702
Electroencephalography and Clinical Neurophysiology, 1979, 46:702--708
© Elsevier/North-Holland Scientific Publishers, Ltd.
SHORT LATENCY SOMATOSENSORY POTENTIALS IN HUMANS DON W. KING and JOSEPH B. GREEN Department of Neurology, Medical College of Georgia, Augusta, Ga. 30901 (U.S.A.)
(Accepted for publication: October 13, 1978) In 1970, Jewett et al. reported small amplitude high frequency potentials which could be detected in human scalp recordings during the first 7 msec following auditory stimulation (Jewett et al. 1970). They proposed that these potentials represented the 'far-field' reflection of distant activity occurring in brain stem auditory pathways. Subsequently, several clinical reports have shown a correlation between abnormalities of these potentials and lesions of the subcortical auditory pathways (Robinson and Rudge 1975; Starr and Achor 1975; Starr 1976; Starr and Hamilton, 1976; Stockard et al. 1976; Stockard and Rossiter 1977). Cracco and Cracco (1976) and Jones (1977) have recently reported short latency potentials in humans following median nerve stimulation. In their averages, these authors included frequencies below 100 c/sec. Based on averages in which frequencies below 100 c/sec were filtered, this report describes a sequence of high frequency somatosensory potentials analogous to the clinically useful auditory potentials. Our data suggest that these components reflect activity of both subcortical and cortical structures. With further clarification, it may become possible to study neural conduction and to localize clinical abnormalities along the entire somatosensory pathway from the periphery to the cerebral cortex. M e t h o d and material
Experiments were carried out in 10 normal subjects (5 male and 5 female) ranging
in age from 16 to 30 years. The subjects were supine t h r o u g h o u t the recording session, several sleeping for short periods of time. Monophasic square pulses (0.2--1.0 msec in duration) were given through a stimulus isolation unit to the median nerve just above the wrist. The intensity was adjusted to produce a vigorous, non-painful twitch of the thumb. The repetition rate was 2/sec for 2000 stimuli. A differential signal was detected by EEG disk electrodes. The active electrode was placed over several locations on the scalp (10-20 system of placement) and neck, and the reference electrode was located over the dorsum of the hand contralateral to stimulation. In 5 subjects, bipolar scalp recordings were also obtained. Total amplication of the signal was 10s; and except when stated otherwise, frequency limits of the amplifiers were set from 100 to 3000 Hz. Computer sampling rates were 20,000--50,000 Hz. The data were permanently recorded with an X-Y plotter; the cursor of the data processor was used to obtain latency values.
Results
Fig. 1 shows averaged responses from 3 subjects recorded between the contralateral central area (C3 electrode) and the left hand. There was minimal variation between repeated averages in the same subject and remarkable similarity among different subjects. All 10 subjects displayed a large positive c o m p o n e n t (Pm) with a peak between 18.9
SHORT LATENCY SOMATOSENSORY POTENTIALS
PI
C 3 -LH
Nm
A
2
,k
/
C3-LH
Nm
C3-LH
-~
,
I0
15
2'o
~5
Fig. 1. Short latency somatosensory evoked potentials in 3 subjects recorded between the left central area (C3) and the left hand following right medium nerve stimulation; duplicate averages superimposed; in this and subsequent figures: P, positive; N, negative ; i, initial ; m, major; time (msec); tracing begins 5 msec after onset of stimulus; relative positivity in grid I results in upward deflection; cal., 0.8 pV.
and 22.3 msec following the stimulus. This was preceded in all subjects by a large negative peak (Nm) occurring between 14.6 and
703
17.3 msec. Preceding the major positive and negative c o m p o n e n t s were an initial positive potential (Pi) and 3 brief negative to positive deflections (P1, P2, P3) (Fig. 1). P2 was the largest of these 3 c o m p o n e n t s and was easily detected in all 10 subjects. P~ was present in all subjects, but poorly defined in t w o (Fig. 13). P3 was well defined in 8 subjects and difficult to identify in the other two (Fig. 41). On the upswing of the major positive peak, one or two additional positive deflections were present in 8 subjects (Fig. 1). Because of lack of uniformity, however, they are not included as definite components in this report. The peak latency values, shown in Table IA, are partly determined by conduction time along peripheral nerve from the stimulating electrode at the wrist to the cervical spinal cord. Consequently, subjects with longer upper extremities have longer latencies for all components. A correction for differing peripheral condition times was made by multiplying the value of the observed peak latency by the ratio of an arbitrary standard of 60 cm to the measured distance in each subject from the stimulating cathode to the C7 spine. In these subjects, the distance varied from 59 to 74 cm with a mean of 65.5 cm. When the correction was made, the coefficient of variation was less than 5% for all c o m p o n e n t s (Table IB). The major positive peak (Pro) was detected over the hemisphere contralateral to stimulation, maximal over the central (C3 or C4) and parietal (P3 or P4) areas (Fig. 21,2). It was absent at the vertex (Cz) and over the ipsilateral hemisphere (Fig. 23,4). On bipolar recordings between the contralateral central area and the vertex, Pm was present (Fig. 32). The c o m p o n e n t s preceding Pm were detected bilaterally over the scalp in recordings referred to the hand (Fig. 2), but were cancelled in bipolar recordings between the contralateral central and vertex regions (Fig. 32). Repetitive stimulation at 20/sec was carried out in 3 subjects. Pi and P1 were unchanged, but Pm was either abolished or decreased by
704
D.W. KING, J.B. GREEN
TABLE I Peak latency values of the short latency somatosensory evoked response in 10 normal subjects. Two recordings in each subject; right median nerve stimulation; C3 referred to the left hand. Corrected value = observed value × 60 (cm)/measured distance (cm). Components
(A) Observed values
Pmaj Nmaj Pi P1 P2 P3
(B) Corrected values
Range (msec)
Mean (msec)
S.D. (msec)
Range (msec)
Mean (msec)
S.D. (msec)
18.9--22.3 14.6--17.3 7.7-- 9.7 10.1--12.8 12.0--14.5 13.3--16.0
20.5 15.7 8.5 11.2 12.9 14.3
1.2 1.0 0.7 0.9 0.9 0.9
17.9--20.5 13.4--14.9 7.3-- 8.2 10.0--10.5 11.3--12.7 12.2--14.3
18.8 14.4 7.8 10.3 11.9 13.1
0.9 0.4 0.3 0.2 0.4 0.4
P~ Pi P, I
p_
C3-LH
Pm
3
I C3-LH 2 / sec
Nm
N
2 2
C3-C z 2 / sec
E
P3-L
5 Cs-LH 20/sec
~b
i~
2b
2's
Fig. 3. Subject 6. 1 : short latency somatosensory potentials recorded between C3 and the left hand following right medium nerve stimulation at 2/sec. 2: recording between C3 and Cz (vertex) at 2/sec. 3 : recording between C3 and left hand at 20/sec.
Cz-LH
,~PI
Nm
C4-L
a
+
B
5
I0
15
20
25
Fig. 2. Subject 3. Short latency potentials recorded between different locations over the scalp and the left hand following right median nerve stimulation.
SHORT LATENCY SOMATOSENSORY POTENTIALS
705
e~ Pi
% I
C3-LH
100 Pi
Nm
2 INION-LH
I
3
CT-LH
-i
~b
{5
~o
5
2'o
2'5
Fig. 4. Subject 8. Short latency potentials following right median nerve stimulation at 2/sec. 1 : C3 to left hand. 2: inion to left hand. 3: base of neck at C 7 spinous process to left hand.
Fig. 5. Subject 9. Short latency potentials recorded between C3 and the left hand following right median nerve stimulation. 1: frequency limits 100--3000 c/sec; cal., 0.8 pV. 2: frequency limits 1--3000 c/sec; cal., 3.2/~V.
greater than 50% in all subjects (Fig. 33). The behavior of P2 and P3 during repetitive stimulation was variable. In 5 of the 10 subjects, recordings referred to the contralateral hand were also obtained
from the inion and the C7 spinous process. P2, P3, Nm, and Pm were not present in these averages (Fig. 42,3). Pi and P1 were present in recordings from the inion, but only Pi was detected at the C7 spine (Fig. 42,3).
TABLE II Comparison of peak latency valuesof Pi, P1, P2 and P3 obtained with frequency limits of 100--3000 c/sec to those of superimposed positive peaks obtained with frequency limits 1--3000 c/sec; two recordings in each subjects; right median nerve stimulation; C3 referred to the left hand. Note that early superimposed potentials do not correspond to Pi, but that later superimposed potentials are almost identical to P1, P2, and P3. Pi Subject 4 Early components (100--3000 c/sec) Superimposed potentials (1--3000 c/sec)
8.1
Subject 9 Early components (100--3000 c/sec) Superimposed potentials (1--3000 c/sec)
9.0
Subject 10 Early components ( 100--3000 c/sec) Superimposed potentials (1--3000 c/sec)
7.7
P]
P2
P3
10.9 11.0
12.0 12.2
13.8 13.8
10.2
11.8 12.0
13.6 13.8
15.3 15.1
8.5
10.5 10.4
12.1 12.0
13.3 13.2
8.9
10.0
706 In 3 subjects, the short latency somatosensory response obtained by filtering frequencies below 100 c/sec was compared with that obtained by setting frequency limits from 1 to 3000 c/sec (Fig. 5). When the lowei frequencies were included, there was a large amplitude early positive potential with 4--5 small amplitude higher frequency potentials superimposed (Fig. 52, arrows), followed in two subjects by a large amplitude negative deflection. The peak latency of the first superimposed potential did not correspond to that of Pi (Fig. 5; Table II). The peak latencies of the final 3 superimposed potentials, however, were almost identical to those of P1, P2, and P3 obtained when frequencies below 100 c/sec were filtered (Fig. 5; Table II). Filtering frequencies below 100 c/sec provided clearer definition of these components (Fig. 5).
Discussion
The data reported here confirm the work of Cracco and Cracco {1976) and Jones (1977) who showed that early potentials can be averaged from the scalp following median nerve stimulation. Our data differ, however, in that frequencies below 100 c/sec were filtered. Cracco and Cracco averaged frequencies between 10 and 2500 c/sec, and Jones set the low frequency response with a time constant of 1 sec. Because of differences in low frequency response, as well as differences in stimulation rate and electrode placement, it is difficult to compare the specific peak latencies reported here with those of other authors. The data reported here suggest that Pi, Nm, and Pin do not correspond in latency to waves obtained without low frequency filtering. On the other hand, in the 3 subjects studied, P1, P2, and P3 did correspond to the later low amplitude potentials superimposed on the initial positive wave. As previously noted, when the low frequencies were filtered, these c o m p o n e n t s became more clearly defined. These data suggest that
D.W. KING, J.B. GREEN filtering frequencies below 100 c/sec, as is done in brain stem auditory potentials, may allow more precise clinical use of the short latency somatosensory response. On the basis of scalp recordings, the origin of the various components cannot be determined with certainty. Nevertheless, the data permit some reasonable speculations. Pm was detected solely over the hemisphere contralateral to stimulation, was maximal over the centroparietal area, and persisted on bipolar recordings between the contralateral central area and vertex. It was decreased or abolished on repetitive stimulation at 20 c/sec. Thus, Pm is a well-localized contralateral response originating in higher order neurons, most likely those of the somatosensory cortex. This component may correspond to the 'primary evoked potential' previously recorded directly from the cortical surface in animals (Marshall 1937) and humans (Jasper et al. 1960). The components preceding Pm were detected in referential recordings from widely distributed locations over the scalp and were abolished in bipolar recordings between the contralateral central area and the vertex. This suggests that these components are the 'far-field' reflection of activity occurring in structures below the cortical level. Pi was present in recordings from the scalp, the inion, and the base of the neck and always occurred well in advance of the major negative deflection recorded from the base of the neck (Fig. 43). If this large negatiw~ deflection can be assumed to represent arrival of the impulse at the spinal cord directly beneath the electrode (Cracco 1973), it is likely that Pi results from activity in the peripheral nerve. PI was present in recordings from the scalp and inion, but not in recordings between the base of the neck and the hand. The peak latency of P1 was remarkably uniform, always occurring within the first millisecond following the major negative deflection recorded from the base of the neck. Thus, the distribution of P1 and its time course suggest that PI
SHORT LATENCY SOMATOSENSORY POTENTIALS reflects activity of first order neurons at their entry into the spinal cord or during their course in the dorsal columns o f the cervical cord. This is in agreement with the findings of Iraqui-Madoz and Wiederholt (1977) who proposed that the first positive potential o f the short latency somatosensory evoked response in cats originated in the posterior columns of the cervical cord or the nucleus cuneatus. P2 and P3 were det ect ed over the scalp but were n o t present in recordings at the inion or the base of the neck. IraquiMadoz and Wiederholt (1977) proposed that the second and third widely distributed c o m p o n e n t s o f the short latency somatosensory response in cats indicated activity in the medial lemniscus and cerebellum respectively. Although we have no evidence in humans for such a specific localization, the time course o f P2 and P3 and the fact t h a t these c o m p o n e n t s could not be r e c or ded at the inion suggest that t he y arise above the cervico-medullary junction. Animal studies using depth recording techniques and clinical correlations with pathological lesions in humans should clarify the origin o f these and other components.
707 R6sum6
Potentiels somato-sensitifs de courte latence chez l 'homme Une s6quence de potentiels de haute fr~quence est m oyenn~e au niveau du scalp chez 10 sujets n o r m a u x au cours des 25 premieres msec apr~s stimulation du nerf m~dian. On not e une c o m p o s a n t e positive de grande amplitude avec une latence de pic situ6e entre 18,9 et 22,3 msec, localis~e au niveau de l'aire somato-sensitive contralat~rale ~ la stimulation. Cette c o m p o s a n t e est pr6c~d6e par un potentiel positif pr~coce survenant ~ la p~riph~rie (latence de pic 7,7--9,7) et d'au moins 3 d~flexions n6gativespositives qui semblent prendre leur origine au niveau de multiples structures sous-corticales. Apr~s correction de la longueur du bras, la variabilit~ entre les sujets est de moins de 5% p o u r routes les composantes. Avec des clarifications ult~rieures, cette m 6t hode pourrait p e r m e t t r e d'~tudier la c o n d u c t i o n nerveuse t o u t au long de la voie somatosensitive. We wish to thank Ms. Bonnie Kovach and Ms. Judy Sitler for technical and clerical assistance.
Summary A sequence o f high f r equency potentials was averaged from the scalp o f 10 normal h u m a n subjects during the first 25 msec following median nerve stimulation. There was a large positive c o m p o n e n t with a peak latency between 18.9 and 22.3 msec localized t o the somatosensory area contralateral t o stimulation. This was preceded by an early positive potential arising peripherally (peak latency 7.7--9.7) and at least 3 negative t o positive deflections which appear t o originate in multiple subcortical structures. When c o r r e c t e d for arm length, intersubject variability was less than 5% for all c o m p o n e n t s . With f u r t h e r clarification, this m e t h o d should allow one to study neural c o n d u c t i o n along the entire somatosensory pathway.
References Cracco, R.Q. Spinal evoked response: peripheral nerve stimulation in man. Electroenceph. clin. Neurophysiol., 1973, 35 : 379--386. Cracco, R.Q. and Cracco, J.B. Somatosensory evoked potential in man; far-field potentials. Electroenceph, clin. Neurophysiol., 1976, 41: 460--466. Iraqui-Madoz, V.J. and Wiederholt, W.C. Far-field somatosensory evoked potentials in the cat: correlation with depth recording. Ann. Neurol., 1977, 1 : 569--574. Jasper, H., Lende, R. and Rasmussen, T. Evoked potentials from the exposed somato-sensory cortex in man. J. nerv. ment. Dis., 1960, 130: 526---537. Jewett, D.L., Romano, M.N. and Williston, J.S. Human auditory evoked potentials: possible brain stem components detected on the scalp. Science, 1970, 167: 1517--1518.
708 Jones, S.J. Short latency potentials recorded from the neck and scalp following median nerve stimulation in man. Electroenceph. clin. Neurophysiol., 1977, 43: 853--863. Marshall, W.H. Cortical representation of tactile sensibility as indicated by cortical potentials. Science, 1937, 85: 388--390. Robinson, K. and Rudge, P. Auditory evoked responses in multiple sclerosis. Lancet, 1975, i: 1164--1166. Starr, A. Auditory brain-stem responses in brain death. Brain, 1976, 99: 543--554. Starr, A. and Achor, L.J. Auditory brain stem responses in neurological disease. Arch. Neuroh (Chic.), 1975, 32: 761--768.
D.W. KING, J.B. GREEN Starr, A. and Hamilton, A.E. Correlation between confirmed sites of neurological lesions and abnormalities of far-field auditory brain stem responses. Electroenceph. clin. Neurophysioh, 1976, 41: 595--608. Stockard, J.J. and Rossiter, V.S. Clinical and pathological correlates of brain stem auditory response abnormalities. Neurology (Minneap.), 1977, 27: 316--325. Stockard, J.J., Rossiter, V.S., Wiederholt, W.C. and Kobayashi, R.M. Brain stem auditory-evoked responses in suspected central pontine myelinolysis. Arch. Neuroh (Chic.), 1976, 33: 726--728.
Electroencephalography and Clinical Neurophysiology, 1979, 46:702--708
© Elsevier/North-Holland Scientific Publishers, Ltd.
SHORT LATENCY SOMATOSENSORY POTENTIALS IN HUMANS DON W. KING and JOSEPH B. GREEN Department of Neurology, Medical College of Georgia, Augusta, Ga. 30901 (U.S.A.)
(Accepted for publication: October 13, 1978) In 1970, Jewett et al. reported small amplitude high frequency potentials which could be detected in human scalp recordings during the first 7 msec following auditory stimulation (Jewett et al. 1970). They proposed that these potentials represented the 'far-field' reflection of distant activity occurring in brain stem auditory pathways. Subsequently, several clinical reports have shown a correlation between abnormalities of these potentials and lesions of the subcortical auditory pathways (Robinson and Rudge 1975; Starr and Achor 1975; Starr 1976; Starr and Hamilton, 1976; Stockard et al. 1976; Stockard and Rossiter 1977). Cracco and Cracco (1976) and Jones (1977) have recently reported short latency potentials in humans following median nerve stimulation. In their averages, these authors included frequencies below 100 c/sec. Based on averages in which frequencies below 100 c/sec were filtered, this report describes a sequence of high frequency somatosensory potentials analogous to the clinically useful auditory potentials. Our data suggest that these components reflect activity of both subcortical and cortical structures. With further clarification, it may become possible to study neural conduction and to localize clinical abnormalities along the entire somatosensory pathway from the periphery to the cerebral cortex. M e t h o d and material
Experiments were carried out in 10 normal subjects (5 male and 5 female) ranging
in age from 16 to 30 years. The subjects were supine t h r o u g h o u t the recording session, several sleeping for short periods of time. Monophasic square pulses (0.2--1.0 msec in duration) were given through a stimulus isolation unit to the median nerve just above the wrist. The intensity was adjusted to produce a vigorous, non-painful twitch of the thumb. The repetition rate was 2/sec for 2000 stimuli. A differential signal was detected by EEG disk electrodes. The active electrode was placed over several locations on the scalp (10-20 system of placement) and neck, and the reference electrode was located over the dorsum of the hand contralateral to stimulation. In 5 subjects, bipolar scalp recordings were also obtained. Total amplication of the signal was 10s; and except when stated otherwise, frequency limits of the amplifiers were set from 100 to 3000 Hz. Computer sampling rates were 20,000--50,000 Hz. The data were permanently recorded with an X-Y plotter; the cursor of the data processor was used to obtain latency values.
Results
Fig. 1 shows averaged responses from 3 subjects recorded between the contralateral central area (C3 electrode) and the left hand. There was minimal variation between repeated averages in the same subject and remarkable similarity among different subjects. All 10 subjects displayed a large positive c o m p o n e n t (Pm) with a peak between 18.9
SHORT LATENCY SOMATOSENSORY POTENTIALS
PI
C 3 -LH
Nm
A
2
,k
/
C3-LH
Nm
C3-LH
-~
,
I0
15
2'o
~5
Fig. 1. Short latency somatosensory evoked potentials in 3 subjects recorded between the left central area (C3) and the left hand following right medium nerve stimulation; duplicate averages superimposed; in this and subsequent figures: P, positive; N, negative ; i, initial ; m, major; time (msec); tracing begins 5 msec after onset of stimulus; relative positivity in grid I results in upward deflection; cal., 0.8 pV.
and 22.3 msec following the stimulus. This was preceded in all subjects by a large negative peak (Nm) occurring between 14.6 and
703
17.3 msec. Preceding the major positive and negative c o m p o n e n t s were an initial positive potential (Pi) and 3 brief negative to positive deflections (P1, P2, P3) (Fig. 1). P2 was the largest of these 3 c o m p o n e n t s and was easily detected in all 10 subjects. P~ was present in all subjects, but poorly defined in t w o (Fig. 13). P3 was well defined in 8 subjects and difficult to identify in the other two (Fig. 41). On the upswing of the major positive peak, one or two additional positive deflections were present in 8 subjects (Fig. 1). Because of lack of uniformity, however, they are not included as definite components in this report. The peak latency values, shown in Table IA, are partly determined by conduction time along peripheral nerve from the stimulating electrode at the wrist to the cervical spinal cord. Consequently, subjects with longer upper extremities have longer latencies for all components. A correction for differing peripheral condition times was made by multiplying the value of the observed peak latency by the ratio of an arbitrary standard of 60 cm to the measured distance in each subject from the stimulating cathode to the C7 spine. In these subjects, the distance varied from 59 to 74 cm with a mean of 65.5 cm. When the correction was made, the coefficient of variation was less than 5% for all c o m p o n e n t s (Table IB). The major positive peak (Pro) was detected over the hemisphere contralateral to stimulation, maximal over the central (C3 or C4) and parietal (P3 or P4) areas (Fig. 21,2). It was absent at the vertex (Cz) and over the ipsilateral hemisphere (Fig. 23,4). On bipolar recordings between the contralateral central area and the vertex, Pm was present (Fig. 32). The c o m p o n e n t s preceding Pm were detected bilaterally over the scalp in recordings referred to the hand (Fig. 2), but were cancelled in bipolar recordings between the contralateral central and vertex regions (Fig. 32). Repetitive stimulation at 20/sec was carried out in 3 subjects. Pi and P1 were unchanged, but Pm was either abolished or decreased by
704
D.W. KING, J.B. GREEN
TABLE I Peak latency values of the short latency somatosensory evoked response in 10 normal subjects. Two recordings in each subject; right median nerve stimulation; C3 referred to the left hand. Corrected value = observed value × 60 (cm)/measured distance (cm). Components
(A) Observed values
Pmaj Nmaj Pi P1 P2 P3
(B) Corrected values
Range (msec)
Mean (msec)
S.D. (msec)
Range (msec)
Mean (msec)
S.D. (msec)
18.9--22.3 14.6--17.3 7.7-- 9.7 10.1--12.8 12.0--14.5 13.3--16.0
20.5 15.7 8.5 11.2 12.9 14.3
1.2 1.0 0.7 0.9 0.9 0.9
17.9--20.5 13.4--14.9 7.3-- 8.2 10.0--10.5 11.3--12.7 12.2--14.3
18.8 14.4 7.8 10.3 11.9 13.1
0.9 0.4 0.3 0.2 0.4 0.4
P~ Pi P, I
p_
C3-LH
Pm
3
I C3-LH 2 / sec
Nm
N
2 2
C3-C z 2 / sec
E
P3-L
5 Cs-LH 20/sec
~b
i~
2b
2's
Fig. 3. Subject 6. 1 : short latency somatosensory potentials recorded between C3 and the left hand following right medium nerve stimulation at 2/sec. 2: recording between C3 and Cz (vertex) at 2/sec. 3 : recording between C3 and left hand at 20/sec.
Cz-LH
,~PI
Nm
C4-L
a
+
B
5
I0
15
20
25
Fig. 2. Subject 3. Short latency potentials recorded between different locations over the scalp and the left hand following right median nerve stimulation.
SHORT LATENCY SOMATOSENSORY POTENTIALS
705
e~ Pi
% I
C3-LH
100 Pi
Nm
2 INION-LH
I
3
CT-LH
-i
~b
{5
~o
5
2'o
2'5
Fig. 4. Subject 8. Short latency potentials following right median nerve stimulation at 2/sec. 1 : C3 to left hand. 2: inion to left hand. 3: base of neck at C 7 spinous process to left hand.
Fig. 5. Subject 9. Short latency potentials recorded between C3 and the left hand following right median nerve stimulation. 1: frequency limits 100--3000 c/sec; cal., 0.8 pV. 2: frequency limits 1--3000 c/sec; cal., 3.2/~V.
greater than 50% in all subjects (Fig. 33). The behavior of P2 and P3 during repetitive stimulation was variable. In 5 of the 10 subjects, recordings referred to the contralateral hand were also obtained
from the inion and the C7 spinous process. P2, P3, Nm, and Pm were not present in these averages (Fig. 42,3). Pi and P1 were present in recordings from the inion, but only Pi was detected at the C7 spine (Fig. 42,3).
TABLE II Comparison of peak latency valuesof Pi, P1, P2 and P3 obtained with frequency limits of 100--3000 c/sec to those of superimposed positive peaks obtained with frequency limits 1--3000 c/sec; two recordings in each subjects; right median nerve stimulation; C3 referred to the left hand. Note that early superimposed potentials do not correspond to Pi, but that later superimposed potentials are almost identical to P1, P2, and P3. Pi Subject 4 Early components (100--3000 c/sec) Superimposed potentials (1--3000 c/sec)
8.1
Subject 9 Early components (100--3000 c/sec) Superimposed potentials (1--3000 c/sec)
9.0
Subject 10 Early components ( 100--3000 c/sec) Superimposed potentials (1--3000 c/sec)
7.7
P]
P2
P3
10.9 11.0
12.0 12.2
13.8 13.8
10.2
11.8 12.0
13.6 13.8
15.3 15.1
8.5
10.5 10.4
12.1 12.0
13.3 13.2
8.9
10.0
706 In 3 subjects, the short latency somatosensory response obtained by filtering frequencies below 100 c/sec was compared with that obtained by setting frequency limits from 1 to 3000 c/sec (Fig. 5). When the lowei frequencies were included, there was a large amplitude early positive potential with 4--5 small amplitude higher frequency potentials superimposed (Fig. 52, arrows), followed in two subjects by a large amplitude negative deflection. The peak latency of the first superimposed potential did not correspond to that of Pi (Fig. 5; Table II). The peak latencies of the final 3 superimposed potentials, however, were almost identical to those of P1, P2, and P3 obtained when frequencies below 100 c/sec were filtered (Fig. 5; Table II). Filtering frequencies below 100 c/sec provided clearer definition of these components (Fig. 5).
Discussion
The data reported here confirm the work of Cracco and Cracco {1976) and Jones (1977) who showed that early potentials can be averaged from the scalp following median nerve stimulation. Our data differ, however, in that frequencies below 100 c/sec were filtered. Cracco and Cracco averaged frequencies between 10 and 2500 c/sec, and Jones set the low frequency response with a time constant of 1 sec. Because of differences in low frequency response, as well as differences in stimulation rate and electrode placement, it is difficult to compare the specific peak latencies reported here with those of other authors. The data reported here suggest that Pi, Nm, and Pin do not correspond in latency to waves obtained without low frequency filtering. On the other hand, in the 3 subjects studied, P1, P2, and P3 did correspond to the later low amplitude potentials superimposed on the initial positive wave. As previously noted, when the low frequencies were filtered, these c o m p o n e n t s became more clearly defined. These data suggest that
D.W. KING, J.B. GREEN filtering frequencies below 100 c/sec, as is done in brain stem auditory potentials, may allow more precise clinical use of the short latency somatosensory response. On the basis of scalp recordings, the origin of the various components cannot be determined with certainty. Nevertheless, the data permit some reasonable speculations. Pm was detected solely over the hemisphere contralateral to stimulation, was maximal over the centroparietal area, and persisted on bipolar recordings between the contralateral central area and vertex. It was decreased or abolished on repetitive stimulation at 20 c/sec. Thus, Pm is a well-localized contralateral response originating in higher order neurons, most likely those of the somatosensory cortex. This component may correspond to the 'primary evoked potential' previously recorded directly from the cortical surface in animals (Marshall 1937) and humans (Jasper et al. 1960). The components preceding Pm were detected in referential recordings from widely distributed locations over the scalp and were abolished in bipolar recordings between the contralateral central area and the vertex. This suggests that these components are the 'far-field' reflection of activity occurring in structures below the cortical level. Pi was present in recordings from the scalp, the inion, and the base of the neck and always occurred well in advance of the major negative deflection recorded from the base of the neck (Fig. 43). If this large negatiw~ deflection can be assumed to represent arrival of the impulse at the spinal cord directly beneath the electrode (Cracco 1973), it is likely that Pi results from activity in the peripheral nerve. PI was present in recordings from the scalp and inion, but not in recordings between the base of the neck and the hand. The peak latency of P1 was remarkably uniform, always occurring within the first millisecond following the major negative deflection recorded from the base of the neck. Thus, the distribution of P1 and its time course suggest that PI
SHORT LATENCY SOMATOSENSORY POTENTIALS reflects activity of first order neurons at their entry into the spinal cord or during their course in the dorsal columns o f the cervical cord. This is in agreement with the findings of Iraqui-Madoz and Wiederholt (1977) who proposed that the first positive potential o f the short latency somatosensory evoked response in cats originated in the posterior columns of the cervical cord or the nucleus cuneatus. P2 and P3 were det ect ed over the scalp but were n o t present in recordings at the inion or the base of the neck. IraquiMadoz and Wiederholt (1977) proposed that the second and third widely distributed c o m p o n e n t s o f the short latency somatosensory response in cats indicated activity in the medial lemniscus and cerebellum respectively. Although we have no evidence in humans for such a specific localization, the time course o f P2 and P3 and the fact t h a t these c o m p o n e n t s could not be r e c or ded at the inion suggest that t he y arise above the cervico-medullary junction. Animal studies using depth recording techniques and clinical correlations with pathological lesions in humans should clarify the origin o f these and other components.
707 R6sum6
Potentiels somato-sensitifs de courte latence chez l 'homme Une s6quence de potentiels de haute fr~quence est m oyenn~e au niveau du scalp chez 10 sujets n o r m a u x au cours des 25 premieres msec apr~s stimulation du nerf m~dian. On not e une c o m p o s a n t e positive de grande amplitude avec une latence de pic situ6e entre 18,9 et 22,3 msec, localis~e au niveau de l'aire somato-sensitive contralat~rale ~ la stimulation. Cette c o m p o s a n t e est pr6c~d6e par un potentiel positif pr~coce survenant ~ la p~riph~rie (latence de pic 7,7--9,7) et d'au moins 3 d~flexions n6gativespositives qui semblent prendre leur origine au niveau de multiples structures sous-corticales. Apr~s correction de la longueur du bras, la variabilit~ entre les sujets est de moins de 5% p o u r routes les composantes. Avec des clarifications ult~rieures, cette m 6t hode pourrait p e r m e t t r e d'~tudier la c o n d u c t i o n nerveuse t o u t au long de la voie somatosensitive. We wish to thank Ms. Bonnie Kovach and Ms. Judy Sitler for technical and clerical assistance.
Summary A sequence o f high f r equency potentials was averaged from the scalp o f 10 normal h u m a n subjects during the first 25 msec following median nerve stimulation. There was a large positive c o m p o n e n t with a peak latency between 18.9 and 22.3 msec localized t o the somatosensory area contralateral t o stimulation. This was preceded by an early positive potential arising peripherally (peak latency 7.7--9.7) and at least 3 negative t o positive deflections which appear t o originate in multiple subcortical structures. When c o r r e c t e d for arm length, intersubject variability was less than 5% for all c o m p o n e n t s . With f u r t h e r clarification, this m e t h o d should allow one to study neural c o n d u c t i o n along the entire somatosensory pathway.
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