Hippocampal electrical correlates of free behavior and behavior induced by stimulation of two hypothalamic-hippocampal systems in the cat

EXPERIMENTAL

NEUROLOGY

49,

%-j28

(1975)

Hippocampal Electrical Correlates Behavior Induced by Stimulation Hippocampal Systems JAMES

Departments Institute,

R.

COLEMAN

of Psychology, Unizlersity

AND

Physiology

of California, Received

of Free Behavior and of Two Hypothalamicin the Cat

DONALD

B.

and Psychiatry, Los Angeles,

March

LINDSLEY

and Brain

1

Research

California90024

18, 1975

Bilateral dorsaland ventral hippocampal and anterior and posterior neocortical electrical activity was recorded in 11 cats during free behavior situations and during stimulation of med’ial and lateral hypothalamic systems which influence hippocampal electrical activity and behavior in distinct and contrasting ways. Behavioral states characterized by alertness, attentiveness, scanning, or investigatory activity were accompanied by hippocampal theta rhythm and by desynchronization of neocortical electrical activity. Behavioral states characterized by relaxed wakefulness, inattentiveness, or drowsiness were accompanied by high voltage irregular patterns of electrical activity in the hippocampus with no organized theta rhythms. Specific behaviors (alerting, orienting, scanning, or investigatory) emerging from relaxed behavioral states occurred concomitantly with hippocampal theta rhythm. Electrical stimulation of the medial hypothalamic system at 100 Hz elicited hippocampal theta rhythm and specific behaviors of alerting, orienting, scanning, and investigatory activity. In contrast, lateral hypothalamic stimulation at 100 Hz caused low voltage desynchronization of hippocampal electrical activity accompanied by postural stability and fixation of gaze. The relation of hippocampal theta rhythm to behavior is discussed in terms of brain stem-hypothalamic-hippocampal systems. INTRODUCTION

The role of the hippocampus with respect to psychological processes and behavior has in recent years become the focus of considerable interest. Various functions have been attributed to the hippocampus, including arousal and attention, perceptual discrimination and decision processes, 1 This work was supported by USPHS Grant NS-8552 to D. B. Lindsley and by Brain Research Institute Mental Health Training Program Traineeship to J. R. C. (USPHS Trainee Grant ST01 MH06415).

Copyright 411 rights

C 1975 by Academic Press, Inc. of reproduction in any form reserved.

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motivation and emotion, learning and memory, and voluntary and automatic control of motor behavior. Many studies, mainly in the rat, have sought to investigate these problems by observing the effects of hippocampal lesions upon behavior, or by stimulating the hippocampus directly and observing behavioral changes. Other studies have looked for correlations between hippocampal electrical activity and behavior, but mainly as changes have occurred under peripheral sensory stimulation or in various types of learning situations. Green and Arduini (13) were the first to study extensively the electrical activity of the hippocampus using primarily acute, unanesthetized, but immobilized rabbit preparations, and also making combined electrophysiological and behavioral observations in a few rabbits, and some observations in acute cat and monkey preparations. They demonstrated that various types of sensory stimulation are accompanied by runs of rhythmic synchronized activity in the hippocampus and usually desynchronized, small amplitude, higher frequency waves in neocortex. Because electrical stimulation in certain regions of the midbrain reticular formation and diencephalon evoked similar rhythmic activity of 3-6/set in the hippocampus along with an activation or arousal-like response neocortically, they referred to the hippocampal rhythmic activity as an “arousal response.” Such rhythmic activity in the hippocampus has been labelled subsequently hippocampa1 theta rhythm. Behaviorally, in rabbits with chronically implanted electrodes, they observed that when the animal appeared alert and showed interest in its environment, regular rhythmic waves (theta activity) of 5-7/set characterized the hippocnmpal electrical activity. Grastyan (8) and Grastyan et al. (g-11, 12) have studied extensively the electrical activity of the hippocampus associated with conditioned behavior, both appetitive and aversive, in the cat. They round hippocampal theta activity associated with the orienting reaction to a conditioned stimulus when it began to be effective in eliciting a conditioned response; later, after consolidation of conditioning, desynchronization of hippocampal activity was more characteristic. Subsequently, Grastyan et al. (9) identified hippocampal activity with motivational factors; theta rhythm with “pull” or approach, and desyncliroiiizatioii \vith avoidance or aversive reactions. Adey and collaborators (l-3, 7) working with cats in a two-choice visual discrimination task, observed theta rhythms in the hippocampus and entorhinal cortex which they associated with approach and decision processes. They interpreted the presence of an organized and stabilized theta rhythm during the approach of the animal to the discriminanda as indicating a goal-directed, alerted state, during which time a decision process and correct discrimination is made. Upoii consolidatioii of the visual dis-

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crimination habit, higher frequency theta activity was revealed by spectral analysis. As the animal prepared to consume the reward, desynchronization of theta activity occurred. Bennett (S), in an effort to resolve some of the differences in results and interpretations emanating from the laboratories of Adey and Grastyan and their collaborators, studied cats in situations involving free and trained behavior in which there were visual and auditory discriminative stimuli. In an approach task he found that theta rhythm appeared only when an animal showed orienting behavior and when behavior reflected an alert or attentive state. In preliminary adaptation to the experimental situation (free behavior) he found that trains of theta rhythms were associatedwith investigative behavior, visual searching, and escape attempts. Although concerned primarily with frequency and phase relationships of theta activity in hippocampal structures, Brown (6) observed during prolonged periods of free behavior in the cat that theta rhythm accompanied orienting and investigatory behavior. Neither Bennett nor Brown in their free behavioral observations in cats made special effort to differentiate brief and specific behavioral episodes and correlate them closely in time with dynamic changesin hippocampal electrical activity. The purpose of the present study was to attempt to identify and to establish close temporal relations between specific spontaneous behaviors, as well as more prolonged behavioral states, and changes in hippocampal and neocortical electrical activity. In addition, such temporal correlations were made between behavior and hippocampal electrical activity induced by electrical stimulation of two hypothalamic-hippocampal systems previously identified in cats by Anchel and Lindsley (4). Stimulation of the more medial of these two pathways in the posterior hypothalamus was found by Anchel and Lindsley to produce theta rhythm in the hippocampus while stimulation of the lateral system caused desynchronization of hippocampal activity. METHODS Eleven adult, male, cats were employed in this study. Each had stimulating electrodes implanted bilaterally in medial (A 11.0, L 0.5, H-4.0)) and lateral (A 11.0, L 3.0, H-5.0) hypothalamic regions. Recording electrodes were implanted bilaterally in dorsal (A 4.0, L 5.5, H + 7.8) and ventral (A 4.0, L 12.5, H + 0.5) regions of the hippocampus. Bilateral neocortical recordings were taken from stainless steel screw electrodes implanted in the skull over anterior (posterior sigmoid gyrus-A 26.0, L 4.0) and posterior (marginal gyrus-A 2.0, I, 2.0) regions. Four small screws embedded in bone over the frontal sinuses were interconnected to form a single ungrounded reference for all recording sites.

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All surgical procedures preliminary to implantation of electrodes were carried out under halothane (Fluothane) anesthesia. The animals were first anesthetized with halothane, intubated with a Xylocaine-coated enclotracheal tube and placed in a Kopf stereotaxic instrument. After surgical procedures were completed, all pressure points and cut surfaces nere infiltrated thoroughly xvith lidocaine (Xylocaine) , a local anesthetic. Before termination of general anesthesia, gallamine triethiodide (Flasedil) was infused via the saphenous vein and the animal placed under artificial respiration. Halothane was replaced by a mixture of nitrous oxide (80%) and oxygen (2O’/o’), prior to final location and testing of hypothalamic stimulating and hippocampal recording electrodes. Hippocampal electrodes were made of insulated, twisted, stainless steel wires (0.23 mm diameter), tips separated by 1.5 mm each bared of insulation for 1 mm. Final recordings (monopolar) were taken with the electrode tip which gave the optimal response \vhen paired with the diffuse frontal sinus reference lead. Stimulating electrodes were twisted, stainless steel wires (0.23 mm diameter) with tips separated by 1 mm and 0.5 mm bared of insulation. Stimulating and recording electrodes were connected to a 25-pin Amphenol connector mounted in an acrylic supporting pedestal held firmly on the skull by anchoring screws. Electrical recording was with a Grass model III-D, eight-channel electroencephalograph. A Grass S4 stimulator and SIC-1 isolation unit provided stimulating pulses at 0.15 msec duration at 100 Hz. Current strength at a given voltage was monitored on a Tektronix 502 oscilloscope. Cats were observed through a one-way window in a sound-proof acoustic chamber (Industrial Acoustics, model 400 A) which held an inner box 84 X 50 X 47 cm with white interior illuminated by 6-14’ bulbs mounted above the plastic observation window of the inner box. Coiled Microdot cable supported by a cantilever system carried insulated and shielded stimulating and recording wires which were attached to a male Amphenol connector fitting into the connector embedded in the skull pedestal. This arrangement permitted the cat to move freely about the 110s. Free behavioral observations were made continuously throughout l-hr repeated sessions estendin g over a period of several days. These observations were begun 2-4 weeks following surgical implantation of electrodes. Discrete behavioral episodes were signalled on the EEG record and recorded verbally on a tape recorder for later transcription to the EEG record. All observable behavior and movements were coded and categorized into brief descriptions, including such items as head and body orientation, posturing, turning, limb or eye movement ; immobility and various stages of sleep were determined by combined behavioral and EEG activity.

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Induced behavioral changes due to electrical stimulation of medial and lateral hypothalamic-hippocampal systems were observed and recorded as in the case of free behavior. Electrical stimulation always began at subthreshold voltage and current values and I\-as increased by 1-V steps to levels optimal for the production of either hippocampal theta rhythm (medial system) or low voltage hippocampal desynchronizet! activit) (lateral system). Each stimulation at 100 Hz was approximately 5 set in duration and was administered periodically only when the animal was quiet and not involved in active behavior. Stimulation inducing behavioral and electrophysiological changes was introduced only during the later observation sessions. Stimulating and recording sites were identified and verified from photographs of histological sections, employing the GuzmanFlores technique (14). In general, for all cats, the histology revealed that the sites for hippocampal recording and hypothalamic stimulating electrodes closely approximated the intended target sites. Figure 1 illustrates for cat E typical photographic enlargements of two wet sections showing in one plane the dorsal and ventral hippocampal recording sites (A) and in another plane the medial and lateral hypothalamic stimulating sites (B). Dorsal hippocampal recording sites were usually in the CA-2 or CA-3 fields. It was found that the lateral tlimension of placement of medial hypothalamic stimulating electrodes \zas rather critical for stimulation only of the medial hypothalamic system eliciting hippocampa! theta waves, and should be confined to within 1.5 mm of the midline (range 0.5-1.5 mm) for the production of hippocampal theta rhythm without concomitant activation of the adjacent lateral hypothalamic system which causes desynchronization of hippocampal electrical activity and low voltage high frequency waves. RESULTS This study had two principal goals and the results will be presented in accordance with the chronological sequence in which these were pursued. The goals may be categorized in the following manner. i. Correlation of hippocampal electrical activity with general behavioral states and with spontaneously occurring specific behazliors in a free behavior situation. This included the identification and demarkation of discrete and specific behaviors and the associated changes in hippocampa! and neocortical electrical activity occurring spontaneously in a free behavior situation against a background of ongoing behavioral states accompanied ___. ___.~__ ~..__~~.___~~. FIG. 1. Typical histology for locating recording and stimulating &es. Cat E. Photographic enlargements of wet brain sections by Guzman-Flores et al. (14) technique showing (A) bilateral dorsal and ventral hippocampal recording sites, (R) bilateral medial and lateral hypothalamic stimulating sites.

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by characteristic patterns of electrical activity. ii. Uelineation oi spxific behaviors induced by selective stimulation of each of two hypothalamichippocampal systems which have contrasting effects upon hippocampal electrical activity. This involved periodic electrical stimulation at 100 Hz of either a medial hypothalamic system, previously determined to be capable of inducing theta rhythm in the hippocampus, or a lateral hypothalamic system stimulation of which produced desynchronized, low voltage, electrical activity in the hippocampus. Electrical Correlates of BehazGoral States and of Spontaneous Specific Behaviors. Continuous behavioral observations and electrical recordings were made in 11 cats, 1 hr per day for several days. Each cat had chronically implanted electrodes for recording dorsal and ventral hippocampal and neocortical electrical activity, Observations of ongoing behavior and specific changes in behavior were made through a one-way vision window of the soundproof chamber and were signalled on the EEG record. In this phase of free behavioral observation the cat was in a bare, whitewalled, lighted box, inside the acoustic chamber, which was devoid of manipulanda or other objects and was without external sources of stimulation except for the continuous and uniform sound of the ventilator fan. When placed in the box on the first day the cats usually spent the first several minutes exploring the box but gradually settled into a relatively immobile posture of sitting, crouching or lying, usually in an awake, eyes-open, relaxed state. In some instances a cat would close its eyes and pass through the various stages of sleep, including neocortical spindle-burst and slow wave stages, and even the stage of rapid eye movement (REM) or paradoxical sleep in which the cat assumed a very relaxed lying or sprawling position. In connection with the brief verbal characterization of the ongoing general behavioral states and the specific, phasic changes in behavior which occur spontaneously it is necessary to provide further explanation of the terms employed in this study. General or ongoing behavior states are conditions in which a cat assumes and maintains, often for a matter of several minutes or more, a given posture such as standing, sitting, crouching or lying on its side. To do so quietly means that there are few if any movements of head, eyes, or body. To be in a relaxed condition means that in whatever body posture the cat may have at the time, its head, limbs and body are not elevated or extended, but instead appear to be in a settled or a lowered posture. Furthermore, the eyelids, though open, are not completely open. The pupils tend to be ovoid rather than slit or round and dilated. In the relaxed state there is no hyperactivity or restless movement of body, head or eyes. An alert posture is considered to be the opposite of relaxed posture, i.e., the body is slightly elevated,

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the limbs more firmly and rigidly extended and the eyelids are held more widely open. Other ongoing states of general behavior are the various stages of sleep ranging from drowsiness, where the eyelids slowly close or fluctuate between open and closed positions and the head tends to droop and not be held upright, to the early stages of sleep with shifts in posture from sitting or crouching tu lying. The relaxed posture exhibited during lying with eyes closed is accompanied by increasing slow wave activity and the onset of spindle bursts in the EEG followed gradually by larger and slower waves (the slow wave sleep stage) in both neocortex and hippophasic REM sleep periods may appear and the campus. Eventually, hippocampal electrical activity then manifests its most rhythmic and regular theta rhythm while the neocortex exhibits low voltage desynchronized activity. Attentive behavior in a human infant or young child is often identified as “observing behavior,” i.e., the head and eyes will follow a slowly moving stimulus, and fixate upon the object if it stops, and then resume tracking of the target if it moves again. In older children and adults, similar behavior may be identified but, in addition, the eyes, head and body posture provide further clues that a fixed or moving target object may be attended to and information derived in the form of learning or modified behavior. In an animal such as the cat it is more difficult to derive evidence which is clearly unequivocal with respect to the construct “attention,” and yet a mouse placed in a cage with a cat enables one to identify orientation posture, crouching behavior, and close eye and head following of the movements of the mouse by the “observing behavior” on the part of the cat. The situation within the experimental chamber does not provide any specific object, either stationary or moving, to attract the attention of the cat, however, the orientation of the body, head and eyes, and the fixed posture of the animal as it gazes straight ahead or in a direction assumed after an orientation of spontaneous nature, clearly suggests that attentive processes are operating. In order to illustrate the characteristic hippocampal electrical activity which is associated with different types of ongoing behavioral states, including sleep stages, and to show the concomitant changes in electrical activity which accompany specific behaviors, five figures representing free behavior in four cats will be presented. The types of ongoing electrical activity, and changes in electrical activity which are associated with these general behavioral states and specific behaviors in the four cats, are typical of the other cats used in this study. Figure 2 illustrates for cat L both behavior states and discrete, phasic behavior changes. In record A, a continuing behavior state, attentive-

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looking, is accompanied by nearly continuous theta rhythm in both dorsal and ventral hippocampal regions and by low voltage desynchronized neocortical electrical activity. Attentive-looking, ,in this instance, consisted in the cat’s head and eyes being oriented steadily to a region of the inner window of the acoustic chamber where there was a weak reflected image of itself. Records B, C, and E illustrate conditions in which the general behavioral state at the start of the record was characterized by a relaxed posture, but midway in the record there was a change in the behavior which is correlated with a shift in the electrical activity of the hippocampus from a high voltage irregular pattern to an organized rhythmic theta activity. In B and C, the behavior shift was from an ongoing relaxed state to alert and attentive states, respectively. In E, a reIaxed ongoing state was suddenly interrupted by a sponaneous orienting-like movement, i.e., sudden head-turning to the right, accompanied by hippocampal theta rhythm. Record D illustrates behavior in which the cat con-

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tinuously turned its head and eyes to different sectors of the box in slow scanning or searching movements as if looking for something. Throughout this continued scanning behavior the hippocampal tracings manifest rhythmic theta activity, while the neocortical tracings show low voltage fast activity superimposed upon large slow waves due to movement artifact. During record F, the cat exhibited orienting-attentive behavior which was accompanied by almost continuous theta activity in the hippocampus. Record A of Fig. 3 shows for cat D a general state of relased wakefulness in which high voltage irregular activity characterizes the hippocampal tracings and partial synchronization the neocortical tracings. In records B-F specific spontaneous behaviors interrupt the ongoing behavior state and are accompanied by changes in hippocampal electrical activity. In records B, C and F the background behavioral state was one of quiet sitting and the specific behavior which occurred was an orientinglike response, i.e., a sudden turning of the head to right, left, up or down, although there was no apparent external stimulus responsible for such a response. In records D and E, the cat was lying quietly at the start and the hippocampus showed an irregular electrical activity which shifted to

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theta rhythm as the cat got up on its feet and began to scan (I>) or investigate (E) the surrounding environment. During investigatory activity a cat typically moves slowly about the box (or may remain in one place) while reaching out with its paw or head, and in some instances manifesting sniffing. In contrast to scanning behavior in which the head and eyes are turned successively to several regions of the box, investigatory behavior usually involves exploration of fairly local regions of the box with attention or observing behavior being closely confined to a restricted area of purview. The records shown in Fig. 4 are from cat E during general behavior states and during specific behaviors, either suddenly and spontaneously initiated or continuously in process. In record A, the cat was in a crouched position of relaxed wakefulness. The hippocampal electrical activity contained both slow and fast components but no persistent theta rhythm. Neocortical records were partially synchronized with low voltage rhythmic activity in evidence. In B, the cat had assumed a lying position and appeared to be completely inattentive to its environment, actual!y close to drowsiness though with eyes open. Hippocampal electrical activity is of the high voltage irregular type and neocortical tracings show electrical

C

LYING

t LOOKING

UP ATTENTIVELY

F

STANDING

INVESTIGATING

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“-mu.\‘+’

FIG. 4. Cat E. Hippocampal and neocortical electrical activity during background behavior states and spontaneous specific changes. (A, B) High voltage, irregular, hippocampal activity during relaxed wake:u!ness and inattentiveness ; theta rhythm accompanies attentive looking (C), continuous scanning (D), orienting-scanning (E), and investigatory (F) behaviors.

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activity of increased amplitude, slowing and irregularity suggesting the approach of drowsiness. In record C, the cat was lying quietly at the start and its hippocampal electrical activity was irregular. Suddenly and spontaneously it raised its head and looked up attentively as if expecting something. With the onset of this type of specific behavior, the hippocampal tracing manifested a regular, rhythmic theta pattern and a similar, low voltage, rhythmic theta-like pattern appeared in the anterior neocortical tracing, but not in the posterior neocortical tracing. Record D, manifesting hippocampal theta rhythm throughout, was obtained while the cat was sitting and continuously turning its head and eyes to different regions of the walls and top of the box as if searching for something. During record E, the cat first made an orienting movement of its head and then began to scan the environment; there were continuous hippocampal theta waves from the start of the orienting movement and on through the scanning period. In record F the cat was standing and moving slowly about the box in an investigatory manner. During this investigatory behavior there were high voltage rhythmic theta waves almost continuously. Records A and B of Fig. 5 for cat B contrast behavioral states of awake relaxation (A) with drowsy relaxation (B) ; the former shows irregular high voltage hippocampal activity and low voltage rhythmic alpha-like waves in neocortical tracings, while the latter shows a greater degree of synchronization of hippocampal slow waves and a flattened or desynchronized pattern in the anterior neocortex (left posterior sigmoid region), but some higher voltage synchrony in the posterior neocortex (right marginal gyrus). Records C and D contrast a general behavioral state of awake alertness (C) with a similar state of awake alertness plus specific scanning behavior (D). In both cases right and left ventral hippocampal tracings show continuous theta rhythm, but only in the scanning situation does the left dorsal hippocampus manifest a high voltage theta rhythm; desynchronization characterizes the neocortical tracings in both conditions. Records E and F are good illustrations of a sudden, spontaneous shift from a drowsy state to an alert state. During the drowsy state hippocampal electrical activity is of the high voltage irregular type, and neocortical tracings manifest synchronous activity of relatively high voltage for neocortex records in the cat. With the onset of the alert state there is a desynchronization of neocortical activity and the ventral hippocampal tracings show well organized rhythmic theta activity. In both cases the left dorsal hippocampal tracing shows low voltage desynchronization. Figure 6 shows for cat B a sequence of records ranging from an awake alert state through drowsy and slow wave sleep stages to REM sleep and In the awake alert state (record finally a return to drowsy wakefulness,

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FIG. 5. Cat B. Hippocampal and neocortical electrical activity during background behavior states and spontaneous specific behavior changes. (A) awake relaxed behavior associated with hippocampal high voltage, irregular activity; (B) drowsy relaxed state shows high voltage, irregular, slow wave activity; (C, D) background states of awake alert and awake alert scanning behaviors accompanied by continuous hippocampal theta rhythms; (E, F) theta rhythm accompanies behavior shift from drowsy to alert state.

A) hippocampal tracings are characterized by slow and somewhat irregular theta rhythm ; neocortical activity is mainly desynchronized and low in voltage. Record B during the drowsy state reveals an arhythmic (nontheta) mixed slow and fast wave pattern with increased synchronization of neocortical activity. Record C during deeper, slow wave sleep, shows hippocampal tracings to be dominated by slower, larger and more irregular activity and neocortical tracings show increased slow wave activity. During REM sleep (records D and E) , there is highly synchronous theta rhythm throughout in the hippocampal tracings and neocortical activity is mainly desynchronized. The REM sleep state lasted 11 min and there is a 7 min interval of REM sleep between records D and E. In record F there is a return from REM sleep to slo’w wave sleep and drowsy wakefulness. The transition from REM sleep to drowsy wakefulness was quite rapid, requiring only about 20 sec. The drowsy wakefulness state was again accompanied by an irregular mixed slow and fast wave pattern of high voltage in the hippocampus.

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Specific Behaviors Associated with Hippocallzpal Theta Rhythm or Desynchronization Elicited by Stilttulation of Medial or Lateral Hypothalamic Systems. Figures 7 and 8 present data from three cats which are representative of the results obtained in all 11 cats when stimulation of either of the two hypothalamic-hippocampal systems induces hippocampal theta rhythm (medial system) or hippocampal desynchronization (lateral system). In addition to these two distinct and contrasting types of effects upon hippocampal electrical activity, it has been our purpose to identify the behavior changes which accompany the induced electrical changes. Stimulation of the medial hypothalamic system, which elicited hippocampal theta rhythm, caused one or more of several types of behavior change similar to those changes in behavior which occurred spontaneously during free behavior observation periods. Figure 7 for cat C contrasts, both electrophysio’logically and behaviorally, the effects of stimulating the medial hypothalamic system with those of stimulating the lateral hypothalamic system. In record A, stimulation of the right medial hypothalamic system at 7 V produced hippocampal A

AWAKE Lps-------.-

ALERT -..-

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D

REM

SLEEP

FIG. 6. Cat B. Hippocampal and neocortical electrical activity during changes of behavioral state from wakefulness to sleep. (A) awake alert state accompanied by low frequency hippocampal theta rhythm; (B) drowsy state characterized by high voltage, irregular, activity in hippocampus ; (C) neocortical slow wave sleep stage accompanied by high voltage, irregular mixed fast and slow wave activity; (D, E) REM-rapid eye movement sleep accompanied by high voltage, highly regular theta. rhythm ; (F) return to drousy wakefulness and irregular hippocampal activity.

COLEMAN

AND

LINDSLEY

FIG. 7. Cat C. Contrasting effects of stimulating right medial and lateral hypothalamic systems upon behavior and upon the electrical activity of the hippocampus and neocortex. (A) Stimulation of right medial hypothalamic system causes bilateral dorsal and ventral hippocampal theta rhythm and alert, orienting and scanning behavior; (B) Stimulation of right lateral hypothalamic system causes desynchronization of electrical activity of ipsilateral dorsal and ventral hippocampus accompanied by fixation of posture and gaze; (C) same as A but with 10 V stimulation which increases frequency of hippocampal theta rhythm and causes cat to become alert, and to investigate and scan.

theta rhythm at 4.5 Hz in both dorsal and ventral hippocampal recording sites and on both the right and left sides. In record B, stimulation of the lateral hypothalamic system caused desynchronization and reduction of amplitude of the ipsilateral hippocampal recordings (right dorsal hippocampus and right ventral hippocampus) but not the contralateral recordings (left ventral hippocampus and left dorsal hippocampus). Such ipsilateral and contralateral differences are usually not observed if higher

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FIG. 8. Cats B and E. Contrasting effects of stimulating medial and lateral hypothalamic systems upon behavior and the electrical activity of the ventral hippocampus and neocortex. (A) Low intensity, just suprathresho~d stimulation of medial hypothalamic system elicits bilaterat ventral hippocampal theta rhythm accompanied by alerting behavior ; (B) L ow intensity stimulation of lateral hypothalamic system elicits low voltage fast activity and desyncl~ronization in ipsilateral ventral hippocampus associated with fixation behavior following an orienting response. (C) Left mediai hypothalamic stimulation produced bilateral ventral hippocampal theta rhythm and orienting and scanning behavior; (U) Left lateral hypothalamic stimulation causes desynchronization of ipsilateral ventral hiypocampal electrical activity and a mixture of theta and fast activity in contralateral ventral hippocampus accompanied by fixation behavior preeeded by an orienting response. stimulating voltages are ttsetl. Neocortically. in record A, medial hypothalamic system stimulation induced IOK level fast activity and sporadic slow wave activity in the visual area (right marginal gyrus) but not in the somatosensory area (left posterior sigmoid gyrus) . In record B, lateral

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hypothalamic system stimulation induced fast, low voltage activity in the visual area (right marginal gyrus) but not in the somatosensory area (left posterior sigmoid gyrus). The above described contrasts in electrophysiological effects in the hippocampus, i.e., theta rhythm with medial hypothalamic system stimulation and desynchronization with lateral hypothalamic system stimulation, were accompanied by contrasting behaviors. In both A and B records stimulation was imposed when the animal was lying quietly and when hippocampal electrical activity was irregular and of moderate amplitude. Against this background medial hypothalamic system stimulation (record A) at 7 V induced higher voltage rhythmic theta activity and caused the animal to manifest alert behavior (lift head slightly and open eyelids wider), followed by an orienting response (turning head to left) and then scanning behavior (continuous head and eye movements as if searching for something). In contrast, lateral hypothalamic system stimulation (record B) at 6 V desynchronized and reduced the amplitude of hippocampal activity (right dorsal hippocampus and right ventral hippocampus) on the side ipsilateral to stimulation (right lateral hypothalamus). The behavioral accompaniment of this electrophysiological change was slight forward extension of the head, stabilization of posture and fixation of gaze. Thus two distinct types of behavior were elicited, depending upon whether the medial or lateral hypothalamic systems were stimulated and whether theta rhythm or desynchronization of hippocampal electrical activity occurred. In record C of Fig. 7, a higher intensity of stimulation (10 V) was administered to the right medial hypothalamic system. This stimulation occurred while cat C was sitting quietly. The background hippocampal electrical activity was irregular and upon stimulation changed to regular and rhythmic theta waves at a frequency of 5.5 Hz. The behavior which accompanied this change in hippocampal electrical activity, i.e., a shift to a theta rhythm pattern, consisted of alerting, investigating and scanning. Thus the behavior change was similar to that elicited by stimulation of the same medial hypothalamic system at 7 V in record A. It should be noted, however, that certain differences in the electrical effects of stimulating the medial hypothalamic system at two different intensities occurred; one, was that the theta rhythm was increased in frequency from 4.5 Hz at 7 V to 5.5 Hz at 10 V. The theta rhythm elicited by the 7 V stimulation did not outlast the period of stimulation, whereas that at 10 V did persist, but the frequency of the theta rhythm was reduced to 4.0 from 5.5 Hz during stimulation. The behavior for the two intensities of stimulation while similar, was more elaborate and involved at the higher intensity of stimulation since the cat manifested investigatory behavior in

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addition to alerting and scanning behavior at the lower intensity. These results with higher intensity stimulation, namely, higher frequency of theta rhythm, greater persistence of theta rhythm, and more extensive investigatory or searching behavior outlasting the period of stimulation, have consistently been found in other cats when the intensity of the medial hypothalamic system was increased well beyond the threshold or slightly suprathreshold levels necessary to evoke hippocampal theta rhythm and behavioral change. The results illustrated in Fig. 8 for cats B and E are similar, both behaviorally and electrophysiologically, to those shown in Fig. 7 for cat C. In Fig. 8A, just suprathreshold stimulation at 4 V in right medial hypothalamic system elicited theta rhythm at 4.0 Hz in both left ventral hippocampus and right ventral hippocampus with an accompanying alerting response, but without orienting and scanning as in cat C (Fig. 7A) in which higher intensity stimulation was used. Stimulation in lateral hypothalamic system (B) at 3 V, only slightly above threshold for producing an effect, caused desynchronization and an increase in low level fast activity, but only ipsilaterally. Changes in neocortical activity were minimal. Despite the low level of stimulation, the associated behavior changes, consisting of orienting and fixation of gaze, were similar to those elicited by lateral hypothalamic system stimulation at higher intensity in cat C. In cat E (Fig. SC), stimulation in medial hypothalamic system at 6 V caused hippocampal theta rhythm at 4.5 Hz which only slightly ouilasted the stimulus and elicited orienting and scanning behavior. Left lateral hypothalamic system stimulation (Fig. SD), also at 6 V, caused desynchronization ipsilaterally in left ventral hippocampus but not contralaterally in right ventral hippocampus, where poorly organized theta-like activity appeared instead of high voltage irregular activity which seemed to be suppressed. Neocortically, low level fast activity was slightly increased. Behaviorally, the cat oriented and fixated, as is characteristic for lateral hypothalamic system stimulation. ~~~+~~~ri~~tio~~ of Remits. During ongoing free behavior states of alertness, attentive?less, scanning, and investigatory activity, often prolonged for one or more minutes, theta rhythm characterizes the electrical activity of both dorsal and ventral hippocampus. The electrical activity of the neocortex typically exhibits desynchronization of ongoing waves and develops fast wave patterns of low voltage. During behavior states of relaxation, inattentivenGess, and drowsinrss, hippocampal electrical activity manifests a high voltage irregular pattern consisting of mixed slow and fast waves. Continuous theta rhythm accompanies behavior states of nlehess and attrntiwwess in which little or no movement is involved, as

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well as those behavior states of scanning and investigatory activity in which continual motor movement plays a part. Thus, theta rhythm is not dependent upon motor movement, Hippocampal theta rhythm is also associated with spontaneously occurring specific behaviors which emerge from a background of ongoing behavior states such as relaxed wakefulness, inattentiveness or drowsiness. The specific behaviors which are accompanied by concomitant runs or bursts of theta rhythm are alerting, orienting, scanning, or inztestigatory activity. During free behavior, stimulation of the medial hypothalamic system, which induces hippocampal theta rhythm and neocortical desynchronization, causes specific behaviors similar to those which occur spontaneously during free behavior, i.e., alerting, orienting, scanning, or investigatory activity. Stimulation of the lateral hypothalamic system, which induces desynchronization of hippocampal electrical activity, causes mainly fixation of gaze and an attentive posture. DISCUSSION The two main goals of this study were, one, to correlate the patterns of electrical activity in the hippocampus with naturally occurring behavior states and spontaneously occurring specific behaviors, and two, to correlate patterns of hippocampal electrical activity, induced by either medial or lateral hypothalamic stimulation, with concomitant changes in behavior. Our results have shown that it is possible to identify patterns of hippocampal electrical activity which are characteristic of ongoing behavior states and also of spontaneously occurring specific behaviors as these occur in a free behavior situation. In addition, stimulation of medial and lateral hypothalamic systems, which induce distinct and contrasting patterns of hippocampal electrical activity, have been found to produce, differentially, specific behmiovs. Together, these two strategies have permitted comparisons between naturally occurring and artificially induced changes in behavior and the changes in electrical activity of the hippocampus which occur concomitantly. The fact that we have found correspondence between the electrical patterns of activity in the hippocampus which accompany naturally occurring specific behaviors and those patterns of electrical activity in the hippocampus which are associated with artificially induced specific behaviors, suggests that the latter method with its immediate and reliable control of hippocampal electrical activity and behavior affords an expedient supplemental approach to the study of hippocampal function and its relationship to behavior. Furthermore, as the functional nature of the

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two hy~thalamic-hippocampal systems becomes better understood and the neuroanatomical origins of these systems in the brainstem become better defined (cf. 17) and related to the developin g conceptions of the brain stem ascending monoamine systems (cf. 15) and the ascending cholinergic pathways (16, 21) , such an approach may hecome very important in attempts to correlate neuroanatomical, neurophysiological, and neurochemical data with behavior. Our results have shown that hippocampal eIectrica1 activity during free behavior in the cat has an irregular pattern and is composed of sporadic slow waves mixed with higher frequency waves n-hen the cat is in awake and drowsibehavior states characterized by relaxation, inattentiveness, ness. In contrast, during awake and alert or attentive behavioral states, theta rhythm dominates hippocampal electrical activity. When specific behaviors such as alerting, orienting, scanning, or investigating occur spontaneously, against a background behavioral state of relaxed quiescence with irregular hippocampal electrical activity, runs or bursts of theta rhythm occur concurrently with these specific behaviors. The occurrence of theta rhythm throughout prolonged behavior states of alertness or attentiveness in the absence of phasic motor activity in the form of head, eye or limb movements seems to indicate that theta rhythm is not exclusively associated with motor movement. Indeed, we, and others (6, 18, 20), have shown that the cat exhibits the most regular and rhythmic theta waves during REM or paradoxical sleep, a condition of complete relaxation devoid of motor activity except for periodic twitches and rapid eye movements. However, our observations have also revealed the concomitance of theta rhythm and specific behaviors described as alerting, orienting, scanning, and investigatory, each of which involves some phasic motor activity. In this sense then, there might be considered some agreement between our observations and those of Vanderwolf (22) in the rat, and also those of Wishaw and Vanderwolf (21) in the rat and cat. WhiIe consistently entertaining the idea that theta rhythm is associated with vo!untary motor movements, Vanderwolf (23) recently reviewed his experience based on rat, rabbit and cat studies and again distinguished between “voluntary” movements which he believes are always accompanied by theta rhythm and “automatic” movements which are usually not accompanied by theta rhythm. Our results, which show theta rhythm durin g immobile behavior states of alertness and attentiveness, seem to indicate that something other than voluntary motor activity is responsible for, or associated with, theta rhythm. W’hen theta rhythms occur in association with specific behaviors such as alerting, orienting, scanning, and investigatory activity, we believe that ascending reticular activation, including proprioceptive feed-

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back, may be enhanced so as to increase afferent synaptic drive upon septal “pacemaker” cells (1’9) and thus initiate and regulate hippocampal theta rhythm. Such ascending reticular system activation is usually associated with arousal reactions, alerting and orienting responses, and manifestations of attentive behavior. However, in our soundproof, isolation chamber, devoid of unique or imposing stimuli, there was a minimum of peripheral stimulation to activate the reticular system. Therefore any such activation would have to be derived from internal physiological or central neural processes, such as drive states, curiosity motives, or changes in attentive states, The point should not be overlooked that attention is necessary to the processing of sensory information in its various levels of complexity and at the same time attention seems to be necessary to the selection and organization of any given voluntary motor activity and related adjustments. The same, however, might be said for motivation. In fact we have no adequate explanation of the neural mechanisms or the physiological processes which underlie the constructs “attention,” “motivation,” “drive behavior. The fact that we cannot now exstates,” and even “voluntary” plain these processes satisfactorily does not mean that they do not exist. It seems likely that they may all be part of the intricate warp and woof of the fabric from which behavior is made, and how to control and isolate one from another is indeed a difficult, if not an impossible, problem. It is of considerable interest that stimulation of the medial hypothalamic system while the cat is in the free behavior situation elicits all of the specific behaviors which the cat spontaneously exhibits without such stimulation and that these specific behaviors of alerting, orienting, scanning and investigatory activity are accompanied by hippocampal theta rhythm. These behaviors and the accompanying theta rhythm can be elicited in this manner upon demand and the stimulation can be repeated time and again with essentially the same results. Increasing the intensity of stimulation to the medial hypothalamic system increased the frequency of the theta rhythm and caused it to outlast the duration of the stimulus. The specific behaviors elicited were essentially the same though prolonged and more elaborate. Such medial hypothalamic stimulation, which produced hippocampal theta rhythm, typically caused desynchronization and activation of neocortical electrical activity, as well as behavioral arousal. The neocortical and behavioral effects accompanying hippocampal theta rhythm, suggest, as Green and Arduini (13) originally proposed, that the ascending reticular activating system is involved in the arousal reaction. Studies by Anchel and Lindsley (4) and Macadar, Chalupa and Lindsley (17) have traced the origins of the medial hypothalamic system to several sites in the mesencephalic and pontine regions of the brain

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stem, such as nucleus reticularis pontis oralis and nucleus locus coeruleus where stimulation at 100 Hz would also elicit hippocampal theta rhythm. In contrast to the effects of stimulation of the medial hypothalamic system, stimulation of the lateral hypothalamic system in the cat caused desynchronization of both bippocampal and neocortical electrical activity with stabilization of posture and fixation of gaze. These effects suggest activation of attentive processes, but in a more focused and restrictive manner than when alerting or scanning behavior was elicited by medial hypothalamic stimulation and accompanied by theta rhythm. Medial hypothalamic stimulation and hippocanipal theta rhythm seem to reflect a dynamic mode of scanning or searching the external environment for information or stimulus cues. In the case of REM sleep, associated with dreaming in humans, a persistent theta rhythm may be associated with a kind of internal scanning and activation of items of past experience. Lateral hypothalamic stimulation, on the other hand, seems to narrow the field of view, whether esternal or internal, and attention is temporarily “locked on” to specific aspects of the environment. The differential and contrasting effects of stimulating the medial and lateral hypothalamic systems upon hippocampal electrical activity and upon behavior suggest that the two systems in conjunction with the hippocampus may comprise a regulating or controlling mechanism governing attentive processes associated with sensory information processing and with the selection and organization of appropriate motor responses. REFERENCES 1. ADEY, W. R. 1967. Hippocampal states and functional relations with corticosubcortical systems in attention and learning, pp. 228-245. In “Progress in brain research: Structure and function of the limbic system.” W. R. Adey and T. Tokizane [Eds.]. Elsevier, Amsterdam. 2. ADEY, W. R., C. W. DUNLOP, and C. E. HENDRIX. 1960. Hippocampal slow waves: Distribution and phase relationships in the course of approach learning. Arch. Neztrol. 3: 74-90. 3. ADEY, W. R., D. 0. WALTER, and D. F. LIXDSLEY. 1962. Subthalamic lesions: Effects on learned behavior and correlated hippocampal and subcortical slowwave activity. Arch. Neztrol. 6: 194-207. 4. ANCHEL, H., and D. B. LINDSLEY. 1972. Differentiation of two reticulo-hypoElcctrocr~ccphalogr. Clirt. thalamic systems regulating hippocampal activity. Neurophysiol. 32 : 209-226. 5. BENNETT, T. L. 1970. Hippocampal EEG correlates of behavior. Elrctrocncephalogr. Clirt. Neurofihysiol. 28 : 17-23. 6. BROWN, B. B. 1968. Frequency and phase of hippocampal theta activity in the spontaneously behaving cat. Electrocmcphalogr. C/in. Ncwophysiol. 24 : 5342. 7. ELAZAR, Z., and W. R. ADEY. 1967. Spectral analysis of low-frequency components in the electrical activity of the hippocampus during learning. E!ectrocncephalogr. Clin. Neztrophysiol. 23 : 225-240.

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