Electroencephalography and Clinical Neurophysiology, 1978, 4 5 : 2 5 9 - - 2 6 7 © Elsevier/North-Holland Scientific Publishers, Ltd.
259
R E C O V E R Y FUNCTION OF S H O R T LATENCY COMPONENTS OF S U R F A C E AND DEPTH R E C O R D E D SOMATOSENSORY EVOKED POTENTIALS IN THE CAT* W.C. WIEDERHOLT
Department of Neurosciences, University of California and the Veteran's Administration Hospital, San Diego, Calif. (U.S.A.) (Accepted for publication: January 23, 1978)
The first event of the somatosensory evoked potential (SEP) recorded from the human scalp with reference to the ear consists of a small positive deflection, followed by a negative deflection and another positive deflection (Goff et al. 1962; Cracco and Bickford 1968; Goff et al. 1969; Cracco 1972a,b; Tamura 1972; Halliday 1975). The third deflection m a y be bilobed (Goff et al. 1962; Debecker and Desmedt 1964; Giblin 1974; Goff et al. 1977). When using median nerve stimulation these potentials correspond to P15, N20, P25, and P30 (Goff et al. 1977). Cracco and Cracco (1976a,b) observed with wrist or knee reference recording that P15 was preceded by two positive potentials (peak latencies 9--11.4 msec and 10.7--13.4 msec). The authors suggested that the earliest potential was generated in peripheral nerves and the second in the brainstem. In previous studies in the rat and in the cat (Wiederholt 1975; Wiederholt and IraguiMadoz 1977; Iragui-Madoz and Wiederholt 1977a,b) we have shown that the surface recorded cortical SEP is preceded by at least three or four small positive potentials. Results of these studies indicate that the early potentials are reflections of activity in the somatosensory pathway of the spinal cord, brainstem, thalamus, and subcortical white matter. Each
* Supported by the Medical Research Service of the Veteran's Administration.
c o m p o n e n t could be characterized b y its response to different rates of continuous stimulation. Late components attenuated at slower rates of stimulation than early components. In the present report we studied recovery function of early surface and depth recorded SEP components b y using paired stimuli with different interstimulus intervals (ISI's). Correlation of latencies and recovery functions of surface recorded potentials with those of depth potentials allowed us to draw further conclusions a b o u t the origin of short latency surface potentials.
Materials and methods
Twelve adult cats weighing 2.5--4.0 kg were used. Animals were anesthetized with intraperitoneal sodium pentobarbital. The initial dose was 40 mg/kg of b o d y weight and increments were given in the course of the experiment to assure adequate levels of anesthesia. Temperature was kept constant at 37°C. Stainless steel screw electrodes were inserted in the exposed skull for surface recording (interelectrode impedances 1--2 kOhm). The anterior electrode was overlying the cortical somatosensory area and the posterior electrode the posterior parietal area. A screw electrode in the nasal bone was used for referential recording. Bipolar depth recordings were obtained from two contacts on a coaxial stainless steel electrode. The
260
W.C. W I E D E R H O L T
center contact had a diameter of 0.1 mm and was exposed 0.25 mm. The shaft contact had a diameter of 0.25 mm and was exposed 0.25 mm. Both were separated by 0.25 mm. Impedances measured 20--30 kOhm. Electrodes were inserted under direct vision into the dorsal columns at C2 or C3 and into post-cruciate cortex. The sensory radiation potential was recorded from a location halfway between post-cruciate cortex and VPL. The following stereotaxic coordinates of Reinoso-Suarez (1961), according to the Horsley-Clarke method, were used: cuneate nucleus, AP - 1 6 , L 1.5, V 1.5; lateral cuneate nucleus, AP - 1 2 , L 4.5, V 3; restiforme body, AP 8, L 5.2, V 5.5; medial lemniscus, AP - 6 , L 1, V 0; VPL, AP 10, L 6, V 11. Adjustments of maximally 1.5 mm had to be made in 70% of placements in order to record a well defined potential. Electrolytic lesions, for verification of location, were within 1 mm of the intended target. Recording was simultaneously done from all surface and depth electrodes in each animal. O u t p u t from the amplifiers was stored on FM magnetic tape.
Stimulation was done with a pair of bare needles inserted subcutaneously in the volar skin of the forepaw. Unilateral and bilateral stimuli were used. Stimulus voltage was adjusted to produce a maximal contralateral cortical SEP. Duration of the stimulus was kept at 0.1 msec. ISI's within pairs of stimuli were 10, 12, 16, 25, 50, 66, 100, 125 and 250 msec. Paired stimuli were given every 0.5 sec because in previous studies we found that early components of the SEP did not attenuate at that rate of stimulation. The recording system had a frequency response from 3 c/sec to 3 kc/sec with 3 dB roll off. Output from the amplifiers or from magnetic tape was fed into a digital laboratory computer and averaged. Duration of the averaging epoch was 33 msec and sampling rate was 0.12 msec/ point. Amplitudes of potentials were measured from peak to peak. At each ISI, 500 consecutive pairs of potentials were averaged and the results are reported as percentage of the ratio of amplitude of test (T) potential to that of control (C) potential (100 X T/C). Data from a minimum of four cats each was obtained for
TABLE I
Latencies o f SEP's. N
Surface p o t e n t i a l s Component I C o m p o n e n t II Component III a n t e r i o r Component IV Cortical SEP
Depth potentials Posterior c o l u m n Cuneate nucleus Lat. c u n e a t e nucleus Medial l e m n i s c u s Inf. cerebellar ped. Cerebellar c o r t e x V P L nucl. o f t h a l a m u s Sensory r a d i a t i o n Somatosensory c o r t e x
L a t e n c i e s (msec) Onset
Peak
7 8 8 8 8
3.8 5.1 6.9 9.3 11.2
± 0.1 -+ 0.2 ± 0.3 ± 0.3 ± 0.5
4.6±0.1 6.1±0.2 8.3±0.4 10.8±0.4 13.0±0.5
5 5 4 8 6 5 5 5 4
4.0 4.3 4.5 5.8 6.0 7.2 6.3 7.7 9.3
± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.4 ± 0.4 ± 0.2 ± 0.2 ±0.3
5.9±0.4 7.1±0.4 7.0±0.4 7.5t0.4 9.1±0.8 13.0±1.8 8.3±0.7 12.3±1.2 15.0±0.9
* Values are mean ± SE; N, n u m b e r o f animals.
R E C O V E R Y FUNCTION OF SOMATOSENSORY POTENTIALS
all depth and surface potentials. At least two complete runs with all ISI's were done in each animal. Means and standard errors were calculated (Table I) and recovery curves were constructed (Fig. 4). In some instances the test potentials were obscured by a large control potential and could n o t be accurately measured.
Results The surface recorded SEP evoked by forepaw stimulation in the cat was complex (Fig. 1) but the subcomponents showed little variability from m o m e n t to m o m e n t or from animal to animal. Four positive deflections
,'SEP'
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3 msec Fig. 1. Surface recordings of average SEP's with bilateral forepaw stimulation. SEP's recorded over the somatosensory area (ANT) and posterior parietal area (POST) with reference (R) to nose. Roman numerals indicate early components. The arrow points to the initial positive deflection of the cortical SEP. Stimulus rate, 2/sec; average of 500 sweeps; sampling rate, 0.12 msec/point. See text for difference between anterior and posterior c o m p o n e n t III.
261
preceded the cortical c o m p o n e n t of the SEP in most animals. In a previous study (IraguiMadoz and Wiederholt 1977b) we have shown that components I and II have identical latencies and similar amplitudes over anterior and posterior head regions. Components I and II were studied at the posterior head location because at the anterior location the potentials evoked by the second stimulus were obscured by those evoked by the first stimulus. Component III could be subdivided into two subcomponents. With unilateral stimulation, one had its amplitude peak over the contralateral somatosensory area while the other was diffusely distributed on both sides of the head. Component IV, like anterior c o m p o n e n t III, was of maximal amplitude over the contralateral somatosensory area. Since components III (anterior) and IV cannot be recorded from posteriorly located electrodes, these potentials were studied as recorded anteriorly. We did not include the diffusely distributed c o m p o n e n t III because it was of very low amplitude and reliable measurements comparing the amplitude of the conditioning potential with that of the test potential were difficult to make. Figs. 2 and 3 show representative samples of the response of the test potential with decreasing ISI's. At an ISI of 25 msec, the second stimulus evoked neither components III and IV nor the surface and depth recorded cortical potential. Evoked potentials recorded directly from the cerebellar cortex could not be reliably identified at ISI's shorter than 25 msec because the conditioning stimulus evoked a series of short and long latency waves which interfered with the identification of the first cerebellar potential. Onset and peak latencies of surface and depth recorded potentials did not change significantly regardless of the ISI. Mean values and standard errors for all latencies are given in Table I. These latency values are in agreement with values obtained in previous studies (IraguiMadoz and Wiederholt 1977a,b). Components I and II showed gradual attenuation of the test potential with progressively decreasing
262
W.C. W I E D E R H O L T
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Fig. 2. R e c o v e r y of surface r e c o r d e d SEP's. P o t e n t i a l s s h o w n are e v o k e d b y t h e s e c o n d o f a pair o f stimuli. P o t e n t i a l s at 500 msec ISI's also serve as c o n t r o l . S w e e p o n s e t c o i n c i d e s w i t h o n s e t o f t e s t s t i m u l u s . A n t e r i o r l y r e c o r d e d t e s t p o t e n t i a l o f 10 m s e c ISI n o t s h o w n b e c a u s e it was o b s c u r e d b y c o n d i t i o n i n g p o t e n t i a l . Average o f 5 0 0 sweeps; s a m p l i n g rate, 0.12 m s e c / p o i n t . V e r t i c a l b a r i n d i c a t e s 4 ~ V for A N T - R a n d 2.5 ~ V for Post-R.
ISI's, b u t both were still present at an ISI of 10 msec. At that ISI, c o m p o n e n t I was less attenuated than c o m p o n e n t II. Anterior c o m p o n e n t III, c o m p o n e n t IV and the surface recorded cortical SEP showed almost identical recovery curves which were distinctly different from those of the first two components (Fig. 4). This difference was quite striking at ISI's of 125 msec or less. Recovery curves of potentials recorded from depth structures are shown in Fig. 4
and were grouped into t w o groups. One group shows activity in presynaptic and postsynaptic sensory pathways including posterior column, medial lemniscus, inferior cerebellar peduncle, and sensory radiation while the other shows activity in nuclear and cortical structures including cuneate nucleus, lateral cuneate nucleus, ventral postero-lateral nucleus of thalamus, somatosensory cortex and cerebellar cortex. The posterior column potential showed little attenuation at any ISI. Potentials
RECOVERY FUNCTION OF SOMATOSENSORY POTENTIALS
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Fig. 3. R e c o v e r y o f S E P ' s r e c o r d e d f r o m dorsal c o l u m n ( D O R S A L C O L ) , m e d i a l l e m n i s c u s ( M E D L E M ) , a n d s e n s o r y r a d i a t i o n ( S E N S O R Y R A D ) . P o t e n t i a l s s h o w n are e v o k e d b y t h e s e c o n d o f a pair of stimuli. P o t e n t i a l s at 500 msec ISI also serve as c o n t r o l . S w e e p o n s e t c o i n c i d e s w i t h o n s e t of test s t i m u l u s . S e n s o r y r a d i a t i o n p o t e n tial at 10 msec n o t s h o w n b e c a u s e it was a b s e n t . Average o f 5 0 0 sweeps; s a m p l i n g rate, 0.12 m s e c / p o i n t .
in the medial lemniscus and the inferior cerebellar peduncle progressively attenuated with ISI's shorter than 125 msec, but in both structures potentials were still clearly present at an ISI of 10 msec. The sensory radiation potential attenuated much more rapidly and the test stimulus did n o t elicit a potential at an ISI of 16 msec. Recovery curves of potentials in cuneate nucleus and lateral cuneate nucleus were n o t clearly different from those in VPL nucleus of thalamus and cerebellar cortex. Nevertheless, the slopes of the curves of the latter two are more alike than those of the former two. Further attenuation of the test potential at ISI's less than 50 msec was more pronounced in VPL and cerebellar cortex compared with medial lemniscus and inferior cerebellar peduncle, respectively. Similarly, at ISI's of 125 msec or less, attenuation of the test potential in the somatosensory cortex was greater than that in the sensory irradiation.
Discussion Recovery curves of surface potentials allowed clear separation of c o m p o n e n t I from c o m p o n e n t II and of these two components, from components III and IV and the cortical potential. This separation of components was optimal at ISI's of 10--50 msec. Recovery curves of surface recorded potentials are very similar to curves obtained with continuous stimulation (Iragui-Madoz and Wiederholt 1977a,b). With continuous stimulation though, recovery occurred at longer ISI's than with paired stimuli which probably is an expression of fatigue associated with prolonged continuous stimulation. The recovery curve of activity in the posterior column showed little if any decrem e n t at any ISI. This is in contrast to recovery curves of potentials in medial lemniscus and inferior cerebellar peduncle which both
264
W.C. WIEDERHOLT
RECOVERY FUNCTIONS OF SURFACE AND DEPTH RECORDED SOMATOSENSORY EVOKED POTENTIALS
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Fig.4. Recovery functionsof surface and
depth
recorded SEP's. Recovery function expressed as percentage of the ratio of amplitude of test (T) potential to that of control (C) potential (100 X T/C). Each point represents the mean value of all animals tested. Vertical bar represents SE which is shown in only one direction for clarity.
showed a progressive and almost identical decrement o f the amplitude of the test potential with ISI's less than 125 msec. This decrement reflects the fact that both structures are separated b y one synapse (cuneate nucleus and lateral cuneate nucleus, respectively) from the site of peripheral stimulation. The decrement of the test potential was even more pronounced when recording from the sensory radiation because of interposition of y e t another synapse, i.e. the VPL. Recovery
curves of activity in nuclear and cortical structures did n o t show the same distinct separation as those from clearly presynaptic or postsynaptic structures. This is probably due to the properties of the depth electrodes which recorded b o t h presynaptic and postsynaptic activity. This interpretation is supported b y the fact that recovery curves of activity in nuclear structures lie somewhat between those for activity in appropriate presynaptic and postsynaptic structures. The different contribution of presynaptic and postsynaptic activity to the potential recorded in nuclear structures probably accounted for the greater variability. Schwartz et al. (1962) showed in the cat that pentobarbital depressed the cortical recorded SEP only at large enough doses to significantly depress brain function. Our cats received pentobarbital 40 mg/kg initially and subsequently 6--10 mg/kg every 2 h. Stimulation was usually begun 4 hours after the animal was anesthetized at which time we assumed a stable anesthetic state. Furthermore, electroencephalographic activity compatible with moderate to light stages of anesthesia was recorded throughout the entire experiment. Comparing our rate of barbiturate administration with that of Schwartz et al. (1962) our cats were most likely at a level of anesthesia at which t h e amplitude of the primary cortical evoked potential was n o t decreased. Mean recovery cycles are difficult to compare because of different schedules of barbiturate administration and different sites of stimulation (radial nerve in Schwartz et al.'s experiments and forepaw in ours). Nevertheless, recovery of all potentials was essentially complete at ISis of 250 msec in our experiments and at 200 msec in Schwartz et al.'s study, except when using the largest dose of pentobarbital. Our schedule of barbiturate administration was very similar to that used by Anderson et al. (1964). The greater decrement of the test potential in the sensory radiation in our experiments was probably due to the fact that both the conditioning stimulus and test stimulus were delivered to
R E C O V E R Y FUNCTION OF SOMATOSENSORY POTENTIALS
the same peripheral structure while Anderson et al. (1964) gave the conditioning stimulus to the ulnar nerve and the test stimulus to the superficial radial nerve. In the present study, recording from all electrodes in each animal was done simultaneously. The response to paired stimuli varied insignificantly from run to run and from animal to animal. These observations suggest that sodium pentobarbital had little, if any, effect on the reported results. Onset and peak latencies of c o m p o n e n t I (3.8/4.6 msec) and of the posterior column potential (4.0/6.4 msec) overlap and recovery curves are very similar. This suggests that c o m p o n e n t I may reflect activity in posterior columns. C o m p o n e n t I fatigued more readily than the posterior column potential, which implies that other activity contributes to c o m p o n e n t I. Likely candidates are potentials in cuneate and lateral cuneate nuclei because their latencies overlapped with those of c o m p o n e n t I. This interpretation is in agreement with our previous observation that c o m p o n e n t I persisted with slightly decreased amplitude after transection at the medullocervical junction. Latencies of c o m p o n e n t II and of potentials in the cuneate nucleus, lateral cuneate nucleus and medial lemniscus, while n o t identical, overlapped. Latencies of potentials in the inferior cerebellar peduncle were significantly longer than those of c o m p o n e n t II. Recovery curves of c o m p o n e n t II and the medial lemniscus potential were identical. Because of similar latencies and recovery curves, it appears that activity in medial lemniscus, cuneate nucleus, and lateral cuneate nucleus could contribute to c o m p o n e n t II. Latencies of anterior c o m p o n e n t III and the VPL potential are almost identical and overlap with those of potentials in the sensory radiation, inferior cerebellar peduncle, and cerebellar cortex. Recovery curve of anterior c o m p o n e n t III is similar to that of the sensory radiation potential and intermediate between that for potentials in VPL and cerebellum and that for the cortical potential. It appears
265
from these data that the major contribution to anterior c o m p o n e n t III is from activity in the VPL and possibly some from sensory radiation and cerebellar cortex potentials. This interpretation is supported by our previous observation that unilateral thalamect o m y abolished c o m p o n e n t III except its presumed cerebellar contribution. Latencies of c o m p o n e n t IV and of the sensory radiation potential overlapped while latencies of activity in the VPL were clearly shorter. Recovery curves of c o m p o n e n t IV and of the sensory radiation were identical. It appears likely that the major contribution to c o m p o n e n t IV is from activity in the sensory radiation. Cortical activity does n o t contribute to c o m p o n e n t IV because we have previously shown that c o m p o n e n t IV persisted after interruption of afferent pathways to the cortex by either blunt cortical undercutting or ablation of the cortex by suctioning. We presume the fifth positive deflection of the surface SEP to be of cortical origin because its latencies overlapped with those of activity recorded directly from the cortex and their recovery functions were similar. Lastly, it was abolished with cortical ablation (Iragui-Madoz and Wiederholt 1977b). In normal human subjects, we studied three low amplitude positive deflections preceding the presumed onset of evoked cortical activity (Wiederholt and Kritchevsky 1977). These three potentials did not change in amplitude or latency with continuous stimulation at 0.5, 4, and 10 stimuli/sec. We assume that these early potentials in man originate in similar subcortical structures as we have demonstrated in the rat and cat. It is not surprising that there is little, if any, change in amplitude and latencies of these early components of the SEP at the above rates of stimulation, because with continuous or paired stimulation in the rat and cat the most striking amplitude changes occurred with ISI's of less than 100 msec. In our human subjects, continuous stimulation at 10 stimul~/sec or faster was uncomfortable b u t in preliminary observations
266 paired stimuli with interpair intervals of 0.5 sec or longer were accepted quite well. In the present study we demonstrated t h a t early components of the SEP in the cat can be differentiated by their different recovery curves. We plan to study early components of the SEP in h u m a n subjects because the technique of paired stimuli m a y provide additional data about their origin. Furthermore, we would like to establish normative data at different ISI's because some disease processes which m a y n o t produce significant changes of the SEP when using long ISI's may well show abnormalities with short ISI's.
Summary Recovery functions of early components of surface and depth recorded somatosensory evoked potentials were studied in the cat. At interstimulus intervals of 50 msec or less differentiation of surface components was optimal. C o m p o n e n t I showed less decrement at all ISI's than did c o m p o n e n t II and recovery curves of both were distinctly different from those of components III (anterior) and IV and the cortical SEP. The latter three potentials had similar recovery curves. Comparisons of latencies and recovery curves of surface recorded potentials with those of depth recorded potentials indicate t h a t c o m p o n e n t I principally reflects activity in posterior column, that potentials in medial lemniscus, cuneate nucleus, and lateral cuneate nucleus contribute to c o m p o n e n t II, that the major generator for the contralateral c o m p o n e n t III is the ventral postero-lateral nucleus of the thalamus, and that c o m p o n e n t IV is largely due to activity in the sensory radiation. The results of this study support previous observations and conclusions about the origin of short latency, surface recorded somatosensory evoked potentials preceding the cortical SEP.
w.c. WIEDERHOLT R6sumd
Fonction de rdcupdration des composantes de courte latence des potentiels dvoquds somatosensitifs enregistrds en surface et en profondeur chez le chat Les fonctions de r6cup6ration des composantes pr6coces des potentiels 6voquds somatosensitifs enregistrds ~ la surface et dans la profondeur ont 6t6 dtudi6s chez le chat. Pour des intervalles inter-stimuli de 50 msec ou moins, la diffdrenciation des composantes de surface s'avdre optimale. La composante I montre moins de diminution pour tous les ISis que n'en montre la composante II et les courbes de r6cupdration de ces 2 composantes diffdrent de fa~on distincte de celle des composantes III (ant6rieures) et IV et du SEP cortical. Ces 3 derniers potentiels ont des courbes de rdcup6ration similaires. La comparaison entre latences et courbes de r6cupdration des potentiels enregistrds en surface et celles des potentiels enregistr6s en profondeur indiquent que la composante I refl~te principalement l'activit6 au niveau de la colonne post6rieure que les potentiels du lemniscus mddian, du noyau cun6en et du n o y a u nunden lat6ral contribuent ~ la composante II, que le gdn6rateur principal de la composante III controlat6rale est le noyau ventral postdrolat~ral du thalamus, et que la composante IV est en grande partie due ~ l'activitd de la radiation sensitive. Les rdsultats de cette 6tude sont en accord avec de pr6c6dentes observations et conclusions concernant l'origine des potentiels dvoqu6s somatosensitifs enregistr6s en surface, de courtes latences ant6rieures au SEP cortical. The author wishes to thank Mr. Byron Budnick for technical assistance and Mrs. Bobbi Wagner for
preparation of the manuscript. References
Andersen, P., Brooks, C. McC., Eccles, J.C. and Sears, T.A. The ventrobasal nucleus of the thalamus: potential fields, synaptic transmission and
RECOVERY FUNCTION OF SOMATOSENSORY POTENTIALS excitability of both presynaptic and postsynaptic components. J. Physiol. (Lond.), 1964, 174: 348--369. Cracco, R.Q. The initial positive potential of the human scalp-recorded somatosensory evoked response. Electroenceph. clin. Neurophysiol., 1972a, 32: 623--629. Cracco, R.Q. Traveling waves of the human scalprecorded somatosensory evoked response. Electroenceph, clin. Neurophysiol., 1972b, 33: 557--566. Cracco, R.Q. and Bickford, R.G. S o m a t o m o t o r and somatosensory evoked responses. Arch. Neurol., 1968, 18: 52--68. Cracco, R.Q. and Cracco, J.B. Subcortical evoked potentials to median nerve stimulation recorded from the human scalp. Electroenceph. clin. Neurophysiol., 1976a, 40: 335. Cracco, R.Q. and Cracco, J.B. Somatosensory evoked potentials in man: far-field potentials. Electroenceph. clin. Neurophysiol., 1976b, 41 : 460--466. Debecker, J. et Desmedt, J.E. Les potentiels ~voqu~s c~r~braux et les potentials de neff sensible chez l'homme. Utilization de l'ordineteur num~rique Mnemotron. Acta neurol, belg., 1964, 64: 1212-1248. Giblin, D.R. Somatosensory evoked potentials in healthy subjects and in patients with lesions of the nervous system. Ann. N.Y. Acad. Sci., 1964, 112: 93--142. Goff, G.D., Matsumiya, Y., Allison, T. and Goff, W.R. The scalp topography of human somatosensory and auditory evoked potentials. Electroenceph, clin. Neurophysiol., 1977, 42: 57--76. Goff, W.R., Matsumiya, Y., Allison, T. and Goff, G.D. Cross-modality comparisons of averaged evoked potentials. In : E. Donchin and D.B. Lindsley (Eds.), Averaged Evoked Potentials. NASA SP-191, Washington, 1969, pp. 95--141.
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Goff, W.R., Rosner, B.S. and Allison, T. Distribution of cerebral somatosensory evoked responses in normal man. Electroenceph. clin. Neurophysiol., 1962, 14: 697--713. Halliday, A.M. Somatosensory evoked responses. In: A. R~mond (Ed.), Handbook of Electroencephalography and Clinical Neurophysiology, Vol. 8, Part A. Elsevier, Amsterdam, 1975: 60--67. Iragui-Madoz, V.J. and Wiederholt, W.C. Far-field somatosensory evoked potentials in the cat: Correlation with depth recording. Ann. Neurol., 1977a, 1: 569--574. Iragui-Madoz, V.J. and Wiederholt, W.C. Far-field somatosensory evoked potentials in the cat. Electroenceph. clin. Neurophysiol., 1977b, 43: 646--657. Reinoso-Suarez, F. Topographischer Hirnatlas der Katze. E. Merck, Darmstadt, 1961. S c h w a r t z , M., Shagass, C., Bittle, R. and Flapan, M. Dose related effects of pentobarbital on somatosensory evoked responses and recovery cycles. Electroenceph. clin. Neurophysiol., 1962, 14: 898--903. Tamura, K. Ipsilateral somatosensory evoked responses in man. Folia psychiat, neurol, jap., 1972, 26: 83--94. Wiederholt, W.C. Far-field reflections of brain stem sensory evoked responses. Trans. Amer. neurol. Ass., 1975, 100: 259--261. Wiederholt, W.C. and Iragui-Madoz, V.J. Far-field somatosensory potentials in the rat. Electroenceph. clin. Neurophysiol., 1977, 42: 456--465. Wiederholt, W.C. and Kritchevsky, M. Early components of the human averaged somatosensory evoked potential (SEP). Excerpta Medica, Amsterdam, 1 9 7 7 , 4 2 7 : 174.
259
R E C O V E R Y FUNCTION OF S H O R T LATENCY COMPONENTS OF S U R F A C E AND DEPTH R E C O R D E D SOMATOSENSORY EVOKED POTENTIALS IN THE CAT* W.C. WIEDERHOLT
Department of Neurosciences, University of California and the Veteran's Administration Hospital, San Diego, Calif. (U.S.A.) (Accepted for publication: January 23, 1978)
The first event of the somatosensory evoked potential (SEP) recorded from the human scalp with reference to the ear consists of a small positive deflection, followed by a negative deflection and another positive deflection (Goff et al. 1962; Cracco and Bickford 1968; Goff et al. 1969; Cracco 1972a,b; Tamura 1972; Halliday 1975). The third deflection m a y be bilobed (Goff et al. 1962; Debecker and Desmedt 1964; Giblin 1974; Goff et al. 1977). When using median nerve stimulation these potentials correspond to P15, N20, P25, and P30 (Goff et al. 1977). Cracco and Cracco (1976a,b) observed with wrist or knee reference recording that P15 was preceded by two positive potentials (peak latencies 9--11.4 msec and 10.7--13.4 msec). The authors suggested that the earliest potential was generated in peripheral nerves and the second in the brainstem. In previous studies in the rat and in the cat (Wiederholt 1975; Wiederholt and IraguiMadoz 1977; Iragui-Madoz and Wiederholt 1977a,b) we have shown that the surface recorded cortical SEP is preceded by at least three or four small positive potentials. Results of these studies indicate that the early potentials are reflections of activity in the somatosensory pathway of the spinal cord, brainstem, thalamus, and subcortical white matter. Each
* Supported by the Medical Research Service of the Veteran's Administration.
c o m p o n e n t could be characterized b y its response to different rates of continuous stimulation. Late components attenuated at slower rates of stimulation than early components. In the present report we studied recovery function of early surface and depth recorded SEP components b y using paired stimuli with different interstimulus intervals (ISI's). Correlation of latencies and recovery functions of surface recorded potentials with those of depth potentials allowed us to draw further conclusions a b o u t the origin of short latency surface potentials.
Materials and methods
Twelve adult cats weighing 2.5--4.0 kg were used. Animals were anesthetized with intraperitoneal sodium pentobarbital. The initial dose was 40 mg/kg of b o d y weight and increments were given in the course of the experiment to assure adequate levels of anesthesia. Temperature was kept constant at 37°C. Stainless steel screw electrodes were inserted in the exposed skull for surface recording (interelectrode impedances 1--2 kOhm). The anterior electrode was overlying the cortical somatosensory area and the posterior electrode the posterior parietal area. A screw electrode in the nasal bone was used for referential recording. Bipolar depth recordings were obtained from two contacts on a coaxial stainless steel electrode. The
260
W.C. W I E D E R H O L T
center contact had a diameter of 0.1 mm and was exposed 0.25 mm. The shaft contact had a diameter of 0.25 mm and was exposed 0.25 mm. Both were separated by 0.25 mm. Impedances measured 20--30 kOhm. Electrodes were inserted under direct vision into the dorsal columns at C2 or C3 and into post-cruciate cortex. The sensory radiation potential was recorded from a location halfway between post-cruciate cortex and VPL. The following stereotaxic coordinates of Reinoso-Suarez (1961), according to the Horsley-Clarke method, were used: cuneate nucleus, AP - 1 6 , L 1.5, V 1.5; lateral cuneate nucleus, AP - 1 2 , L 4.5, V 3; restiforme body, AP 8, L 5.2, V 5.5; medial lemniscus, AP - 6 , L 1, V 0; VPL, AP 10, L 6, V 11. Adjustments of maximally 1.5 mm had to be made in 70% of placements in order to record a well defined potential. Electrolytic lesions, for verification of location, were within 1 mm of the intended target. Recording was simultaneously done from all surface and depth electrodes in each animal. O u t p u t from the amplifiers was stored on FM magnetic tape.
Stimulation was done with a pair of bare needles inserted subcutaneously in the volar skin of the forepaw. Unilateral and bilateral stimuli were used. Stimulus voltage was adjusted to produce a maximal contralateral cortical SEP. Duration of the stimulus was kept at 0.1 msec. ISI's within pairs of stimuli were 10, 12, 16, 25, 50, 66, 100, 125 and 250 msec. Paired stimuli were given every 0.5 sec because in previous studies we found that early components of the SEP did not attenuate at that rate of stimulation. The recording system had a frequency response from 3 c/sec to 3 kc/sec with 3 dB roll off. Output from the amplifiers or from magnetic tape was fed into a digital laboratory computer and averaged. Duration of the averaging epoch was 33 msec and sampling rate was 0.12 msec/ point. Amplitudes of potentials were measured from peak to peak. At each ISI, 500 consecutive pairs of potentials were averaged and the results are reported as percentage of the ratio of amplitude of test (T) potential to that of control (C) potential (100 X T/C). Data from a minimum of four cats each was obtained for
TABLE I
Latencies o f SEP's. N
Surface p o t e n t i a l s Component I C o m p o n e n t II Component III a n t e r i o r Component IV Cortical SEP
Depth potentials Posterior c o l u m n Cuneate nucleus Lat. c u n e a t e nucleus Medial l e m n i s c u s Inf. cerebellar ped. Cerebellar c o r t e x V P L nucl. o f t h a l a m u s Sensory r a d i a t i o n Somatosensory c o r t e x
L a t e n c i e s (msec) Onset
Peak
7 8 8 8 8
3.8 5.1 6.9 9.3 11.2
± 0.1 -+ 0.2 ± 0.3 ± 0.3 ± 0.5
4.6±0.1 6.1±0.2 8.3±0.4 10.8±0.4 13.0±0.5
5 5 4 8 6 5 5 5 4
4.0 4.3 4.5 5.8 6.0 7.2 6.3 7.7 9.3
± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.4 ± 0.4 ± 0.2 ± 0.2 ±0.3
5.9±0.4 7.1±0.4 7.0±0.4 7.5t0.4 9.1±0.8 13.0±1.8 8.3±0.7 12.3±1.2 15.0±0.9
* Values are mean ± SE; N, n u m b e r o f animals.
R E C O V E R Y FUNCTION OF SOMATOSENSORY POTENTIALS
all depth and surface potentials. At least two complete runs with all ISI's were done in each animal. Means and standard errors were calculated (Table I) and recovery curves were constructed (Fig. 4). In some instances the test potentials were obscured by a large control potential and could n o t be accurately measured.
Results The surface recorded SEP evoked by forepaw stimulation in the cat was complex (Fig. 1) but the subcomponents showed little variability from m o m e n t to m o m e n t or from animal to animal. Four positive deflections
,'SEP'
IV,'
inl"
/
i
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. .
:'./ P O S T - R ".-..,'~'
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~,.
.
~,,.
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~"
3 msec Fig. 1. Surface recordings of average SEP's with bilateral forepaw stimulation. SEP's recorded over the somatosensory area (ANT) and posterior parietal area (POST) with reference (R) to nose. Roman numerals indicate early components. The arrow points to the initial positive deflection of the cortical SEP. Stimulus rate, 2/sec; average of 500 sweeps; sampling rate, 0.12 msec/point. See text for difference between anterior and posterior c o m p o n e n t III.
261
preceded the cortical c o m p o n e n t of the SEP in most animals. In a previous study (IraguiMadoz and Wiederholt 1977b) we have shown that components I and II have identical latencies and similar amplitudes over anterior and posterior head regions. Components I and II were studied at the posterior head location because at the anterior location the potentials evoked by the second stimulus were obscured by those evoked by the first stimulus. Component III could be subdivided into two subcomponents. With unilateral stimulation, one had its amplitude peak over the contralateral somatosensory area while the other was diffusely distributed on both sides of the head. Component IV, like anterior c o m p o n e n t III, was of maximal amplitude over the contralateral somatosensory area. Since components III (anterior) and IV cannot be recorded from posteriorly located electrodes, these potentials were studied as recorded anteriorly. We did not include the diffusely distributed c o m p o n e n t III because it was of very low amplitude and reliable measurements comparing the amplitude of the conditioning potential with that of the test potential were difficult to make. Figs. 2 and 3 show representative samples of the response of the test potential with decreasing ISI's. At an ISI of 25 msec, the second stimulus evoked neither components III and IV nor the surface and depth recorded cortical potential. Evoked potentials recorded directly from the cerebellar cortex could not be reliably identified at ISI's shorter than 25 msec because the conditioning stimulus evoked a series of short and long latency waves which interfered with the identification of the first cerebellar potential. Onset and peak latencies of surface and depth recorded potentials did not change significantly regardless of the ISI. Mean values and standard errors for all latencies are given in Table I. These latency values are in agreement with values obtained in previous studies (IraguiMadoz and Wiederholt 1977a,b). Components I and II showed gradual attenuation of the test potential with progressively decreasing
262
W.C. W I E D E R H O L T
ANT-R
sooJ
!ii
POST-R
j,'V",
i !
--
j
.J
'./.,
__.._
,
25,
I0
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.~
Fig. 2. R e c o v e r y of surface r e c o r d e d SEP's. P o t e n t i a l s s h o w n are e v o k e d b y t h e s e c o n d o f a pair o f stimuli. P o t e n t i a l s at 500 msec ISI's also serve as c o n t r o l . S w e e p o n s e t c o i n c i d e s w i t h o n s e t o f t e s t s t i m u l u s . A n t e r i o r l y r e c o r d e d t e s t p o t e n t i a l o f 10 m s e c ISI n o t s h o w n b e c a u s e it was o b s c u r e d b y c o n d i t i o n i n g p o t e n t i a l . Average o f 5 0 0 sweeps; s a m p l i n g rate, 0.12 m s e c / p o i n t . V e r t i c a l b a r i n d i c a t e s 4 ~ V for A N T - R a n d 2.5 ~ V for Post-R.
ISI's, b u t both were still present at an ISI of 10 msec. At that ISI, c o m p o n e n t I was less attenuated than c o m p o n e n t II. Anterior c o m p o n e n t III, c o m p o n e n t IV and the surface recorded cortical SEP showed almost identical recovery curves which were distinctly different from those of the first two components (Fig. 4). This difference was quite striking at ISI's of 125 msec or less. Recovery curves of potentials recorded from depth structures are shown in Fig. 4
and were grouped into t w o groups. One group shows activity in presynaptic and postsynaptic sensory pathways including posterior column, medial lemniscus, inferior cerebellar peduncle, and sensory radiation while the other shows activity in nuclear and cortical structures including cuneate nucleus, lateral cuneate nucleus, ventral postero-lateral nucleus of thalamus, somatosensory cortex and cerebellar cortex. The posterior column potential showed little attenuation at any ISI. Potentials
RECOVERY FUNCTION OF SOMATOSENSORY POTENTIALS
SENSORY RAD
MED LEM
DORSAL COL
263
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+15msec
~.,~.. " , ~ :
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Fig. 3. R e c o v e r y o f S E P ' s r e c o r d e d f r o m dorsal c o l u m n ( D O R S A L C O L ) , m e d i a l l e m n i s c u s ( M E D L E M ) , a n d s e n s o r y r a d i a t i o n ( S E N S O R Y R A D ) . P o t e n t i a l s s h o w n are e v o k e d b y t h e s e c o n d o f a pair of stimuli. P o t e n t i a l s at 500 msec ISI also serve as c o n t r o l . S w e e p o n s e t c o i n c i d e s w i t h o n s e t of test s t i m u l u s . S e n s o r y r a d i a t i o n p o t e n tial at 10 msec n o t s h o w n b e c a u s e it was a b s e n t . Average o f 5 0 0 sweeps; s a m p l i n g rate, 0.12 m s e c / p o i n t .
in the medial lemniscus and the inferior cerebellar peduncle progressively attenuated with ISI's shorter than 125 msec, but in both structures potentials were still clearly present at an ISI of 10 msec. The sensory radiation potential attenuated much more rapidly and the test stimulus did n o t elicit a potential at an ISI of 16 msec. Recovery curves of potentials in cuneate nucleus and lateral cuneate nucleus were n o t clearly different from those in VPL nucleus of thalamus and cerebellar cortex. Nevertheless, the slopes of the curves of the latter two are more alike than those of the former two. Further attenuation of the test potential at ISI's less than 50 msec was more pronounced in VPL and cerebellar cortex compared with medial lemniscus and inferior cerebellar peduncle, respectively. Similarly, at ISI's of 125 msec or less, attenuation of the test potential in the somatosensory cortex was greater than that in the sensory irradiation.
Discussion Recovery curves of surface potentials allowed clear separation of c o m p o n e n t I from c o m p o n e n t II and of these two components, from components III and IV and the cortical potential. This separation of components was optimal at ISI's of 10--50 msec. Recovery curves of surface recorded potentials are very similar to curves obtained with continuous stimulation (Iragui-Madoz and Wiederholt 1977a,b). With continuous stimulation though, recovery occurred at longer ISI's than with paired stimuli which probably is an expression of fatigue associated with prolonged continuous stimulation. The recovery curve of activity in the posterior column showed little if any decrem e n t at any ISI. This is in contrast to recovery curves of potentials in medial lemniscus and inferior cerebellar peduncle which both
264
W.C. WIEDERHOLT
RECOVERY FUNCTIONS OF SURFACE AND DEPTH RECORDED SOMATOSENSORY EVOKED POTENTIALS
100 80" 60' 40 i
• • • -
20
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¢UNEATENUCL. L*r c~r~TE~,L VPLOFTHALAMUS SOMATOSENSORCORTEx Y
2~o
~o
INTERSTIMULUS INTERVAL (log msec)
Fig.4. Recovery functionsof surface and
depth
recorded SEP's. Recovery function expressed as percentage of the ratio of amplitude of test (T) potential to that of control (C) potential (100 X T/C). Each point represents the mean value of all animals tested. Vertical bar represents SE which is shown in only one direction for clarity.
showed a progressive and almost identical decrement o f the amplitude of the test potential with ISI's less than 125 msec. This decrement reflects the fact that both structures are separated b y one synapse (cuneate nucleus and lateral cuneate nucleus, respectively) from the site of peripheral stimulation. The decrement of the test potential was even more pronounced when recording from the sensory radiation because of interposition of y e t another synapse, i.e. the VPL. Recovery
curves of activity in nuclear and cortical structures did n o t show the same distinct separation as those from clearly presynaptic or postsynaptic structures. This is probably due to the properties of the depth electrodes which recorded b o t h presynaptic and postsynaptic activity. This interpretation is supported b y the fact that recovery curves of activity in nuclear structures lie somewhat between those for activity in appropriate presynaptic and postsynaptic structures. The different contribution of presynaptic and postsynaptic activity to the potential recorded in nuclear structures probably accounted for the greater variability. Schwartz et al. (1962) showed in the cat that pentobarbital depressed the cortical recorded SEP only at large enough doses to significantly depress brain function. Our cats received pentobarbital 40 mg/kg initially and subsequently 6--10 mg/kg every 2 h. Stimulation was usually begun 4 hours after the animal was anesthetized at which time we assumed a stable anesthetic state. Furthermore, electroencephalographic activity compatible with moderate to light stages of anesthesia was recorded throughout the entire experiment. Comparing our rate of barbiturate administration with that of Schwartz et al. (1962) our cats were most likely at a level of anesthesia at which t h e amplitude of the primary cortical evoked potential was n o t decreased. Mean recovery cycles are difficult to compare because of different schedules of barbiturate administration and different sites of stimulation (radial nerve in Schwartz et al.'s experiments and forepaw in ours). Nevertheless, recovery of all potentials was essentially complete at ISis of 250 msec in our experiments and at 200 msec in Schwartz et al.'s study, except when using the largest dose of pentobarbital. Our schedule of barbiturate administration was very similar to that used by Anderson et al. (1964). The greater decrement of the test potential in the sensory radiation in our experiments was probably due to the fact that both the conditioning stimulus and test stimulus were delivered to
R E C O V E R Y FUNCTION OF SOMATOSENSORY POTENTIALS
the same peripheral structure while Anderson et al. (1964) gave the conditioning stimulus to the ulnar nerve and the test stimulus to the superficial radial nerve. In the present study, recording from all electrodes in each animal was done simultaneously. The response to paired stimuli varied insignificantly from run to run and from animal to animal. These observations suggest that sodium pentobarbital had little, if any, effect on the reported results. Onset and peak latencies of c o m p o n e n t I (3.8/4.6 msec) and of the posterior column potential (4.0/6.4 msec) overlap and recovery curves are very similar. This suggests that c o m p o n e n t I may reflect activity in posterior columns. C o m p o n e n t I fatigued more readily than the posterior column potential, which implies that other activity contributes to c o m p o n e n t I. Likely candidates are potentials in cuneate and lateral cuneate nuclei because their latencies overlapped with those of c o m p o n e n t I. This interpretation is in agreement with our previous observation that c o m p o n e n t I persisted with slightly decreased amplitude after transection at the medullocervical junction. Latencies of c o m p o n e n t II and of potentials in the cuneate nucleus, lateral cuneate nucleus and medial lemniscus, while n o t identical, overlapped. Latencies of potentials in the inferior cerebellar peduncle were significantly longer than those of c o m p o n e n t II. Recovery curves of c o m p o n e n t II and the medial lemniscus potential were identical. Because of similar latencies and recovery curves, it appears that activity in medial lemniscus, cuneate nucleus, and lateral cuneate nucleus could contribute to c o m p o n e n t II. Latencies of anterior c o m p o n e n t III and the VPL potential are almost identical and overlap with those of potentials in the sensory radiation, inferior cerebellar peduncle, and cerebellar cortex. Recovery curve of anterior c o m p o n e n t III is similar to that of the sensory radiation potential and intermediate between that for potentials in VPL and cerebellum and that for the cortical potential. It appears
265
from these data that the major contribution to anterior c o m p o n e n t III is from activity in the VPL and possibly some from sensory radiation and cerebellar cortex potentials. This interpretation is supported by our previous observation that unilateral thalamect o m y abolished c o m p o n e n t III except its presumed cerebellar contribution. Latencies of c o m p o n e n t IV and of the sensory radiation potential overlapped while latencies of activity in the VPL were clearly shorter. Recovery curves of c o m p o n e n t IV and of the sensory radiation were identical. It appears likely that the major contribution to c o m p o n e n t IV is from activity in the sensory radiation. Cortical activity does n o t contribute to c o m p o n e n t IV because we have previously shown that c o m p o n e n t IV persisted after interruption of afferent pathways to the cortex by either blunt cortical undercutting or ablation of the cortex by suctioning. We presume the fifth positive deflection of the surface SEP to be of cortical origin because its latencies overlapped with those of activity recorded directly from the cortex and their recovery functions were similar. Lastly, it was abolished with cortical ablation (Iragui-Madoz and Wiederholt 1977b). In normal human subjects, we studied three low amplitude positive deflections preceding the presumed onset of evoked cortical activity (Wiederholt and Kritchevsky 1977). These three potentials did not change in amplitude or latency with continuous stimulation at 0.5, 4, and 10 stimuli/sec. We assume that these early potentials in man originate in similar subcortical structures as we have demonstrated in the rat and cat. It is not surprising that there is little, if any, change in amplitude and latencies of these early components of the SEP at the above rates of stimulation, because with continuous or paired stimulation in the rat and cat the most striking amplitude changes occurred with ISI's of less than 100 msec. In our human subjects, continuous stimulation at 10 stimul~/sec or faster was uncomfortable b u t in preliminary observations
266 paired stimuli with interpair intervals of 0.5 sec or longer were accepted quite well. In the present study we demonstrated t h a t early components of the SEP in the cat can be differentiated by their different recovery curves. We plan to study early components of the SEP in h u m a n subjects because the technique of paired stimuli m a y provide additional data about their origin. Furthermore, we would like to establish normative data at different ISI's because some disease processes which m a y n o t produce significant changes of the SEP when using long ISI's may well show abnormalities with short ISI's.
Summary Recovery functions of early components of surface and depth recorded somatosensory evoked potentials were studied in the cat. At interstimulus intervals of 50 msec or less differentiation of surface components was optimal. C o m p o n e n t I showed less decrement at all ISI's than did c o m p o n e n t II and recovery curves of both were distinctly different from those of components III (anterior) and IV and the cortical SEP. The latter three potentials had similar recovery curves. Comparisons of latencies and recovery curves of surface recorded potentials with those of depth recorded potentials indicate t h a t c o m p o n e n t I principally reflects activity in posterior column, that potentials in medial lemniscus, cuneate nucleus, and lateral cuneate nucleus contribute to c o m p o n e n t II, that the major generator for the contralateral c o m p o n e n t III is the ventral postero-lateral nucleus of the thalamus, and that c o m p o n e n t IV is largely due to activity in the sensory radiation. The results of this study support previous observations and conclusions about the origin of short latency, surface recorded somatosensory evoked potentials preceding the cortical SEP.
w.c. WIEDERHOLT R6sumd
Fonction de rdcupdration des composantes de courte latence des potentiels dvoquds somatosensitifs enregistrds en surface et en profondeur chez le chat Les fonctions de r6cup6ration des composantes pr6coces des potentiels 6voquds somatosensitifs enregistrds ~ la surface et dans la profondeur ont 6t6 dtudi6s chez le chat. Pour des intervalles inter-stimuli de 50 msec ou moins, la diffdrenciation des composantes de surface s'avdre optimale. La composante I montre moins de diminution pour tous les ISis que n'en montre la composante II et les courbes de r6cupdration de ces 2 composantes diffdrent de fa~on distincte de celle des composantes III (ant6rieures) et IV et du SEP cortical. Ces 3 derniers potentiels ont des courbes de rdcup6ration similaires. La comparaison entre latences et courbes de r6cupdration des potentiels enregistrds en surface et celles des potentiels enregistr6s en profondeur indiquent que la composante I refl~te principalement l'activit6 au niveau de la colonne post6rieure que les potentiels du lemniscus mddian, du noyau cun6en et du n o y a u nunden lat6ral contribuent ~ la composante II, que le gdn6rateur principal de la composante III controlat6rale est le noyau ventral postdrolat~ral du thalamus, et que la composante IV est en grande partie due ~ l'activitd de la radiation sensitive. Les rdsultats de cette 6tude sont en accord avec de pr6c6dentes observations et conclusions concernant l'origine des potentiels dvoqu6s somatosensitifs enregistr6s en surface, de courtes latences ant6rieures au SEP cortical. The author wishes to thank Mr. Byron Budnick for technical assistance and Mrs. Bobbi Wagner for
preparation of the manuscript. References
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Goff, W.R., Rosner, B.S. and Allison, T. Distribution of cerebral somatosensory evoked responses in normal man. Electroenceph. clin. Neurophysiol., 1962, 14: 697--713. Halliday, A.M. Somatosensory evoked responses. In: A. R~mond (Ed.), Handbook of Electroencephalography and Clinical Neurophysiology, Vol. 8, Part A. Elsevier, Amsterdam, 1975: 60--67. Iragui-Madoz, V.J. and Wiederholt, W.C. Far-field somatosensory evoked potentials in the cat: Correlation with depth recording. Ann. Neurol., 1977a, 1: 569--574. Iragui-Madoz, V.J. and Wiederholt, W.C. Far-field somatosensory evoked potentials in the cat. Electroenceph. clin. Neurophysiol., 1977b, 43: 646--657. Reinoso-Suarez, F. Topographischer Hirnatlas der Katze. E. Merck, Darmstadt, 1961. S c h w a r t z , M., Shagass, C., Bittle, R. and Flapan, M. Dose related effects of pentobarbital on somatosensory evoked responses and recovery cycles. Electroenceph. clin. Neurophysiol., 1962, 14: 898--903. Tamura, K. Ipsilateral somatosensory evoked responses in man. Folia psychiat, neurol, jap., 1972, 26: 83--94. Wiederholt, W.C. Far-field reflections of brain stem sensory evoked responses. Trans. Amer. neurol. Ass., 1975, 100: 259--261. Wiederholt, W.C. and Iragui-Madoz, V.J. Far-field somatosensory potentials in the rat. Electroenceph. clin. Neurophysiol., 1977, 42: 456--465. Wiederholt, W.C. and Kritchevsky, M. Early components of the human averaged somatosensory evoked potential (SEP). Excerpta Medica, Amsterdam, 1 9 7 7 , 4 2 7 : 174.
