EXPERIMENTAL

n’EPROLOGY

51,

22-40 (1976)

Long-Latency Corticocortical Evoked Responses in Squirrel Monkey Frontal Cortex ELEANOR

Dcpartnmt School

H.

BOYD,

EUGENE

of Pharnzacology of Mcdicirw axd

S.

BOYD,

AND

L. E.

BROWN

1

and Toxicology, University of Rochestrr Dtvktistry, Rochester, New York 14642

Early (5 to 15 msec) and late (120 to 180 msec) responses are evoked bilaterally in postarcuate cortex of unanesthetized squirrel monkeys by electrical stimulation of postcentral somatosensory cortex. Sectioning the precentral corpus callosum abolishes both early and late responses to contralateral stimulation and leaves unchanged the responses recorded at the same site to ipsilateral stimulation, suggesting that generation of the late response depends on generation of the early response in the same frontal lobe. Units in postarcuate cortex fire in relation to the early and late responses, but are silent in the period between them. Other investigators have suggested that late components of the direct cortical response and postinhibitory rebound of cortical neurons are dependent on input to cortex from the thalamus. Our data indicate, rather, that it is input to cortex from the mesencephalon, probably feeding through the thalamus, which is necessary. Late responses are selectively depressed or even abolished by bilateral lesions in and near the mesencephalic reticular formation (MRF) and by general anesthetic agents, but the convulsant stimulants, picrotoxin and pentylenetetrazol, can induce recovery of late responses after lesions or administration of anesthetic agents. The data are consistent with the interpretation that neurons in postarcuate cortex are excited to fire during the early surface response, undergo long-duration postsynaptic inhibition such as is generated in cortical neurons by activation of many cortical afferents, and then show postinhibitory rebound if input from the mesencephalic reticular formation has not been interrupted by lesions or anesthetic agents, or if the convulsants substitute for this input.

INTRODUCTION Previous neurophysiologic investigations in the anesthetized squirrel monkey (7, 9, 44) have demonstrated that relatively short latency (5 to 15 msec) corticocortical responses are evoked in frontal lobe postarcuate cortex 1 This investigation was supported in part by a USPHS Grant to the University of Rochester. 22 Copyright All rights

F’ress, IX. Q 1976 by Academic in any form reserved. of reproduction

General Research Support

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[areas 4 and 6, motor and premotor cortex (39, 42, 48) ] by electrical stimulation of ipsilateral and contralateral postcentral somatosensory cortex. Those responses have also been demonstrated in the unanesthetized cerueau isolt and enctplzale isolb squirrel monkey (9-12). In addition to those relatively short-latency corticocortical responses, a longer-latency (120 to 180 msec) response has been demonstrated in unanesthetized monkeys with brain stem transections caudal to the mesencephalon (10). The effects of some neuropharmacologic agents on both those early and late responses have been described previously (8, 10-12). Other investigators (14, 26) have suggested that late components of the direct cortical response, which are similar to the late responses we have recorded in squirrel monkeys, are dependent on input to cortex from the thalamus. The present paper describes experiments designed to elucidate further the mechanism of generation of late corticocortical responses evoked in frontal lobe by stimulation of postcentral somatosensory cortex. These experiments include determination of the effects on those responses of lesions of thalamus, mesencephalon, and corpus callosum, and the relation of cortical unit firing to the surface responses. METHODS The experiments were carried out in the unanesthetized squirrel monkey, S&z&i sciureus (500 to 1000 g, either sex), prepared under diethyl ether anesthesia with cannulae (i) in the trachea to permit artificial respiration, monitoring, and maintenance of expired carbon dioxide at 4.0 + 0.5% (Beckman LB-l medical gas analyzer) ; (ii) in the femoral artery to monitor arterial blood pressure continuously (Statham pressure transducer and Grass polygraph) ; and (iii) in the femoral vein to allow a) injection of urea (30% urea in 5% dextrose, 1 ml plus 1 ml/3 hr) to minimize cerebral edema (24) ; b) infusion of methoxamine (3-15 pg/ min) in animals needing a pressor agent to sustain blood pressure ; and c) injection of gallamine triethiodide (3 mg/kg plus 1.5 mg/kg/45 min) in animals in which a convulsant agent was studied and in some animals in which it was needed for efficient artificial respiration. Methoxamine and gallamine have previously been shown to be without effect on the early and late surface evoked responses (12). Because the general anesthetic agents which we have tested (ether, pentobarbital, and chloralose) abolish the late response under study (12), painful input to mesencephalon and telencephalon was eliminated by sectioning the brain stem caudal to the mesencephalon but rostra1 to the trigeminal nerve. A craniotomy was performed and a spatula was passed through the parietal lobe, then ventrally, rostra1 to the tentorium cerebelli,

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through the rostra1 pans on the dorsal surface of the brain stem and, depending on the angle, through the pons or rostra1 medulla ventrally. Examination of the sections after killing the animal revealed that most sections were pretrigeminal and essentially complete bilaterally. To prevent pain in the event sectioning was incomplete or too far caudal, wounds and pressure points of the stereotaxic instrument were infiltrated with lidocaine which was supplemented every 2 hr, a rate which was found previously, in enc6phale isol& preparations not given gallamine, adequate to maintain a synchronous electrocorticogram (ECoG), indicative of a pain-free state, and adequate to prevent somatic and autonomic signs of pain. Anesthesia was discontinued and experiments were begun 2 to 3 hr later when the evoked responses were fully recovered from suppression by ether (12). The dura was removed and a mineral oil bath was formed over exposed cortex within a rubber dental dam cemented to the scalp. Bath and rectal temperatures were maintained at 37 * 1 C. One or more monopolar ECoGs were recorded continuously from prefrontal cortex. Experiments were terminated if background brain electrical activity deteriorated or if mean arterial blood pressure fell below 60 mm Hg for an appreciable time. A pair of silver ball stimulating electrodes, 1 to 2 mm apart, was placed just postcentrally in the forelimb/trunk area of somatosensory cortex, as defined by Benjamin and Welker (6). Stimuli were provided by a Grass S88 stimulator, isolation unit, and constant current unit. A Devices Digitimer was used with the timing circuits of the S88 to control delivery of stimuli which were monophasic, O.l-msec duration pulses ranging from a fraction of a milliampere up to 5 mA. Stimuli were delivered at the rate of l/4 set, a rate found to be sufficiently low to avoid diminution of the late responses with repeated stimulation. ECoG and evoked cortical activity were recorded by monopolar silver ball electrodes against a reference electrode placed on deflected dura or on a 0.5 cm2 piece of saline-saturated gauze over the occipital skull. These placements of the reference electrode were verified many times as truly indifferent by recording, during stimulation, an unresponsive cortical area against the reference. Recordings of cortical units were made with commericaIly available (Transidyne) tungsten microelectrodes inserted through a plexiglass pressor foot to reduce cortical pulsations. All responses were amplified with Grass P511 preamplifiers, with frequency responses set at 0.15 Hz and 2 kHz for surface and microelectrode field potential recordings, and at 30 Hz and 10 kHz for unit recordings. Data were recorded on a four-channel FM magnetic tape recorder (Hewlett Packard) for subsequent analysis with a LAB-8 digital computer (Digital Equipment Corp. j In view of the variability of evoked responses in this semi-intact nervous system, seres of 16 or more surface evoked

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responses were averaged. Analysis of unit data consisted of the formation of poststimulus-time histograms and of stimulus-linked raster displays of dots (each dot representing a spike) so that any changes in unit behavior with time during successive stimulus presentations could be detected. Histograms and rasters consisted of 500 or 1000 I-msec bins. A windowing technique was used to discriminate unit spikes. When activity of a single unit was analyzed, the spikes were used to trigger the oscilloscope and the waveform of the spikes was examined for consistency of amplitude and shape. Lesions were made using a spatula after coagulation of interfering blood vessels, or by passing metered amounts of current (Grass LM-5 d-c lesion maker) for measured times through insulated concentric bipolar stainless steel electrodes (outer barrel, 23-gauge tubing; inner strut, 0.254 mm wire protruding about 1 mm), placed with the aid of the stereotaxic atlas of Emmers and Akert (19). To control for nonspecific depression, which could be produced by the lesions, (i) responses unaffected by the lesion were recorded from the same cortical site and/or (ii) 1 to 2 hr recovery, with maintenance of vital signs, was allowed before any lesion was judged to have had an effect on the evoked response. An exception to this was when picrotoxin (Mann Research Laboratories) or pentylenetetrazol (Metrazol, Knoll) were given to induce recovery after some brain stem lesions. At the end of each experiment, the animal was killed with an overdose of pentobarbital and the brain was perfused with 0.97% saline followed by 10% formalin. The extent of all experimental lesions was determined histologically in frozen sections cut at 2.5 ,urn and stained with cresyl violet and Luxol fast blue ( 15). RESULTS Wuzeforutl and Distribution

of the Evoked Responses

Experiments in three unanesthetized monkeys verified the distribution of the early responses found previously (9, 44) in anesthetized monkeys and showed that distribution of late responses was coextensive with that of early responses (Figs. 1 and 2). The individual early response typically consisted of a biphasic, surface positive-negative waveform, or a monophasic surface positive waveform, although occasionally, ipsilateral to the stimulus, the waveform was triphasic with a yet later positivity (Figs. 5-7). Latency-to-initial-peak positivity ranged from 5 msec to 10 msec. The late response was typically also biphasic, surface positive-negative, or, occasionally, triphasic with a later positive component, with latency-to-initialpeak positivity varying among animals from 120 msec to 180 msec. The

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FIG. 1. Map of early responses evoked on dorsolateral cortical surface of squirrel monkey by stimulating (5 mA, at arrows) postcentral somatosensory cortex. Each trace is mounted on the diagram at approximate site where it was recorded and is an average of 16 responses, compensated for changes in the preparation with time, with reference to changes at a fixed recording site. In this and subsequent figures, positivity at the cortical surface recording electrode, relative to the remote reference electrode, is indicated by a downward deflection from baseline. Calibration: ImV, 10 msec. (Monkey L 15).

of responses included ipsilateral and contralateral frontal lobe postarcuate areas (areas 4 and 6) and ipsilateral and contralateral parietal lobe near the stimulus site and its homBotopicpoints, and especially around the dimple labeled “s. PC.” by Rosabal (39). Typically the responses decreasedmarkedly in amplitude 1 mm from each of the optimum recording sites and became monophasic negative before falling to zero yet farther away. Extensive mapping of the dorsolateral cortical surface to determine sites which, when stimulated, would evoke early and late responsesin postarcuate cortex was done in two monkeys. Distribution of effective stimulus sites was similar for eliciting early and for eliciting late responsesin both ipsilateral [Figs. 3 and 4, and (10, Fig. 2) ] and contralateral (not illustrated) postarcuate cortex. These experiments indicate a discreteness of effective stimulus sites. They showed that stimulation of some cortical sites near the distribution

postarcuate recording sites was less effective than stimulation of more distant pericentral cortical areas. Effective cortical stimulus sites included

ipsilateral and contralateral pericentral somatosensory and motor sensory cortex, the homotopic frontal lobe site (not illustrated), and, in some monkeys (10, Fig. 2)) ipsilateral and contralateral superior temporal gyrus

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FIG. 2. Map of late responses evoked as described in Fig. 1, showing distribution similar to that of early responses. Early responses and stimulus artifacts are truncated in many traces. Calibrations : 1 mV, 100 msec. (Monkey L 15). in or near auditory cortex (31) and ipsilateral and contralateral prestriate cortex near visual cortex (16). These experiments suggested that any cortical stimulus evoking an early response in postarcuate cortex would also evoke a late response at the same site.

FIG. 3. Map of cortical sites where stimulation evoked early responses in ipsilateral postarcuate cortex. Responses, which were all recorded at site labeled “R,” are mounted on the cortical diagram at the respective stimulus sites. Calibrations: 1 mV, 10 msec. (Monkey L 15).

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FIG. 4. Map of cortical sites where stimulation evoked late responses in ipsilateral postarcuate cortex, at site labeled “R.” As in Fig. 3, responses are mounted at the respective stimulus sites. Calibrations: 1 mV, 100 msec. (Monkey L 15).

Efjects

of Brain

Lesions

on Late Responses

Effects of Cutting Precentrd Corpm Callosum Experimental brain lesions previously reported (9, 44) s h owed that the pathway for early responses, evoked bilaterally in frontal postarcuate cortex by stimulating

somatosensory cortex, runs rostrally from somatosensory cortex, in underlying white matter, to the postarcuate area ipsilateral to the stimulus, then crosses in the precentral corpus callosum to the postarcuate area contralateral to the stimulus. In the present study, stimulating electrodes were placed on both left and right somatosensory cortex and recording electrodes on both left and right postarcuate cortex so that ipsilateral and contralateral early and late responses could be recorded in each hemisphere. Precentral corpus callosum was selectively transected in two monkeys. In both monkeys, this procedure immediately abolished both early and late responses evoked by contralateral stimuli, and left unaffected early and late responses evoked at the same sites by ipsilateral stimuli. One of those experiments

is illustrated

in Fig.

5, showing

that ipsilateral

early

responses

were slightly depressed and the ipsilateral late responses were actually slightly increased, but that both had recovered essentially to control amplitude 1 hr after lesioning. Persistence of ipsilateral responses showed that abolition of the contralateral responses was not due to some nonspecific factor associated with the lesion. Histologic examination of these brains showed

monkeys.

that

the lesion

was essentially

restricted

to corpus

callosum

in both

Sectioning of precentral corpus callosum was complete in one

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monkey, but somewhat incomplete (germ, rostrum, and fine tissue bridges intact) in the other. Efects of Rostra1 Brain Stem Lesions. Eighteen squirrel monkeys successfully prepared with brain stem transections at the level of the mesencephalon or more rostrally, uniformly showed early but not late responses, whereas 88 monkeys successfully prepared with brain stem transections caudal to the mesencephalon uniformly showed both early and late responses, In four monkeys prepared with more caudal brain stem transections and showing late responses, a second, experimental, brain stem transection was made with a spatula, to determine more precisely what brain stem strucMONKEY

LIZ

LEFT

RIGHT

POST-ARCUATE

POST-ARCUATE

PRELESION

:: !i ;:

. LEFT

STIMULATION

: i : ‘G

i :: j \r

POSTLESION

SOMATOSENSORY

RIGHT SOMATOSENSORY

STIMULATION n ; ‘: ; i.

-. ,r”2-

l-..r

-

: \ ; v

-.-I

FIG. 5. Sectioning of precentral corpus callosum abolished both early and late responses evoked in left and right postarcuate cortex by stimulation (5 mA) of contralateral somatosensory cortex, but left mostly unchanged the early and late responses evoked at same sites by stimulation (5 mA) of ipsilateral somatosensory cortex. Recording and stimulating sites are specified. In this figure and in Figs. 6 and 7, each early response is shown to left of each stimulus-recording sequence on a different time scale (calibration: 10 msec) from that used to the right to show the late response (calibration : 100 msec). Early responses and stimulus artifacts are truncated in displays of late responses. Each trace is average of 32 responses. The postlesioning responses were recorded 1 hr after sectioning. Voltage calibration: 0.5 mV.

30 MONKEY

BOYD,

L56

IPSILATERAL

BOYD

POST-ARCUATE

AiVD

BROWN

CONTRALATERAL

POST-ARCUATE

FIG. 6. A brain stem transection tunneling rostrally and ventrally through the diencephalon abolished late responses in postarcuate cortex both ipsilateral and contralateral to the stimulus, but did not significantly change early responses recorded at either site. Only a long-lasting biphasic surface positive-negative shift remained after lesioning where the late responses had been. “Ipsilateral” and “contralateral” are related to the hemisphre to which stimuli (5 mA) were applied. “After lesion” refers to the immediate postlesioning period. Each trace is average of 32 responses. Calibration : 1.3 mV for ipsilateral, 1.0 mV for contralateral responses; 10 and 100 msec.

tures were necessary for generation of late cortical responses. Late responses were selectively abolished in all four monkeys with only slight or transient effects on early responses. Histologic examination showed that those lesions, which tunneled rostrally and ventrally, completely transected the diencephalon with relatively little involvement of the basal ganglia more laterally. One of these experiments is illustrated in Fig. 6. These monkeys showed ECoG changes typical of the cervea~ isolb, i.e., they developed high-voltage, low-frequency ECoG activity. Three other monkeys received bilateral electrolytic lesions, which were confined to mesencephalic reticular formation (MRF) and surrounding tissue (superior and inferior colliculi, nucleus of lateral lemniscus), and did not extend rostrally into the diencephalon. In all three monkeys, late responses were either abolished or markedly reduced in amplitude, whereas early responses recorded at the same postarcuate sites were only slightly or transiently affected. In two animals, the lesions caused ECoG changes as in the diencephalic animals described above. One of these experiments is illustrated in Fig. 7. By itself, the lesion on the right was mostly without

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--

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FIG. 7. Abolition of the late, but not the early, response, by bilateral electrolytic lesions of the mesencephalic reticular formation. The lesions, which extended about 2 mm to 2.5 mm rostrocaudally, are diagrammed in the inset coronal section of the brain stem. The administration of picrotoxin was followed by development of a discrete, triphasic, positive-negative-positive waveform corresponding to the late response before lesioning. Contralateral responses were affected similarly by the lesions and by picrotoxin. Each trace is average of 32 responses. Stimulation: 2 mA, left somatosensory cortex. Calibrations : 0.5 mV; 10 and 100 msec (early and late responses, respectively).

but when the lesion on the left was made, late responseswere abolished bilaterally with only minimal and transient effects on early responses. The postlesion recovery period in this animal was terminated when both ipsilateral and contralateral early responses had recovered prelesioning amplitude, so that the effects of picrotoxin could be studied (see below). Bilateral electrolytic lesions of the mesencephalicreticular formation were attempted in a fourth monkey, but only lesions on the left extended into the mesencephalicreticular formation ; the lesion on the right was confined to superior and inferior colliculi. This monkey showed no significant changes in early or late responsesor ECoG.

effect on any response,

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In none of the eight operated animals was there any correlation between disappearance of late responses and blood pressure changes (which were variable in direction and usually small). Reversal

of the Effect of Brain Stem Lesions

by Convulsant

Drugs

In animals in which late responses were small or nonexistent, because of preparatory or experimental lesions involving mesencephalon or more rostra1 brain stem structures, administration of picrotoxin (0.5 to 1.0 mg/ kg; in five of eight monkeys in which it was tried) or pentylenetetrazol ( 15 to 20 mg/kg ; in both monkeys in which it was tried) was followed, within a few minutes, by the appearance of late responses and an increase in early responses. In animals given picrotoxin, this occurred before, and in

FIG. 8. Poststimulus time histograms. A, of a positive-negative biphasic unit, and B, of a mostly monophasic negative unit recorded simultaneously, about 2 mm deep in postarcuate cortex, showing two different patterns of response to electrical stimulation (5 mA) of contralateral postcentral somatosensory cortex. The unit in B fired early and showed a rebound burst of firing after the silent period, whereas the unit in A showed only the silent period without early or rebound firing. Poststimulustime histograms are based on 115 stimulus presentations. C is a single sweep of activity recorded by the microelectrode versus a silver ring in the pressor foot on the cortical surface. In Figs. 8-10, ordinates of the histograms show the number of spikes in each 1-msec bin. In Figs. 8 and 9, dashed vertical lines in the histograms indicate time of stimulus presentation; negativity recorded by the microelectrode is indicated by a downward deflection. Calibrations: 0.4 mV, 12.5 msec. (Monkey L 127).

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L118

++-?I MONKEY

LIZ0

FIG. 9. Correlation of repetitive burst activity of cortical neurons with repetitive waves in the average field potential (L118B, LlZOC) in two different monkeys. The unit in monkey L118 was about 2 mm deep in cortex and was biphasic negativepositive; that in LIZ0 was about 0.5 mm deep and was biphasic positive-negative, The unit in monkey L118, but not that in L120, fired early as well as late. The data for monkey L118 were based on 94 stimulus presentations ; those for monkey L120 on 146, except for the raster (B), which shows (top to bottom) the responses to the frrst 125 stimuli. Calibrations : 0.625 mV (Monkey L118) ; 0.250 mV (Monkey L120) ; 125 msec.

given pentylenetetrazol, this occurred after, a generalized seizure. An experiment in which picrotoxin induced recovery of late responsesis shown in Fig. 7. The ability of these drugs to promptly reverse the effects of lesions is further indication that lesions did not in some way cause nonspecific cortical depressionor death of cortical neurons.

animals

Correlation of Unit Responseswith Surface Evoked Responses The response patterns of neurons in postarcuate cortex were studied by extracellular recording in 15 monkeys. Micrometer readings indicated that most of the stimulus-related units studied were 1.5 to 2.0 mm deep in cortex, where the largest neurons, most easily sampled by an extracellular microelectrode (47), are situated. The most consistent observation was the long period of unit silence between early and late surface responses.Even neurons which were otherwise uninfluenced by the stimulus (Fig. 8A) and injured neurons displaying the characteristic high frequency “death burst” of firing were often silent in this period. Examples are shown in Figs. 8 to 10. Frequently the silent period correlated with a positive wave in the field potential (Figs. 8 and 9).

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POST-ARCUbTE

BOYD

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CONTRIIL4TERIIL

POST-ARCUATE

FIG. 10. “Resetting” of late cortical surface response and late cortical unit activity by appropriately timed stimuli. In monkey L25 (left column), from top to bottom, one, three, four, six, or ten stimulus pulses (5 mA, 60 msec between pulses) were delivered to contralateral somatosensory cortex with resultant stepwise postponement or resetting of the late response. Each trace is average of 16 responses recorded in postarcuate cortex. In monkey L31 (right column), traces in upper parts of A and B are average of 169 cortical surface responses; the poststimulus-time histograms in lower parts of A and B show, on the same time scale, multiunit activity recorded concomitantly with a microelectrode in cortex nearby; in A, single pulses (5 mA) were presented; in B, paired stimuli, 50 msec apart, reset the late surface response and the late unit firing. Stimulus artifacts and early responses are truncated in the surface records. Calibrations: 1 mV, 100 msec.

After this silent period, many, but not aI1, neurons displayed a rebound burst of firing (Figs. 8B, 9, and lO--Monkey L31), which coincided in time with the late responserecorded on the cortical surface. However, some neurons did not rebound but only resumed their previous level of firing (Fig. 8A). Many neurons, both those showing a low level of background firing (Figs. 8B and lO--Monkey L31) and those which were mostly silent except in relation to the stimulus (both monkeys in Fig. 9), increased their firing at a time which coincided with the early surface response. However, someneurons which rebounded during the late surface responseeither did not fire at all during the early surface response (Fig. 9, Monkey LEO) or did not fire consistently during the early surface response (the small unit in Fig. SC).

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There was a definite tendency for unit firing to be most pronounced when surface early and late responses were going from the initial positive phase to the negative phase. This relation of unit firing to late responses can be seen in Figs. 8 to 10. The time scales in these figures are too compressed to illustrate the relation of unit firing to early responses. In addition to firing during surface early and late responses, unit firing also occurred during the later repetitive waves which were observed in surface and field potential recordings of several of these monkeys, and which have been found to be accentuated by picrotoxin and other neuropharmacologic agents (8, 12). Fig. 9 shows two examples of the relation of unit firing to later waves. “Resetting”

of Late Responses

by Repetitive

Stivmlation

It has been shown previously (10) that the late response is postponed or “reset” by the second of a pair of stimulus pulses appropriately timed to evoke a second early response of appreciable amplitude before the late response to the first stimulus occurred. Those experiments have now been extended to nine monkeys, seven of which showed resetting, and to trains of up to 10 pulses, 50 msec or 60 msec between pulses. A typical experiment is illustrated in Fig. l&Monkey L25. As with appropriately timed pairs of pulses, the late response was postponed or “reset” in each case until a relatively constant time after the last stimulus pulse. Figure 10 (Monkey L31) shows that the late rebound burst of unit firing is also “reset.” DISCUSSION The ipsilateral responses described here come under the heading of what has been called most recently the direct cortical response, a cortical surface potential evoked in a number of species by cortical electrical stimulation near the recording electrode (1, 14, 26, 28, 35). With weak stimulation, the direct cortical response consists of a short-latency, monophasic negative waveform which decrements rapidly from the stimulus site. With increasing stimulus strength, an initial positivity is brought in, along with later components and the response can be recorded at a greater distance from the stimulus site. A discrete, long-latency, positive-negative waveform, similar to late responses described here in squirrel monkeys, has been recorded, especially in unanesthetized preparations (14, 46). Brooks and Enger (14) have referred to this as a “thalamic wave,” citing as evidence for the thalamic origin the study of Dempsey and Morison (18). Gardner and Gartside (21, 22) evoked a similar late component in the microelectrode field potential, concomitant with a late burst of unit firing, by stimulating cortex of urethane-anesthetized rats. This late response, like ours (12), was augmented by pentylenetetrazol and by another convulsant,

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bicuculline, which, like picrotoxin, blocks inhibition mediated by gammaaminobutyric acid. Because they were able to record in the thalamic ventrobasal complex early and late responses simultaneously with cortical responses, they, too, concluded that the late component of the direct cortical response resulted from activity evoked in thalamus. Late contralateral responses in the squirrel monkey described here bear some resemblance to the late interhemispheric corticocortical evoked response described in cats by Rutledge and Kennedy (40, 41) and in sheep by Meyerson (32), and termed the interhemispheric delayed response. Both are corticocortical responses and follow the generation of an earlier transcallosal response, and both depend on the mesencephalon (Rutledge and Kennedy believe that the interhemispheric delayed response was relayed from cortex to mesencephalon and back to cortex). However, responses in monkeys are of longer latency, both when measured from the stimulus and when measured from the early response; they are depressed or abolished by a wide range of doses of chloralose [ 10, 20, or 50 mg/kg (12)], the anesthetic agent used by Rutledge and Kennedy and by Meyerson ; and they depend on generation of the early response at the same site (that is, the contralateral early and late responses in squirrel monkeys were abolished by cutting the corpus callosum, a procedure which, in cats and sheep, selectively abolished early responses but left the late interhemispheric delayed response). Late responses in squirrel monkeys bear some resemblance to the “repetitive response” recorded by Morison and Dempsey (33) in the cortex of cats. That repetitive response was similar to the late response in squirrel monkeys in form and latency, and in the fact that both can be reset in their timing by a second stimulus. However, the “repetitive response” was observed best under light pentobarbital anesthesia, and a wide range (2 to 20 mg/kg) of doses of pentobarbital depresses to the point of abolition the late response in squirrel monkey (10, 12). The results described here can be explained most simply if some of a population of neurons in postarcuate cortex fired during the early surface long-duration inhibitory response, and this firing set up a widespread, process (perhaps by recurrent collateral postsynaptic inhibitory mechanisms) from which the same or other neurons in the population rebounded and fired again. Later repetitive waves which were sometimes observed would then be associated with repetitions of this sequence. Surface early, late, and repetitive responses would then represent either summed postsynaptic potentials in underlying neurons [the usual explanation for cortical surface potentials (17, 34, 37) ] or an envelope of synchronously generated action potentials such as is associated with synchronous firing of thalamocortical aff erents (34).

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The model described above would explain the loss of late and early contralateral responses when the corpus callosum was cut, and would explain the fact that a late response was evoked only in areas of cortex in which a given stimulus also evoked an early response. The resetting of the late response, which was observed only when the second stimulus was appropriately timed to evoke an early response of its own, would be due to prolongation of inhibition by the early response of other cells in the population to the second stimulus. Although there is no direct evidence that the silent period between the early and late responses described here is due to postsynaptic inhibition, two lines of evidence lend support to this interpretation. First, positive field potentials, such as those recorded in this study, have been recorded in the ventrobasal complex of thalamus (3) in relation to inhibitory postsynaptic potentials in surrounding neurons. Second, activation of a variety of cortical inputs (27, 29, 36, 45), including activation of corticocortical and callosal pathways and activation of cortical elements by direct cortical stimulation (23, 26, 28, 38), has been reported to produce long-duration silent periods in pyramidal tract and nonpyramidal tract cortical neurons. Intracellular recording has shown that many of these silent periods are associated with hyperpolarizing potentials and postsynaptic inhibition [but see (13) 1. The data indicate that bilateral lesions in or near mesencephalic reticular formation, or transection of diencephalon, selectively reduce or abolish late responses without appreciably affecting early responses generated at the same cortical recording sites. The role of mesencephalon in generation of late responses could be to provide tonic input to bring to threshold, or add excitatory bias to, cortical neurons during the postinhibitory rebound period. Although postinhibitory rebound (also called postanodal exaltation) has been postulated by Anderson and Sears (4) to account for synchronous repetitive firing of neurons in the thalamic ventrobasal complex, and is generally accepted as a synchronizing mechanism, Maekawa and Purpura (30) have questioned the adequacy of this mechanism alone for bringing neurons to threshold repetitively [see also Discussion in (2) 1. Thus, addition of excitatory bias from the mesencephalic reticular formation could be necessary. Evidence consistent with the interpretation that postinhibitory rebound alone is inadequate for repetitive firing of cortical neurons is provided by the observation of KrnjeviC et al. (26) and Creutzfeldt et al. (17) that postinhibitory rebound of single neurons is not observed in isolated cortical slabs. Those investigators, and Renaud and Kelly (38) have suggested that input, perhaps from distant cortical sites or from thalamus (as suggested for late components of the direct cortical response) is necessary. Our data suggest, rather, that it is input from the mesencephalic reticular formation, probably feeding through thalamus,

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