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Reticular modulation of higher auditory centers in monkey
EXPERIMENTAL
NEUROLOGY
Reticular
18,
161-176
Modulation
DAVID
Department
SYMMES
(1967)
of Higher in Monkey AND
KENNETH
V.
of Physiology, Yale University New Haven, Connecticut Received
February
Auditory
Centers
ANDERSONI
School 06510
of Medicine,
3, 1967
Evoked responses to clicks and shocks at several levels of the auditory system were averaged during control periods and during periods of intermittent electrical stimulation of the mesencephalic reticular formation in en&hale isolt monkeys. The following features of reticular modulation of higher auditory structures were observed: reduced amplitude of short latency cortical responses; augmented amplitude and shorter latency of secondary or late cortical discharges; facilitated thalamic transmission, probably accounting for shortened cortical recovery cycles for paired clicks; and lack of effect of reticular activation on relay function of the inferior colliculus. Introduction
The problem of central regulation of transmission in the afferent systems remains a complex and baffling one despite a considerable number of recent investigations and resulting hypotheses, the latter generally untested. The most frequently used indicator of transmitted information, and hence dependent variable in studies on central regulation, is the slow wave or massed evoked potential. Despite the fact that such potentials are in fact some sort of crude average themselves, marked variability in the amplitude of various components is seen in both chronic and acute animal preparations with recordings throughout the sensory system of interest. Separate analysis of the factors producing this variability is under way (1, 6, 12, 16, 2.1, 23), utilizing in general three refinements upon earlier techniques: namely, response averaging, to improve waveform definition at the cost of elimination of some part of response variability; careful control of the independent variables suspected of bearing causal relationships to response amplitude; and placement of electrodes at multiple levels within the sensory system in order to detect independent modulating influences upon transmission. The major modulating influence upon evoked potentials appears to be the level of activity, whether spontaneous or induced, in the reticular formation 1 This research has been supported by U.S.P.H.S. We are grateful to R. G. Galambos for many helpful address is Department of Anatomy, Emory University, 161
grants NB-02681 and MH-07136. suggestions. Dr. Anderson’s present. Georgia 30304.
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(2). The present studies were undertaken to clarify several questions with respect to the effects of increased reticular activity on transmission in the auditory system, about which a considerable number of conflicting reports have appeared. Among these questions are: the direction of changes,if any, in the several components of the auditory evoked potential recorded at the cortex; the contribution to these effects of influences exerted at the thalamic and tectal levels; and the concurrent changesin recovery cycles to paired clicks and central shocks. We have chosen to study these questions in acute primate preparations, both becauseof relevance to behavioral studies from our laboratory and intrinsic advantages of this approach which will be mentioned where appropriate below. Methods Surgical Preparations. The most satisfactory preparation for these experiments was the acute encdphale isold monkey (M. mulatta) prepared under ether anesthesia.Twenty-six monkeys were prepared in this manner, seven under thiopental sodium anesthesiainstead of ether, and two under ether replaced by Flaxedil rather than spinal section. The Penthothal-prepared animals gave results comparable to the etherized ones despite a longer delay before good behavioral and EEG arousal could be obtained (2-3 hr as opposed to 0.5-l hr for ether) after the anesthetic was discontinued. After infiltration with 2% Xylocaine of infraorbital, ear canal, and upper incisor areas the monkey was placed in a Kopf heavy duty stereotaxic instrument. Spinal transection was performed just rostra1 to Cl rootlets with a blunt spatula and artificial respiration begun. Craniotomy was immediately performed, a section of dura extending over approximately Q of the hemisphere surface removed, and a warm mineral oil bath set up. In three preparations the parietal operculum was aspirated exposing the auditory cortex prior to application of oil. Anesthesia was continued until all surgical procedures were completed. We believe that there are several unique advantages to a carefully prepared and maintained enckphale isolt monkey, in contrast to an earlier report (20). The greatest of these is the clear and reliable behavioral arousability of the animal which permits better control over the level of arousal than EEG and pupil size alone (although these are useful). In the aroused state such preparations exhibit vigorous facial responsesand visual following similar to the behavior of a caged monkey. When properly maintained with local anesthetic at pressure points and protected from extraneous stimuli, periods of drowsiness occur, marked by absence of spontaneous facial movements, partly or completely closed eyes, and a more synchronized EEG. Such behavioral signswere useful not only in detecting fluctuations among relatively high levels of arousal but in monitoring the general physiological state of the preparation. We found that elevated thresholds for arousal (to pin
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prick or electrical stimulation of the brain stem) and loss of healthy color from the exposed cortex were sensitive indicators of deterioration and warned of falling blood pressure. The following procedures were useful in extending the working life of these preparations (maximum 14 hr, average 8 hr): maintenance of body and cortical temperature between 35 and 39C via fluid-filled heating elements; irregular augmented inspiration to prevent ate&a&; saline injections at about 100 ml every 3 hr. Recording Evoked Potentials. Electrodes were placed in various structures stereotaxically and oriented for best responses. The electrodes were of concentric bipolar design with 32gauge enamel-insulated wire (tip) inside Teflon-insulated 26-gauge stainless steel tubing (barrel). Approximately 0.5 mm of tip and exposed barrel section were separated vertically by 0.5-l mm. In all but three cases electrodes were driven through parietal operculum to auditory cortex and adjusted for best amplitude and latency of evoked response, either to click or to shocks in lower auditory structures. With these arrangements the location of the tip with respect to cortical anatomy is unknown, but in the experiments with direct visualization of cortex the best records were obtained with the barrel at or near the cortical surface and the tip in lower cortical layers. Responses thus obtained were characterized by an initial barrel-positive deflection. The evoked responses obtained from electrodes placed through intact parietal operculum resembled these and the convention was adopted of displaying all evoked responses with the initiaI deflection upward. Records from subcortical structures were generally led off utilizing the bipolar configuration, although clarification of field effects was often facilitated by recording against a remote indifferent electrode. Evoked responses were led off to Tektronix FM 122 preamplifiers (bandwidth 8-10 000 cycle/s&), monitored on a Tektronix 561 oscilloscope, passed through an additional stage of amplification, and fed to a Mnemotron CAT 400B waveform totaller equipped with SOO-kc/set carrier frequency. The accumulated average responsewaveforms were written out on a Moseley 7590 plotter and the entire system calibrated for amplitude of the original signal. Acoustic Stimulation. Click stimuli were generated by driving a single Audivox 9C hearing aid receiver with O.l-msec rectangular pulses and presented to the animal through a short coupler and hollow ear bar. An amplitude level was selected below the maximum power output of the transducer and defined as zero attenuation. This level was approximately 70 db above the threshold for minimal detectable evoked responses.Attenuation below the zero level was obtained by appropriate reduction of the driving pulses, and was employed to estimate the magnitude of changes in evoked potential amplitude after the method of Desmedt (5). All responsesto clicks reported herein were obtained from auditory structures contralateral to the stimulated ear. Electrical Stimulation. Rectangular unidirectional pulses were generated
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by a specially designed four-channel stimulator and delivered to the preparation via ELS constant-current isolation units. Trains of pulses 700 msec in duration consisting of pulses 0.25 msec in width at 200 pulse/set were delivered to the mesencephalic reticular formation (MRF) at a repetition rate of 1 per 1.5-2 sec. Current flowing between tip and barrel of the stimulating electrode did not exceed a peak value of 0.5 mamp. In the interval between trains, and following the end of each train by 200 to 500 msec, test stimuli consisting of single or paired clicks or shocks were delivered to the auditory system. Twenty-five to one hundred such cycles were repeated in a series, occupying in some cases more than 3 min. These procedures have the advantages of reducing the likelihood of adaptation of MRF to continuous stimulation over long time periods, and eliminating the interference of MRF shock artifacts and MRF evoked responses with responses evoked by test stimuli. Control series were obtained just prior to every MRF series by presenting the identical stimuli without MRF stimulation. At least 5 min (and frequently more) were allowed to pass between the end of one MRF series and the next control series and extraneous sensory stimuli were carefully excluded. Direct electrical stimulation of subcortical auditory structures was carried out with the same bipolar concentric electrodes in many cases which had been placed on the basis of recording evoked potentials of maximum amplitude. The stimulating pulses were unidirectional single or paired shocks limited to O.l-mamp current between tip and barrel. As a matter of procedure the strength of shock to a subcortical structure was adjusted to give a response higher in the system on the same order of magnitude as that evoked by click. This procedure held the unphysiological aspects of direct stimulation of the afferent system to a minimum. At the conclusion of each experiment the animals were anesthetized with barbiturate and a number of small d-c lesions (0.25 mamp for 10 set) placed to assist in locating exact sites from which records were taken. After perfusion and fixation with 10% formalin the brains were removed, sectioned on a freezing microtome at 25 11 and stained generally with the stain of Kluver and Barrera. Results
General Findings. From a total of thirty-five monkeys prepared for these experiments twenty-eight met our criteria for maintained excellent physiological condition and proved to have accurately placed electrodes. The classes of data secured from these is indicated in Table 1. In general, behavioral arousal and effects on evoked auditory activity were obtained together from MRF electrodes, and could be obtained with electrodes on either side of the midline. No systematic attempt was made to explore the brain stem for sites of maximum effect, although all sites giving rise to reliable arousal with
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TABLE SUMMARY
OF EXPERIMENTS
Responses evoked by clicks in cortex in RAD in MGB in IC Responses evoked by shocks IC to MGB IC to RAD IC to cortex MGB to RAD MGB to cortex RAD to cortex
ON
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1 MODULATION
OF HIGHER
AUDITORY
CENTERS
No. of expts. 23
5 9 11
minimal motor components (eye movements, grimacing) were close to that shown in Fig. 1. Typical recording and stimulating locations within the auditory system are also shown in Fig. 1. In connection with the averaged responsedata to be presented, many changesin evoked potentials were clearly visible on the monitor oscilloscopeto the unaided eye, and on someoccasions effects could be seenwhich dissipated after the first few evoked responsesin the seriestotalled by the computer. The final average in such casesmight show little or no change from control. Such short-lived effects of MRF stimulation may well be significant but were excluded from consideration in the present study. Inferior CoZZ~cuZus (ZC) . Very little effect of MRF stimulation was seenon click-evoked IC responses.In nine of eleven preparations, IC responseswere not significantly altered (Fig. ZA). In two preparations moderate reductions in amplitude of both first and second evoked responses(not exceeding 10 equivalent db) were seen. The latter may have been the result of spread of stimulating current to regions having efferent connections with the olivocochlear system (5). The lack of MRF modulation of IC activity in monkey was also evident in comparison of cortical responsesevoked by IC shock with cortical responsesevoked by MGB shock (Fig. 2B, C) . The augmentation of response obtained with IC initiated activity is approximately the sameon a per cent of control basis as the augmentation obtained with MGB evoked activity, suggesting that the facilitation in MGB accounts for both effects. The recovery cycle of responsesin IC to paired clicks was short and uncomplicated (Fig. 6). The amplitude of the secondevoked responsereached that of the first responsewith delays as short as 30 msec in most cases. Medial Geniculate Body (MGB). The results of our studies of click-
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evoked responses in the MGB are somewhat difficult to interpret because of the problem of response deterioration. Of seventeen verified placements in MGB (in pars principalis or at the border with pars magnocellularis) fourteen yielded click-evoked responses which were relatively small (50 yv or
FIG. 1. Electrode placements typical of may be seen, in some cases coinciding with the arrow. Upper left = IC; right = MGB;
those used in study. Sma!l marker lesions recording or stimulating sites as marked by lower left = RAD; right z MRF.
less) and decreased in amplitude by 50% or more within 30 min of placing the electrode. For this reason satisfactory average response data to clicks could be obtained in only nine preparations, and within that series some variability of MRF ‘effect was found. The largest and most frequently observed effect was augmentation of both responses to paired clicks (Fig. 2D), which was seen in four preparations. In two preparations response augmenta-
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tion was confined to the second response and was relatively slight, and in three other preparations responses were not affected by MRF stimulation (Fig. 2E). While the mechanism responsible for the above-mentioned spontaneous decay of response amplitude is not understood, our data suggest that it is a local process near the recording electrode. We observed that cortical responses
FIG. 2. Averaged evoked responses with concurrent MRF stimulation at right of each pair and control records on left. A: responses from IC evoked by click pairs, showing little effect. B: cortical response evoked by shocks to MGB, which show approximately equal augmentation as do cortical responses to IC shocks in C. D, E: responses from MGB evoked by click pairs. F: responses in RAD evoked by shocks to MGB. G: responses in MGB evoked by shocks to IC. Vertical bars = 25 pv. Time scale given by stimulus artifacts, which are separated by 20 msec in A, B, C, F, and by 30 msec in D, E, G. In these and all succeeding figures 0.6 msec should be deducted from latency measurements to correct for sound transmission in the stimulating system.
to click could remain at control levels during deterioration of the MGB response, suggesting grossly normal relay function of the MGB. It was extremely difficult, however, to obtain any improvement in the MGB response by relocating the electrode and efforts to improve the response often had the opposite effect. Evoked responses from the other (ipsilateral) MGB could be recorded with normal amplitude, however, providing further evidence that the deterioration was not general. If the negative results of MRF stimulation on MGB responses to clicks may be regarded as contaminated by
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a deteriorating condition of the MGB, the evidence as a whole leads to the conclusion that the principal MRF effect in the thalamic relay is one of facilitation. No examples of evoked response decrease were observed in the nine preparations studied. The largest effects were seen on the most stable responses, and they were in the direction of augmentation, especially the second response of a pair. No consistent data on recovery cycles of clickevoked MGB activity could be obtained due to the difficulty of maintaining adequate responses over the time needed to test many click separations. The general conclusion that MRF activation facilitates transmission through the thalamus is supported by the results of studies on centrallyevoked responses. Responses recorded in the auditory radiations (RAD) from shocks to the MGB always showed large and consistent increases (Fig. 2F), with somewhat less consistent results when responses were recorded at the cortex. The latter recordings were characterized by either marked increase of both responses with more pronounced effects on the second (four of seven preparations, illustrated in Figs. 2C & 3D) or increase of the second response only (three preparations, illustrated in Fig. 3C). Responses in MGB followBAD--ctx
MOR--ctx
FIG. 3. Effects of MRF stimulation on averaged cortical responses evoked by paired shocks to auditory pathway. Same display as in Fig. 2. Responses initiated in auditory radiations (RAD) are reduced while those initiated in MGB are augmented in the same preparations. Vertical bars = 50 pv. All stimulus artifacts separated by 20 msec.
ing IC stimulation were always powerfully facilitated (Fig. 2G), and the effect appears due more to thalamic (postsynaptic) than collicular (presynaptic) influences, as discussed above. Auditary Cortex. The effects of reticular activation on cortical responses
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to clicks are different for the several components of the evoked response complex. The most prominent feature of MRF modulation (seen in nineteen of twenty-three preparations) is reduction in amplitude of the initial surface positive deflection. Records typical of those we obtained are shown in Fig. 4, in which the components as described by Teas and Kiang may be identified (23). In Fig. 4A, the evoked response to a single click in a drowsy but readily arousable monkey is shown, and the early positive peak (ER,) with a latency of 12.5 msec is reduced to approximately 75% of control amplitude by concurrent MRF stimulation. The reduction in amplitude in this case is
FIG. 4. Effects of MRF stimulation on averaged cortical responses evoked by clicks. Same display as in Fig. 2. A: single click. B: paired clicks, 25msec separation, no second response in control. C: paired clicks, 2O-msec separation. D: paired clicks, 40-msec separation. E: paired clicks, 84Lmsec separation. F: augmentation of ER, in second response of C, clarified by subtraction of average response to single click. Vertical bars = 50 pv. Time scale the same in all records.
equivalent to about 25db input attenuation. In other experiments reductions up to 35 equivalent db were observed. These estimates of the magnitude of response reduction were made with the aid of data such as that presented in Fig. 6 (left), based on averaged responses to clicks of varying intensities. We found it practicable to work over the range of only the first 40 db of attenuation, and this fact coupled with the somewhat lower zero attenuation level in our studies (referred to evoked response threshold) reconciles the curve in Fig. 6 reasonably well with that obtained by Teas and Kiang for the chronic cat (23). The MRF effects on the later components are also evident. At the same time that the amplitude of ER1 was reduced (Fig. 4A), the latency of a
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second positive peak (ER,) decreased and the amplitude increased to approximately 1.50% of control. A third deflection (ER.?), surface negative in sign and more diffuse, was also augmented by MRF stimulation. In many records the late components are hard to identify with certainty as to peak amplitude and latency, but in general the changes in them were not as great as for the ERr component. The succeeding records in Fig. 4 reveal another aspect of reticular modulation of cortical evoked potentials, namely, the simultaneous augmentation of the ERr component in evoked responses to the second of two closely spaced clicks. This reticular-induced shortening of recovery cycles was seen only for click separations less than about 80 msec. We found recovery cycles at the longer intervals to be very unstable, but were able to verify that no reticular facilitation of the second primary responses could be obtained. Cortical responses obtained from shocks to the auditory radiations were reduced to a degree comparable or greater than that seen when click stimuli were used (Fig. 3). No example of augmentation of the ER1 component was observed to single or paired clicks at any separation. It was consistently possible in our experiments to obtain opposite effects of MRF activation on responses recorded in the cortex from RAD stimulation and from MGB stimulation in the same preparations (Fig. 3) and these observations lead to the conclusion that shock delivered within the MGB initiates substantial intrathalamic (presynaptic) neuronal activity which is under the influence of the reticular activating system. Changes in evoked potentials induced by MRF stimulation as shown in Fig. 4 could also be observed during arousal of the animal by external means, a preferred method if not for the difficulty of sustaining a reasonably constant level of arousal to a single stimulus. Heightened arousal was obtained in one case by bringing a glove close to the monkey’s face during the averaging period. The effects on the evoked potential were identical to those obtained with electrical stimulation (Fig. SA) . Study of animals anesthetized for surgery with thiopental sodium permitted us to reach the following conclusions about the effects of light barbiturate anesthesia: (i) the late components only of the evoked response are diminished (Fig. SB); (ii) effects of MRF stimulation on evoked potentials are abolished in close correlation with disappearance of signs of behavioral arousal; and (iii) recovery cycles to paired clicks are lengthened (Figs. 5B & 6). These findings have previously been reported from several laboratories. Finally, the possibility that increased blood ilow through auditory structures, particularly cortex, could be a contributing factor to the MRF effects was investigated. Invar and Siiderburg (14) demonstrated in this connection that cortical blood flow could increase in some forms of arousal without
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FIG. 5. A: effect of arousal from threatening stimuli on cortical evoked response to paired clicks separated by 20 msec. Control record on left. B: effect of light barbiturate anesthesia on cortical response to click and on recovery cycle. Left record 2 hours following 20 mg/kg thiopental sodium, animal unarousable. Right record 25 hours later, 40 msec. C: marked elevation of systemic animal drowsy awake. Click separation, blood pressure has little effect on evoked responses. Left record control, right record averaged during period indicated. Vertical bars = SO uv.
FIG. 6. Left, relative amplitude of early positive click attenuation. Mean of four preparations. Right, in second evoked response as a function of click preparations. For cortex, single preparation under
deflection (ER,) as a function of relative amplitude of same deflection separation. For IC, mean of four two conditions as in Fig. 5B.
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systemic blood pressure elevation. Figure 5C illustrates the results of an experiment in which femoral artery blood pressure was greatly elevated by intravenous injection of epinephrine 1: 10 000. During the pressor response evoked potentials were averaged and compared with controls obtained just prior to the injection. No significant effects were observed, suggesting indirectly that elevated cortical blood flow does not in itself produce changes in evoked responses. Discussion
The main findings presented here support the proposition that the fast components of sensory discharges in the auditory system of the monkey are little changed by reticular activation at the level of the IC, greatly facilitated at the level of the MGB, and moderately inhibited or reduced in amplitude at the level of the sensory cortex. The amplitude of the primary cortical response to a given click, then, should be viewed as the result of a balance of opposed modulating influences located in the cortex and the thalamic relay. Longer latency components of click-evoked responses are prominent in records from the auditory cortex, but seem in the best maintained enctphale isole’preparations to be influenced somewhatlessby reticular activation than are the primary components. The effect of reticular activation on late components (both ER? surface-positive and ER3 surface-negative) is clearly to augment them, and in our observations can become the most conspicuousconsequenceof MRF stimulation in preparations in which the cortex is in less than optimal condition (revealed by a blanched appearance and deterioration of the amplitude of ER1). We have thus found circumstances in which the effects of reticular activation are primarily on the late components of the evoked response,in agreement with others (10, 13, 23). Support for the proposed relative independenceof IC relay function from arousal level is available in the form of incidental observations in two reports on long-term changesin IC-evoked responses(7, 1l), and in a recent study in which such independence could be obtained after destruction of middle ear muscles(21). That IC has more than simple relay function is, of course, established and the possibility exists that a substantial portion of the ascending auditory input in the lateral lemniscusbypassesthe IC, or at least synapsesin very deep layers (see 15, for discussion). Our observations are that sites scattered throughout all areas of the primate IC yield clickevoked responses,and initiate rostrally directed volleys when stimulated directly, which are relatively unaffected by reticular activation. The evidence that reticular activation facilitates thalamic transmission while simultaneously inhibiting fast intracortical responsesto afferent volleys is in complete agreement with recent work on chronic cats undergoing span-
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taneous fluctuations in arousal level (3, 4, 9). We observed no sign of reticular facilitation of cortical primary responses initiated by shocks in the thalamic radiations, in agreement with these workers and others (8). Cortical responses to click pairs did, however, reveal the shortened recovery cycle during reticular activation frequently reported (1, 19, 22). In view of the absence of this effect in tests utilizing shock pairs to the auditory radiations, and the very potent facilitation found in the MGB, especially of the second of two evoked responses, we conclude that the shortened cortical recovery cycle in the auditory cortex is a passive effect dependent on altered input. Facilitation on the order of 200% increase of amplitude has been observed for responses evoked in IC and recorded in the radiations or in the MGB, and for responses evoked in the MGB and recorded in the radiations. Less potent effects were observed in cortical records to MGB and IC shocks, (five of twelve preparations demonstrated facilitation confined to the second response) as might be expected if an opposing inhibitory process were activated in the cortex. The exact duration of the critical interval through which thalamic facilitation can override cortical inhibition remains uncertain despite extensive investigation in the present experiments. In rare cases thalamic facilitation appears to be continuously dominant, since both evoked responses are augmented by reticular activation. The much more common finding, however, is that augmentation of the fast components is only seen in the response to the second of two clicks and then only when the click separation is less than the critical interval. Our data suggest that the critical interval lies between 50 and 80 msec, a figure considerably smaller than that reported by others using, for the most part, periodic stimuli. The interval may be related to the first peak of recovered excitability in the plot of relative amplitude of second response against click separation. All workers seem to agree that a peak of excitability is reached (possibly slightly supernormal) between 50 and 80 msec following click presentation (18, 19). Our observations are that at click separations longer than this cortical inhibition is normally dominant and equivalent amplitude reduction of both responses follows reticular activation. It is possible, however, that certain intervals coinciding with succeeding peaks of excitability may produce augmented second responses. In our experiments the shape of the recovery function beyond click separations of about 100 msec was so variable within and between animals that selection of one particular set of oscillations in excitability would be arbitrary. A question is raised by the above argument as to whether so-called cortical recovery cycles to peripheral stimuli are not in fact largely thalamic recovery cycles. Careful study of radiation initiated cortical recovery cycles clearly reveals a different sort of function (8). Evidence to clarify this point is un-
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fortunately not available in the literature and could not be obtained in the present study, primarily due to the difficulty we had maintaining stable clickevoked responses in the thalamus. It is important as the technology of response averaging advances to obtain a consistency of nomenclature and consequent precision of description of the components of auditory evoked responses. The cortical evoked responses we have recorded resemble in all respects those recorded by Teas and Kiang (23), and we find their carefully documented report of the correlation between components of the evoked response complex and organic variables generally applicable. When records are published, for example, presenting primary click-evoked responses having latencies to the first peak of 45 msec (Fig. 3 in 22), we have difficulty in reconciling the findings with our own. We have not obtained evidence that reticular activation differentially affects various subregions within the auditory cortex, although the methods used precluded a systematic study. The largest effects were obtained on potentials having the short latency characteristics described by Pribram, Rosner and Rosenblith (17), but the difference between the effects on these and on longer latency responses (14-17 msec to first peak) was slight and not qualitative. Considerable attention has been given in the literature to the apparently opposite effects of reticular activation on cortical evoked responses initiated peripherally by physiological stimulation of receptors and on those responses evoked centrally by shocks to the afferent pathway (1). The former are reported as reduced (early components, single flash or click) and the latter as augmented. Although the problem is somewhat clarified by locating the facilitation at the thalamic level and the inhibition at the cortical level, the basic phenomenon remains unexplained. In our observations this finding was encountered repeatedly. It appears necessary to include in an account of reticular modulation of single cortical evoked potentials, then, the observation that facilitation is most prominent when, and in fact is rarely seen unless, highly synchronized afferent volleys are present. In the case of evoked potentials recorded in the thalamus, however, facilitation is revealed more readily since there is no opposing inhibitory process. Reticular activation at this level facilitates both peripherally and centrally originating volleys, the latter more strikingly. References 1.
2.
3.
factors influencing the evoked potentials of the BREMER, F. 1961. Neurogenic cerebral cortex, pp. 259-278. In “Sensory Communication.” W. A. Rosenblith red.]. Wiley, New York. BREMER, F., and V. BONNET. 1950. Interprbtation des r&&ions rhythmiques prolong&e dans aires sensorielles de l’tcorce c&brale. ElectroencephoEog. Clin. Neurophysiol. 2: 329-400. DAGNINO, N., E. FAVALE, C. LOEB, and M. MANFREDI. 1965. Responses evok&
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8. 9.
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Il. 12.
13.
14. 15. 16.
17.
18.
19.
20. 21.
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by stimulation of the acoustic pathway during the sleep-wakefulness cycle. Experentiu 21: 459-460. DAGNINO, N., E. FAVALE, C. LOEB, and M. MANFREDI. 1965. Sensory transmission in the geniculostriate system of the cat during natural sleep and arousal. J. Neurophysiol. 28: 443-456. DESMEDT, J. E. 1962. Auditory-evoked potentials from cochlea to cortex as influenced by activation of the efferent olivo-cochlear bundle. J. Acoust. Sot. Am. 34: 1478-1496. DESMEDT, J. E., and G. LA GRUTTA. 1957. The effect of selective inhibition of pseudocholinesterase on the spontaneous and evoked activity of the cat’s cerebral cortex. J. Physiol. London 196: 20-40. DUN~P, C. W., R. W. WEBSTER, and R. H. DAY. 1964. Amplitude changes of evoked potentials at the inferior colliculus during acoustic habituation. J. Auditory Res. 4: 159-169. EVARTS, E. V., T. C. FLEMING, and P. R. HUTTENLOCHER. 1960. Recovery cycle of visual cortex of the awake and sleeping cat. Am. J. Physiol. 199: 373-376. FAVALE, E., C. LOEB, and M. MANFREDI. 1963. Le risposte corticali somatiche durante il sonno profondo: particulare comportamento della risposte radiatocorticali. Boll. Sot. Ital. Biol. Sper. 39: 430-432. FUSTER, J. M., and R. F. DOCTER. 1962. Variations of optic evoked potentials as a function of reticular activity in rabbits with chronically implanted electrodes. J. Neurophysiol. 25: 324-336. GALLIN, D. 1965. Background and evoked activity in the auditory pathway: effects of noise-shock pairing. Science 149: 761-763. Physiological and psychological aspects of selective perception, HORN, G. 1965. pp. 155-216. In “Advances in the Study of Behavior.” D. S. Lehrman, R. A. Hinde, and E. Shaw [eds.]. Academic Press, New York. HUTTENLOCHER, P. R. 1960. Effects of state of arousal on click responses in the mesencephalic reticular formation. EZectroencepha!og. Clin. Neurophysiol. 12: 819-827. 1958. Cortical blood flow related to EEG INGVAR, D. H., and V. S~DERBERC. patterns evoked by stimulation of the brain stem. Acta Physiol. Stand. 42: 130-143. JANE, J. A., R. B. MASTERSON, and I. T. DIAMOND. 1965. The function of the tectum for attention to auditory stimuli in the cat. J. Camp. Neural. 126: 165-192. JANE, J. A., G. D. SMIRNOV, and H. H. JASPER. 1962. Effects of distraction upon simultaneous auditory and visual evoked potentials. Electroencephalog. Clin. Neurophysiol. 14: 344-358. PRIBRAM, K. H., B. S. ROSNER, and W. A. ROSENBLITH. 1954. Electrical responses to acoustic clicks in monkey: extent of neocortex activated. J. Neurophysiol. 17: 336-344. ROSENZWEIG, M. R., and W. A. ROSENBLITH. 1953. Responses to successive auditory stimuli at the cochlea and at the auditory cortex. Psychol. Monogr. 67: No. 13: l-26. Effect of different states of a!ertness on SCHWARTZ, M., and C. SHAGASS. 1962. somatosensory and auditory recovery cycles. Electroencephalog. Clin. Neurophysiol.
14: 11-20. SEGUNDO, J. P., on electrocortical STARR, A. 1964. auditory pathway
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NAQUET, and P. BUSER. 1955. Effects of cortical stimulation activity in monkeys. J. Neurosurg. 18: 236-245. Influence of motor activity on click-evoked responses in the of waking cats. Ezgtl. Neztrul. 10: 191-204.
176 22.
23.
SYMMES M., and M. DEMETRESCU. acoustic stimuli. Electroencephalog. TEAS, D. C., and N. Y-s. KIANG. cortex. Exptl. Neural. 10: 91-119.
STERIADE,
AND
ANDERSON
1962. Reticular facilitation of responses to Clin. Neurophysiol. 14: 21-36. 1964. Evoked responses from the auditory
NEUROLOGY
Reticular
18,
161-176
Modulation
DAVID
Department
SYMMES
(1967)
of Higher in Monkey AND
KENNETH
V.
of Physiology, Yale University New Haven, Connecticut Received
February
Auditory
Centers
ANDERSONI
School 06510
of Medicine,
3, 1967
Evoked responses to clicks and shocks at several levels of the auditory system were averaged during control periods and during periods of intermittent electrical stimulation of the mesencephalic reticular formation in en&hale isolt monkeys. The following features of reticular modulation of higher auditory structures were observed: reduced amplitude of short latency cortical responses; augmented amplitude and shorter latency of secondary or late cortical discharges; facilitated thalamic transmission, probably accounting for shortened cortical recovery cycles for paired clicks; and lack of effect of reticular activation on relay function of the inferior colliculus. Introduction
The problem of central regulation of transmission in the afferent systems remains a complex and baffling one despite a considerable number of recent investigations and resulting hypotheses, the latter generally untested. The most frequently used indicator of transmitted information, and hence dependent variable in studies on central regulation, is the slow wave or massed evoked potential. Despite the fact that such potentials are in fact some sort of crude average themselves, marked variability in the amplitude of various components is seen in both chronic and acute animal preparations with recordings throughout the sensory system of interest. Separate analysis of the factors producing this variability is under way (1, 6, 12, 16, 2.1, 23), utilizing in general three refinements upon earlier techniques: namely, response averaging, to improve waveform definition at the cost of elimination of some part of response variability; careful control of the independent variables suspected of bearing causal relationships to response amplitude; and placement of electrodes at multiple levels within the sensory system in order to detect independent modulating influences upon transmission. The major modulating influence upon evoked potentials appears to be the level of activity, whether spontaneous or induced, in the reticular formation 1 This research has been supported by U.S.P.H.S. We are grateful to R. G. Galambos for many helpful address is Department of Anatomy, Emory University, 161
grants NB-02681 and MH-07136. suggestions. Dr. Anderson’s present. Georgia 30304.
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(2). The present studies were undertaken to clarify several questions with respect to the effects of increased reticular activity on transmission in the auditory system, about which a considerable number of conflicting reports have appeared. Among these questions are: the direction of changes,if any, in the several components of the auditory evoked potential recorded at the cortex; the contribution to these effects of influences exerted at the thalamic and tectal levels; and the concurrent changesin recovery cycles to paired clicks and central shocks. We have chosen to study these questions in acute primate preparations, both becauseof relevance to behavioral studies from our laboratory and intrinsic advantages of this approach which will be mentioned where appropriate below. Methods Surgical Preparations. The most satisfactory preparation for these experiments was the acute encdphale isold monkey (M. mulatta) prepared under ether anesthesia.Twenty-six monkeys were prepared in this manner, seven under thiopental sodium anesthesiainstead of ether, and two under ether replaced by Flaxedil rather than spinal section. The Penthothal-prepared animals gave results comparable to the etherized ones despite a longer delay before good behavioral and EEG arousal could be obtained (2-3 hr as opposed to 0.5-l hr for ether) after the anesthetic was discontinued. After infiltration with 2% Xylocaine of infraorbital, ear canal, and upper incisor areas the monkey was placed in a Kopf heavy duty stereotaxic instrument. Spinal transection was performed just rostra1 to Cl rootlets with a blunt spatula and artificial respiration begun. Craniotomy was immediately performed, a section of dura extending over approximately Q of the hemisphere surface removed, and a warm mineral oil bath set up. In three preparations the parietal operculum was aspirated exposing the auditory cortex prior to application of oil. Anesthesia was continued until all surgical procedures were completed. We believe that there are several unique advantages to a carefully prepared and maintained enckphale isolt monkey, in contrast to an earlier report (20). The greatest of these is the clear and reliable behavioral arousability of the animal which permits better control over the level of arousal than EEG and pupil size alone (although these are useful). In the aroused state such preparations exhibit vigorous facial responsesand visual following similar to the behavior of a caged monkey. When properly maintained with local anesthetic at pressure points and protected from extraneous stimuli, periods of drowsiness occur, marked by absence of spontaneous facial movements, partly or completely closed eyes, and a more synchronized EEG. Such behavioral signswere useful not only in detecting fluctuations among relatively high levels of arousal but in monitoring the general physiological state of the preparation. We found that elevated thresholds for arousal (to pin
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prick or electrical stimulation of the brain stem) and loss of healthy color from the exposed cortex were sensitive indicators of deterioration and warned of falling blood pressure. The following procedures were useful in extending the working life of these preparations (maximum 14 hr, average 8 hr): maintenance of body and cortical temperature between 35 and 39C via fluid-filled heating elements; irregular augmented inspiration to prevent ate&a&; saline injections at about 100 ml every 3 hr. Recording Evoked Potentials. Electrodes were placed in various structures stereotaxically and oriented for best responses. The electrodes were of concentric bipolar design with 32gauge enamel-insulated wire (tip) inside Teflon-insulated 26-gauge stainless steel tubing (barrel). Approximately 0.5 mm of tip and exposed barrel section were separated vertically by 0.5-l mm. In all but three cases electrodes were driven through parietal operculum to auditory cortex and adjusted for best amplitude and latency of evoked response, either to click or to shocks in lower auditory structures. With these arrangements the location of the tip with respect to cortical anatomy is unknown, but in the experiments with direct visualization of cortex the best records were obtained with the barrel at or near the cortical surface and the tip in lower cortical layers. Responses thus obtained were characterized by an initial barrel-positive deflection. The evoked responses obtained from electrodes placed through intact parietal operculum resembled these and the convention was adopted of displaying all evoked responses with the initiaI deflection upward. Records from subcortical structures were generally led off utilizing the bipolar configuration, although clarification of field effects was often facilitated by recording against a remote indifferent electrode. Evoked responses were led off to Tektronix FM 122 preamplifiers (bandwidth 8-10 000 cycle/s&), monitored on a Tektronix 561 oscilloscope, passed through an additional stage of amplification, and fed to a Mnemotron CAT 400B waveform totaller equipped with SOO-kc/set carrier frequency. The accumulated average responsewaveforms were written out on a Moseley 7590 plotter and the entire system calibrated for amplitude of the original signal. Acoustic Stimulation. Click stimuli were generated by driving a single Audivox 9C hearing aid receiver with O.l-msec rectangular pulses and presented to the animal through a short coupler and hollow ear bar. An amplitude level was selected below the maximum power output of the transducer and defined as zero attenuation. This level was approximately 70 db above the threshold for minimal detectable evoked responses.Attenuation below the zero level was obtained by appropriate reduction of the driving pulses, and was employed to estimate the magnitude of changes in evoked potential amplitude after the method of Desmedt (5). All responsesto clicks reported herein were obtained from auditory structures contralateral to the stimulated ear. Electrical Stimulation. Rectangular unidirectional pulses were generated
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by a specially designed four-channel stimulator and delivered to the preparation via ELS constant-current isolation units. Trains of pulses 700 msec in duration consisting of pulses 0.25 msec in width at 200 pulse/set were delivered to the mesencephalic reticular formation (MRF) at a repetition rate of 1 per 1.5-2 sec. Current flowing between tip and barrel of the stimulating electrode did not exceed a peak value of 0.5 mamp. In the interval between trains, and following the end of each train by 200 to 500 msec, test stimuli consisting of single or paired clicks or shocks were delivered to the auditory system. Twenty-five to one hundred such cycles were repeated in a series, occupying in some cases more than 3 min. These procedures have the advantages of reducing the likelihood of adaptation of MRF to continuous stimulation over long time periods, and eliminating the interference of MRF shock artifacts and MRF evoked responses with responses evoked by test stimuli. Control series were obtained just prior to every MRF series by presenting the identical stimuli without MRF stimulation. At least 5 min (and frequently more) were allowed to pass between the end of one MRF series and the next control series and extraneous sensory stimuli were carefully excluded. Direct electrical stimulation of subcortical auditory structures was carried out with the same bipolar concentric electrodes in many cases which had been placed on the basis of recording evoked potentials of maximum amplitude. The stimulating pulses were unidirectional single or paired shocks limited to O.l-mamp current between tip and barrel. As a matter of procedure the strength of shock to a subcortical structure was adjusted to give a response higher in the system on the same order of magnitude as that evoked by click. This procedure held the unphysiological aspects of direct stimulation of the afferent system to a minimum. At the conclusion of each experiment the animals were anesthetized with barbiturate and a number of small d-c lesions (0.25 mamp for 10 set) placed to assist in locating exact sites from which records were taken. After perfusion and fixation with 10% formalin the brains were removed, sectioned on a freezing microtome at 25 11 and stained generally with the stain of Kluver and Barrera. Results
General Findings. From a total of thirty-five monkeys prepared for these experiments twenty-eight met our criteria for maintained excellent physiological condition and proved to have accurately placed electrodes. The classes of data secured from these is indicated in Table 1. In general, behavioral arousal and effects on evoked auditory activity were obtained together from MRF electrodes, and could be obtained with electrodes on either side of the midline. No systematic attempt was made to explore the brain stem for sites of maximum effect, although all sites giving rise to reliable arousal with
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OF EXPERIMENTS
Responses evoked by clicks in cortex in RAD in MGB in IC Responses evoked by shocks IC to MGB IC to RAD IC to cortex MGB to RAD MGB to cortex RAD to cortex
ON
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5 9 11
minimal motor components (eye movements, grimacing) were close to that shown in Fig. 1. Typical recording and stimulating locations within the auditory system are also shown in Fig. 1. In connection with the averaged responsedata to be presented, many changesin evoked potentials were clearly visible on the monitor oscilloscopeto the unaided eye, and on someoccasions effects could be seenwhich dissipated after the first few evoked responsesin the seriestotalled by the computer. The final average in such casesmight show little or no change from control. Such short-lived effects of MRF stimulation may well be significant but were excluded from consideration in the present study. Inferior CoZZ~cuZus (ZC) . Very little effect of MRF stimulation was seenon click-evoked IC responses.In nine of eleven preparations, IC responseswere not significantly altered (Fig. ZA). In two preparations moderate reductions in amplitude of both first and second evoked responses(not exceeding 10 equivalent db) were seen. The latter may have been the result of spread of stimulating current to regions having efferent connections with the olivocochlear system (5). The lack of MRF modulation of IC activity in monkey was also evident in comparison of cortical responsesevoked by IC shock with cortical responsesevoked by MGB shock (Fig. 2B, C) . The augmentation of response obtained with IC initiated activity is approximately the sameon a per cent of control basis as the augmentation obtained with MGB evoked activity, suggesting that the facilitation in MGB accounts for both effects. The recovery cycle of responsesin IC to paired clicks was short and uncomplicated (Fig. 6). The amplitude of the secondevoked responsereached that of the first responsewith delays as short as 30 msec in most cases. Medial Geniculate Body (MGB). The results of our studies of click-
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evoked responses in the MGB are somewhat difficult to interpret because of the problem of response deterioration. Of seventeen verified placements in MGB (in pars principalis or at the border with pars magnocellularis) fourteen yielded click-evoked responses which were relatively small (50 yv or
FIG. 1. Electrode placements typical of may be seen, in some cases coinciding with the arrow. Upper left = IC; right = MGB;
those used in study. Sma!l marker lesions recording or stimulating sites as marked by lower left = RAD; right z MRF.
less) and decreased in amplitude by 50% or more within 30 min of placing the electrode. For this reason satisfactory average response data to clicks could be obtained in only nine preparations, and within that series some variability of MRF ‘effect was found. The largest and most frequently observed effect was augmentation of both responses to paired clicks (Fig. 2D), which was seen in four preparations. In two preparations response augmenta-
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tion was confined to the second response and was relatively slight, and in three other preparations responses were not affected by MRF stimulation (Fig. 2E). While the mechanism responsible for the above-mentioned spontaneous decay of response amplitude is not understood, our data suggest that it is a local process near the recording electrode. We observed that cortical responses
FIG. 2. Averaged evoked responses with concurrent MRF stimulation at right of each pair and control records on left. A: responses from IC evoked by click pairs, showing little effect. B: cortical response evoked by shocks to MGB, which show approximately equal augmentation as do cortical responses to IC shocks in C. D, E: responses from MGB evoked by click pairs. F: responses in RAD evoked by shocks to MGB. G: responses in MGB evoked by shocks to IC. Vertical bars = 25 pv. Time scale given by stimulus artifacts, which are separated by 20 msec in A, B, C, F, and by 30 msec in D, E, G. In these and all succeeding figures 0.6 msec should be deducted from latency measurements to correct for sound transmission in the stimulating system.
to click could remain at control levels during deterioration of the MGB response, suggesting grossly normal relay function of the MGB. It was extremely difficult, however, to obtain any improvement in the MGB response by relocating the electrode and efforts to improve the response often had the opposite effect. Evoked responses from the other (ipsilateral) MGB could be recorded with normal amplitude, however, providing further evidence that the deterioration was not general. If the negative results of MRF stimulation on MGB responses to clicks may be regarded as contaminated by
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a deteriorating condition of the MGB, the evidence as a whole leads to the conclusion that the principal MRF effect in the thalamic relay is one of facilitation. No examples of evoked response decrease were observed in the nine preparations studied. The largest effects were seen on the most stable responses, and they were in the direction of augmentation, especially the second response of a pair. No consistent data on recovery cycles of clickevoked MGB activity could be obtained due to the difficulty of maintaining adequate responses over the time needed to test many click separations. The general conclusion that MRF activation facilitates transmission through the thalamus is supported by the results of studies on centrallyevoked responses. Responses recorded in the auditory radiations (RAD) from shocks to the MGB always showed large and consistent increases (Fig. 2F), with somewhat less consistent results when responses were recorded at the cortex. The latter recordings were characterized by either marked increase of both responses with more pronounced effects on the second (four of seven preparations, illustrated in Figs. 2C & 3D) or increase of the second response only (three preparations, illustrated in Fig. 3C). Responses in MGB followBAD--ctx
MOR--ctx
FIG. 3. Effects of MRF stimulation on averaged cortical responses evoked by paired shocks to auditory pathway. Same display as in Fig. 2. Responses initiated in auditory radiations (RAD) are reduced while those initiated in MGB are augmented in the same preparations. Vertical bars = 50 pv. All stimulus artifacts separated by 20 msec.
ing IC stimulation were always powerfully facilitated (Fig. 2G), and the effect appears due more to thalamic (postsynaptic) than collicular (presynaptic) influences, as discussed above. Auditary Cortex. The effects of reticular activation on cortical responses
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to clicks are different for the several components of the evoked response complex. The most prominent feature of MRF modulation (seen in nineteen of twenty-three preparations) is reduction in amplitude of the initial surface positive deflection. Records typical of those we obtained are shown in Fig. 4, in which the components as described by Teas and Kiang may be identified (23). In Fig. 4A, the evoked response to a single click in a drowsy but readily arousable monkey is shown, and the early positive peak (ER,) with a latency of 12.5 msec is reduced to approximately 75% of control amplitude by concurrent MRF stimulation. The reduction in amplitude in this case is
FIG. 4. Effects of MRF stimulation on averaged cortical responses evoked by clicks. Same display as in Fig. 2. A: single click. B: paired clicks, 25msec separation, no second response in control. C: paired clicks, 2O-msec separation. D: paired clicks, 40-msec separation. E: paired clicks, 84Lmsec separation. F: augmentation of ER, in second response of C, clarified by subtraction of average response to single click. Vertical bars = 50 pv. Time scale the same in all records.
equivalent to about 25db input attenuation. In other experiments reductions up to 35 equivalent db were observed. These estimates of the magnitude of response reduction were made with the aid of data such as that presented in Fig. 6 (left), based on averaged responses to clicks of varying intensities. We found it practicable to work over the range of only the first 40 db of attenuation, and this fact coupled with the somewhat lower zero attenuation level in our studies (referred to evoked response threshold) reconciles the curve in Fig. 6 reasonably well with that obtained by Teas and Kiang for the chronic cat (23). The MRF effects on the later components are also evident. At the same time that the amplitude of ER1 was reduced (Fig. 4A), the latency of a
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second positive peak (ER,) decreased and the amplitude increased to approximately 1.50% of control. A third deflection (ER.?), surface negative in sign and more diffuse, was also augmented by MRF stimulation. In many records the late components are hard to identify with certainty as to peak amplitude and latency, but in general the changes in them were not as great as for the ERr component. The succeeding records in Fig. 4 reveal another aspect of reticular modulation of cortical evoked potentials, namely, the simultaneous augmentation of the ERr component in evoked responses to the second of two closely spaced clicks. This reticular-induced shortening of recovery cycles was seen only for click separations less than about 80 msec. We found recovery cycles at the longer intervals to be very unstable, but were able to verify that no reticular facilitation of the second primary responses could be obtained. Cortical responses obtained from shocks to the auditory radiations were reduced to a degree comparable or greater than that seen when click stimuli were used (Fig. 3). No example of augmentation of the ER1 component was observed to single or paired clicks at any separation. It was consistently possible in our experiments to obtain opposite effects of MRF activation on responses recorded in the cortex from RAD stimulation and from MGB stimulation in the same preparations (Fig. 3) and these observations lead to the conclusion that shock delivered within the MGB initiates substantial intrathalamic (presynaptic) neuronal activity which is under the influence of the reticular activating system. Changes in evoked potentials induced by MRF stimulation as shown in Fig. 4 could also be observed during arousal of the animal by external means, a preferred method if not for the difficulty of sustaining a reasonably constant level of arousal to a single stimulus. Heightened arousal was obtained in one case by bringing a glove close to the monkey’s face during the averaging period. The effects on the evoked potential were identical to those obtained with electrical stimulation (Fig. SA) . Study of animals anesthetized for surgery with thiopental sodium permitted us to reach the following conclusions about the effects of light barbiturate anesthesia: (i) the late components only of the evoked response are diminished (Fig. SB); (ii) effects of MRF stimulation on evoked potentials are abolished in close correlation with disappearance of signs of behavioral arousal; and (iii) recovery cycles to paired clicks are lengthened (Figs. 5B & 6). These findings have previously been reported from several laboratories. Finally, the possibility that increased blood ilow through auditory structures, particularly cortex, could be a contributing factor to the MRF effects was investigated. Invar and Siiderburg (14) demonstrated in this connection that cortical blood flow could increase in some forms of arousal without
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FIG. 5. A: effect of arousal from threatening stimuli on cortical evoked response to paired clicks separated by 20 msec. Control record on left. B: effect of light barbiturate anesthesia on cortical response to click and on recovery cycle. Left record 2 hours following 20 mg/kg thiopental sodium, animal unarousable. Right record 25 hours later, 40 msec. C: marked elevation of systemic animal drowsy awake. Click separation, blood pressure has little effect on evoked responses. Left record control, right record averaged during period indicated. Vertical bars = SO uv.
FIG. 6. Left, relative amplitude of early positive click attenuation. Mean of four preparations. Right, in second evoked response as a function of click preparations. For cortex, single preparation under
deflection (ER,) as a function of relative amplitude of same deflection separation. For IC, mean of four two conditions as in Fig. 5B.
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systemic blood pressure elevation. Figure 5C illustrates the results of an experiment in which femoral artery blood pressure was greatly elevated by intravenous injection of epinephrine 1: 10 000. During the pressor response evoked potentials were averaged and compared with controls obtained just prior to the injection. No significant effects were observed, suggesting indirectly that elevated cortical blood flow does not in itself produce changes in evoked responses. Discussion
The main findings presented here support the proposition that the fast components of sensory discharges in the auditory system of the monkey are little changed by reticular activation at the level of the IC, greatly facilitated at the level of the MGB, and moderately inhibited or reduced in amplitude at the level of the sensory cortex. The amplitude of the primary cortical response to a given click, then, should be viewed as the result of a balance of opposed modulating influences located in the cortex and the thalamic relay. Longer latency components of click-evoked responses are prominent in records from the auditory cortex, but seem in the best maintained enctphale isole’preparations to be influenced somewhatlessby reticular activation than are the primary components. The effect of reticular activation on late components (both ER? surface-positive and ER3 surface-negative) is clearly to augment them, and in our observations can become the most conspicuousconsequenceof MRF stimulation in preparations in which the cortex is in less than optimal condition (revealed by a blanched appearance and deterioration of the amplitude of ER1). We have thus found circumstances in which the effects of reticular activation are primarily on the late components of the evoked response,in agreement with others (10, 13, 23). Support for the proposed relative independenceof IC relay function from arousal level is available in the form of incidental observations in two reports on long-term changesin IC-evoked responses(7, 1l), and in a recent study in which such independence could be obtained after destruction of middle ear muscles(21). That IC has more than simple relay function is, of course, established and the possibility exists that a substantial portion of the ascending auditory input in the lateral lemniscusbypassesthe IC, or at least synapsesin very deep layers (see 15, for discussion). Our observations are that sites scattered throughout all areas of the primate IC yield clickevoked responses,and initiate rostrally directed volleys when stimulated directly, which are relatively unaffected by reticular activation. The evidence that reticular activation facilitates thalamic transmission while simultaneously inhibiting fast intracortical responsesto afferent volleys is in complete agreement with recent work on chronic cats undergoing span-
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taneous fluctuations in arousal level (3, 4, 9). We observed no sign of reticular facilitation of cortical primary responses initiated by shocks in the thalamic radiations, in agreement with these workers and others (8). Cortical responses to click pairs did, however, reveal the shortened recovery cycle during reticular activation frequently reported (1, 19, 22). In view of the absence of this effect in tests utilizing shock pairs to the auditory radiations, and the very potent facilitation found in the MGB, especially of the second of two evoked responses, we conclude that the shortened cortical recovery cycle in the auditory cortex is a passive effect dependent on altered input. Facilitation on the order of 200% increase of amplitude has been observed for responses evoked in IC and recorded in the radiations or in the MGB, and for responses evoked in the MGB and recorded in the radiations. Less potent effects were observed in cortical records to MGB and IC shocks, (five of twelve preparations demonstrated facilitation confined to the second response) as might be expected if an opposing inhibitory process were activated in the cortex. The exact duration of the critical interval through which thalamic facilitation can override cortical inhibition remains uncertain despite extensive investigation in the present experiments. In rare cases thalamic facilitation appears to be continuously dominant, since both evoked responses are augmented by reticular activation. The much more common finding, however, is that augmentation of the fast components is only seen in the response to the second of two clicks and then only when the click separation is less than the critical interval. Our data suggest that the critical interval lies between 50 and 80 msec, a figure considerably smaller than that reported by others using, for the most part, periodic stimuli. The interval may be related to the first peak of recovered excitability in the plot of relative amplitude of second response against click separation. All workers seem to agree that a peak of excitability is reached (possibly slightly supernormal) between 50 and 80 msec following click presentation (18, 19). Our observations are that at click separations longer than this cortical inhibition is normally dominant and equivalent amplitude reduction of both responses follows reticular activation. It is possible, however, that certain intervals coinciding with succeeding peaks of excitability may produce augmented second responses. In our experiments the shape of the recovery function beyond click separations of about 100 msec was so variable within and between animals that selection of one particular set of oscillations in excitability would be arbitrary. A question is raised by the above argument as to whether so-called cortical recovery cycles to peripheral stimuli are not in fact largely thalamic recovery cycles. Careful study of radiation initiated cortical recovery cycles clearly reveals a different sort of function (8). Evidence to clarify this point is un-
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fortunately not available in the literature and could not be obtained in the present study, primarily due to the difficulty we had maintaining stable clickevoked responses in the thalamus. It is important as the technology of response averaging advances to obtain a consistency of nomenclature and consequent precision of description of the components of auditory evoked responses. The cortical evoked responses we have recorded resemble in all respects those recorded by Teas and Kiang (23), and we find their carefully documented report of the correlation between components of the evoked response complex and organic variables generally applicable. When records are published, for example, presenting primary click-evoked responses having latencies to the first peak of 45 msec (Fig. 3 in 22), we have difficulty in reconciling the findings with our own. We have not obtained evidence that reticular activation differentially affects various subregions within the auditory cortex, although the methods used precluded a systematic study. The largest effects were obtained on potentials having the short latency characteristics described by Pribram, Rosner and Rosenblith (17), but the difference between the effects on these and on longer latency responses (14-17 msec to first peak) was slight and not qualitative. Considerable attention has been given in the literature to the apparently opposite effects of reticular activation on cortical evoked responses initiated peripherally by physiological stimulation of receptors and on those responses evoked centrally by shocks to the afferent pathway (1). The former are reported as reduced (early components, single flash or click) and the latter as augmented. Although the problem is somewhat clarified by locating the facilitation at the thalamic level and the inhibition at the cortical level, the basic phenomenon remains unexplained. In our observations this finding was encountered repeatedly. It appears necessary to include in an account of reticular modulation of single cortical evoked potentials, then, the observation that facilitation is most prominent when, and in fact is rarely seen unless, highly synchronized afferent volleys are present. In the case of evoked potentials recorded in the thalamus, however, facilitation is revealed more readily since there is no opposing inhibitory process. Reticular activation at this level facilitates both peripherally and centrally originating volleys, the latter more strikingly. References 1.
2.
3.
factors influencing the evoked potentials of the BREMER, F. 1961. Neurogenic cerebral cortex, pp. 259-278. In “Sensory Communication.” W. A. Rosenblith red.]. Wiley, New York. BREMER, F., and V. BONNET. 1950. Interprbtation des r&&ions rhythmiques prolong&e dans aires sensorielles de l’tcorce c&brale. ElectroencephoEog. Clin. Neurophysiol. 2: 329-400. DAGNINO, N., E. FAVALE, C. LOEB, and M. MANFREDI. 1965. Responses evok&
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by stimulation of the acoustic pathway during the sleep-wakefulness cycle. Experentiu 21: 459-460. DAGNINO, N., E. FAVALE, C. LOEB, and M. MANFREDI. 1965. Sensory transmission in the geniculostriate system of the cat during natural sleep and arousal. J. Neurophysiol. 28: 443-456. DESMEDT, J. E. 1962. Auditory-evoked potentials from cochlea to cortex as influenced by activation of the efferent olivo-cochlear bundle. J. Acoust. Sot. Am. 34: 1478-1496. DESMEDT, J. E., and G. LA GRUTTA. 1957. The effect of selective inhibition of pseudocholinesterase on the spontaneous and evoked activity of the cat’s cerebral cortex. J. Physiol. London 196: 20-40. DUN~P, C. W., R. W. WEBSTER, and R. H. DAY. 1964. Amplitude changes of evoked potentials at the inferior colliculus during acoustic habituation. J. Auditory Res. 4: 159-169. EVARTS, E. V., T. C. FLEMING, and P. R. HUTTENLOCHER. 1960. Recovery cycle of visual cortex of the awake and sleeping cat. Am. J. Physiol. 199: 373-376. FAVALE, E., C. LOEB, and M. MANFREDI. 1963. Le risposte corticali somatiche durante il sonno profondo: particulare comportamento della risposte radiatocorticali. Boll. Sot. Ital. Biol. Sper. 39: 430-432. FUSTER, J. M., and R. F. DOCTER. 1962. Variations of optic evoked potentials as a function of reticular activity in rabbits with chronically implanted electrodes. J. Neurophysiol. 25: 324-336. GALLIN, D. 1965. Background and evoked activity in the auditory pathway: effects of noise-shock pairing. Science 149: 761-763. Physiological and psychological aspects of selective perception, HORN, G. 1965. pp. 155-216. In “Advances in the Study of Behavior.” D. S. Lehrman, R. A. Hinde, and E. Shaw [eds.]. Academic Press, New York. HUTTENLOCHER, P. R. 1960. Effects of state of arousal on click responses in the mesencephalic reticular formation. EZectroencepha!og. Clin. Neurophysiol. 12: 819-827. 1958. Cortical blood flow related to EEG INGVAR, D. H., and V. S~DERBERC. patterns evoked by stimulation of the brain stem. Acta Physiol. Stand. 42: 130-143. JANE, J. A., R. B. MASTERSON, and I. T. DIAMOND. 1965. The function of the tectum for attention to auditory stimuli in the cat. J. Camp. Neural. 126: 165-192. JANE, J. A., G. D. SMIRNOV, and H. H. JASPER. 1962. Effects of distraction upon simultaneous auditory and visual evoked potentials. Electroencephalog. Clin. Neurophysiol. 14: 344-358. PRIBRAM, K. H., B. S. ROSNER, and W. A. ROSENBLITH. 1954. Electrical responses to acoustic clicks in monkey: extent of neocortex activated. J. Neurophysiol. 17: 336-344. ROSENZWEIG, M. R., and W. A. ROSENBLITH. 1953. Responses to successive auditory stimuli at the cochlea and at the auditory cortex. Psychol. Monogr. 67: No. 13: l-26. Effect of different states of a!ertness on SCHWARTZ, M., and C. SHAGASS. 1962. somatosensory and auditory recovery cycles. Electroencephalog. Clin. Neurophysiol.
14: 11-20. SEGUNDO, J. P., on electrocortical STARR, A. 1964. auditory pathway
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NAQUET, and P. BUSER. 1955. Effects of cortical stimulation activity in monkeys. J. Neurosurg. 18: 236-245. Influence of motor activity on click-evoked responses in the of waking cats. Ezgtl. Neztrul. 10: 191-204.
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1962. Reticular facilitation of responses to Clin. Neurophysiol. 14: 21-36. 1964. Evoked responses from the auditory