of the lateral gyrus, MSS-middle suprasylvian gyrus, MSS (A)-anterior portion of MSS, MSS (P)-posterior portion of MSS, NCM-nucleus centralis medialis of the thalamus, P-gyrus proreus, PS-posterior sigmoid gyrus, PSS-posterior suprasylvian gyrus, R-right, REF OCC-occipital reference electrode.
1 P-OCC
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demonstrated by the observation that early in the development of the syndrome the bursts in different gyri often occurred asynchronously. Also in the experiments on isolated cortical slabs (36) in which the suprasylvian gyrus was included in the isolated region, typical bursts appeared in cortex outside the slab, but not within it. Cooling
and Lesions of the ITP
In seven animals an attempt was made to reversibly interrupt the rostra1 outflow of the thalamus by cooling the ITP. In normal animals before penicillin administration, recruiting responses, induced by stimulation of
Before
ASS-REF
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Cooling
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B
During
Cooling
ITP 6
y
MSS PMSS PSS R
ASS MSS PMSS
FIG. 2. Horizontal thalamus.
Effect bars
of cooling the inferior thalamic indicate duration of stimulation
peduncle ITP of the nucleus
on recruiting responses. centralis medialis of the
290
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FIG. 3. Effect of cooling the inferior thalamic peduncle (ITP) on generalized epileptic bursts. I-EEG record before (A) and during(B) cooling. II-Number of epileptic bursts per minute in three cats (A,B,C) before, during, and after cooling the ITP (cooling periods indicated by stippling).
the nucleus centralis medialis and recorded over the suprasylvian gyrus, disappeared or were markedly reduced during cooling of the ITP. This effect was associated with desynchronization of the background activity (Fig. 2). During ITP cooling, the cats exhibited pupillary dilatation, and in instances when the paralysis induced by gallamine was incomplete, there was shivering which suggested the possibility of an activation of thermogenic thermoregulatory activity caused by cooling of the preoptic area which lies very close to the ITP.
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In three of seven animals which received i.m. penicillin, bilaterally synchronous epileptic bursts disappeared during ITP cooling; in the remaining four the spike and wave activity was markedly reduced. Simultaneously there was desynchronization of the background activity of the EEG (Fig. 3, I). In a few instances, the epileptic bursts reappeared before the end of cooling. However, more often desynchronization and absence of epileptic bursts outlasted the end of cooling. The effect of cooling on the epileptic bursts was a reversible and reproducible phenomenon as seen in Fig. 3, II. In view of the striking effects produced by ITP cooling we were surprised to find that the epileptic activity induced by i.m. penicillin remained unaltered in five cats in which the ITP was destroyed bilaterally by thermocoagulation (Fig. 4). Only in one cat the incidence of the bursts appeared slightly reduced after the lesion. In three additional animals, lesions of the ITP failed to produce any consistent modification of the recruiting responses or spindles induced by i.v. barbiturates or by bilateral mesencephalic reticular formation lesions. The observation that actual bilateral destruction of the ITP was incapable of reproducing the abolition or reduction of recruiting responses and epileptic discharges achieved by cooling this pathway in the same anatomical location indicated that the observed cortical electrophysiologic effects produced by cooling of the ITP were not the consequence of blockade of transmission through this pathway, but that cooling in this region was exerting its effect through some brain structures other than the
292 1
ASS -REF OCC -i~~~~~~~Z MSS (A) I +-J-44....,.,,.._.
FIG. 4. Generalized peduncle.
PELLEGRINI
Before
epileptic
Bilateral
bursts
before
AND
GLOOR
ITP
Lesion
and after bilateral
lesion
of the inferior
thalamic
ITP. The desynchronization of the EEG observed during ITP cooling suggested that the abolition or reduction of recruiting responses and epileptic discharges may have been mediated indirectly through the brain stem reticular formation. Possibly thermogenic thermoregulatory mechanisms in the preoptic area, which is adjacent to the ITP and projects to the brain stem reticular formation (12), may have been activated by inadvertent cooling of this very thermosensitive region. To test this hypothesis, the ITP
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was cooled bilaterally in animals with bilateral lesions in the rostra1 midbrain reticular formation. In none of the five animals with such mesencephalic lesions were the epileptic bursts abolished during ITP cooling (Fig. 5), except for a short period at the time of a brief initial phase of residual desynchronization of the EEG immediately after the onset of cooling. However, there was in three of these animals some reduction (sometimes of amplitude only) of the epileptic bursts. Spontaneous spindle activity after midbrain reticular formation lesions was always reduced to a variable degree during ITP cooling, but disappeared in only one animal. The degree of reduction of spindle activity seemed to be proportional to the degree of residual desynchronization of the background activity induced by ITP cooling. As in intact animals the effects induced by ITP cooling were reversible. Bilateral
Symmetricul
Partial
Thalamic
Lesions
Anterior Thalamic Lesions. These involved the anterior pole of the thalamus bilaterally and extended back to the midthalamic region. They involved a number of specific and unspecific nuclei as shown in Fig. 6. In five of these animals the anterior thalamic radiations and the ITP were almost completely destroyed. Figure 6 also shows the effect of these lesions on the generalized cortical epileptic activity. No reduction in the number of epileptiform bursts was noted in four of these animals. In at least three the number of bursts increased. It is possible, however, that this increase could have been related to the still continuing buildup of the epileptic activity produced by the resorption of the i.m. penicillin depot. There was generally, however, some decrease in burst duration of about 47 to 76%. In two cats (Fig. 6, Nos. 54 and 55) there was a marked reduction in the number of epileptic discharges and some seemed to be transformed into bursts of rhythmic sharp or sharp and slow waves which were not entirely characteristic of the discharges of feline generalized penicillin epilepsy. In one of these animals (No. 55), the reduction in the number of bursts was most likely related to the inadvertent induction of spreading depression by mechanical injury caused by the insertion of the thermocoagulation electrode into the brain, because the EEG became flat and spindles began to appear instead of epileptic bursts, a change which was identical to that deliberately induced in other animals by cortical application of KC1 or localized mechanical injury to the cortex (15). In the other cat (No. 54) spreading depression was not a factor and the reduction of epileptiform activity remains unexplained since the lesions did not differ in size or location from those obtained in other animals of this group. Lesions of the Massa Intermedia. Part of the massa intermedia had already been destroyed in animals with anterior thalamic lesions. In two
started
FIG. 5. Cat with bilateral lesion of the midbrain reticular formation. Effect of cooling the inferior thalamic peduncle on generalized epileptic bursts. There is a transient suppression of the bursts during a short desynchronization of the EEG.
’ Cooling
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additional cats a more selective and complete destruction of the massa intermedia was carried out as shown in Fig. 7. In one cat, the lesion spared about 1 mm of the posterior border of the massa intermedia. There was no consistent decrease in the frequency or duration of the epileptic bursts in these cats (Fig. 7). Lesions of the Nucleus Lateralis Posterior. Such lesions were made bilaterally in 10 cats. Figure 8 shows a representative example. The nucleus was never completely destroyed. Conversely, adjacent nuclei were also partially involved to a variable degree, particularly the pulvinar, the nucleus lateralis dorsalis, the posterior part of the nucleus medialis dorsalis, and the lateral wing of the nucleus centralis. Bilateral lesions of the nucleus lateralis posterior in all animals produced a profound effect upon the epileptic bursts, particularly on those occurring in the middle suprasylvian gyrus (Fig. 9). These effects, however, varied from animal to animal. In four cats, bursts typical for feline generalized penicillin epilepsy were no longer seen after such lesions. In three of these cats some bursts in the form of sharp or slow waves were still recorded and probably represented abortive forms of the typical bursts, and in the fourth animal only continuous slow-wave activity with superimposed random spikes and sharp waves was seen. In six animals epileptic bursts were still present, at least on one side, but the form of the complexes recorded from the middle suprasylvian gyrus was modified compared to that seen before, the spike of the complex being reduced in voltage and/or its duration increased (Fig. 10, left side, L MSS). The only exception was the animal with the smallest lesion of the nucleus lateralis posterior in which the shape of the epileptiform complexes appeared unmodified on the side of the smaller lesion. In three animals the epileptic bursts were increased in number on one or both sides, whereas in the other three they were reduced. In one cat of this group (Fig. 9, No. 57) insertion of the thermocoagulation probe seemed to have induced spreading depression, which made the results difficult to evaluate. In all six animals with persistence of epileptic bursts in some form, their duration was decreased, except in one cat (Fig. 9, No. 68) in which they were increased. In only three cats was it possible to accurately evaluate the effects of lesions of the lateral posterior nucleus on the epileptic activity occurring outside the suprasylvian gyrus, as, e.g., in the ectosylvian and posterior sigmoid gyri. In one of these three cats there was on the right side selective abolition of the epileptic bursts originating from the suprasylvian gyrus, and those occurring from neighboring gyri, although modified, continued to occur (Fig. 10, R). In the other two cats, however, the epileptic bursts generated by three different gyri (including the suprasylvian) were similarly
DIENCEPHALIC
II
LESIONS:
EFFECT ON EPILEPSY
Left
297
Right
Percent FIG.
6-Continued.
affected. Conversely, in one cat, bilateral lesions of the lateral geniculate bodies failed to reduce the incidence or modify the shape of the spike and wave bursts recorded from the lateral gyri to which the lateral geniculate bodies project. Foref’s Field Lesions. Bilateral symmetrical lesions in Forel’s fields were made in three cats (Fig. 11). These lesions, in addition to Forel’s fields, destroyed parts of the ventromedial thalamus and the medial edge of the cerebral peduncle. None of these animals showed any decrease in the incidence of the epileptic bursts, but in one there was some reduction in burst duration (Fig. 11). DISCUSSION These experiments demonstrate that no partial, symmetric diencephalic lesions were capable of reliably abolishing the bilaterally synchronous generalized bursts which characterize feline generalized penicillin
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epilepsy. Only two procedures used in this study did, under certain circumstances, abolish these bursts, or reduce or modify them significantly. These were: (i) bilateral cooling (but not bilateral lesions) of the ITP, and (ii) symmetrical large lesions of the nucleus lateralis posterior of the thalamus. Role of the Inferior Thalamic Peduncle in Mediating Recruiting Responses, Spindles, and Epileptic Bursts of Generalized Penicillin Epilepsy The paradoxical observation was made that bilateral cooling, but not bilateral interruption of the ITP by destructive lesions, was capable of abolishing both the recruiting responses in normal animals, and the generalized bilateral synchronous epileptic discharges in cats which had received a large i.m. injection of penicillin. Although this observation again indicates that the typical discharges of feline generalized penicillin epilepsy depend on thalamocortical mechanisms involved in the elaboration of cortical recruiting responses and spindles, the failure of anatomical destruction of the ITP to abolish both phenomena makes it unlikely that blockade of transmission through this pathway was responsible for their disappearance in response to ITP cooling. Cooling in this region most likely induced these changes by some indirect effect which was mediated through the preoptic area situated near the ITP. It is likely that cooling of the ITP produces small changes in the local preoptic temperature. Because this region is highly thermosensitive (5,7,30,41) by virtue of its involvement in thermoregulation, its function is greatly affected even by small brain temperature changes in this region (14, 2 1, 23, 28, 3 1, 34). Cooling of this region activates thermogenic mechanisms (3, 4) probably by removing a tonic inhibition exerted upon the posterior hypothalamus as postulated by
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FIG. 9. Lesions of nucleus lateralis posterior showing the effect on epileptic bursts in 10 animals. For further explanations see Legend to Fig. 6.
Benzinger (7) and demonstrated by Nutik (35). De demonstrated that this inhibition extended to formation. This agrees with Bremer’s observations area has hypnogenic properties and acts in a way that of the reticular formation of the brain stem.
Armond and Fusco (12) the midbrain reticular (8- 10) that the preoptic which is antagonistic to
FIG. 10. EEG of a cat before (A) and after (B) bilateral lesion of the lateral posterior nucleus. Note abolition of epileptic burst in the right suprasylvian gyrus and modification of the burst in the left suprasylvian gyrus. Those from the right ectosylvian gyrus were modified after the lesion whereas those from the left ectosylvian gyrus were virtually unaffected.
DIENCEPHALIC
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EFFECT
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303
EPILEPSY TALA
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Right 50
100%
0
100%
Percent
FIG. Forel’s to Fig.
11. Lesions field lesions 6.
of Forel’s on epileptic
fields. bursts
I-extent in three
of a typical bilateral lesion. animals. For further explanations
II-effect of see Legend
The most likely explanation for the effects on recruiting responses and generalized epileptic discharges seen in our study is therefore that inadvertent cooling of the preoptic area, close to the ITP, removed the inhibition exerted upon the posterior hypothalamus and upper midbrain
304
PELLEGRINI
AND
GLOOR
reticular formation, thus indirectly producing activation of the ascending reticular system. This explanation is supported by our observation that whenever in the intact animal cooling of the ITP was carried out and recruiting responses or generalized epileptic discharges disappeared, the EEG became desynchronized. It is well known that activation of the ascending reticular system blocks both recruiting responses (32, 39) and epileptic discharges of feline generalized penicillin epilepsy (17, 19,46). A similar explanation for the suppression of spindles after orbitofrontal lesions was given by Robertson and Lynch (42). The interpretation given for our findings is supported by the fact that cooling of the ITP lost much of its effectiveness in abolishing recruiting responses and generalized epileptiform discharges in animals with bilateral lesions of the midbrain reticular formation. The residual effectiveness of ITP cooling observed after such lesions seemed to be related to a residual desynchronization of the cortical EEG induced by ITP cooling. This probably indicates that part of the reticular formation rostra1 to the lesion still responded to ITP cooling. Our findings and interpretation conflict with those of Skinner and Lindsley (44), Skinner (43), and Villablanca et al. (47) who postulated on the basis of cooling and lesion experiments an important role of the ITP in the mediation of spindles, recruiting responses, and thalamically evoked spike and wave discharges, although the latter authors were unable to demonstrate such an effect upon spike and wave discharges produced by chlorambucil. Even though the effects of ITP cooling in these experiments could be explained on the basis of an indirectly induced arousal effect, the discrepancy of findings with regard to the effect of ITP lesions is more difficult to explain. It may be related to the fact that only spindles in anteriorly situated cortical regions, including particularly the sigmoid gyri, are dependent on the rostra1 thalamic outflow through the ITP, whereas those to more posterior cortical regions, particularly the suprasylvian gyri, are independent of this system (6, 24). Role of the Nucleus Lateralis Posterior in the Genesis of Generalized Epileptic Discharges Induced by Penicillin Large symmetrical lesions of the lateralis posterior nuclei were the only partial thalamic lesions which produced a significant modification or reduction of the epileptic bursts of feline generalized penicillin epilepsy. This may be explained partially by the fact that all our recordings included the region of the middle suprasylvian gyrus to which this nucleus projects (18, 22,48) whereas the coverage of other gyri varied from experiment to experiment. Also important is the fact that the suprasylvian gyrus to which
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the nucleus lateralis posterior projects is the cortical region in which the epileptic bursts in this model of generalized epilepsy usually first appear and continue to predominate even after the discharges have become generalized. One of the main thalamocortical connections involved in the triggering of epileptic discharges in this model may therefore involve thalamocortical pathways connecting the lateralis posterior to the cortex of the suprasylvian gyrus. The notion of multiple facultative thalamic pacemakers of spindle activity (1, 2), or at least that of two spindle generating systems (11, 24), must be considered in any explanation of the thalamocortical mechanism of triggering epileptic bursts in this model. The frequent multifocal character of the initial bursts as penicillin epilepsy develops may reflect the existence of a facultative multifocal pacemaker mechanism. Quesney et al. (40) and Gloor et al. (16) showed that spindles, recruiting responses, and the epileptic bursts of feline generalized penicillin epilepsy can be triggered by electrical stimulation of the nucleus lateralis posterior. However, electrical stimulation of the midline and intralaminar nuclei as well as of the neostriatum was equally or sometimes even more effective. It is possible therefore that stimulation of the latter nuclei exerts its effect via the nucleus lateralis posterior, at least with regard to the activation of the most prominent discharges which usually occupy the suprasylvian gyrus. The main vehicle for the transmission of the thalamocortical volleys responsible for the triggering of epileptic discharges in feline generalized penicillin epilepsy may thus be the “dorsal spindle system” of Horvath and Buser (24). Epileptic discharges occurring in regions other than the suprasylvian gyrus could possibly be mediated by other thalamocortical segments, the midline and intralaminar nuclei acting as a synchronizing pacemaker mechanism of the multiple thalamic spindle generators as was postulated by Purpura and Cohen (38). If this were true, bilateral lesions of the nucleus lateralis posterior should not abolish or otherwise impair the epileptiform discharges occurring outside the projection areas of this nucleus. This was, however, only observed once in three animals with bilateral lesions of the lateralis posterior nuclei in which adequate EEG coverage of gyri adjacent to the suprasylvian gyrus was carried out. Conversely, bilateral lesions of the lateral geniculate nucleus in one animal failed to affect the occurrence of epileptic bursts in its cortical projection area, the gyrus lateralis. It is therefore difficult to avoid the conclusion that in some way the nucleus lateralis posterior must play an important role in the initiation or mediation of epileptic discharges of feline generalized penicillin epilepsy, at least within its own projection area. This conclusion is consonant with the fact that, in those animals with the smaller subtotal thalamectomies described in the study by Pellegrini et al. (36) where most
306
PELLEGRINI
AND GLOOR
of the nucleus lateralis posterior was spared, the epileptic discharges were least affected. Effects of Other Bilateral
Partial
Diencephalic
Lesions
All partial diencephalic lesions other than those involving the nucleus lateralis posterior left the generalized bilaterally synchronous discharges of feline generalized penicillin epilepsy essentially unaffected. Massa intermedia lesions were completely ineffective in spite of the fact that stimulation in this region readily precipitates bilaterally synchronous epileptic bursts (16,40). This indicates that the neuronal systems triggering spontaneous bursts must be widely distributed throughout the thalamus, even though from some regions they can be more easily activated by electrical stimulation than from others. For many cortical regions on the lateral convexity the thalamocortical projections of the nucleus lateralis posterior may be the most important, even though not the exclusive, final common path mediating the thalamocortical volleys triggering the epileptic bursts [“dorsal spindle system” of Horvath and Buser (24)]. In view of the alleged effectiveness of bilateral Forel’s field lesions as a method of treatment of generalized epilepsy (29, 33), it is of some interest that lesions in this region proved to be completely ineffectual in this experimental model. REFERENCES 1. ANDERSEN, P., S. A. ANDERSSON, AND T. LIMO. 1967. Some factors involved in the thalamic control of spontaneous barbiturate spindles. J. Physiol. (London) 192: 257-281. 2. ANDERSEN, P., S. A. ANDERSON, AND T. LOMO. 1967. Nature of thalamocortical relations during spontaneous barbiturate spindle activity. J. Physiol. (London) 192: 283-307. 3. ANDERSSON, B. 1964. Hypothalamic temperature and thyroid activity. Pages 35-49 in M. P. CAMERON AND M. O’CONNOR, Eds., Brain-Thyroid Relationships with Special Reference to Thyroid Disorders, Ciba Foundation Study Group No. 18. Churchill, London. 4. ANDERSSON, B., C. C. GALE, AND J. W. SUNDSTEN. 1962. Effects of chronic central cooling on alimentation and thermoregulation. Acra Physiol. Stand. 55: 177-188. 5. ANDERSSON, B., R. GRANT, AND S. LARSSON. 1956. Central control of heat loss mechanisms in the goat. Acta Physiof. Stand. 37: 261-280. 6. BELLAVANCE, A. 1969. Contribution ci l’ktude de l’influence du cortex orbitofrontal sar le syst&ne recrufant thalamique. These Maitrise, Universitt de Montreal. 7. BENZINGER, T. H. 1969. Heat regulation: homeostasis of central temperature in man. Physiol.
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4: 671-759.
8. BREMER, F. 1973. Preoptic Biol. 111: 85-111. 9. BREMER, F. 1975. A further the preoptic hypnogenic 10. BREMER, F. 1977. Cerebral
hypnogenic area and reticular activating system. Arch.
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study of the inhibitory processes induced by the activation of structure. Arch. Ital. Biol. 113: 79-88. hypnogenic centers. Ann. Neural. 2: l-6.
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11. BUSER, P.. AND F. E. HORVATH. 1972. Thalamo-caudate-cortical relationships in synchronized activity. II. Further differentiation between spindle systems by cooling and lesions in the mesencephalon. Bruin Res. 39: 43-60. 12. DE ARMOND, S. J., AND M. M. Fusco. 1971. The effect of preoptic warming on the arousal system of the mesencephalic reticular formation. Ex~. Neural. 33: 653-670. 13. DEMPSEY, E. W., AND R. S. MORRISON. 1942. The production ofrhythmically recurrent cortical potentials after localized thalamic stimulation. Am. J. Physiol. 135: 293-300. 14. EISENMAN. J. S., AND D. C. JACKSON. 1967. Thermal response patterns of septal and preoptic neurons in cats. E.up. Nruml. 19: 33-45. 15. GLOOR, P.. A. PELLEGRINI, AND G. K. Kosro~ou~os. 1979. Effects of changes in cortical excitability upon the epileptic bursts in generalized penicillin epilepsy of the cat. Elec~troenceph. C/in. Neurophysiol. 46: 274-289. 16. GLOOR. P.. L. F. QUESNEY. AND H. ZUMSTEIN. 1977. Pathophysiology of generalized penicillin epilepsy in the cat: the role of cortical and subcortical structures. II. Topical application of penicillin to the cerebral cortex and to subcortical structures. Elrctroencrph.
C/in.
Neurophysiol.
43: 79-94.
17. GLOOR. P., AND G. TESTA. 1974. Generalized penicillin epilepsy in the cat: effects of intracarotid and intravertebral pentylenetetrazol and amobarbital injections. Electrnenceph. C/in. Nuurophy.siol. 36: 499-5 15. 18. GRAYBIEL, A. M. 1970. Some thalamocortical projections of the pulvinar-posterior system of the thalamus in the cat. Bruin Ras. 22: 13 I- 136. 19. GUBERMAN, A., AND P. GLOOR. 1974. Cholinergic drug studies of generalized penicillin epilepsy in the cat. Bruin RPS. 78: 203-222. 20. HABLITZ. J. J. 1978. Chronically isolated cerebral hemisphere in the cat: effects of parenteral administration of penicillin. Epilrpsia 19: 637-642. 21. HARDY, J. D.. R. F. HELLON, AND K. SUTHERLAND. 1964. Temperature-sensitive neurones in the dog’s hypothalamus. J. Physiol. (London) 175: 242-253. 22. HEATH, C. J.. AND E. G. JONES. 1971. The anatomical organization of the suprasylvian gyrus of the cat. E~&J. Ancrt. E~tt#,icklunRsgesch. 45: I-61. 23. HELLON. R. F. 1967. Thermal stimulation of hypothalamic neurones in unanaesthetized rabbits. J. Physiol. (London) 193: 381-395, 24. HORVATH. F. E., AND P. BUSER. 1972. Thalamo-caudate-cortical relationships in synchronized activity. I. Differentiation between ventral and dorsal spindle systems. Brain
Res.
39: 21-41.
25. JASPER. H. 1949. Diffuse projection systems: the integrative action of the thalamic reticular system. Electroenc,eph. C/in. Newophysiol. 1: 405-420. 26. JASPER, H., AND C. AJMONE-MARSAN. 1954. A Steraotcrxic. Atl[rs of the Dicncrphtrlon of the Cut. National Research Council of Canada, Ottawa. 27. JASPER, H. H.. AND J. DROOGLEEVER-FORTUYN. 1946. Experimental studies on the functional anatomy of petit ma1 epilepsy. RFS. Puhl. Ass. Res. Nrrv. Merit. Div. 26: 272-298. 28. JELL, R. M.. AND P. GLOOR. 1972. Distribution of thermosensitive and nonthermosensitive preoptic and anterior hypothalamic neurons in unanesthetized cats, and effects of some anesthetics. Can. J. P12ysiol. Phurmacol. 50: 890-901. 29. JINNAI, D.. AND A. NISHIMOTO. 1963. Stereotaxic destruction of Fore1 H field for the treatment of epilepsy. Neurochirurgie 6: 164-176. 30. MAGOUN. H. W.. F. HARRISON, J. R. BROBECK, AND S. W. RANSON. 1938. Activationof heat loss mechanisms by local heating of the brain. J. Neurophysi~d. 1: IOI- 114. 31. MITRA. J., P. GLOOR. AND R. JELL.. 1972. Effect ofnasopharyngeal temperature change on preoptic and anterior hypothalamic thermo-sensitive units. Con. J. Physiol. Phurmtrt~ol.
50: 445-457.
308
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AND GLOOR
32. MORUZZI, G., AND H. W. MAGOUN. 1949. Brain stem reticular formation and activation of the EEG. Electroenceph. Clin. Neurophysiol. 1: 455-473. 33. MUKAWA, J. 1964. Effects of brainstem and subcortical lesions on corticogenic epileptic convulsion with special reference to Forel-H field. Acta Med. Ukayama 18: 153- 171. 34. NAKAYAMA, T., H. R. HAMMEL, J. D. HARDY, AND J. A. EISENMAN. 1963. Thermal stimulation of electrical activity of single units of the preoptic region. Am. J. Physiol.
‘204: 1122- 1126. 35. NUTIK, S. L. 1973. Posterior 36. 37.
hypothalamic neurons responsive to preoptic region thermal stimulation. J. Neurophysiol. 36: 238-249. PELLEGRINI, A., J. MUSGRAVE, AND P. GLOOR. 1979. The role of afferent input of subcortical origin in the genesis of the bilaterally synchronous epileptic discharges of feline generalized penicillin epilepsy. Exp. Neurol. 63: 155-173. PRINCE, D. A, AND D. FARRELL. 1969. “Centrencephalic” spike wave discharges following parenteral penicillin injection in the cat. Neurology (Minneapolis) 19:
309-3 10. 38. PURPURA, D. P., AND B. COHEN.
1962. Intracellular recording from thalamic neurons during recruiting responses. J. Neurophysiol. 25: 621-635. 39. PURPURA, D. P., J. G. MCMURTRY, AND K. MAEKAWA. 1966. Synaptic events in ventrolateral thalamic neurons during suppression of recruiting responses by brain stem reticular stimulation. Brain Res. 1: 63-76. 40. QUESNEY, L. F., P. GLOOR, E. KRATZENBERG, AND H. ZUMSTEIN. 1977. Pathophysiology of generalized penicillin epilepsy in the cat: the role of cortical and subcortical structures. I. Systemic application of penicillin. Elecrroenceph. C/in. Neurophysiol.
42: 640-655. 41. RANSON, S. W. 1940. Regulation of body temperature. Res. Publ. Ass. Res. Nerv. 42. 43. 44. 45. 46.
47. 48.
Merit. Dis. 20: 342-399. ROBERTSON, R. T., AND G. S. LYNCH. 1971. Orbitofrontal modulation of EEG spindles. Brain Res. 28: 562-566. SKINNER, J. E. 1971. Abolition of several forms of synchronization during blockade in the inferior thalamic peduncle. Electroenceph. Clin. Neurophysiol. 31: 211-221. SKINNER, J. E., AND D. B. LINDSLEY. 1967. Electrophysiological and behavioral effects of blockade of the nonspecific thalamo-cortical system. Brain Res. 6: 95- 118. STARZL, T. E., AND H. W. MAGOUN. 1951. Organization of the diffuse thalamic projection system. J. Neurophysiol. 14: 133- 146. TESTA, G., AND P. GLOOR. 1974. Generalized penicillin epilepsy in the cat: Effect of midbrain cooling. Electroenceph. C/in. Neurophysiol. 36: 517-524. VILLABLANCA, J., J. SCHLAG, AND R. MARCUS. 1970. Blockingofexperimentalspikeand wave by a localized forebrain lesion. Epilepsia 11: 163-177. WALLER, W. H., AND R. W. BARRIS. 1937. Relationships of thalamic nuclei to the cerebral cortex in the cat. J. Comp. Neurol. 67: 317-341.
NEUROLOGY
66,285-308
(1979)
Effects of Bilateral Partial Diencephalic Lesions on Cortical Epileptic Activity in Generalized Penicillin Epilepsy in the Cat ANDREA Montreul
Neurological
PELLEGRINI
AND PIERRE GLOOR’
Institute, 3801 University QlrPhec H3A 2B4, Canuda Received
April
Street,
MontrPul.
23. 1979
The effects of bilateral symmetric partial diencephalic lesions on the epileptic discharges of generalized penicillin epilepsy in the cat were studied. Bilateral cooling of the inferior thalamic peduncle (ITP) reversibly abolished recruiting responses and epileptic discharges, whereas bilateral destructive lesions of the ITP were without effect. The effect of ITP cooling on these electrocortical phenomena is most likely due to inadvertent cooling of the nearby preoptic area. Preoptic cooling releases posterior hypothalamic and mesencephalic reticular mechanisms from inhibition and induces an electrocortical arousal response which is inimical to the elaboration of recruiting responses and of the epileptic bursts of generalized feline penicillin epilepsy. Of all other bilateral partial diencephalic lesions. only those of nucleus lateralis posterior exerted a pronounced effect on the epileptic discharges in this model. Even these lesions, however. rarely produced total abolition of the discharges, but reduced or modified them considerably. This may be related to the fact that the epileptic discharges in this model predominate in the cortical projection area of this nucleus. Bilateral lesions of Forel’s fields had no effect on the epileptic discharges. We conclude that the thalamocortical volleys responsible for triggering the epileptic discharges of feline generalized penicillin epilepsy probably arise from multiple potential pacemakers which are widely distributed throughout the thalamus. It is, however, possible that the thalamocortical projection ofthe nucleus lateralis posterior, possibly through the “dorsal spindle system” of F. E. Horvath and P. Buser (1972, Brain Res. 39: 21-41), represents the principal, though not the exclusive, final common path through which the thalamocortical volleys precipitating the epileptic bursts in this model are mediated. Abbreviations: EEG-electroencephalogram, ITP-inferior thalamic peduncle. ’ This work was supported by grant MT-3140 awarded by the Medical Research Council of Canada to Dr. Gloor, to whom reprint requests should be addressed. Dr. Pellegrini’s present address is Clinica Neurologica dell’Unitersit8, Via Giustiniani 1, 35100 Padova. Italy. 285 0014-4886/79/l 10285-24$02.00/O Copyright 0 1979 by Academic Press, Inc. All rights of reproduction in any form rewrved.
286
PELLEGRINI
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GLOOR
INTRODUCTION In a previous study (36), we demonstrated that the characteristic 3 to 6/s spike and wave, or multiple spike or sharp wave bursts of generalized penicillin epilepsy of the cat do not occur in cortex that has been completely or nearly completely disconnected from all thalamic inputs, although such deafferented cortex retains the capability of producing widespread, synchronous epileptiform activity of a different form, even without the influence of penicillin. Similar results were recently reported by Hablitz (20). These observations suggested that the spontaneous bursts of feline generalized penicillin epilepsy are as dependent on thalamocortical volleys as those evoked by single shock or low-frequency electrical stimulation (16,40). The thalamocortical volleys inducing these spontaneous epileptic bursts seem to be the same as those which mediate recruiting responses and spontaneous spindles in the normal cat (15, 16, 36, 40). In the light of these findings, it appeared of interest to determine whether more restricted bilateral and symmetric thalamic or other diencephalic lesions, particularly in regions which have been considered important for the initiation or the mediation of spindles and recruiting responses, would abolish the typical epileptic bursts of feline generalized penicillin epilepsy. Among these regions, the midline and intralaminar thalamic nuclei (13, 16, 25, 27, 40, 49, as well as the inferior thalamic peduncle (ITP) (43, 44), seemed to be of particular interest. METHODS Fourty-two acute experiments were carried out on cats of either sex weighing 2.5 to 4.5 kg. The surgical, anesthetic, and analgesic procedures were the same as those described by Quesney er al. (40). Generalized penicillin epilepsy was induced by the i.m. injection of 300,000 to 400,000 IU/kg sodium penicillin G. Bipolar and monopolar electroencephalograms (EEGs) were recorded from the dura or directly from cortex with an 8-channel Elema-Schonander Mingograf or an 8-channel Model T Offner electroencephalograph. The reference electrode was placed in the neck muscles. In some experiments the ITP was cooled bilaterally with two stereotaxically implanted thermodes, of the type used by Testa and Gloor (46), each having its tip placed in the region of the ITP where it emerges from the rostra1 pole of the thalamus and forms a relatively compact bundle [stereotaxic coordinates F + 13.5, L +3, H -3.5, according to the atlas of Jasper and Ajmone-Marsan (26)]. The technique of perfusion with coolant was that used by Testa and Gloor (46), except that in some animals, instead of supercooled methylbutane, water at t0.5”C was perfused through the
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thermode. With this method, the brain temperature at the tip of the thermode could not be lowered to less than + 10°C. When cooling was done with supercooled methylbutane, the temperature at the tip of the thermode was never allowed to be less than + 1°C. After generalized epileptic discharge had been induced by the i.m. injection of penicillin, diencephalic lesions were made with a Wyss Model C coagulator using a radio frequency current through an electrode with an uninsulated tip of 1 mm equipped with a small thermocouple. The temperature at the tip of the probe was kept at +75”C for 3 min; this produced a lesion about 3 mm in diameter. Larger confluent lesions were made by multiple insertions of the thermocoagulating probe in adjacent regions. Stereotaxic coordinates for the lesions were chosen from the atlas of Jasper and Ajmone-Marsan (26). In several groups of cats symmetrical destruction of the following diencephalic areas was carried out: ITP, anterior thalamus, massa intermedia, nucleus lateralis posterior of the thalamus, and Forel’s fields. In some cats in which the ITP was cooled, midbrain lesions were made bilaterally at F +2, either by electrocoagulation or by a stereotaxically inserted leucotome. At the end of the experiments, the animals were killed with an overdose of anesthetic agent, and the brain was perfused and fixed in 10% formalin solution. The location of the thermodes or the site and extent of the lesions was identified in 15pm serial histologic sections stained with cresyl violet. In some experiments electrical stimulations were carried out with stereotaxically implanted bipolar concentric 35gauge stainless-steel electrodes with an interelectrode distance of 0.5 mm and a resistance of 40 to 60 ka. RESULTS The bilaterally synchronous epileptic bursts induced by i.m. penicillin were similar to those described by earlier investigators (17, 37, 40). They consisted of 3 to 6/s spike and wave, or multiple spike or sharp wave bursts. One aspect to which little attention was paid in previous investigations, but which proved to be important for the interpretation of some of the results reported in this study, was the mode of development and the topographical distribution of these bursts. They most often appeared first in the middle suprasylvian gyrus and sometimes appeared to remain confined to that gyrus. Only later did they develop in other gyri as they gradually became generalized (Fig. 1). Even then, however, the epileptic bursts usually predominated in the suprasylvian gyrus. That the lower voltage bursts in the other gyri could not be attributed to volume conduction of larger bursts occurring in the suprasylvian gyrus was
REF-
99
min
after
Penicillin
IM
110
min
after
Penicillin
IM 135
min
after
Penicillin
IM 167
min
after
IPOX
Penicillin
IM 35b
FIG. 1. Gradual development of generalized epileptic activity after intramuscular injection of penicillin: Epileptic bursts first appear in the middle suprasylvian gyri bilaterally and still predominate after the epileptic discharges have become generalized. Abbreviations in this and subsequent figures: AES-anterior ectosylvian gyrus, AS-anterior sigmoid gyrus, ASS-anterior suprasylvian gyrus, C-cingulate gyrus, ITP-inferior thalamic peduncle, L-left, MES-middle ectosylvian gyrus, ML-midportion of the lateral gyrus, MSS-middle suprasylvian gyrus, MSS (A)-anterior portion of MSS, MSS (P)-posterior portion of MSS, NCM-nucleus centralis medialis of the thalamus, P-gyrus proreus, PS-posterior sigmoid gyrus, PSS-posterior suprasylvian gyrus, R-right, REF OCC-occipital reference electrode.
1 P-OCC
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EPILEPSY
demonstrated by the observation that early in the development of the syndrome the bursts in different gyri often occurred asynchronously. Also in the experiments on isolated cortical slabs (36) in which the suprasylvian gyrus was included in the isolated region, typical bursts appeared in cortex outside the slab, but not within it. Cooling
and Lesions of the ITP
In seven animals an attempt was made to reversibly interrupt the rostra1 outflow of the thalamus by cooling the ITP. In normal animals before penicillin administration, recruiting responses, induced by stimulation of
Before
ASS-REF
OCC
Cooling
---~~~,wvJ,h~~Mv~~
B
During
Cooling
ITP 6
y
MSS PMSS PSS R
ASS MSS PMSS
FIG. 2. Horizontal thalamus.
Effect bars
of cooling the inferior thalamic indicate duration of stimulation
peduncle ITP of the nucleus
on recruiting responses. centralis medialis of the
290
PELLEGRINI
AND GLOOR
FIG. 3. Effect of cooling the inferior thalamic peduncle (ITP) on generalized epileptic bursts. I-EEG record before (A) and during(B) cooling. II-Number of epileptic bursts per minute in three cats (A,B,C) before, during, and after cooling the ITP (cooling periods indicated by stippling).
the nucleus centralis medialis and recorded over the suprasylvian gyrus, disappeared or were markedly reduced during cooling of the ITP. This effect was associated with desynchronization of the background activity (Fig. 2). During ITP cooling, the cats exhibited pupillary dilatation, and in instances when the paralysis induced by gallamine was incomplete, there was shivering which suggested the possibility of an activation of thermogenic thermoregulatory activity caused by cooling of the preoptic area which lies very close to the ITP.
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In three of seven animals which received i.m. penicillin, bilaterally synchronous epileptic bursts disappeared during ITP cooling; in the remaining four the spike and wave activity was markedly reduced. Simultaneously there was desynchronization of the background activity of the EEG (Fig. 3, I). In a few instances, the epileptic bursts reappeared before the end of cooling. However, more often desynchronization and absence of epileptic bursts outlasted the end of cooling. The effect of cooling on the epileptic bursts was a reversible and reproducible phenomenon as seen in Fig. 3, II. In view of the striking effects produced by ITP cooling we were surprised to find that the epileptic activity induced by i.m. penicillin remained unaltered in five cats in which the ITP was destroyed bilaterally by thermocoagulation (Fig. 4). Only in one cat the incidence of the bursts appeared slightly reduced after the lesion. In three additional animals, lesions of the ITP failed to produce any consistent modification of the recruiting responses or spindles induced by i.v. barbiturates or by bilateral mesencephalic reticular formation lesions. The observation that actual bilateral destruction of the ITP was incapable of reproducing the abolition or reduction of recruiting responses and epileptic discharges achieved by cooling this pathway in the same anatomical location indicated that the observed cortical electrophysiologic effects produced by cooling of the ITP were not the consequence of blockade of transmission through this pathway, but that cooling in this region was exerting its effect through some brain structures other than the
292 1
ASS -REF OCC -i~~~~~~~Z MSS (A) I +-J-44....,.,,.._.
FIG. 4. Generalized peduncle.
PELLEGRINI
Before
epileptic
Bilateral
bursts
before
AND
GLOOR
ITP
Lesion
and after bilateral
lesion
of the inferior
thalamic
ITP. The desynchronization of the EEG observed during ITP cooling suggested that the abolition or reduction of recruiting responses and epileptic discharges may have been mediated indirectly through the brain stem reticular formation. Possibly thermogenic thermoregulatory mechanisms in the preoptic area, which is adjacent to the ITP and projects to the brain stem reticular formation (12), may have been activated by inadvertent cooling of this very thermosensitive region. To test this hypothesis, the ITP
DIENCEPHALIC
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293
was cooled bilaterally in animals with bilateral lesions in the rostra1 midbrain reticular formation. In none of the five animals with such mesencephalic lesions were the epileptic bursts abolished during ITP cooling (Fig. 5), except for a short period at the time of a brief initial phase of residual desynchronization of the EEG immediately after the onset of cooling. However, there was in three of these animals some reduction (sometimes of amplitude only) of the epileptic bursts. Spontaneous spindle activity after midbrain reticular formation lesions was always reduced to a variable degree during ITP cooling, but disappeared in only one animal. The degree of reduction of spindle activity seemed to be proportional to the degree of residual desynchronization of the background activity induced by ITP cooling. As in intact animals the effects induced by ITP cooling were reversible. Bilateral
Symmetricul
Partial
Thalamic
Lesions
Anterior Thalamic Lesions. These involved the anterior pole of the thalamus bilaterally and extended back to the midthalamic region. They involved a number of specific and unspecific nuclei as shown in Fig. 6. In five of these animals the anterior thalamic radiations and the ITP were almost completely destroyed. Figure 6 also shows the effect of these lesions on the generalized cortical epileptic activity. No reduction in the number of epileptiform bursts was noted in four of these animals. In at least three the number of bursts increased. It is possible, however, that this increase could have been related to the still continuing buildup of the epileptic activity produced by the resorption of the i.m. penicillin depot. There was generally, however, some decrease in burst duration of about 47 to 76%. In two cats (Fig. 6, Nos. 54 and 55) there was a marked reduction in the number of epileptic discharges and some seemed to be transformed into bursts of rhythmic sharp or sharp and slow waves which were not entirely characteristic of the discharges of feline generalized penicillin epilepsy. In one of these animals (No. 55), the reduction in the number of bursts was most likely related to the inadvertent induction of spreading depression by mechanical injury caused by the insertion of the thermocoagulation electrode into the brain, because the EEG became flat and spindles began to appear instead of epileptic bursts, a change which was identical to that deliberately induced in other animals by cortical application of KC1 or localized mechanical injury to the cortex (15). In the other cat (No. 54) spreading depression was not a factor and the reduction of epileptiform activity remains unexplained since the lesions did not differ in size or location from those obtained in other animals of this group. Lesions of the Massa Intermedia. Part of the massa intermedia had already been destroyed in animals with anterior thalamic lesions. In two
started
FIG. 5. Cat with bilateral lesion of the midbrain reticular formation. Effect of cooling the inferior thalamic peduncle on generalized epileptic bursts. There is a transient suppression of the bursts during a short desynchronization of the EEG.
’ Cooling
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295
additional cats a more selective and complete destruction of the massa intermedia was carried out as shown in Fig. 7. In one cat, the lesion spared about 1 mm of the posterior border of the massa intermedia. There was no consistent decrease in the frequency or duration of the epileptic bursts in these cats (Fig. 7). Lesions of the Nucleus Lateralis Posterior. Such lesions were made bilaterally in 10 cats. Figure 8 shows a representative example. The nucleus was never completely destroyed. Conversely, adjacent nuclei were also partially involved to a variable degree, particularly the pulvinar, the nucleus lateralis dorsalis, the posterior part of the nucleus medialis dorsalis, and the lateral wing of the nucleus centralis. Bilateral lesions of the nucleus lateralis posterior in all animals produced a profound effect upon the epileptic bursts, particularly on those occurring in the middle suprasylvian gyrus (Fig. 9). These effects, however, varied from animal to animal. In four cats, bursts typical for feline generalized penicillin epilepsy were no longer seen after such lesions. In three of these cats some bursts in the form of sharp or slow waves were still recorded and probably represented abortive forms of the typical bursts, and in the fourth animal only continuous slow-wave activity with superimposed random spikes and sharp waves was seen. In six animals epileptic bursts were still present, at least on one side, but the form of the complexes recorded from the middle suprasylvian gyrus was modified compared to that seen before, the spike of the complex being reduced in voltage and/or its duration increased (Fig. 10, left side, L MSS). The only exception was the animal with the smallest lesion of the nucleus lateralis posterior in which the shape of the epileptiform complexes appeared unmodified on the side of the smaller lesion. In three animals the epileptic bursts were increased in number on one or both sides, whereas in the other three they were reduced. In one cat of this group (Fig. 9, No. 57) insertion of the thermocoagulation probe seemed to have induced spreading depression, which made the results difficult to evaluate. In all six animals with persistence of epileptic bursts in some form, their duration was decreased, except in one cat (Fig. 9, No. 68) in which they were increased. In only three cats was it possible to accurately evaluate the effects of lesions of the lateral posterior nucleus on the epileptic activity occurring outside the suprasylvian gyrus, as, e.g., in the ectosylvian and posterior sigmoid gyri. In one of these three cats there was on the right side selective abolition of the epileptic bursts originating from the suprasylvian gyrus, and those occurring from neighboring gyri, although modified, continued to occur (Fig. 10, R). In the other two cats, however, the epileptic bursts generated by three different gyri (including the suprasylvian) were similarly
DIENCEPHALIC
II
LESIONS:
EFFECT ON EPILEPSY
Left
297
Right
Percent FIG.
6-Continued.
affected. Conversely, in one cat, bilateral lesions of the lateral geniculate bodies failed to reduce the incidence or modify the shape of the spike and wave bursts recorded from the lateral gyri to which the lateral geniculate bodies project. Foref’s Field Lesions. Bilateral symmetrical lesions in Forel’s fields were made in three cats (Fig. 11). These lesions, in addition to Forel’s fields, destroyed parts of the ventromedial thalamus and the medial edge of the cerebral peduncle. None of these animals showed any decrease in the incidence of the epileptic bursts, but in one there was some reduction in burst duration (Fig. 11). DISCUSSION These experiments demonstrate that no partial, symmetric diencephalic lesions were capable of reliably abolishing the bilaterally synchronous generalized bursts which characterize feline generalized penicillin
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epilepsy. Only two procedures used in this study did, under certain circumstances, abolish these bursts, or reduce or modify them significantly. These were: (i) bilateral cooling (but not bilateral lesions) of the ITP, and (ii) symmetrical large lesions of the nucleus lateralis posterior of the thalamus. Role of the Inferior Thalamic Peduncle in Mediating Recruiting Responses, Spindles, and Epileptic Bursts of Generalized Penicillin Epilepsy The paradoxical observation was made that bilateral cooling, but not bilateral interruption of the ITP by destructive lesions, was capable of abolishing both the recruiting responses in normal animals, and the generalized bilateral synchronous epileptic discharges in cats which had received a large i.m. injection of penicillin. Although this observation again indicates that the typical discharges of feline generalized penicillin epilepsy depend on thalamocortical mechanisms involved in the elaboration of cortical recruiting responses and spindles, the failure of anatomical destruction of the ITP to abolish both phenomena makes it unlikely that blockade of transmission through this pathway was responsible for their disappearance in response to ITP cooling. Cooling in this region most likely induced these changes by some indirect effect which was mediated through the preoptic area situated near the ITP. It is likely that cooling of the ITP produces small changes in the local preoptic temperature. Because this region is highly thermosensitive (5,7,30,41) by virtue of its involvement in thermoregulation, its function is greatly affected even by small brain temperature changes in this region (14, 2 1, 23, 28, 3 1, 34). Cooling of this region activates thermogenic mechanisms (3, 4) probably by removing a tonic inhibition exerted upon the posterior hypothalamus as postulated by
ZF VlVl
DIENCEPHALIC
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FIG. 9. Lesions of nucleus lateralis posterior showing the effect on epileptic bursts in 10 animals. For further explanations see Legend to Fig. 6.
Benzinger (7) and demonstrated by Nutik (35). De demonstrated that this inhibition extended to formation. This agrees with Bremer’s observations area has hypnogenic properties and acts in a way that of the reticular formation of the brain stem.
Armond and Fusco (12) the midbrain reticular (8- 10) that the preoptic which is antagonistic to
FIG. 10. EEG of a cat before (A) and after (B) bilateral lesion of the lateral posterior nucleus. Note abolition of epileptic burst in the right suprasylvian gyrus and modification of the burst in the left suprasylvian gyrus. Those from the right ectosylvian gyrus were modified after the lesion whereas those from the left ectosylvian gyrus were virtually unaffected.
DIENCEPHALIC
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FIG. Forel’s to Fig.
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of Forel’s on epileptic
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I-extent in three
of a typical bilateral lesion. animals. For further explanations
II-effect of see Legend
The most likely explanation for the effects on recruiting responses and generalized epileptic discharges seen in our study is therefore that inadvertent cooling of the preoptic area, close to the ITP, removed the inhibition exerted upon the posterior hypothalamus and upper midbrain
304
PELLEGRINI
AND
GLOOR
reticular formation, thus indirectly producing activation of the ascending reticular system. This explanation is supported by our observation that whenever in the intact animal cooling of the ITP was carried out and recruiting responses or generalized epileptic discharges disappeared, the EEG became desynchronized. It is well known that activation of the ascending reticular system blocks both recruiting responses (32, 39) and epileptic discharges of feline generalized penicillin epilepsy (17, 19,46). A similar explanation for the suppression of spindles after orbitofrontal lesions was given by Robertson and Lynch (42). The interpretation given for our findings is supported by the fact that cooling of the ITP lost much of its effectiveness in abolishing recruiting responses and generalized epileptiform discharges in animals with bilateral lesions of the midbrain reticular formation. The residual effectiveness of ITP cooling observed after such lesions seemed to be related to a residual desynchronization of the cortical EEG induced by ITP cooling. This probably indicates that part of the reticular formation rostra1 to the lesion still responded to ITP cooling. Our findings and interpretation conflict with those of Skinner and Lindsley (44), Skinner (43), and Villablanca et al. (47) who postulated on the basis of cooling and lesion experiments an important role of the ITP in the mediation of spindles, recruiting responses, and thalamically evoked spike and wave discharges, although the latter authors were unable to demonstrate such an effect upon spike and wave discharges produced by chlorambucil. Even though the effects of ITP cooling in these experiments could be explained on the basis of an indirectly induced arousal effect, the discrepancy of findings with regard to the effect of ITP lesions is more difficult to explain. It may be related to the fact that only spindles in anteriorly situated cortical regions, including particularly the sigmoid gyri, are dependent on the rostra1 thalamic outflow through the ITP, whereas those to more posterior cortical regions, particularly the suprasylvian gyri, are independent of this system (6, 24). Role of the Nucleus Lateralis Posterior in the Genesis of Generalized Epileptic Discharges Induced by Penicillin Large symmetrical lesions of the lateralis posterior nuclei were the only partial thalamic lesions which produced a significant modification or reduction of the epileptic bursts of feline generalized penicillin epilepsy. This may be explained partially by the fact that all our recordings included the region of the middle suprasylvian gyrus to which this nucleus projects (18, 22,48) whereas the coverage of other gyri varied from experiment to experiment. Also important is the fact that the suprasylvian gyrus to which
DIENCEPHALIC
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the nucleus lateralis posterior projects is the cortical region in which the epileptic bursts in this model of generalized epilepsy usually first appear and continue to predominate even after the discharges have become generalized. One of the main thalamocortical connections involved in the triggering of epileptic discharges in this model may therefore involve thalamocortical pathways connecting the lateralis posterior to the cortex of the suprasylvian gyrus. The notion of multiple facultative thalamic pacemakers of spindle activity (1, 2), or at least that of two spindle generating systems (11, 24), must be considered in any explanation of the thalamocortical mechanism of triggering epileptic bursts in this model. The frequent multifocal character of the initial bursts as penicillin epilepsy develops may reflect the existence of a facultative multifocal pacemaker mechanism. Quesney et al. (40) and Gloor et al. (16) showed that spindles, recruiting responses, and the epileptic bursts of feline generalized penicillin epilepsy can be triggered by electrical stimulation of the nucleus lateralis posterior. However, electrical stimulation of the midline and intralaminar nuclei as well as of the neostriatum was equally or sometimes even more effective. It is possible therefore that stimulation of the latter nuclei exerts its effect via the nucleus lateralis posterior, at least with regard to the activation of the most prominent discharges which usually occupy the suprasylvian gyrus. The main vehicle for the transmission of the thalamocortical volleys responsible for the triggering of epileptic discharges in feline generalized penicillin epilepsy may thus be the “dorsal spindle system” of Horvath and Buser (24). Epileptic discharges occurring in regions other than the suprasylvian gyrus could possibly be mediated by other thalamocortical segments, the midline and intralaminar nuclei acting as a synchronizing pacemaker mechanism of the multiple thalamic spindle generators as was postulated by Purpura and Cohen (38). If this were true, bilateral lesions of the nucleus lateralis posterior should not abolish or otherwise impair the epileptiform discharges occurring outside the projection areas of this nucleus. This was, however, only observed once in three animals with bilateral lesions of the lateralis posterior nuclei in which adequate EEG coverage of gyri adjacent to the suprasylvian gyrus was carried out. Conversely, bilateral lesions of the lateral geniculate nucleus in one animal failed to affect the occurrence of epileptic bursts in its cortical projection area, the gyrus lateralis. It is therefore difficult to avoid the conclusion that in some way the nucleus lateralis posterior must play an important role in the initiation or mediation of epileptic discharges of feline generalized penicillin epilepsy, at least within its own projection area. This conclusion is consonant with the fact that, in those animals with the smaller subtotal thalamectomies described in the study by Pellegrini et al. (36) where most
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of the nucleus lateralis posterior was spared, the epileptic discharges were least affected. Effects of Other Bilateral
Partial
Diencephalic
Lesions
All partial diencephalic lesions other than those involving the nucleus lateralis posterior left the generalized bilaterally synchronous discharges of feline generalized penicillin epilepsy essentially unaffected. Massa intermedia lesions were completely ineffective in spite of the fact that stimulation in this region readily precipitates bilaterally synchronous epileptic bursts (16,40). This indicates that the neuronal systems triggering spontaneous bursts must be widely distributed throughout the thalamus, even though from some regions they can be more easily activated by electrical stimulation than from others. For many cortical regions on the lateral convexity the thalamocortical projections of the nucleus lateralis posterior may be the most important, even though not the exclusive, final common path mediating the thalamocortical volleys triggering the epileptic bursts [“dorsal spindle system” of Horvath and Buser (24)]. In view of the alleged effectiveness of bilateral Forel’s field lesions as a method of treatment of generalized epilepsy (29, 33), it is of some interest that lesions in this region proved to be completely ineffectual in this experimental model. REFERENCES 1. ANDERSEN, P., S. A. ANDERSSON, AND T. LIMO. 1967. Some factors involved in the thalamic control of spontaneous barbiturate spindles. J. Physiol. (London) 192: 257-281. 2. ANDERSEN, P., S. A. ANDERSON, AND T. LOMO. 1967. Nature of thalamocortical relations during spontaneous barbiturate spindle activity. J. Physiol. (London) 192: 283-307. 3. ANDERSSON, B. 1964. Hypothalamic temperature and thyroid activity. Pages 35-49 in M. P. CAMERON AND M. O’CONNOR, Eds., Brain-Thyroid Relationships with Special Reference to Thyroid Disorders, Ciba Foundation Study Group No. 18. Churchill, London. 4. ANDERSSON, B., C. C. GALE, AND J. W. SUNDSTEN. 1962. Effects of chronic central cooling on alimentation and thermoregulation. Acra Physiol. Stand. 55: 177-188. 5. ANDERSSON, B., R. GRANT, AND S. LARSSON. 1956. Central control of heat loss mechanisms in the goat. Acta Physiof. Stand. 37: 261-280. 6. BELLAVANCE, A. 1969. Contribution ci l’ktude de l’influence du cortex orbitofrontal sar le syst&ne recrufant thalamique. These Maitrise, Universitt de Montreal. 7. BENZINGER, T. H. 1969. Heat regulation: homeostasis of central temperature in man. Physiol.
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