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The effects of electroconvulsive shock on the flash visual evoked potential in the rat
Electroencephalography and clinical Neurophysiology 104 (1997) 180–187
The effects of electroconvulsive shock on the flash visual evoked potential in the rat Nigel A. Shaw* Department of Physiology, School of Medicine, University of Auckland, Private Bag 92019, Auckland, New Zealand Accepted for publication: 16 December 1996
Abstract The effects of electroconvulsive shock (ECS) on the flash visual evoked potential (FVEP) were studied in the awake albino rat. Immediately after the induction of generalised seizure activity, the FVEP was totally abolished although accidentally averaged rhythmic epileptiform activity was often present in the trace. During the second recording, a potential had reappeared but this response was suspected of being a superior colliculus FVEP masquerading as a cortical response. By the third recording, the genuine cortical FVEP had returned, albeit with an abnormally large amplitude. The waveform subsequently remained significantly distorted although it had regained an approximately normal morphology within 6–7 min of the administration of ECS. It was not possible to identify the principal site of action of ECS but it was concluded that ECS may impact on activity generated at more than one location within the optic pathways. The present findings are compared with a number of previous animal and human studies where the FVEP was apparently preserved following ECS and attempts are made to explain the discrepancy in results. The relevance of the present findings for understanding the pathophysiology of electrical stunning and of the loss or impairment of consciousness during generalised epileptic seizures is also discussed. 1997 Elsevier Science Ireland Ltd. Keywords: Electroconvulsive shock; Flash visual evoked potential; Generalised seizure activity; Rat; Superior colliculus; Visual cortex
1. Introduction A number of procedures can be employed to artificially induce generalised seizure activity (GSA). These include physiological, chemical and mechanical methods. However, the simplest and most common technique is simply to pass a brief electric current through the brain. This will induce tonic-clonic seizures which closely mimic a spontaneous grand mal convulsion as well as an abrupt but brief loss of consciousness and retrograde amnesia. When this procedure is used as a treatment for severe depressive illness and other psychiatric and possible neurological disorders, it is called electroconvulsive therapy (ECT). When used to study the effects of seizure activity on experimental animals, it is normally labelled electroconvulsive shock (ECS). When
* Corresponding author. Tel.: +64 9 3737599; fax: +64 9 3737499.
0168-5597/97/$17.00 1997 Elsevier Science Ireland Ltd. All rights reserved PII S0921-884X(97)9602 5-2
used to render animals insensible prior to their slaughter in abattoirs, it is described as electrical stunning. Despite its widespread use, the mechanisms of action of ECS/ECT are still basically unknown. What is conspicuously missing is a knowledge of the acute physiology of GSA. One method for obtaining this information is by the recording of evoked potentials (EPs). EPs may be recorded from different sensory systems and can provide discrete snap shots of changes in cerebral activity during the acute ictal and post-ictal periods. Of the many kinds of EPs, probably the easiest to obtain following the induction of GSA by ECS is the flash visual evoked potential (FVEP). The FVEP is the response which is elicited in the occipital cortex by a flash of light. There have now been a number of investigations of the effects of ECT on the FVEP in patients (e.g. Small and Small, 1971; Kriss et al., 1980) and of ECS on the FVEP in animals (e.g. Myslobodsky and Kofman, 1982; Gregory and Wotton, 1985). It is a feature of virtually all these studies that, while there was invariably some alteration to its
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waveform, the FVEP was always basically preserved during the initial recording following ECS or ECT. It is also notable that when the FVEP was recorded after the administration of convulsant drugs, it was similarly reported to be retained following the onset of epileptiform activity (e.g. Rodin et al., 1966; Mirsky and Tecce, 1968; Burchiel et al., 1976). What is anomalous about such findings is that if there is only limited disruption of neurotransmission within a sensory pathway following ECS, then it becomes more difficult to explain why ECS possesses its various therapeutic, anesthetic and amnesic properties. In addition, it has also been demonstrated that ECS will totally abolish the equivalent response to the FVEP in the somatosensory system, i.e. the cortical somatosensory EP (Shaw, 1985). At least two explanations are possible for this discrepancy. First, it is conceivable that the somatosensory and visual systems are differentially sensitive to the effects of ECS. More likely, however, the apparent preservation of the FVEP following ECS was due to the conditions under which it was recorded. For instance, the concurrent medication administered during the modified ECT treatment may have played a protective role for the EP waveform. In addition, any loss of the FVEP following ECS may have gone undetected because of a delay between the onset of GSA and the actual start of the initial recording. The purpose of the present study was to attempt to resolve these contradictory claims regarding the vulnerability of cortical EPs to ECS. This was done by recording the FVEP under near identical conditions to those existing when the effects of ECS were examined on the somatosensory EP.
2. Methods and materials Subjects were 25 adult male albino rats (300–350 g). One week prior to the experiment, each subject had four small stainless steel screws inserted while under pentobarbital anesthesia (60 mg/kg.) Screws did not penetrate the dura and were insulated and secured with dental acrylic. One screw was implanted over the visual cortex (6–7 mm posterior to bregma and 3–4 mm lateral to the sagittal suture). Two screws were inserted sequentially along the nasal bone. A fourth electrode was inserted elsewhere in the dorsal skull. On the experimental day, each animal was initially curarised with a dose of d-tubocurarine chloride (4 mg/kg). As soon as signs of neuromuscular blockade became apparent, the subject was connected to a respirator and artificially ventilated at a rate of 50 strokes per min, each of 15 ml. Animals were attached to the respirator via a mask constructed of the mouthpiece end of a balloon which fitted snugly over the animal’s snout. The mask was secured to the animal by lodging the bottom part behind the upper incisors and the top half over the more distal of the nasal bone screws. ECG electrodes were attached and the stroke rate and volume of the ventilator were adjusted (if necessary) to maintain a normal heart rate (350–400 BPM). EP
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electrodes were then attached. The active lead was connected to the screw inserted over the visual cortex. The reference lead was connected to the second of the nasal bone screws. The ground lead was connected to the remaining skull screw. Several baseline FVEPs were then obtained to ensure that the waveform could be reproducibly recorded. FVEPs were recorded using a Medelec MS6. Analysis time was 100 ms and sampling interval 100 ms. Thirty-two responses were averaged to obtain each FVEP. Bandpass of the amplifiers was set to 3.2–320 Hz (Shaw, 1992a). Stimuli were light pulses delivered by a Grass model PS 22 photic stimulator. Duration of each flash was 10 ms and stimulation rate was 2/s. Flash intensity was set to step 8 on the 1–16 scale. In order to muffle the click which accompanied each light flash (Shaw, 1992b), small rubber plugs were inserted into each ear. FVEPs were recorded following monocular stimulation of the eye contralateral to the cortical electrode. Recordings were made in a totally darkened room. Following the baseline recordings, GSA was induced by transmitting a brief electric current (80 mA for 600 ms) via miniature bulldog clips attached to the animal’s ears. The insides of the clips were filled with electrode paste. This magnitude of ECS will induce tonic-clonic seizures in the awake non-paralysed animal lasting approximately 1 min accompanied by a period of unconsciousness and loss of reflex activity for up to 3 min (Shaw, 1985). The first recording was made between 0 and 30 s after the induction of GSA (0 min). Because of the delay in reconnecting the animal to the recording equipment plus the temporary overloading of the averaging equipment by the high voltage epileptiform activity, the first FVEP recording usually did not begin until 10–15 s had elapsed following the administration of ECS. A second FVEP was recorded between 30 and 60 s (1 min), and subsequent FVEPs were obtained at 2, 3, 4, 5, 6, 7, 8, 9 and 10 min. The animal was then immediately euthanised with an overdose of pentobarbital. The technique of chemically paralysing and artificially ventilating an awake animal was a necessary condition to obtain the present information and is similar to that previously employed to study the effects of convulsant drugs on EPs (e.g. Rodin et al., 1966; Burchiel et al., 1976). It allowed the effects of ECS on EPs to be quantified free from the contaminating influences of EMG movement artifact and sedative or anesthetic drugs. In various combinations, these factors appear to have confounded a number of previous investigations of ECS and EPs. There is also no reason to suspect that neuromuscular blockade is in any respect a discomforting experience. Any restriction on the use of curariform agents in awake animals is simply to protect them from stressful or painful procedures being inflicted on the false assumption that because they are immobilised, they must also be insensitive. In the present experiment, no such stimuli were inflicted on the subjects. Further evidence and discussion that short term neuromuscular blockade causes distress in neither humans (Smith et al., 1947) nor animals (Foutz et al., 1983) is available elsewhere.
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3. Results Six examples of the effects of ECS on the FVEP waveform are illustrated in Figs. 1–3. Examples of the normal FVEP recorded from the awake rat are shown in the baseline (before ECS) traces. The FVEP consists of an initial positivity (P1) with a peak latency which occurs at just under 30 ms following the onset of the photic stimulus. P1 is followed by a negative trough and then by a secondary positive complex. Later components of the FVEP which may persist for several 100 ms were not studied in the present experiment. P1 is generated by post-synaptic activity arising within the neurons of inner granular layer 4 of the visual cortex. The primary negative and secondary positive components are thought to represent more superficial depolarisation and hyperpolarisation, respectively (Dyer et al., 1987). As the
Fig. 2. Two further examples of the effects of ECS on the FVEP. The amplitude scale at the bottom of subject 3 applies to all the tracings. Note the different amplitude scales for the FVEPs recorded from subject 4.
Fig. 1. Two examples of the effects of electroconvulsive shock (ECS) on the flash visual evoked potential (FVEP). The baseline (before ECS) potential was recorded just prior to the induction of generalised seizure activity. Subsequent FVEPs were recorded at the times indicated after ECS. In the baseline examples, the primary positive component of the FVEP (P1) is identified with its actual latency (ms) in parentheses. In the remaining FVEPs, only the latency of P1 is indicated. Note the different amplitude scales for the potentials recorded from both subjects. In this and the subsequent two figures, the recordings made at 7 and 9 min are not illustrated.
principal concern of the present experiment was confined to whether or not the FVEP was totally abolished following ECS, only the behaviour of the P1 component needed to be systematically analysed. It can be seen from the 6 traces recorded at 0 min in Figs 1–3 that activity of some type was usually present during the tonic phase of the seizure. This mostly took the form of rhythmic activity of variable amplitude. These traces appeared to be generated by the serendipitous averaging of high frequency spikes which became accidentally timelocked to the photic stimulus. Averaging just 32 responses was usually insufficient for the epileptiform activity to be effectively cancelled out. There was, however, no evidence that the FVEP itself had been preserved during the tonic period. Nonetheless, by the second recording which was made between 30 and 60 s, a FVEP had invariably reappeared. The overall amplitude of this potential was markedly attenuated in comparison with the baseline recording, but the latency of the primary positivity was only minimally increased. It will be subsequently argued that the potential recorded at 1 min was not, in fact, a genuine cortical FVEP. Nevertheless, there was little doubt that the response obtained at 2 min represented the re-emergence of the
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increased latency which subsequently declines. Quite the contrary was observed here, where the latency of the putative P1 response unexpectedly jumped 4–8 ms between the 1 min and 2 min recordings. The simplest means of resolving this discrepancy is to assume that comparisons are being erroneously made between waveforms which have quite different generators. Furthermore, if the waveform obtained at 1 min was indeed a genuine cortical FVEP, then it is difficult to explain why it did not share in the augmentation of the P1 component displayed by the succeeding FVEPs for the next 2–3 min. Finally, if the present interpretation is correct, then it implies that the FVEP waveforms recorded between 2 and 4 min are probably an amalgam of activity from both the genuine and imitation cortical FVEPs. Group data for all 25 animals studied is summarised in Fig. 4. This confirms that the amplitude and latency trends displayed by the individual subjects in Figs. 1–3 were, in general, typical of the group as a whole. When calculating the amplitude of the P1 component, the baseline employed was the preceding shallow negativity which had a latency of approximately 20 ms. Under other circumstances, it might have been more appropriate to use the large negative trough which follows P1. However, as the illustrations in Figs. 1–3
Fig. 3. Two more examples of the effects of ECS on the FVEP. The amplitude scale at the bottom of each subject applies to all their traces.
authentic cortical FVEP, albeit with a marked increase in both the latency and amplitude of P1. The waveform remained significantly distorted and abnormal for the next 2–3 min due mostly to the persisting dilation of the P1 component. Thereafter, there followed a quite rapid decline in the amplitude of P1 associated with a decrease in its latency, such that within 6 min the FVEP had regained a more or less normal morphology. It is notable that even by the end of the recording period at 10 min, overall amplitude of the waveform had not usually been completely restored and the latency of P1 remained slightly prolonged. It is also clear from Figs. 1–3 that, while all 6 subjects displayed this basic pattern of change following ECS, there was considerable inter-animal variability in the timing and extent of these modifications to their waveforms This was also true for the other 19 animals studied. Superficially, the waveform recorded at 1 min after ECS had the appearance of a diminutive FVEP. The principal reason why it is claimed that this response was not a bona fide cortical FVEP is because of the aberrant behaviour of the latency of its primary positive component (P1). Normally, when an EP component is lost, it returns with an
Fig. 4. Mean latency and amplitude (±1 SD) of the P1 component of the FVEP at the times indicated following ECS. Note that at the first post-ECS recording (0 min), P1 was absent in every subject.
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reveal, using this component as the baseline would not adequately capture the complex voltage changes which P1 underwent after ECS. Following ECS, the FVEP was lost in all subjects (0 min). By the second recording (1 min), a waveform had unfailingly reappeared with a latency and amplitude (at least of P1) not too dissimilar to that of the baseline values. By the third recording (2 min), the mean latency of P1 had abruptly increased more than 6 ms in comparison with the baseline and this was associated with a prominent increase in the mean amplitude of P1 of more than 150%. By the fifth recording (4 min) both the latency and amplitude of P1 had begun to decline and near stable values were achieved by 6–7 min. While its amplitude returned to near normal values, the latency of P1 remained slightly prolonged when the recordings terminated at 10 min. The quite atypical character of the mean P1 component recorded at 1 min tends to reinforce the impression gained from the individual examples that this is probably a foreign response. As mentioned above, only the initial positivity (P1) of the FVEP was formally analysed. However, an inspection of some of the examples in Figs. 1–3 during the recovery phase (2–4 min) might give the impression that the latency of the primary negativity was markedly increased in latency when compared to that of P1. Nevertheless, a more detailed examination of the relevant FVEPs suggests that this appearance is misleading. As subjects 1, 2, 3, 4 and 6 show, the primary negativity is not normally a homogeneous component but is usually bifurcated into a major primary subcomponent with a latency of 35–40 ms followed by a secondary, relatively positive, low amplitude subcomponent occurring between 50 and 55 ms. During the post-ictal period when the P1 component is enlarged, the primary negativity is so distorted that it temporarily permits the secondary negativity to assume a more dominant role. This creates the false impression of a large latency increase. It is notable that in the single example where the primary negativity is not bifurcated (subject 5), there is no indication that this component is proportionally more prolonged than that of P1. In summary, there is no evidence that activity presumed to be generated in the more superficial layers of the visual cortex is any more sensitive to ECS than that which arises at deeper levels.
4. Discussion 4.1. The loss of the FVEP by ECS Contrary to several previous reports, the present findings have demonstrated that the FVEP is totally abolished by a single dose of ECS, at least during the tonic phase of the seizure. There is thus no conflict between the results of GSA on the FVEP and those on the homologous potential generated in the somatosensory system (Shaw, 1985). There is also a possible alternative explanation for the loss of the
FVEP which must be acknowledged. It may be that averaging just 32 epochs of high amplitude paroxysmal activity may not be sufficient to extract an EP which might be buried within it. Under these conditions, the apparent loss of the FVEP would be an artifact caused by a failure of the averaging process rather than by the effects of ECS on sensory transmission. Be that as it may, such an interpretation seems unlikely. The FVEP is a comparatively high voltage response which would not be readily swamped by the background epileptiform activity. Moreover, where this activity has been effectively cancelled out, as in subject 4 (Fig. 2), not a vestige of the FVEP can be detected in the trace. How long the FVEP actually remains absent is uncertain. If it is supposed that the waveform recorded during the clonic phase (between 30–60 s) is not a genuine cortical response, it would indicate that the FVEP must be lost for between 1 and 2 min. This would further mean that the FVEP is abolished for a period almost twice as long as the somatosensory EP was following ECS (Shaw, 1985). It is also assumed that the diffuse high amplitude hypersynchronous epileptiform activity causes generalised impairment of neurotransmission, at least at the cortical level, and so should prevent the formation of responses such as the FVEP. As Small and her colleagues (Small et al., 1978) have pointed out, any apparent retention of the FVEP during the ictal period implies that some cortical neurons must remain unrecruited and so uninvolved in seizure activity. The present findings suggest that it is probably not necessary to invoke these more complex qualifications. 4.2. The possible role of the collicular FVEP If the response recorded during the clonic phase of the seizure (30–60 s) is not an authentic FVEP, then the question must be raised as to where is its site of origin. In theory, such a potential could be a far field reflection of activity arising from the retina, optic pathway or the thalamus. It is also conceivable that the response might represent an early, rather more robust constituent of the primary positivity (P1) which becomes unveiled after other elements composing P1 have been shorn away by the action of ECS. Even so, the behaviour of the potential remains much more consistent with a far field rather than a near field cortical origin. The most likely candidate to generate a spurious cortical FVEP under such circumstances is the superior colliculus. If a coronal section of the rat’s brain is examined (Thompson, 1978), it can be seen that the superior colliculus is a large structure which directly underlies much of the visual cortex. In man and higher mammals, the superior colliculus seems to operate simply as an ocular reflex center. By contrast, in an animal such as the rat, the superior colliculus is a much more sophisticated structure which still seems to retain, and thereby share with the occipital cortex, the function of visual perception. In rodents, the optic nerve fibres project retinal images directly onto the superior colliculus in a strictly topographic manner (Smith, 1972; Brodal, 1981)
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and as a consequence it would be expected to generate a collicular FVEP. Using intracranial recordings, this prediction has been confirmed by many studies (e.g. Dyer and Annau, 1977; Hetzler and Berger, 1984). It is also notable that when the cortical FVEP and collicular FVEP have been recorded from the same animal (e.g. Woolley, 1976; Hetzler and Oaklay, 1981; Hetzler et al., 1981), not only were their waveforms basically congruent, but each also possessed an initial positive component in close temporal proximity to each other. It is therefore not unreasonable to surmise that in the temporary absence of the cortical FVEP, the normally masked collicular FVEP could briefly take its place, thereby creating the illusion that the early components of the cortical FVEP, at least, had returned during the clonic phase of the seizure. 4.3. The post-ictal enhancement of the FVEP The brief rise in the voltage of the P1 component of the FVEP during the post-ECS period is presumably analogous to similar findings reported for epileptic patients. For instance, Broughton et al. (1969) described an enlargement of the FVEP in patients with photosensitive epilepsy, while Halliday and Halliday (1980) reported a comparable finding in those with progressive myoclonic epilepsy. Likewise, a series of animal studies have found that the FVEP may become enhanced following the administration of convulsant drugs (Rodin et al., 1966; Mirsky and Tecce, 1968; Burchiel et al., 1976). As Jones (1993) has recently observed, the mode of action of this paroxysmal increase in amplitude remains unknown. Originally, Halliday and Halliday (1980) suggested it was a function of epileptiform activity generated in diffusely projecting reticular pathways. An alternative explanation is that the amplification is simply a consequence of the potential arising within a neuronal population of heightened excitability (Mirsky and Tecce, 1968). The voltage of epileptiform activity may not be uniformly distributed across the cortex and it has been shown that abnormally enlarged waveforms seem to be localized to regions of higher intensity discharges in the background EEG (Mirsky and Tecce, 1968). At least part of the increase in the amplitude of P1 might also simply be accounted for by the absence or attenuation of the following negative potential.
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tration of ECT, in a manner similar to the present experiment. If their findings are reviewed, it is apparent that the FVEP waveform was deteriorating due to the methohexitone anesthesia even before the application of ECT. Following ECT, the average waveform was still present but poorly defined. It is, however, virtually impossible to disentangle the interacting effects of ECT and barbiturate anesthesia on the waveform during the ictal period. In particular it is unclear to what extent any anticonvulsant properties of methohexitone may have safeguarded the waveform from the destructive effects of ECT. This is a problem which besets and confounds all such clinical studies of ECT and EPs, as is explicitly acknowledged by these authors. With regard to the animal studies, Myslobodsky and Kofman (1982) obtained FVEPs from the awake rat but did not begin recording until after seizure activity had subsided. This presumably accounts for why the waveform appeared to be preserved apart from some minor lingering abnormalities and evidence of post-ictal enhancement of the early components. Rather more difficult to explain are the findings of Gregory and Wotton (1985) who recorded FVEPs from awake restrained sheep. Animals were administered a dose of ECS comparable to that routinely used in abattoirs to stun a sheep. Unlike Myslobodsky and Kofman, Gregory and Wotton began their recordings immediately after the induction of GSA but still managed to obtain a well-defined response during the tonic-clonic phase in two-thirds of their subjects. The most parsimonious explanation for the seeming preservation of the FVEP during the ictal period is that the recordings from the sheep had also been contaminated by far field collicular activity. This assumes that the superior colliculus plays the same functional role in the ruminant brain as it does in the rodent brain. Further, the averaging time for each potential recorded from the sheep lasted 50 s, unlike the more limited recording periods employed in the present experiment. In practice, this meant that the first postictal recording extended throughout the tonic and clonic phases of the seizure. Judging by the present findings in the rat, it would be expected that averaging throughout this period would at least yield a visual response of some type, although not necessarily a genuine cortical FVEP. This may not be an entirely satisfactory explanation as the superior colliculus in the sheep does not lie as close to the visual cortex as it does in the rat (Skinner, 1974) and the paper contains other incongruous data.
4.4. Comparison to previous studies of ECS and the FVEP 4.5. The physiology of electrical stunning No single cause can explain why previous investigations have failed to detect the abolition of the FVEP waveform after the administration of ECS or ECT. In some cases, the explanation is readily available. For instance, Small and Small (1971) delayed recording from their patients for at least an hour after ECT. Under such circumstances, it is hardly surprising that no loss of waveform was reported. In contrast, Kriss et al. (1980) made serial recordings from patients which began immediately after the adminis-
The present results are also of relevance to the enduring and ethically significant problem of the mechanism of action by which ECS actually stuns a subject. One possibility is that ECS acts in a manner similar to the conventional anesthetic agents such as the barbiturates by inhibiting or impairing synaptic transmission within the specific and nonspecific (reticular) pathways of the brainstem and midbrain. The effects of barbiturates such as pentobarbital on the
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FVEP of the rat have been well documented (Hetzler and Oaklay, 1981; Shaw, 1992a). While amplitudes and latencies of some components may be altered and secondary waves abolished, the early part of the waveform was preserved even under a surgical level of barbiturate anesthesia. This is in contrast to the present findings where ECS temporarily obliterated the entire waveform and therefore indicates the involvement of an entirely different mode of action than that of depression of a center controlling wakefulness. A second possibility is that ECS causes unconsciousness because the seizure activity it induces interferes with the normal processes of the centrencephalic integrating center. The centrencephalon is a functional system devised by Penfield and Jasper (1954) to explain how seizures become disseminated and generalised. According to the centrencephalic theory, volleys of abnormal electrical discharges emanating from thalamic reticular nuclei are propagated diffusely to the cortex where they serve to recruit and regulate epileptiform activity. Excitatory bursts arising within the reticulo-thalamo-cortical projection pathways would effectively disrupt their role of maintaining a state of cortical alertness and a loss of consciousness would rapidly ensue (Gastaut and Fischer-Williams, 1959). One of the difficulties with this theory is that it does not clearly predict what changes would be expected to occur to the morphology of cortical EPs following the induction of GSA by ECS. Presumably the later components would be lost but the fate of the earlier responses such as the P1 of the FVEP is uncertain. Therefore, the most that could be claimed for the centrencephalic theory is that the total disappearance of the FVEP is probably not incompatible with it. A third possibility is that epileptiform activity simply blocks or overrides the sensory signal at some level within the brain. Such an occlusion would cause functional deafferentation of the cortex and so render the subject unconscious. This theory would predict that cortical EPs must be totally abolished following ECS and so the current findings are most compatible with it. Previous recordings where the FVEP was seemingly preserved following the administration of ECS have raised the disturbing possibility that electrical stunning of animals in abattoirs may actually result in little or no loss of consciousness (e.g. Gregory and Wotton, 1985). The present results have established that this concern is probably unwarranted. 4.6. ECS and generalised epilepsy The present findings are also relevant to the persistent question of the pathophysiological mechanisms underlying the behavioural and cognitive deficits associated with spontaneous generalised seizures. It is likely that the same basic mode of action which is responsible for electrical stunning can also account for the impairment or loss of consciousness which is a hallmark of grand mal or petit mal epilepsy. Both appear to involve a similar interference or disruption of sensory processing by epileptiform activity (Mirsky and
Tecce, 1968). If this hypothesis is correct, it would be expected that both electrical stunning and generalised epilepsy should result in the attenuation or disappearance of the FVEP. Such a prediction is consistent with the report by Orren (1978) who recorded FVEPs from patients with classical petit mal epilepsy immediately prior to the appearance of spike and wave discharges. Although the FVEP was never entirely abolished during this brief period, individual components were lost and the waveform was markedly suppressed. In an earlier study, Orren also managed to obtain technically high quality FVEPs during bursts of spike and wave activity from petit mal patients (quoted by Mirsky et al., 1986). She reported that the same components of the FVEP which disappeared following ECS in the present experiment were also lost in the majority of her subjects. Similarly, Mirsky and his colleagues (Mirsky et al., 1973) recorded FVEPs from widespread locations within the visual system during drug-induced spike and wave activity in the monkey. In this instance, it was observed that not only the cortical FVEP was susceptible to GSA, but also EPs generated more caudally in the lateral geniculate body, optic pathway and retina. Nevertheless, the more centrally generated the response, the greater was the alteration to its waveform by paroxysmal activity. Not all such reports are as congruous with the present findings as those of Orren and Mirsky. For example, Burchiel et al. (1976) using the feline penicillin model described an enhancement of the FVEP following the onset of spike and wave activity. Ostensibly, such results represent a challenge to the understanding that the absences of petit mal epilepsy are underlain by a reduction or inhibition of sensory input (Mirsky, 1978). Reasons for these discrepant findings may include differences in species, techniques for inducing GSA and modes of neurophysiological action (Mirsky et al., 1986.) In the present experiment, ECS precipitated an increase in FVEP amplitude of the same magnitude as that reported by Burchiel et al. (1976). Nonetheless, this enhancement was confined quite discretely to the post-ictal period when reflex activity and consciousness were being rapidly regained. During the convulsion itself, the FVEP waveform appeared to be completely demolished. The current results may therefore allow the respective roles of suppression and enhancement of EPs to be more clearly defined than is possible with other models of experimental GSA. A final controversy concerns the identity of the primary site of action of GSA at which the disturbances of awareness, consciousness and attention are initiated or mediated. Extrapolating from their respective animal models of druginduced epilepsy, Mirsky (1978) and Gloor (1978) reached quite different conclusions. For Gloor, the principal site of impact of GSA resided within the cortex, whereas Mirsky located it within the mesopontine reticular formation. The present findings, where GSA was induced electrically, do little to resolve this matter. The loss of the cortical FVEP does not necessarily imply that the effects of ECS specifically target cortical function. Interference of sensory trans-
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mission at any level within the visual pathways could account for the loss of the waveform. The equivocal nature of the waveform recorded during the clonic phase of the seizure (1 min) further confounds any interpretation. Even if the origin of the putative far field FVEP was established with certainty, there still remains the doubt over the extent to which it is itself abolished during the acute ictal period (0 min). While there was clearly no sign of this response in the initial trace recorded after ECS, this does not necessarily mean that it was lost, along with the cortical potential. It could, for instance, simply indicate that the high voltage epileptiform activity acted as a barrier to block transmission of the potential from the superior colliculus or elsewhere to the surface of the brain. Be that as it may, if the far field potential is genuinely lost during the tonic phase, then the most parsimonious explanation for the sequential return of the waveforms is that ECS may have a primary site of action at more than one location within the visual system. It is of interest that Mirsky et al. (1973) using topical and systemic application of convulsant drugs came to a broadly similar conclusion. Acknowledgements This research was supported by the Maurice and Phyllis Paykel Trust. The author thanks Paul Hill, Jack Sinclair and Bruce Smaill for their advice and support, and Jane Utting for typing the manuscript. References Brodal, A. Neurological Anatomy, 3rd edn. Oxford University Press, New York, 1981. Broughton, R., Meier-Ewert, K.-H. and Ebe, M. Evoked visual, somatosensory and retinal potentials in photosensitive epilepsy. Electroenceph. clin. Neurophysiol., 1969, 27: 373–386. Burchiel, K.J., Myers, R.R. and Bickford, R.G. Visual and auditory evoked responses during penicillin-induced generalized spike-and-wave activity in cats. Epilepsia, 1976, 17: 293–311. Dyer, R.S. and Annau, Z. Flash evoked potentials from rat superior colliculus. Pharmacol. Biochem. Behav., 1977, 6: 453–459. Dyer, R.S., Jensen, K.F. and Boyes, W.K. Focal lesions of visual cortex: effects on visual evoked potentials in rats. Exp. Neurol, 1987, 95: 100– 115. Foutz, A.S., Dauthier, C. and Kerdelhue, B. b-Endorphin plasma levels during neuromuscular blockade in unanesthetized cat. Brain Res., 1983, 263: 119–123. Gastaut, H. and Fischer-Williams, M. The physiopathology of epileptic seizures. In: J. Field, H.W. Magoun and V.E. Hall (Eds.), Handbook of Physiology, Section 1: Neurophysiology, Vol I. American Physiological Society, Washington DC, 1959, pp. 329–363. Gloor, P. Generalized epilepsy with bilateral synchronous spike and wave discharge: new findings concerning its physiological mechanisms. Electroenceph. clin. Neurophysiol. (Suppl.), 1978, 34: 245–249. Gregory, N.G. and Wotton, S.B. Sheep slaughtering procedures. IV. Responsiveness of the brain following electrical stunning. Br. Vet. J., 1985, 41: 74–81.
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Halliday, A.M. and Halliday, E. Cerebral somatosensory and visual evoked potentials in different clinical forms of myoclonus. In: J.E. Desmedt (Ed.), Clinical Uses of Cerebral, Brainstem and Spinal Somatosensory Evoked Potentials, Prog. Clin. Neurophysiol., Vol. 7. Karger, Basel, 1980, pp. 292–310. Hetzler, B.E. and Berger, L.K. Ketamine-induced modification of photic evoked potentials in the superior colliculus of hooded rats. Neuropharmacology, 1984, 23: 473–476. Hetzler, B.E. and Oaklay, K.E. Dose effects of pentobarbital on evoked potentials in visual cortex and superior colliculus of the albino rat. Neuropharmacology, 1981, 20: 969–978. Hetzler, B.E., Heilbronner, R.L., Griffin, J. and Griffin, G. Acute effects of alcohol on evoked potentials in visual cortex and superior colliculus of the rat. Electroenceph. clin. Neurophysiol., 1981, 51: 69–79. Jones, S.J. Somatosensory evoked potentials II: clinical observations and applications. In: A.M. Halliday (Ed.), Evoked Potentials in Clinical Testing, 2nd edn. Churchill Livingstone, Edinburgh, 1993, pp. 1–466. Kriss, A., Halliday, A.M., Halliday, E. and Pratt, R.T.C. Evoked potentials following unilateral ECT. II. The flash evoked potential. Electroenceph. clin. Neurophysiol., 1980, 48: 490–501. Mirsky, A.F. Epilepsy, attentiveness and consciousness: recent contributions from behavioral and physiological investigations. Electroenceph. clin. Neurophysiol. (Suppl.), 1978, 34: 269–275. Mirsky, A.F. and Tecce, J.J. The analysis of visual evoked potentials during spike and wave EEG activity. Epilepsia, 1968, 9: 211–220. Mirsky, A.F., Bloch, S., Tecce, J.J., Lessell, S. and Marcus, E. Visual evoked potentials during experimentally induced spike-wave activity in monkeys. Electroenceph. clin. Neurophysiol., 1973, 35: 25–37. Mirsky, A.F., Duncan, C.C. and Myslobodsky, M.S. Petit mal epilepsy: a review and integration of recent information. J. Clin. Neurophysiol., 1986, 3: 179–208. Myslobodsky, M.S. and Kofman, O. Unilateral versus bilateral electroconvulsive shock in albino rats: comparison of behavioral symptomatology and neocortical reactivity changes. Biol. Psychiatry, 1982, 17: 363–380. Orren, M.M. Evoked potential studies in petit mal epilepsy. Electroenceph. clin. Neurophysiol. (Suppl.), 1978, 34: 251–257. Penfield, W. and Jasper, H. Epilepsy and the Functional Anatomy of the Human Brain. Little, Brown, Boston, 1954. Rodin, E., Gonalez, S., Calwell, D. and Laginess, D. Photic evoked potentials during induced epileptic seizures. Epilepsia, 1966, 7: 202–214. Shaw, N.A. Effect of electroconvulsive shock on the somatosensory evoked potential in the rat. Exp. Neurol., 1985, 90: 566–579. Shaw, N.A. The effects of low-pass filtering on the flash visual evoked potential of the albino rat. J. Neurosci. Methods, 1992a, 44: 233–240. Shaw, N.A. Auditory potentials elicited by the Grass photic stimulator in the rat. Physiol. Behav., 1992b, 52: 401–403. Skinner, J.E. Neuroscience: a Laboratory Manual. Saunders, Philadelphia, 1974. Small, I.F. and Small, J.G. Electroencephalographic (EEG), evoked potential, and direct current (DC) responses with unilateral electroconvulsive treatment (ECT). J. Nerv. Ment. Dis., 1971, 152: 396–404. Small, J.G., Small, I.F. and Milstein, V. Electrophysiology of EST. In: M.A. Lipton, A. DiMascio and K.F. Killam (Eds.), Psychopharmacology: a Generation of Progress. Raven Press, New York, 1978, pp. 759– 769. Smith, C.G. Basic Neuroanatomy, 2nd edn. University of Toronto Press, Toronto, 1972. Smith, S.M., Brown, H.O., Toman, J.E.P. and Goodman, L.S. The lack of cerebral effects of d-tubocurarine. Anesthesiology, 1947, 8: 1–14. Thompson, R. A Behavioral Atlas of the Rat Brain. Oxford University Press, New York, 1978. Woolley, D.E. Some aspects of the neurophysiological basis of insecticide action. Fed. Proc., 1976, 35: 2610–2617.
The effects of electroconvulsive shock on the flash visual evoked potential in the rat Nigel A. Shaw* Department of Physiology, School of Medicine, University of Auckland, Private Bag 92019, Auckland, New Zealand Accepted for publication: 16 December 1996
Abstract The effects of electroconvulsive shock (ECS) on the flash visual evoked potential (FVEP) were studied in the awake albino rat. Immediately after the induction of generalised seizure activity, the FVEP was totally abolished although accidentally averaged rhythmic epileptiform activity was often present in the trace. During the second recording, a potential had reappeared but this response was suspected of being a superior colliculus FVEP masquerading as a cortical response. By the third recording, the genuine cortical FVEP had returned, albeit with an abnormally large amplitude. The waveform subsequently remained significantly distorted although it had regained an approximately normal morphology within 6–7 min of the administration of ECS. It was not possible to identify the principal site of action of ECS but it was concluded that ECS may impact on activity generated at more than one location within the optic pathways. The present findings are compared with a number of previous animal and human studies where the FVEP was apparently preserved following ECS and attempts are made to explain the discrepancy in results. The relevance of the present findings for understanding the pathophysiology of electrical stunning and of the loss or impairment of consciousness during generalised epileptic seizures is also discussed. 1997 Elsevier Science Ireland Ltd. Keywords: Electroconvulsive shock; Flash visual evoked potential; Generalised seizure activity; Rat; Superior colliculus; Visual cortex
1. Introduction A number of procedures can be employed to artificially induce generalised seizure activity (GSA). These include physiological, chemical and mechanical methods. However, the simplest and most common technique is simply to pass a brief electric current through the brain. This will induce tonic-clonic seizures which closely mimic a spontaneous grand mal convulsion as well as an abrupt but brief loss of consciousness and retrograde amnesia. When this procedure is used as a treatment for severe depressive illness and other psychiatric and possible neurological disorders, it is called electroconvulsive therapy (ECT). When used to study the effects of seizure activity on experimental animals, it is normally labelled electroconvulsive shock (ECS). When
* Corresponding author. Tel.: +64 9 3737599; fax: +64 9 3737499.
0168-5597/97/$17.00 1997 Elsevier Science Ireland Ltd. All rights reserved PII S0921-884X(97)9602 5-2
used to render animals insensible prior to their slaughter in abattoirs, it is described as electrical stunning. Despite its widespread use, the mechanisms of action of ECS/ECT are still basically unknown. What is conspicuously missing is a knowledge of the acute physiology of GSA. One method for obtaining this information is by the recording of evoked potentials (EPs). EPs may be recorded from different sensory systems and can provide discrete snap shots of changes in cerebral activity during the acute ictal and post-ictal periods. Of the many kinds of EPs, probably the easiest to obtain following the induction of GSA by ECS is the flash visual evoked potential (FVEP). The FVEP is the response which is elicited in the occipital cortex by a flash of light. There have now been a number of investigations of the effects of ECT on the FVEP in patients (e.g. Small and Small, 1971; Kriss et al., 1980) and of ECS on the FVEP in animals (e.g. Myslobodsky and Kofman, 1982; Gregory and Wotton, 1985). It is a feature of virtually all these studies that, while there was invariably some alteration to its
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waveform, the FVEP was always basically preserved during the initial recording following ECS or ECT. It is also notable that when the FVEP was recorded after the administration of convulsant drugs, it was similarly reported to be retained following the onset of epileptiform activity (e.g. Rodin et al., 1966; Mirsky and Tecce, 1968; Burchiel et al., 1976). What is anomalous about such findings is that if there is only limited disruption of neurotransmission within a sensory pathway following ECS, then it becomes more difficult to explain why ECS possesses its various therapeutic, anesthetic and amnesic properties. In addition, it has also been demonstrated that ECS will totally abolish the equivalent response to the FVEP in the somatosensory system, i.e. the cortical somatosensory EP (Shaw, 1985). At least two explanations are possible for this discrepancy. First, it is conceivable that the somatosensory and visual systems are differentially sensitive to the effects of ECS. More likely, however, the apparent preservation of the FVEP following ECS was due to the conditions under which it was recorded. For instance, the concurrent medication administered during the modified ECT treatment may have played a protective role for the EP waveform. In addition, any loss of the FVEP following ECS may have gone undetected because of a delay between the onset of GSA and the actual start of the initial recording. The purpose of the present study was to attempt to resolve these contradictory claims regarding the vulnerability of cortical EPs to ECS. This was done by recording the FVEP under near identical conditions to those existing when the effects of ECS were examined on the somatosensory EP.
2. Methods and materials Subjects were 25 adult male albino rats (300–350 g). One week prior to the experiment, each subject had four small stainless steel screws inserted while under pentobarbital anesthesia (60 mg/kg.) Screws did not penetrate the dura and were insulated and secured with dental acrylic. One screw was implanted over the visual cortex (6–7 mm posterior to bregma and 3–4 mm lateral to the sagittal suture). Two screws were inserted sequentially along the nasal bone. A fourth electrode was inserted elsewhere in the dorsal skull. On the experimental day, each animal was initially curarised with a dose of d-tubocurarine chloride (4 mg/kg). As soon as signs of neuromuscular blockade became apparent, the subject was connected to a respirator and artificially ventilated at a rate of 50 strokes per min, each of 15 ml. Animals were attached to the respirator via a mask constructed of the mouthpiece end of a balloon which fitted snugly over the animal’s snout. The mask was secured to the animal by lodging the bottom part behind the upper incisors and the top half over the more distal of the nasal bone screws. ECG electrodes were attached and the stroke rate and volume of the ventilator were adjusted (if necessary) to maintain a normal heart rate (350–400 BPM). EP
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electrodes were then attached. The active lead was connected to the screw inserted over the visual cortex. The reference lead was connected to the second of the nasal bone screws. The ground lead was connected to the remaining skull screw. Several baseline FVEPs were then obtained to ensure that the waveform could be reproducibly recorded. FVEPs were recorded using a Medelec MS6. Analysis time was 100 ms and sampling interval 100 ms. Thirty-two responses were averaged to obtain each FVEP. Bandpass of the amplifiers was set to 3.2–320 Hz (Shaw, 1992a). Stimuli were light pulses delivered by a Grass model PS 22 photic stimulator. Duration of each flash was 10 ms and stimulation rate was 2/s. Flash intensity was set to step 8 on the 1–16 scale. In order to muffle the click which accompanied each light flash (Shaw, 1992b), small rubber plugs were inserted into each ear. FVEPs were recorded following monocular stimulation of the eye contralateral to the cortical electrode. Recordings were made in a totally darkened room. Following the baseline recordings, GSA was induced by transmitting a brief electric current (80 mA for 600 ms) via miniature bulldog clips attached to the animal’s ears. The insides of the clips were filled with electrode paste. This magnitude of ECS will induce tonic-clonic seizures in the awake non-paralysed animal lasting approximately 1 min accompanied by a period of unconsciousness and loss of reflex activity for up to 3 min (Shaw, 1985). The first recording was made between 0 and 30 s after the induction of GSA (0 min). Because of the delay in reconnecting the animal to the recording equipment plus the temporary overloading of the averaging equipment by the high voltage epileptiform activity, the first FVEP recording usually did not begin until 10–15 s had elapsed following the administration of ECS. A second FVEP was recorded between 30 and 60 s (1 min), and subsequent FVEPs were obtained at 2, 3, 4, 5, 6, 7, 8, 9 and 10 min. The animal was then immediately euthanised with an overdose of pentobarbital. The technique of chemically paralysing and artificially ventilating an awake animal was a necessary condition to obtain the present information and is similar to that previously employed to study the effects of convulsant drugs on EPs (e.g. Rodin et al., 1966; Burchiel et al., 1976). It allowed the effects of ECS on EPs to be quantified free from the contaminating influences of EMG movement artifact and sedative or anesthetic drugs. In various combinations, these factors appear to have confounded a number of previous investigations of ECS and EPs. There is also no reason to suspect that neuromuscular blockade is in any respect a discomforting experience. Any restriction on the use of curariform agents in awake animals is simply to protect them from stressful or painful procedures being inflicted on the false assumption that because they are immobilised, they must also be insensitive. In the present experiment, no such stimuli were inflicted on the subjects. Further evidence and discussion that short term neuromuscular blockade causes distress in neither humans (Smith et al., 1947) nor animals (Foutz et al., 1983) is available elsewhere.
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3. Results Six examples of the effects of ECS on the FVEP waveform are illustrated in Figs. 1–3. Examples of the normal FVEP recorded from the awake rat are shown in the baseline (before ECS) traces. The FVEP consists of an initial positivity (P1) with a peak latency which occurs at just under 30 ms following the onset of the photic stimulus. P1 is followed by a negative trough and then by a secondary positive complex. Later components of the FVEP which may persist for several 100 ms were not studied in the present experiment. P1 is generated by post-synaptic activity arising within the neurons of inner granular layer 4 of the visual cortex. The primary negative and secondary positive components are thought to represent more superficial depolarisation and hyperpolarisation, respectively (Dyer et al., 1987). As the
Fig. 2. Two further examples of the effects of ECS on the FVEP. The amplitude scale at the bottom of subject 3 applies to all the tracings. Note the different amplitude scales for the FVEPs recorded from subject 4.
Fig. 1. Two examples of the effects of electroconvulsive shock (ECS) on the flash visual evoked potential (FVEP). The baseline (before ECS) potential was recorded just prior to the induction of generalised seizure activity. Subsequent FVEPs were recorded at the times indicated after ECS. In the baseline examples, the primary positive component of the FVEP (P1) is identified with its actual latency (ms) in parentheses. In the remaining FVEPs, only the latency of P1 is indicated. Note the different amplitude scales for the potentials recorded from both subjects. In this and the subsequent two figures, the recordings made at 7 and 9 min are not illustrated.
principal concern of the present experiment was confined to whether or not the FVEP was totally abolished following ECS, only the behaviour of the P1 component needed to be systematically analysed. It can be seen from the 6 traces recorded at 0 min in Figs 1–3 that activity of some type was usually present during the tonic phase of the seizure. This mostly took the form of rhythmic activity of variable amplitude. These traces appeared to be generated by the serendipitous averaging of high frequency spikes which became accidentally timelocked to the photic stimulus. Averaging just 32 responses was usually insufficient for the epileptiform activity to be effectively cancelled out. There was, however, no evidence that the FVEP itself had been preserved during the tonic period. Nonetheless, by the second recording which was made between 30 and 60 s, a FVEP had invariably reappeared. The overall amplitude of this potential was markedly attenuated in comparison with the baseline recording, but the latency of the primary positivity was only minimally increased. It will be subsequently argued that the potential recorded at 1 min was not, in fact, a genuine cortical FVEP. Nevertheless, there was little doubt that the response obtained at 2 min represented the re-emergence of the
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increased latency which subsequently declines. Quite the contrary was observed here, where the latency of the putative P1 response unexpectedly jumped 4–8 ms between the 1 min and 2 min recordings. The simplest means of resolving this discrepancy is to assume that comparisons are being erroneously made between waveforms which have quite different generators. Furthermore, if the waveform obtained at 1 min was indeed a genuine cortical FVEP, then it is difficult to explain why it did not share in the augmentation of the P1 component displayed by the succeeding FVEPs for the next 2–3 min. Finally, if the present interpretation is correct, then it implies that the FVEP waveforms recorded between 2 and 4 min are probably an amalgam of activity from both the genuine and imitation cortical FVEPs. Group data for all 25 animals studied is summarised in Fig. 4. This confirms that the amplitude and latency trends displayed by the individual subjects in Figs. 1–3 were, in general, typical of the group as a whole. When calculating the amplitude of the P1 component, the baseline employed was the preceding shallow negativity which had a latency of approximately 20 ms. Under other circumstances, it might have been more appropriate to use the large negative trough which follows P1. However, as the illustrations in Figs. 1–3
Fig. 3. Two more examples of the effects of ECS on the FVEP. The amplitude scale at the bottom of each subject applies to all their traces.
authentic cortical FVEP, albeit with a marked increase in both the latency and amplitude of P1. The waveform remained significantly distorted and abnormal for the next 2–3 min due mostly to the persisting dilation of the P1 component. Thereafter, there followed a quite rapid decline in the amplitude of P1 associated with a decrease in its latency, such that within 6 min the FVEP had regained a more or less normal morphology. It is notable that even by the end of the recording period at 10 min, overall amplitude of the waveform had not usually been completely restored and the latency of P1 remained slightly prolonged. It is also clear from Figs. 1–3 that, while all 6 subjects displayed this basic pattern of change following ECS, there was considerable inter-animal variability in the timing and extent of these modifications to their waveforms This was also true for the other 19 animals studied. Superficially, the waveform recorded at 1 min after ECS had the appearance of a diminutive FVEP. The principal reason why it is claimed that this response was not a bona fide cortical FVEP is because of the aberrant behaviour of the latency of its primary positive component (P1). Normally, when an EP component is lost, it returns with an
Fig. 4. Mean latency and amplitude (±1 SD) of the P1 component of the FVEP at the times indicated following ECS. Note that at the first post-ECS recording (0 min), P1 was absent in every subject.
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reveal, using this component as the baseline would not adequately capture the complex voltage changes which P1 underwent after ECS. Following ECS, the FVEP was lost in all subjects (0 min). By the second recording (1 min), a waveform had unfailingly reappeared with a latency and amplitude (at least of P1) not too dissimilar to that of the baseline values. By the third recording (2 min), the mean latency of P1 had abruptly increased more than 6 ms in comparison with the baseline and this was associated with a prominent increase in the mean amplitude of P1 of more than 150%. By the fifth recording (4 min) both the latency and amplitude of P1 had begun to decline and near stable values were achieved by 6–7 min. While its amplitude returned to near normal values, the latency of P1 remained slightly prolonged when the recordings terminated at 10 min. The quite atypical character of the mean P1 component recorded at 1 min tends to reinforce the impression gained from the individual examples that this is probably a foreign response. As mentioned above, only the initial positivity (P1) of the FVEP was formally analysed. However, an inspection of some of the examples in Figs. 1–3 during the recovery phase (2–4 min) might give the impression that the latency of the primary negativity was markedly increased in latency when compared to that of P1. Nevertheless, a more detailed examination of the relevant FVEPs suggests that this appearance is misleading. As subjects 1, 2, 3, 4 and 6 show, the primary negativity is not normally a homogeneous component but is usually bifurcated into a major primary subcomponent with a latency of 35–40 ms followed by a secondary, relatively positive, low amplitude subcomponent occurring between 50 and 55 ms. During the post-ictal period when the P1 component is enlarged, the primary negativity is so distorted that it temporarily permits the secondary negativity to assume a more dominant role. This creates the false impression of a large latency increase. It is notable that in the single example where the primary negativity is not bifurcated (subject 5), there is no indication that this component is proportionally more prolonged than that of P1. In summary, there is no evidence that activity presumed to be generated in the more superficial layers of the visual cortex is any more sensitive to ECS than that which arises at deeper levels.
4. Discussion 4.1. The loss of the FVEP by ECS Contrary to several previous reports, the present findings have demonstrated that the FVEP is totally abolished by a single dose of ECS, at least during the tonic phase of the seizure. There is thus no conflict between the results of GSA on the FVEP and those on the homologous potential generated in the somatosensory system (Shaw, 1985). There is also a possible alternative explanation for the loss of the
FVEP which must be acknowledged. It may be that averaging just 32 epochs of high amplitude paroxysmal activity may not be sufficient to extract an EP which might be buried within it. Under these conditions, the apparent loss of the FVEP would be an artifact caused by a failure of the averaging process rather than by the effects of ECS on sensory transmission. Be that as it may, such an interpretation seems unlikely. The FVEP is a comparatively high voltage response which would not be readily swamped by the background epileptiform activity. Moreover, where this activity has been effectively cancelled out, as in subject 4 (Fig. 2), not a vestige of the FVEP can be detected in the trace. How long the FVEP actually remains absent is uncertain. If it is supposed that the waveform recorded during the clonic phase (between 30–60 s) is not a genuine cortical response, it would indicate that the FVEP must be lost for between 1 and 2 min. This would further mean that the FVEP is abolished for a period almost twice as long as the somatosensory EP was following ECS (Shaw, 1985). It is also assumed that the diffuse high amplitude hypersynchronous epileptiform activity causes generalised impairment of neurotransmission, at least at the cortical level, and so should prevent the formation of responses such as the FVEP. As Small and her colleagues (Small et al., 1978) have pointed out, any apparent retention of the FVEP during the ictal period implies that some cortical neurons must remain unrecruited and so uninvolved in seizure activity. The present findings suggest that it is probably not necessary to invoke these more complex qualifications. 4.2. The possible role of the collicular FVEP If the response recorded during the clonic phase of the seizure (30–60 s) is not an authentic FVEP, then the question must be raised as to where is its site of origin. In theory, such a potential could be a far field reflection of activity arising from the retina, optic pathway or the thalamus. It is also conceivable that the response might represent an early, rather more robust constituent of the primary positivity (P1) which becomes unveiled after other elements composing P1 have been shorn away by the action of ECS. Even so, the behaviour of the potential remains much more consistent with a far field rather than a near field cortical origin. The most likely candidate to generate a spurious cortical FVEP under such circumstances is the superior colliculus. If a coronal section of the rat’s brain is examined (Thompson, 1978), it can be seen that the superior colliculus is a large structure which directly underlies much of the visual cortex. In man and higher mammals, the superior colliculus seems to operate simply as an ocular reflex center. By contrast, in an animal such as the rat, the superior colliculus is a much more sophisticated structure which still seems to retain, and thereby share with the occipital cortex, the function of visual perception. In rodents, the optic nerve fibres project retinal images directly onto the superior colliculus in a strictly topographic manner (Smith, 1972; Brodal, 1981)
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and as a consequence it would be expected to generate a collicular FVEP. Using intracranial recordings, this prediction has been confirmed by many studies (e.g. Dyer and Annau, 1977; Hetzler and Berger, 1984). It is also notable that when the cortical FVEP and collicular FVEP have been recorded from the same animal (e.g. Woolley, 1976; Hetzler and Oaklay, 1981; Hetzler et al., 1981), not only were their waveforms basically congruent, but each also possessed an initial positive component in close temporal proximity to each other. It is therefore not unreasonable to surmise that in the temporary absence of the cortical FVEP, the normally masked collicular FVEP could briefly take its place, thereby creating the illusion that the early components of the cortical FVEP, at least, had returned during the clonic phase of the seizure. 4.3. The post-ictal enhancement of the FVEP The brief rise in the voltage of the P1 component of the FVEP during the post-ECS period is presumably analogous to similar findings reported for epileptic patients. For instance, Broughton et al. (1969) described an enlargement of the FVEP in patients with photosensitive epilepsy, while Halliday and Halliday (1980) reported a comparable finding in those with progressive myoclonic epilepsy. Likewise, a series of animal studies have found that the FVEP may become enhanced following the administration of convulsant drugs (Rodin et al., 1966; Mirsky and Tecce, 1968; Burchiel et al., 1976). As Jones (1993) has recently observed, the mode of action of this paroxysmal increase in amplitude remains unknown. Originally, Halliday and Halliday (1980) suggested it was a function of epileptiform activity generated in diffusely projecting reticular pathways. An alternative explanation is that the amplification is simply a consequence of the potential arising within a neuronal population of heightened excitability (Mirsky and Tecce, 1968). The voltage of epileptiform activity may not be uniformly distributed across the cortex and it has been shown that abnormally enlarged waveforms seem to be localized to regions of higher intensity discharges in the background EEG (Mirsky and Tecce, 1968). At least part of the increase in the amplitude of P1 might also simply be accounted for by the absence or attenuation of the following negative potential.
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tration of ECT, in a manner similar to the present experiment. If their findings are reviewed, it is apparent that the FVEP waveform was deteriorating due to the methohexitone anesthesia even before the application of ECT. Following ECT, the average waveform was still present but poorly defined. It is, however, virtually impossible to disentangle the interacting effects of ECT and barbiturate anesthesia on the waveform during the ictal period. In particular it is unclear to what extent any anticonvulsant properties of methohexitone may have safeguarded the waveform from the destructive effects of ECT. This is a problem which besets and confounds all such clinical studies of ECT and EPs, as is explicitly acknowledged by these authors. With regard to the animal studies, Myslobodsky and Kofman (1982) obtained FVEPs from the awake rat but did not begin recording until after seizure activity had subsided. This presumably accounts for why the waveform appeared to be preserved apart from some minor lingering abnormalities and evidence of post-ictal enhancement of the early components. Rather more difficult to explain are the findings of Gregory and Wotton (1985) who recorded FVEPs from awake restrained sheep. Animals were administered a dose of ECS comparable to that routinely used in abattoirs to stun a sheep. Unlike Myslobodsky and Kofman, Gregory and Wotton began their recordings immediately after the induction of GSA but still managed to obtain a well-defined response during the tonic-clonic phase in two-thirds of their subjects. The most parsimonious explanation for the seeming preservation of the FVEP during the ictal period is that the recordings from the sheep had also been contaminated by far field collicular activity. This assumes that the superior colliculus plays the same functional role in the ruminant brain as it does in the rodent brain. Further, the averaging time for each potential recorded from the sheep lasted 50 s, unlike the more limited recording periods employed in the present experiment. In practice, this meant that the first postictal recording extended throughout the tonic and clonic phases of the seizure. Judging by the present findings in the rat, it would be expected that averaging throughout this period would at least yield a visual response of some type, although not necessarily a genuine cortical FVEP. This may not be an entirely satisfactory explanation as the superior colliculus in the sheep does not lie as close to the visual cortex as it does in the rat (Skinner, 1974) and the paper contains other incongruous data.
4.4. Comparison to previous studies of ECS and the FVEP 4.5. The physiology of electrical stunning No single cause can explain why previous investigations have failed to detect the abolition of the FVEP waveform after the administration of ECS or ECT. In some cases, the explanation is readily available. For instance, Small and Small (1971) delayed recording from their patients for at least an hour after ECT. Under such circumstances, it is hardly surprising that no loss of waveform was reported. In contrast, Kriss et al. (1980) made serial recordings from patients which began immediately after the adminis-
The present results are also of relevance to the enduring and ethically significant problem of the mechanism of action by which ECS actually stuns a subject. One possibility is that ECS acts in a manner similar to the conventional anesthetic agents such as the barbiturates by inhibiting or impairing synaptic transmission within the specific and nonspecific (reticular) pathways of the brainstem and midbrain. The effects of barbiturates such as pentobarbital on the
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FVEP of the rat have been well documented (Hetzler and Oaklay, 1981; Shaw, 1992a). While amplitudes and latencies of some components may be altered and secondary waves abolished, the early part of the waveform was preserved even under a surgical level of barbiturate anesthesia. This is in contrast to the present findings where ECS temporarily obliterated the entire waveform and therefore indicates the involvement of an entirely different mode of action than that of depression of a center controlling wakefulness. A second possibility is that ECS causes unconsciousness because the seizure activity it induces interferes with the normal processes of the centrencephalic integrating center. The centrencephalon is a functional system devised by Penfield and Jasper (1954) to explain how seizures become disseminated and generalised. According to the centrencephalic theory, volleys of abnormal electrical discharges emanating from thalamic reticular nuclei are propagated diffusely to the cortex where they serve to recruit and regulate epileptiform activity. Excitatory bursts arising within the reticulo-thalamo-cortical projection pathways would effectively disrupt their role of maintaining a state of cortical alertness and a loss of consciousness would rapidly ensue (Gastaut and Fischer-Williams, 1959). One of the difficulties with this theory is that it does not clearly predict what changes would be expected to occur to the morphology of cortical EPs following the induction of GSA by ECS. Presumably the later components would be lost but the fate of the earlier responses such as the P1 of the FVEP is uncertain. Therefore, the most that could be claimed for the centrencephalic theory is that the total disappearance of the FVEP is probably not incompatible with it. A third possibility is that epileptiform activity simply blocks or overrides the sensory signal at some level within the brain. Such an occlusion would cause functional deafferentation of the cortex and so render the subject unconscious. This theory would predict that cortical EPs must be totally abolished following ECS and so the current findings are most compatible with it. Previous recordings where the FVEP was seemingly preserved following the administration of ECS have raised the disturbing possibility that electrical stunning of animals in abattoirs may actually result in little or no loss of consciousness (e.g. Gregory and Wotton, 1985). The present results have established that this concern is probably unwarranted. 4.6. ECS and generalised epilepsy The present findings are also relevant to the persistent question of the pathophysiological mechanisms underlying the behavioural and cognitive deficits associated with spontaneous generalised seizures. It is likely that the same basic mode of action which is responsible for electrical stunning can also account for the impairment or loss of consciousness which is a hallmark of grand mal or petit mal epilepsy. Both appear to involve a similar interference or disruption of sensory processing by epileptiform activity (Mirsky and
Tecce, 1968). If this hypothesis is correct, it would be expected that both electrical stunning and generalised epilepsy should result in the attenuation or disappearance of the FVEP. Such a prediction is consistent with the report by Orren (1978) who recorded FVEPs from patients with classical petit mal epilepsy immediately prior to the appearance of spike and wave discharges. Although the FVEP was never entirely abolished during this brief period, individual components were lost and the waveform was markedly suppressed. In an earlier study, Orren also managed to obtain technically high quality FVEPs during bursts of spike and wave activity from petit mal patients (quoted by Mirsky et al., 1986). She reported that the same components of the FVEP which disappeared following ECS in the present experiment were also lost in the majority of her subjects. Similarly, Mirsky and his colleagues (Mirsky et al., 1973) recorded FVEPs from widespread locations within the visual system during drug-induced spike and wave activity in the monkey. In this instance, it was observed that not only the cortical FVEP was susceptible to GSA, but also EPs generated more caudally in the lateral geniculate body, optic pathway and retina. Nevertheless, the more centrally generated the response, the greater was the alteration to its waveform by paroxysmal activity. Not all such reports are as congruous with the present findings as those of Orren and Mirsky. For example, Burchiel et al. (1976) using the feline penicillin model described an enhancement of the FVEP following the onset of spike and wave activity. Ostensibly, such results represent a challenge to the understanding that the absences of petit mal epilepsy are underlain by a reduction or inhibition of sensory input (Mirsky, 1978). Reasons for these discrepant findings may include differences in species, techniques for inducing GSA and modes of neurophysiological action (Mirsky et al., 1986.) In the present experiment, ECS precipitated an increase in FVEP amplitude of the same magnitude as that reported by Burchiel et al. (1976). Nonetheless, this enhancement was confined quite discretely to the post-ictal period when reflex activity and consciousness were being rapidly regained. During the convulsion itself, the FVEP waveform appeared to be completely demolished. The current results may therefore allow the respective roles of suppression and enhancement of EPs to be more clearly defined than is possible with other models of experimental GSA. A final controversy concerns the identity of the primary site of action of GSA at which the disturbances of awareness, consciousness and attention are initiated or mediated. Extrapolating from their respective animal models of druginduced epilepsy, Mirsky (1978) and Gloor (1978) reached quite different conclusions. For Gloor, the principal site of impact of GSA resided within the cortex, whereas Mirsky located it within the mesopontine reticular formation. The present findings, where GSA was induced electrically, do little to resolve this matter. The loss of the cortical FVEP does not necessarily imply that the effects of ECS specifically target cortical function. Interference of sensory trans-
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mission at any level within the visual pathways could account for the loss of the waveform. The equivocal nature of the waveform recorded during the clonic phase of the seizure (1 min) further confounds any interpretation. Even if the origin of the putative far field FVEP was established with certainty, there still remains the doubt over the extent to which it is itself abolished during the acute ictal period (0 min). While there was clearly no sign of this response in the initial trace recorded after ECS, this does not necessarily mean that it was lost, along with the cortical potential. It could, for instance, simply indicate that the high voltage epileptiform activity acted as a barrier to block transmission of the potential from the superior colliculus or elsewhere to the surface of the brain. Be that as it may, if the far field potential is genuinely lost during the tonic phase, then the most parsimonious explanation for the sequential return of the waveforms is that ECS may have a primary site of action at more than one location within the visual system. It is of interest that Mirsky et al. (1973) using topical and systemic application of convulsant drugs came to a broadly similar conclusion. Acknowledgements This research was supported by the Maurice and Phyllis Paykel Trust. The author thanks Paul Hill, Jack Sinclair and Bruce Smaill for their advice and support, and Jane Utting for typing the manuscript. References Brodal, A. Neurological Anatomy, 3rd edn. Oxford University Press, New York, 1981. Broughton, R., Meier-Ewert, K.-H. and Ebe, M. Evoked visual, somatosensory and retinal potentials in photosensitive epilepsy. Electroenceph. clin. Neurophysiol., 1969, 27: 373–386. Burchiel, K.J., Myers, R.R. and Bickford, R.G. Visual and auditory evoked responses during penicillin-induced generalized spike-and-wave activity in cats. Epilepsia, 1976, 17: 293–311. Dyer, R.S. and Annau, Z. Flash evoked potentials from rat superior colliculus. Pharmacol. Biochem. Behav., 1977, 6: 453–459. Dyer, R.S., Jensen, K.F. and Boyes, W.K. Focal lesions of visual cortex: effects on visual evoked potentials in rats. Exp. Neurol, 1987, 95: 100– 115. Foutz, A.S., Dauthier, C. and Kerdelhue, B. b-Endorphin plasma levels during neuromuscular blockade in unanesthetized cat. Brain Res., 1983, 263: 119–123. Gastaut, H. and Fischer-Williams, M. The physiopathology of epileptic seizures. In: J. Field, H.W. Magoun and V.E. Hall (Eds.), Handbook of Physiology, Section 1: Neurophysiology, Vol I. American Physiological Society, Washington DC, 1959, pp. 329–363. Gloor, P. Generalized epilepsy with bilateral synchronous spike and wave discharge: new findings concerning its physiological mechanisms. Electroenceph. clin. Neurophysiol. (Suppl.), 1978, 34: 245–249. Gregory, N.G. and Wotton, S.B. Sheep slaughtering procedures. IV. Responsiveness of the brain following electrical stunning. Br. Vet. J., 1985, 41: 74–81.
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