Hippocampal Electrical Activity Research Articles

Influences of hypothalamic stimulation upon septal and hippocampal electrical activity in the cat

Spontaneous patterns of hippocampal EEG and septal cell activity were studied in immobilized cats, and the influences of high frequency stimulation of medial hypothalamus (MH) and lateral hypothalamus (LH) were determined. Septal cells were divided into 3 classes on the basis of their discharge patterns: (1), rhythmic bursting (2), non-rhythmic bursting and (3), non-bursting, and the relationship of these discharge patterns to hippocampal theta rhythm was analyzed. Rhythmic bursting cells displayed close frequency and phase relations to hippocampal theta rhythm and were located chiefly in the diagonal band of Broca. Cells of the other two categories were found both within and outside of the diagonal band region. High frequency stimulation of medial hypothalamus evoked theta rhythm in hippocampus and bursts of repetitive firing at theta frequency from rhythmic bursting septal cells, whereas lateral hypothalamic stimulation desynchronized hippocampal EEG activity and disrupted rhythmic bursting patterns in the septum. The majority (59%) of rhythmic bursting cells ceased bursting and fired continuously at an increased rate during high frequency stimulation of lateral hypothalamus, while other (33%) rhythmic bursting cells ceased bursting and were either completely inhibited or fired at a markedly reduced rate. Of the rhythmic bursting cells tested with high frequency stimulation of lateral hypothalamus 73% responded with short latencies (1.7–4.5 msec). No rhythmic bursting cells were found which responded to medial hypothalamic stimulation at fixed latencies. Brain stem-hypothalamic pathways which mediate these differential effects upon septum and hippocampus are discussed.

Effects of subtotal hippocampal lesions upon hippocampal electrical activity

Hippocampal electrical activity was recorded during spontaneous behavior in 20 rats before and after the production of electrolytic lesions of the hippocampus. Electrographic seizures resulted from one-third of the lesions but hippocampal activity returned to normal within 24 hr and showed no further abnormalities in 1–5 months of observation. Hippocampal electrographic seizure activity did not result in any observed behavioral seizures in any rat. Electrolytic lesions of the neocortex overlying hippocampus caused neither behavioral seizures nor electrographic seizures in the hippocampus during the period of recording.

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

Bilateral dorsal and ventral hippocampal and anterior and posterior neocortical electrical activity was recorded in 11 cats during free behavior situations and during stimulation of medial 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.

Pathway mediating hypotension elicited by stimulation of the amygdala in the rat.

Cardiovascular responses were elicited from 372 histologically verified sites in the amygdala and closely adjacent structures in 82 rats under urethan or chloralose anesthesia. Arterial hypotension was elicited at low current intensities from the medial, central, lateral, and basal nuclei of rats under either anesthetic; the hypotension was still elicited in artificially respired paralyzed rats. Stimulation of the cortical nucleus produced variable changes in arterial pressure. The hypotension elicited under urethan was not found to be correlated with changes in the frequency of electrical activity of hippocampus, amygdala, or septum or to be associated with electrical seizures. Hypertension was usually elicited from the medial nucleus at high current intensities and was attributed to current spread to the cortex ventral to the medial nucleus from which hypertension was elicited at low current intensities. Stimulation of the central, basal, and lateral nuclei under chloralose elicited bradycardia. Lesions of the stria terminalis had no effect on the hypotension, whereas lesions of the ipsilateral medial forebrain bundle abolished this response.

Frequency analysis of rat hippocampal electrical activity

Hippocampal electrical activity was recorded in free-moving rats and behavioral observations carried out simultaneously. The hippocampal EEG was analysed by an EEG spectrograph giving the amplitudes of 20 frequencies in the 2–30 Hz band in the form of spectrograms. Twenty spectrograms per sec were generated, often showing considerable fluctuations even during any one behavioral pattern. Therefore mean spectrograms of several behavioral patterns were constructed by averaging 256 spectrograms, which corresponds to an analysis time of 12.8 sec per behavioral pattern. This was also performed, in modified form, for the hippocampal EEG accompanying sleep. Spectrograms could be classified into three groups. The first group includes the spectrograms, composed of a theta EEG, made during patterns of voluntary behavior (walking, rearing, sniffing objects) and paradoxical sleep. The spectrograms accompanying patterns of automatic behavior (grooming head and body, eating, sniffing air) belong to the second group whereas those made during immobile behavior (sitting, lying, slow wave sleep) form the third group. In using this method the results of Vanderwolf and Whishaw were generally confirmed in that there exists a strong temporal relationship between hippocampal electrical activity and behavior.

HIPPOCAMPAL RHYTHMIC, SLOW ACTIVITY AND HEART RATE UNDER FOOD AND WATER DEPRIVATION IN THE RAT

The effect of food and water deprivation upon hippocampal rhythmic theta activity and heart rate were investigated in albino rats. With regard to hippocampal electrical activity, it was specifically intended in the present study to see whether frequency of theta wave would change with deprivation. The results of this experiment indicated that no uniform change in frequency of theta wave occurred during first and second days of deprivation. With longer (at least for three days) of deprivation, however, theta frequency tended to decrease for all Ss. Heart rate was found to decrease during deprivation as compared with during control sessions. These findings were discussed in terms of an activation theory (12) and hippocampal function (6, 7).

Hippocampal electrical activity during long-continued avoidance performance: Effects of fatigue

Rats were trained to avoid electric shock by jumping out of a box to a height of 35.5 or 48 cm. Hippocampal slow wave activity was then recorded continuously during a single session in which avoidance testing was continued to the point of extreme fatigue. Rhythmical slow activity (RSA) always appeared prior to a successful avoidance response and rose in frequency as the instant of jump initiation approached. As the rats became fatigued and failed to avoid, this anticipatory upward shift in frequency failed to occur.

Spectral analysis of hippocampal slow wave activity during feeding and drinking in the rabbit

The dorsal hippocampal electrical activity was recorded in rabbits and correlated with discrete components of consummatory responses during drinking and eating, as well as during immobile attention. Power-spectral and autocorrelation analyses revealed that chewing was accompanied by two dominant frequencies, one of low power from 0.5 to 3.0 Hz, and one in the theta range of relatively low power and frequency. Theta rhythms accompanied all other components of ingestive behavior, with an increasing order of frequencies from licking of water, eating of liquid food, biting of solid foods, to immobile attention, whereas power of theta remained invariant across these conditions.

Effects of medial septal lesions on the hippocampal electrical activity and the orienting response to auditory stimulation in drinking rats

Medial septal lesions (MSL) abolished hippocampal theta activity and attenuated the overall hippocampal potential level, while frontal cortical lesions (FCL) had no effect on theta rhythm. A decrease in the overall potential level was observed only in some FCL rats. Although spontaneous behavioral patterns, including drinking, face-washing, searching, body movements, awake-and-resting, and slow-wave sleep, were not altered either by MSL or FCL during the 15 min observation, orienting responses to tones, especially searching-type, were reduced during drinking in MSL rats, while no such reductions were observed in FCL and no-lesion control rats. Auditory evoked potentials were not changed either by MSL or FCL. Spontaneous behavioral patterns were not different in FCL rats whether or not the overall hippocampal potential level was decreased.

Hippocampal theta rhythm: Intra-hippocampal formation contributions

The intra-hippocampal organization of theta rhythm was examined in chronically implanted rats. Small lesions were placed in the posterior aspects of the hippocampal formation and the subsequent effect of the lesion ascertained electrophysiologically. Lesions of the subiculum significantly diminished the amount of theta rhythm present and the effect appeared to be permanent. Lesions of the lateral entorhinal region produced a large, but transient, increase in the amount of theta rhythm present. The electrophysiological consequences of these lesions were organized in ‘lamellae’ as previously suggested. In addition, the effects of the discrete lesions were expressed ipsilaterally only. The fiber connections underlying these effects were reviewed and the subiculum was postulated to be a critical region for the maintenance of normal ‘on-going’ hippocampal electrical activity. The increase in theta rhythm following lateral entorhinal lesions was postulated to be result of the loss of high-frequency influences from other neural regions. It was concluded that, although paced by the septal regions, hippocampal theta rhythm is dependent upon intra-hippocampal formation maintenance and modulation. In particular, the present data indicate the importance of the subiculum in this maintenance. Furthermore, the data suggest an important role of the hippocampal formation in the organization of theta rhythm.

Electroencephalographic correlates of behaviors elicited by electrical stimulation of the septum: Seizure induced feeding

Recordings of rat neocortical and hippocampal EEG were taken during and following electrical stimulation of the medial septum. Low intensity stimulation which did not produce abnormal alterations of the hippocampal or cortical EEG resulted in behaviors such as hyperactivity, shaking, grooming, and yawning. As the stimulation intensity was increased, the effects on the electrical activity of both structures became more and more pronounced until prolonged, bilateral seizures followed by depression of the hippocampal electrical activity occurred. In all such instances, voracious eating was observed subsequently. The results indicate that care must be taken to distinguish the activation of neural mechanisms by artificial means through the use of such techniques as electrical and chemical stimulation, from that which occurs during or following pathological seizure activity.

Inhalation of cannabis smoke in rats

A new inhalation apparatus for rats is described for administrating Cannabis smoke in a standardized, controlled fashion. Electroencephalographic data indicated that, 15 min after smoke administration, cortical and hippocampal electrical activity was similar to that seen 25 min after interperitoneal injections of Δ 9-tetrahydrocannabinol (THC). A comparison of 30 mg/kg THC given orally, 4 mg/kg THC injected interperitoneally, and an estimated 17.5 mg/kg THC inhaled in Cannabis smoke indicated that all three modes of drug administration resulted in a significant reduction of open-field activity with the injection route being the most effective.

Neural pathway associated with the EEG arousal response by olfactory stimulation

In trigeminal deafferented encéphale isolé and intact cats immobilized with gallamine triethiodide, and midpontine pretrigeminal cats, an arousal effect of olfactory stimulation on the neocortical and hippocampal electrical activities was studied. The goal was to explore the desynchronizing system for the olfactory arousal and to determine the olfactory pathway to this desynchronizing system. 1. 1. Neocortical and hippocampal arousal responses were elicited either by odor or by electrical stimulation of the olfactory bulb. The latter stimulation occasionally provoked seizure discharges in the prepyriform cortex and hippocampus. Phenobarbital markedly affected the olfactory arousal response and abolished it in doses of 6–10 mg/kg. 2. 2. In a high cerveau isolé cat, in which the brain stem was transected at the border between the diencephalon and the mesencephalon, either blowing of odorized air into the nostril or electrical stimulation of the olfactory bulb failed to elicit the EEG arousal response, though electrical stimulation of the nuclei ventralis anterior and centralis medialis of the thalamus still provoked a neocortical desynchronization. This is interpreted as meaning that the mesencephalic reticular formation plays an important role in producing the olfactory arousal. 3. 3. Among anatomically proposed olfactory pathways to the mesencephalon, a lesion of the medial forebrain bundle led to a complete disappearance of the olfactory arousal, while it was not abolished by lesions of the fornix and the stria terminalis. It is concluded that the olfactory arousal response is produced by projection of olfactory impulses to the mesencephalic reticular formation through the medial forebrain bundle.

Hippocampal theta activity and the attention component of discrimination learning

To assess the possibility that the occurrence of hippocampal theta during learning reflects the role of this structure in mediating attention or orienting responses, hippocampal electrical activity was monitored while chronically implanted cats learned and were shifted from a CRF to a cued DRL schedule in an operant conditioning apparatus. The end of the DRL was signaled by the onset of a panel of lights, and it was hypothesized that if theta is a correlate of attention to environmental responses, then as learning progressed and the animals attended to this relevant environmental cue, the incidence of theta between bar-presses should increase. Bennett and Gottfried (1970) previously reported that learning and performance of a noncued DRL are accompanied by hippocampal desynchronization. The results supported the hypothesis, and in conjunction with results previously reported by Bennett and Gottfried the present findings support the notion that the critical difference between those tasks in which learning and performance are correlated with theta versus those accompanied by hippocampal desynchronization is whether the animal can master the problem by attending to environmental cues. Some broader implications of these results for theories of hippocampal functioning are discussed.

Hippocampal Electrical Activity in Spontaneous Behavior and Active Avoidance Conditioning in Rats

The purpose of this study is to examine previous theories of hippocampal electrical activity in relation to behavior with special emphasis on VANDERWOLF's hypothesis, based on new data with rats as subjects, in which hippocampal activity was studied during spontaneous behavior patterns and active avoidance conditioning with MILLER-MOWRER box.The main results were : (1) Hippocampal EEG during spontaneous behavior.Theta rhythms of 79 Hz were clearly observed during such behaviors as searching and rearing without locomotion and walking forward or around, as well as during head and body movements without locomotion. When animals were awake but motionless, hippocampal EEG consisted of desynchronized activity was also observed.(2) Hippocampal EEG during active avoidance learning.Hippocampal theta activity was not produced by the presentation of CS (2000 Hz pure tone) in habituation trials. In avoidance training, CS elicited a clear train of theta activity (79 Hz) starting a few seconds before the occurrence of avoidance responses. In the final state of avoidance learning, theta activity occurred immediately after the onset of CS and theta frequency increased to 911 Hz just before shuttling to the next compartment.From these and other results, hippocampal theta activity was shown to be associated not only with voluntary movement as suggested by VANDERWOLF but also with arousal level or general attention level.

Hippocampal electrical activity in the monkey during delayed alternation problems

1. 1. Electrical activity from hippocampus and inferotemporal cortex was recorded from monkeys ( Macaca mulatta) during the learning of two delayed alternation problems: go/no-go alternation and classical spatial alternation. 2. 2. Power spectrum analysis clearly revealed 3–5 c/sec theta rhythm in the electrical activity of the primate hippocampus and the appearance of systematic differentiation in this theta bandwidth over the course of learning. Early in learning there were no differences between the trial categories of the go/no-go problem ( i.e., go correct, no-go correct, go incorrect, no-go incorrect) or between problems. As criterion was approached, however, theta rhythm became increasingly more prominent on no-go as against go trials. Theta on left-right alternation more closely resembled no-go trial activity. On go trials, theta activity tended to diminish as learning advanced. 3. 3. Theta rhythm on incorrect trials of the go/no-go problem was consistent with the nature of the animal's response. 3–5 c/sec activity was similar on go correct and no-go incorrect trials near and at criterion, and theta on these trial categories was clearly different from no-go correct, go incorrect, and left-right correct and incorrect trial electrical activity. 4. 4. These effects all took place in the period immediately following stimulus onset. Hippocampal electrical activity recorded later in each trial did not show any systematic differences between trial categories or problems. 5. 5. The EEG from inferotemporal cortex differed from hippocampal activity in the greater proportion of fast activity and in the absence of any of the effects observed in the hippocampal leads. 6. 6. The prominent occurrence of theta on no-go trials and the close similarity of the hippocampal power spectra on no-go and left-right alternation trials suggests that these responses demand a high degree of modulation or control by an active hippocampus. From the appearance of the theta effects early in each trial, it appears that the hippocampus is involved in the anticipatory programming of intended acts, and the suggestion is that a fast time calculation of an intention to inhibit a response is made at the beginning of the trial. The data of this study suggest that hippocampal theta reflects response bias, conceived as a program organizing observing and instrumental responding.

Hippocampal physiology: Experiments on regulation of its electrical activity, on the mechanism of seizures, and on a hypothesis of learning

A review is made of experiments which show that hippocampal electrical activity is dependent on synaptic facilitation and regulated by extracellular potassium levels, and the relationship this has to hippocampal seizures. Later, the relation of hippocampal RNA to electrical activity and to [K+]0 levels is analyzed. Finally, a hypothesis on the role of the hippocampus in learning, supported by these findings, is considered.

Effect of trifluoperazine and chlorpromazine on hippocampal electrical activity

Effect of trifluoperazine and chlorpromazine on hippocampal electrical activity

Interrelationships of hippocampal EEG and visual evoked responses during appetitive behavior in rats

Summary Averaged visual evoked responses (VERs) at the cortex to flashes and hippocampal (HPC) electrical activity were examined during different phases of bar-pressing for sucrose solution on a fixed-ratio schedule. Systematic variations were found in amplitude of late components of VERs. The late components were largest during reinforcing consummatory behavior, smallest during the postreinforcement period, and intermediate during bar-pressing. Further analysis of VERs within each bar-press sequence revealed increased amplitude of late components as the animal progressed through the sequence. These orderly changes in VERs related closely to hippocampal EEG which also varied systematically during different phases of appetitive behavior. The average frequency of synchronous HPC activity related inversely to amplitude of VER late components. Analysis of synchronous HPC activity during 1-sec epochs before and after each flash revealed strong negative correlations between these measures and late negative and afterdischarge components of the VER. These findings, which indicate significant interrelationships among sensory evoked response, hippocampal EEG activity and associated behavioral patterns of activity, were viewed in terms of the plasticity of operation of electrophysiological arousal systems.

Time course of retroactive effects of hippocampal stimulation on learning

Rats bearing chronically implanted monopolar hippocampal electrodes received conditioned emotional response (CER) training, in which a tone or light (the CS) was paired with foot shock. Different groups of subjects received 30 sec of low-level or 10 sec of higher-level hippocampal stimulation 0, 10, 20, 30, 40, or 120 sec after each CS-foot shock pairing. Subjects were then deprived of water, and strength of the CER was determined in a session in which licking from a spout was permitted and degree of suppression of licking in the presence of the CS was noted. Low-level hippocampal stimulation did not significantly affect CER acquisition. Higher-level stimulation completely blocked acquisition when delivered immediately after each CER trial and produced partial but significant interference at delays of 10–40 sec. Stimulation given 2 min after each trial was ineffective. These results are interpreted as reflecting a brief temporal gradient of retroactive interference with memory consolidation processes, produced by disruption of normal hippocampal electrical activity.