Analysis of somato-sensory, auditory and visual averaged transcortical and scalp responses in the monkey

488

Electroencephalography and Clinical Neurophysiology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

ANALYSIS OF SOMATO-SENSORY, AUDITORY AND VISUAL AVERAGED TRANSCORTICAL AND SCALP RESPONSES IN TIlE MONKEY 1 WILLIAM B. HARDIN JR. AND VINCENT F. CASTELLUCC!

Department of Neurology, Washington University School of Medicine, St. Louis, Mo. 63110 ( U,S.A.) (Accepted for publication: September 29, 1969)

The origin of the averaged late surface negative component of the vertex response to somato-sensory, auditory or photic stimtdation has not been demonstrated satisfactorily, yet many clinical studies have made use of this response. The latency or amplitude of this wave component is said to change in relation to sleep (Weitzman and Kremen 1965), selective attentioJ,~ and discrimination (Haider et al. 1964; Satterfield 1965),, judgment and decision (Davis 1964; Bartlett and White 1965; Donchin and Lindsley 196~; Ruchin and John 1966), reaction time (Morrell and Morrell 1966), auditory perception (Cody and Bickford 1965), and intelligence in children (Ertl, personal communication). Although there have been many investigations of latency response components that are shorter than 50 msec (Adrian and Matthews 1934; Bartley and Heinbecker 1938; Forbes and Morison 1939), much less interest has been shown in the longer latency responses of waking, unanesthetized animals and man. The technique of averaging has encouraged investigation of the vertex potential in man (Dawson 1954; Barlow 1957), but only a few studies of the long latency components have been undertaken in the unanesthetized monkey (Rosner et al. 1963; Hughes and Mazurowski 1964; Spinelli 1967). Such studies are necessary to establish the sources and distributions of the potential. The present study describes the variability in wave shapes and latencies of the averaged vertex z This investigation was supported by grants NB 05378-07 and NB 14513 from The National Institutes of Neurological Diseasesand Blindness, U.S. Public Health Service.

responses to median nerve, auditory and photic stimuli as recorded with referential leads from the scalp and dural surface of the squirrel monkey. The difference between t.he visually evc,ked vertex response and those responses ,,ough s,omato-sensory and auditory ~xcited *"stimulation are analyzed, the distribution of transcortical averaged evoked responses (AERs) is examined and primary receiving area responses are compared with vertex responses. METHODS AND MATERIAL

Averaged potentials were recorded from forty-five adult male and female squirrel monkeys (Salmiri sciureus) weighing more than 700 g. All animals were anesthetized initially with Surital or inhalation ethel. A polyethylene tube was inserted into a femoral vein for the administration of fluids and Flaxedil (gallamine triethiodide). A tracheostomy was performed and the animals were artificially respired. Whenever interim surfi~erywas required, the animals were temporarily re-anesthetized. Skin incisions were infiltrated with local anesthetic and kept moist with physiological saline-soaked cotton. During photic stimulation the eyelids were retracted gently with small blunt hooks, and rb~ corneas were bathed periodically with an ophthalmic solution. The somato-sensory stimulus (shock) was a 0.1 msec square wave pulse delivered through a Grass S-4 stimulator and an isolation unit to bipolar electrodes applied to the median nerve at the wrist. Before administration of the neuromuscular blocking agent, the stimulus intensity Electroenceph. din. Neurophysiol., 19/0, 28:488-498

489

SQUIR~L MONKEY AVERAGED RESPONSES

was adjusted to provide a slight thumb move-

ment with each stimulus. The auditory stimulus, click, consisted of a 1.0 msec pulse delivered to a GrassAudiomonitor (Model AM-30) positioned 75 cm in front of the supine reclining monkey so as to produce a loud click. Care was taken to prevent spread of mechanical vibrations of the audio-amplifier unit from reaching the monkey; a foam rubber insulating pad was placed beneath the unit which was in turn placed on a stand apart from the table where the monkey lay. A Grass photostimulator positioned 20 cm directly in front of the monkey was used to deliver brief photic flashes of 15 psec duration. In order to insure that the faint sound made by the photic stimulator would not interfere with the purely flash-evoked potential, the monkey's eyes were taped with black electrician's tape, after which several control runs were made. No evoked potential could be elicited under these conditions. The animal was given an initial dose of 4 mg/kg of Flaxedil and periodic maintenance doses as needed to abolish the effect of spontaneous movement and short latency myogenic potentials on the record (Bickford 1964; Mast 1965). Three to 4 h were allowed for rccovery from the anesthetic as gauged by the replacement of fast activity in the vertex EEG with an 8-13 c/sec pattern. Periodically, the effect of Flaxedil was allowed to lessen so that the be. havior of the awake, slightly moving animal could be related to the EEG patterns observed in the paralytic state. Mechanical stimulation (tapping and touch) served to keep the animal attentive. Although the monkey's precise level of attentive"~!~s could not be assessed direcjly, his EEG gave as indirect evidence of appropriate recording ~onditions. Whenever the EEG depressed or de~ynchronized in response to tactile stimulation, conditions were believed optimal for averaging the vertex potential. In one group of twenty monkeys, a vertex scalp-to-nose referential recording system was used, and each monkey lay unrestrained and in a semi-reClining position. In a second group of four monkeys, photic responses only were evaluated with the use of vertex transcortical surface-to-nose, white matter-to-nose, and transocular recording systems before and after complete optic nerve transection. In a third

group of twenty-one monkeys, transcortical, surface-to-nose, and white matter-to-nose recordings were made of all three stimulus modalities at selected areas over the entire hemisphere convexity. For the optic nerve transection and transcortical survey, monkeys were restrained gently in a stereotaxic device in a manner that did not interfere with vision or hearing. Each of the three different stimuli were given one modality at a time, with 15 min between runs. Four runs of 50 responses at inter-stimulus intervals of 10 sec were averaged for each stimulus modality. The background EEG was monitored continuously with a Grass Polygraph, Model 1'-4. Evoked responses were averaged on line by a LINC computer, and all of the sampled brain activity was stored simultaneously on magnetic tape for off-line playback at different sweep speeds. Each animal was sacrificed at the end of the experiment with an overdose of Surital.

~m_

P3

NIl. 1 Comparative appearances of scalp AERs recorded for 250 ~ with vertex-to-nosereferentialelectrodesystems to median nerve shock (S), click (C), and flash (F) stimulation. Deflections are desijp~atodP (positive) and N (negative).Doublefinesindicatethe.standarddeviations of peak latencies. Dotted fine in (F) shows alternative appearance of the flash AER. £[eetroeneeph. olin. Neurophysiol., 197@~,28:488-498

490

W. B. HARDIN JR. AND V. F. CASTELLUCCI TABLE I Mean peak laten¢ies and amplitude ranses for deflections Shock msec

Click /~V

msec

Flash ~V

msec

/~V

Pl

11 (-4- 3)*

20-75

19 (-4- 3)

10-50

14 ( ± 3)

10-30

N~

16 (4- 4)

Small

29 ( ~ 7)

Small

30(-4-- 8)

75-150

P

55

Small

n

76

Small

P2

36(-!- 6)

50-150

55(-4- 8)

50-150

115 (-t-20)

30-75 Small

N:,

92 (-i-20)

50-150

.'.-'. ( ± 12)

50-150

140(±15)

Pa

158 (.-]:30)

10-50

152 (-4-24)

10-40

160(-1-25)

Small

244 (-4-26)

50-100

Ns * Standard deviations are provided where more than ten observations were made.

SHOCK

lpos. I0 Honke/s

71Polnte

Fig. 2 Distribution of AERs to contralateral median nerve shock, Upper tracin8 of each pair recorded durato-white matter (transcortic~IL Lower tracin8 of each pair recorded white matter-to,nose. Dura-tonose (not shown) is polarity reversed with respect to all lower tracings. Minor differences were found with ipsilateral as compared to contralateral stimulation. Ctdibration: 50 I,V and I00 msec.

Electroenceph. clin. Neurophysiol., 19/0, 28:488-498

SQUIRREL, MONKEY AVERAGED RESPONSES

491

of the AERs to shock, cSck and flash in awake monkeys who had received Flaxedil.

RESULTS

General

Response to median nerve stimulation (shock)

For purposes of analysis, positive and negative components of the averaged evoked responses (AERs) were numbered successively, beginning with the initial positive deflection, Pz (Fig. 1). Peak latencies of various components were calculated with their standard deviations from the mean and ranges in amplitude (Table I). The shapes and peak latencies of vertex scalpto-nose AERs in the unanesthetized, unrestrained monkey were virtually identical to those of the vertex dural surface-to-nose AERs of the stereotaxically restrained animals. Amplitudes of the former, however, were lower. Restraint per se appeared to have virtually no affect on the forms

The total duration of the early complex (Pl, Nz and P~) wa~ ~tpproximately 58 msec (Fig. I and 2). Pz averaged 20-75 /~V whereas Ps averaged 50-150/~V when'recorded transcortically at or near the vertex. In contrast to Ps, Pz was always sharply peaked. Ns was the most prominent component in the long latency group. The third positive deflection, Ps (so designated when evident at amplitudes exc~ding 30/~V), occurred upon the upswing t:o..-- N2. No consistent deflections were ever seen which had a mean peak latency beyond 250 msec. Seventy-one cortical points were studied in

SHOCK

transcortical

C contralateral i

ipsilaterai

\

TPO. vC vi aC ai

inUeetion firet peek 59_ 11.6 9,6 14,9 4.9 9.6 7,9 i6,1 2sm. ee

i ~ e , o o a I l l i o ~ . o o

..

J

eo

i . l e o o a o o

Fig. 3 Comparison of contralateral (C) AERJ with ipsilateral 0) recorded tramcortically to median nerve shock. Vertex(v) and arm area (a). Vertical white lines indicatefirst infloctionand first peak latencies. Amplitudes adjusted to similar heightsfor display. Electroenceph. din. Neurophysiol., 1970, 28:488--498

492

W . B . HARDIN JR. A/~D V. F. CP,STELLUCCI

ten monkeys with transcortical leads, and the contralateral AERs were compared with those with ipsilateral shock (Fig. 2 and 3). lpsilateral AERs were of slightly lower amplitude and longer latency than contralateral AERs, but otherwise they were similar in form and cortical distribution. Thus Px through Ps were welldeveloped rostral to the Sylvi~n fissure but were negligible or absent posterior to it (Fig. 2). Contralateral peak latency for Px over the arm area, Sm-l, was 9.5 msec in contrast to the ipsilate,'al (a i) Pl (15.1 msec)(Benjamin and Welker 1957; Blomquist and Lorenzini 1965). The longer latency waves Ps through Pa were virtually the same with either contralateral or ipsilateral stimulation. In all instances where a transcortically recorded AER occurred, dural surface-tonose and white matter-to-nose records showed

~

~ii

'

,

iil

polarity reversal with respect to each other indicating local cortical generation of bo~h early and late waves.

Respo~ !~ o,df~ory ~tim,dation (click) The designations given the deflections of the AER to click were the same as those given for AER to shock bemuse the forms were similar (Fig. 1). The duration of the early complex (P1, Nt and Pg) was approximately 70 msec. P1 through Ps had amplitudes comparable to those with shock (Table I). Again, no peaks could be identified exceeding 250 msec in latency. Fifty-two cortical points were studied in eight monkeys with transcortical leads. Click-evoked AERs were well developed all over the frontal convexity as well as over the superior temporal gyras in and near the primary auditory receiving

i ¸:

: .....

..........

:~

:¸¸¸

g Monkege

62 Polnte

CLICK |

lpos. -

~ ~ ~

~i:

• ~,~i~

~

Fig, 4 Distributiouof A E R s to clickstimulation,Upper tracingof each pair recorded alum=to-whitematter (transcortical).Lower tracingof each pair recorded white matter,to=nose.Dura-to=nose (not shown) ispolarityreversedwith respectto alllower tracings~Calibration:50/4V and I00 msec,

Electroenceph. clm. Neurophyswl., 19"/0,28:488=498

SQUIRRELMONKEYAVERAGEDRESPONSES area, as described by Massopust et aL (1968). The averaged peak latency of Px at the cortex subjacent to the vertex was 23.3 msec (Fig. 4). This was 6.9 msec later than the Px recorded from the superior temporal gyms near the prim~ry auditory receiving cortex, Dura-to-noseand white matter-to-nose recordings of the entire AEK complex showed polarity reversal, which indicates that the waves were generated locally for as long as 250 msec. The transcortical AER resembled the corresponding dura-to-no~e AER in all recordings using shock or click, Response to photic stimulation (flash) Deflections of the AER to flash, recorded referentially with vertex-to-nose leads, could be designated Px through Ha in spite of some dissimilarities of shape when compared to shock and click (Fig. 1; Table I). The wave complex was composed of an initial positive ( P l ) ~ o n a n d a large, 75-150 /~V negative Oql)deflecti~m lasting a5 ± 5 msec. Within the trough of /41 were several small positive peaks with latencies of approximately 19, 30 and 45 msec. Long latency deflections were more variable in their appearance by comparison with shock and click AERs. For example, a distinct small amplitude positive-negative deflection (labelled p and n, respectively) appeared in ten monkeys (Fig. 1). The origin of the p--n deflection remained obscure. It may represent an individual variation in the extent te which visual area responses spread by volume conduction to the vertex, or a late response ofthe retinal generator. In ten animals, partially inclusive of the former group, a second, invariably small, negative wave (Ha) appeared on the downo swing of Ps and was followed occasionally by a small Ps wave, which probably corresponded to the late positive component of the other two modalities. Thereafter a slow negativity (Ns) developed. The characteristic appearance and latencies of deflections that make up both the early complex and P9 were identical in dura-to-nose and transocular recordings. They persisted even after complete optic nerve trans6cfion (Fig. 5). 1"he constancy of p , n and Ns in the tufa-to-nose record likewise bore no relationship to the presence or absence of the optic nerves. Although

493

Ns and Ps were evident in vertex-to-nose records in approximately half of the intact animals, they were never seen in transocular or cornea-tonose combinations or, in the optic nerve, transected dura-to-nose records. Vertex transcortical records differed f r o m all referential combinations in showing neither early complex nor long latency deflections either before or after optic nerve transection (Fig. 5, B, E), indicating that no deflections to flash originated from cottex immediately subjacent to the vertex scalp lead.

,0

i":/:]:~

dura to nos~. S E

.....) ;

C F

G cornea t.o

retro-ocular

.....

100 ins

/

pos I

Fig. 5 AERs to photic stimulation displayedfor 250 reset, illustrating results before and after optic nerve transection. Electrode positions correspond to the designated potentials indicated on the left. Vertical calibrations beside eac__h!~-pc~.~-:50/~V. Seventy-nine cortical points were studied in fourteen monkeys with transcortical recordings dm'ing photic stimulation. In contrast to shock and click, there were virtually no flash AERs over the entire frontal lobe convexity or superior temporal cyrus (Fig. 6). However, the striate and peristriate regions (Cowey 1964) demonstrated well-developed cortically generated responses (Fig. 7). Referential montages (dura-to-nose and white matter-to-nose) recorded large AERs all over the cortical surface, but these proved to be merely variations of the cornea-to-nose response. Only when the cortical electrode was Electroenceph. din. NeurophysioL,1970, 28:488-498

494

W. B. HARDIN JR. AND V. F. CASTELLUCCI

,

I

HH

,

~I~]~r~

FLASH

Fig. 6 Distribution of AERs to flashstimulation. See legendof Fig. 4. Dura-to-nose(not shown)is partially polarity reversedto lower tracing only in the occipital cortex. Significant transcorti~l response plus polarity reversal indicates local generation of potential. See also Fig. 7. Calibration: 50 I~Vand 100 ms~. 4'

placed over the visual areas was the AER modified by the addition of cortically generated potentials. DISCUSSION

For median nerve shock and for click, the relatively simple response pattern and lack of development of potentials beyond 250 msec in monkeys contrast mark,,dly with the averaged evoked responses of man where the response is more prolonged and complex (Cody and Bickford 1965; Satterfield 1965; Davis et al. 1966). It is clear that the AERs in the awake unanesthetized squirrel monkey have a similar appearance whether they are recorded referential-

ly from the scalp vertex or transcortically within 3 mm of the vertex. Our transcortical surveys of the dorsal convexity of the brain with ipsilateral and contralateral shock and with click reveal that the major AER waves (PI, Ps, Ns and Ps) can be produced by frontal cortex. Hughes and Mazurowski (1964), in their study of the K complex in rhesus monkeys, found touch and sound ("clap") responses to be greater on the mesial hemisphere over the cingulate gyrus than over the frontal lobe convexity. Amplitudes were enhanced i'f the unanesthetized animal was allowed t o become drowsy. Their recordings were referential, and latencies were not measured. Nevertheless, their data concur with our findings that the major Electroenceph. clin. Neuwphysiol., 19/0, 28:488--498

SQUIRREL MONKEY AVERAGED RESPONS~

495

FLASH I tfHnortleel 3 dare te nee

6 gblte inciter te aeee

65

"

65 ms

396 me

\! t

/ I

\ "

tf /

i

.~\\\'~1~

65,,

3,6,,

65 me

3 9 6 me

! Fig. 7 Comparison of typical vertex AERs to those from the primary visual receiving area with flash stimulus. Note that tracings 3 and 6 are not polarity reversed at vertex, but show partial polarity reversal at visual area. (Intermixture of local potentials from visual cortex plus distant contribution from retinae makes judgment of polarity reversal difficult.) Calibration: 50 t~V, 65 and 396 reset.

portion of the frontal lobe is productive of long latency sound and somato-sensory responses in monkeys. Rosner et al. (1963), in his study of AERs in the cebus monkey, suggested that the first and second surface positive responses (peak latencies of 12 and 30 reset) represent responses dependent upon primary projection and ascending reticular (diffuse projection) systems, respectively. If the late Ps wave is indeed relayed by the diffuse proje~,tion system, then our findings bear upon the postulate in that Ps is not generated in the posterior part of the brain with somato.sensory and auditory stimuli nor in the anterior part of the brain with photic stimuli. Thus the ascending reticular system appears to have some anterior-posterior selectivity, at least as far as the squirrel monkey is concerned. Only with the median nerve shock, which

could be given both ipsilaterally and contralaterally, were the responses of one hemisphere clearly separated and compared with those of the other (Fig. 4). The inflection and peak latenties of the transcortical response, PI, occurred 2-4 msec earlier with contralateral than with ipsilateral stimulation. This difference in latency is no doubt the result of synaptio delay and transcallosal conduction time. The lower amplitudes and less sharp peaks of ipsilateral responses are compatible with an additional relay and cerisequ©nt temporal dispersion. The similarity of transcortically recorded shock and click responses at the vertex as well as elsewhere in the frontal lobe of the squirrel monkey implies a similar mechanism of generation. It also suggests that a closer functional relation exists between somato-scnsory and auditory sensibility than for either with visual sensibility. Electroenceph. clin. Neurophysiol., 1970, 28:488-498

496

W. B. H A R D I N JR. A N D V. F. CASTELLUCCI

Likewise the shape of the photic response is, of itself, indicative of a different mechanism of development compared to shock and click. There are several reasons for concluding that the major components of the vertex scalp-to-nose response to photic stimuli do not originate from immediately underlying brain. First, the early complex strongly resembles the single flash electroretinogram (ERG) reported in the cat and guinea pig (Wolin e t al. 1964), and in man (Heck and Rendahl 1957). In this connection, Brindley and Hamasaki (1962) demonstrated that transection of the optic nerves did not alter the ERG of the cat, Second, transcortical records obtained near the vertex are either fiat or else show no consistently reappearing deflections (Fig. 5-7). Third, the vertex response referred to the nose is nearly identical in form and latency with that obtained by transocular recording. Fourth, the entire complex reverses polarity, depending upon which active electrode, corneal or retro-ocuiar, is used against the nose. Fifth, refmential vertex recordings obtained after complete optic nerve transection differ from those of the intact animal only in the disappearance of the variable Ns and Pa waves. Thus, although Na and Ps may represent activity from the striate cortex transmitted to the vertex by volume conduction, the remaining deflections originate from the retinal generator. Our scalp vertex-to-nose records resemble those of Arfel {1967), Who used a similar recording combination to examine photic flash responses in comatose man. He too believed these responses to originate exclusively in the retina. Our transcortical AERs, taken from the striate and peristriate area, demonstrate the visual cortex response uncontaminated by retinal potentials. Nevertheless, they are consistent with the monopolar records taken from the same area of the squirrel monkey by Cowey (1964). The initial latency of his first positive deflection (30-40 msec) was the same as ours. It is noteworthy that this reference electrode was placed on the temporal dura to avoid distortion from the retinal generator. We agree in part with those who maintain that specific projection areas can be distinguished from cortical association areas with monopolar recording systems insofar as early latency and

greater amplitude of the initial positive cesponse are concerned; this has been demonstrated with the use of averaged evoked responses to median nerve shock in the squirrel monkey by Rosner et al. (1963), to an auditory stimulus inthe cat by Celesia (1968)and to photic stimulation in the squirrel monkey by Cowey (1964). However, Goldring et al. (1967)demonstrated in the awake unanesthetized cat that association area responses to peripheral stimuli vary considerably from one animal to another and from time to time in the same animal. Therefore, under conditions of referential recording of AERs over specific projection cortex, the contribution of the association cortex to the "primary area" response will be capricious, not to mention the probable contribution from generators located anywhere within the hemisphere, cortical as well as subcortical (Kelley et al. 1965). The difficulty of assigning small averaged responses in a large volume conductor such as a brain to the site of a "critical" electrode serves to re-emphasize the importance of recording with bipolar electrodes across small distances (Kelly et al. 1965). Transcortica! recording roger.her with demonstration of polarity reversal is necessary to make such an inference. We believe, therefore, that virtually all of the major components of the average vertex response for click and median nerve shock are generated in vertex cortex and the entire frontal lobe convexity, and probably mesial surface, of the squirrel monkey. This is not the case for the averaged photic response, since neither early components nor those with latencies as long as 400 msec can be found with transcortical recordings anywhere near the vertex or frontal lobe. SUMMARY

Averaged evoked responses (AERs) to median nerve, auditory and photic stimulation were recorded with scalp vertex-to-nose or vertex transcortical electrodes in forty-five unanesthetized squirrel monkeys, Potentials were designated as follows: Px (I 1-14 msec), Nx (16-30 msec), P2 (36-115 msec), N2 (92-140 msec), P s (158160 msec) and Ns (200-270 msec). The AERs of all three modalities were compared with respect to peak amplitudes and wave shapes in the same Electroenceph. din. Neurophysiol., 1970, 28:488-498

SQUIRREL MONKEYAVERAGEDRESPONSES

and different monkeys, and a characteristic pattern for each modality was constructed from the total group. The photic AER differed clearly from the other two, which were similar. The referentially recorded vertex-to-nose AER was identical to the single flash electroretinogram. There was no response to flash when the AER was recorded transcortically at the vertex. Transcortical recordings were made of AERs from 202 cortical points in twenty-one of the forty-five monkeys using shock, click and photic stimuli. The frontal cortex and vertex produced locally generated long latency waves to shock and click but not to flash. Only the occipital cortex was productive of true photic responses. The study of long latency AERs from small brained animals, such as the squirrel monkey and cat, is greatly simplified if bipolar electrodes are used to record across small distance, This method helps to eliminate the confusion that can arise from the unwanted addition of potentials arising from generators away from the presumed recording site. ~SUMt

497

l'6clair quand la REM est enregistr6e au vertex par d6rivation transcorticale. Les REMs par enregistrements transcorticaux ont 6t6 obtenus/t partir de 202 points du cortex, sur vingt-et-un des quarante-cinq singes en utilisant des chocs, des clics et la stimulation photique. Le cortex frontal et le vertex produisent des ondes locales de longue latence au choc et au clic mais pas/t l'6clair. Seul le cortex occipital produit de v6ritables r6ponses/t la lumi~re. L'6tude des REMs de longue latence chez des animaux/~ petit cerveau tels que le singe6cureuil et le chat est grandement simplifi6e si des 61ectrodes bipolaires sont utilis6es pour des enregistrements/t faible distance. Cette m6thode aide/t 61iminer la confusion qui peut survenir du fait de l'addition non d6sir6e de potentiels provenant de g6n6rateurs 61oign6s du sii~ge pr(:sum(: d'enregistrement. The authors wish to thank Dr. William M. landau and Dr. James L. O'Learyfor their helpful suggestions during the preparation of this paper.

REFERENCES

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