Physiology & Behavior, Vol. 16, pp. 489--492. Pergamon Press and Brain Research Pubi., 1976. Printed in the U.S.A.

Theta EEG Activity and Memory Processes in Rats

THOMAS NICHOLAS, GARY GALBRAITH 2 , AND DONALD J. LEWIS

Department o f Psychology, University o f Southern California Los Angeles CA 90007

(Received 20 June 1975)

NICHOLAS, T. W., G. GALBRAITH AND D. J. LEWIS. Them EEG activity and memory processes in rats. PHYSIOL. BEHAV. 16(4) 489-492, 1976. - Rodent EEG theta activity recorded in the 30 min period after ECS treatment has previously been correlated with passive avoidance retention. In the present experiment rats were trained and tested in an appetitive maze, thereby avoiding the possible confounding of footshock-produced EEG arousal. Our results produced the same relationship between theta and retention, but since ECS was given 7 days after training, we conclude that theta activity is not related only to memory consolidation. Patterns of theta during reinstatement were also found to be more similar to the pattern recorded late in learning when the reinstatment took place in the original start box. The data support the notion of cue-dependent amnesia, and the hypothesis that ECS acts by inhibiting the memory which is active at the time of ECS.

Amnesia Electroconvulsive shock Electrophysiology Memory processes Theta

IT has been reported by Landfield, McGaugh and Tusa [5] that rodent electroencephalographic (EEG) activity in the theta frequency range ( 4 - 9 Hz) may be a correlate of the memory storage process. After training and treatment in a 1-trial passive-avoidance task they compared a group receiving electroconvulsive shock (ECS) and a group not receiving ECS. The ECS group had significantly lower theta activity and was amnesic. That report was quickly criticized [4] with the suggestion that the apparent correlation between EEG theta and memory might rather reflect a relationship between theta activity and arousal resulting from the footshock required to train the animals in the task.

A test of these notions might better be made in an appetitive task, thereby avoiding the criticism of foot- shock-produced arousal. Also it is desirable to use a task in which memory consolidation processes are clearly separated from memory retrieval processes. It has previously been shown [ 1, 9, 10] that a cue-dependent amnesia is produced even long after learning if ECS is paired with important cues from a prior learning. For example, a retention deficit occurs when ECS is given 7 days after training in the start box of an appetitive maze, but not when ECS is given in a wooden box very different from the maze start box [1]. Presumably, the start box provides cues which evoke the memory of the learned experience, and when this memory coincides with ECS, amnesia for the memory results. The deficit in the maze task is not likely to be due to fear induced by ECS, since animals receiving ECS to extin- guished start box cues show no deficit [9].

If theta is a crucial correlate only of memory storage, as previously suggested [5], then theta should not appear at the time of retrieval, or should not relate in any systematic manner to the pattern of theta recorded during rrlemory storage. If however, theta is a correlate of both storage and retrieval, then it should also appear at the time of retrieval in much the same form as during storage. If so, theta patterns at the time of reinstatement that are more similar to those patterns recorded at the time of storage should signal a more effective reinstatement and, hence, greater amnesic effects.

METHOD

Animals and Apparatus An appetitive spatial maze was used; it was made up of a

start box, a goal box and 4 choice boxes each containing a right and left passage. The apparatus is a modified Krechevsky maze (K-maze) described elsewhere [9]. Eighteen animals were first pretrained to run the maze and obtain food in the goal box. Then either the right or left path was blocked in each choice box, and the animals were trained to learn 1 of 2 specific patterns of right and left choices. After the animals learned the maze to a criterion of 1 error in 3 successive trials, they were returned to their home cages for a retention interval of 7 days. During this period they were handled daily and placed in a dissimilar, or, neutral, box. On the seventh day half of the animals were returned individually to the start box of the maze and given ECS treatment (SB-ECS) there. The other half received ECS in the neutral box (NB-ECS). Transpinnate

1This project was supported by Research grants GB-12372 and BMS74-19492 from the National Science Foundation, D. J. Lewis, Principle Investigator. The research was performed while the senior author was a predoctoral trainee under a National Institute of Mental Health Grant, MH 10554.

2 Now at UCLA Neuropsychiatric Institute, Pacific State Hospital, Pomona, California.

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ECS was an average current of 55 mA for 0.5 sec. Twenty four hours later each animal was required to relearn the maze to the original learning criterion.

P r o c e d u r e

Brain electrical activity was recorded from stainless steel screws anchored in the skull and bonded with dental acrylic to an assembly of Amphenol connectors as in Landfield e t al. [5]. A Grass Model IV EEG unit amplified and displayed EEG activity, which was also recorded on an FM tape recorder (Honeywell 7600). There were 3 different segments of the EEG data obtained in the experiment:

(1) Pre-ECS EEG. Brief EEG records (5 sec duration) were obtained at 4 times before ECS was administered. The EEG records were monitored continuously on the EEG unit after placing the animal in the start box (SB) or neutral box (NB). As soon as the activity had settled down, i.e., movement artifact were absent or nearly so, the tape recording was begun. The digitized 5 sec data epochs (processed off-line) were later displayed on a computer CRT screen and observed for amplifier blocking or any distortion of the data that would unduly effect a spectral analysis in the 4 - 9 Hz band. (a) Training Phase. During a recording trial in the maze each animal was connected to the recording leads, placed in the SB, and the SB gate slightly raised. After the EEG was recorded the animal was removed from the SB and replaced in its cage to await the next training trial. One EEG record was taken after the first training trial for each animal; a second record was taken late in training immediately after the trial in which each animal ran the maze 2 consecutive times with only one error. (b) Retention Phase. A third EEG record was taken from all animals on the sixth day of the retention period. The animals were connected to the recording leads, and a record was taken when they were placed in the NB during handling. (c) Reinstatement Phase. During the reinstatment period (immediately before ECS), a fourth EEG record was taken from the animals placed in the SB of the K-maze. Cortical EEG records were taken from the control animals under identical conditions, except that they were placed in the NB and given ECS there.

(2) Early Seizure Pattern. Some authors have previously suggested that correlates of memory processes may exist in the various portions of the first minutes of the Post-ECS EEG [2, 6, 14, 16, 17, 18]. Therefore, the duration, frequency and amplitude of the seizure patterns in the experiment were examined by measuring the paper EEG record during the first 3 min following ECS with a millimeter ruler.

(3) Post ECS EEG. The 30 min period following ECS was continuously recorded on magnetic tape. This post ECS EEG was analyzed for theta frequencies with a bandpass filter and digital logic circuit. A score for each animal for the amount of theta was taken by counting the presence or absence of theta frequencies during 5 four min sampling periods, beginning 4 rain after ECS treatment.

RESULTS AND DISCUSSION

The tape recorded 5 second blocks of pre-ECS EEG waveforms were digitized (128 samples/sec) and power spectra computed by fast Fourier analyses for each of the 5 sec blocks. In order to obtain an overall index of the similarity of the EEG waveforms from 2 periods, the absolute deviation was computed between the power values

at each frequency in the band (4 to 9 Hz). The resulting 6 absolute deviation measures were then summed, and this summed deviation score was taken for each animal as a gross measure of the amount of disparity between any 2 EEG waveforms. Thus, for any animal, 2 EEG waveforms which had basically similar power spectra at each frequency in the theta band would show a smaller deviation score than 2 EEG waveforms with discrepant power values. Deviation EEG scores were computed for each animal between the early and late learning periods; between the late learning and handling periods; and between early learning and handling periods. The hypothesis that the EEG patterns obtained during K maze training (early and late) and handling were different from each other was tested by computing a Friedman Two-way analysis of variance by ranks [15] over the deviation scores from the 3 periods. The test failed to show a reliable difference (p = 0.654) among the EEG of the different periods.

In an analysis of behavioral scores, a Mann Whitney U test [15] over the errors made in original learning to criterion for SB-ECS and NB-ECS animals failed to reach statistical significance (p>0.10); thus there were no pre- treatment differences between the 2 groups in their ability to learn the maze. In contrast, a Mann Whitney U test [ 15 ] of the K maze relearning errors showed that the nine SB-ECS animals made significantly (p<0.05) more errors in relearning than did the 9 NB-ECS animals (Median SB-ECS = 3 errors; median NB-ECS = 0 errors). Thus, a cue- dependent amnesia was produced for SB-ECS animals, but not for NB-ECS animals.

The amount of theta within the 30 min post-ECS period is displayed in Fig. 1. Also included in Fig. 1 is the amount of theta for another group of animals (N = 7) that was trained like the 2 ECS groups, but which was placed in the SB at the end of the 7 day retention interval, and was not given ECS (SB-NECS). This additional control group displayed a higher amount of theta at each sampling period than either of the 2 ECS groups. A Mann Whitney U Test [15], over the theta scores in sampling Period 1 revealed that the amount of theta at that time is essentially the same for the NB-ECS and SB-ECS groups. At sampling Period 5, however, the NB-ECS group had a significantly greater amount of theta (p<0.05) than did the SB-ECS group. It is evident from an inspection of Fig. 1 that the data reconstructs the basic relationship between retention and amount of theta reported in Landfield e t al. [5] in that amnesic animals show less theta than nonamnesic animals. However, since in the present experiment ECS treatment is separated from learning by a 7 day period, the hypothesis that theta is involved only in the memory storage process is not supported.

Our data also failed to indicate that the early ECS seizure pattern is a correlate of memory processes at the time of retrieval. In the present data Mann Whitney U Tests over the primary afterdischarge, isoelectric periods, and secondary afterdischarge all failed to reject the hypothesis of no difference between SB-ECS and NB-ECS animals.

An important assumption for a cue-dependent amnesia is that at the time of ECS a memory is active and open to the amnesic effects of the ECS. If it could be shown that EEG records obtained 7 days after learning during memory reinstatement were somehow related to the EEG record during the late learning period in the maze, then the assumption underlying a cue-dependent amnesia might gain electrophysiological support. If the EEG of start box

EEG AND MEMORY 491

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FIG. 1. Median cumulative amount of theta (4-9 Hz) recorded in the 30 min period after ECS or NECS treatment. Sampling periods were 4 min long, and every other second (120 sec in each 4 min period) was examined for theta.

reinstatment is an analogue of the EEG of learning, then a comparison of those two waveforms should show less deviation than a comparison of the EEG in late learning and the control condition for reinstatement, the neutral box. Thus a deviation score was computed between the rein- statment EEG and the late learning EEG for SB-ECS and NB-ECS animals.

A Mann Whitney U Test [15] of the deviation EEG scores fell just short (p = 0.065) of the accepted criterion of statistical significance. Siegal [ 15] has shown that the randomization test may be more powerful than the U test for data of this kind, and the randomization test, in fact, showed that the deviation EEG scores for SB-ECS and NB-ECS animals differed in the predicted direction (/9<0.025). Thus, we conclude that EEG theta activity during reinstatement is more similar to EEG during late learning when the reinstatement occurs in the identical start box of the maze, rather than in the neutral box. The group data thus suggests that the pattern of EEG theta activity is meaningfully related to both storage and retrieval, and is a function of the particular cues that reinstate the original memory.

In the spatial maze task, where ECS is administered 7 days after learning, amnesia is considered to be a memory retrieval deficit, not a memory storage failure. If memory

storage can be considered to be complete 7 days after the animals demonstrate learning of the multitrial maze task, then a consolidation theory would make no prediction concerning amount of theta and retention at that time [12,13]. However the present data show that when ECS is given 7 days after learning, there exists the same re- lationship between amount of theta and retention as when ECS is given immediately after learning. John [3] has presented an argument that during the retrieval of a memory a unique pattern of electrical activity is displayed throughout the brain. In the context of the present experiment, theta activity may be associated with the retrieval of a memory of learning in the appetitive spatial maze. ECS administered at the time of retrieval of the maze memory suppressed those frequencies in the post ECS EEG for animals whose later relearning performance was inferior. This conclusion applies equally to the Landfield et al. [5] paper if the assumption is made that the consolidation of simple tasks is extremely quick [7, 8, 1 1 ]. Thus, the data are electrophysiological support for the interpretation that ECS can be expected to produce amnesia for a memory whenever ECS follows immediately upon the activation of that memory, regardless of the temporal interval between original learning and ECS treatment.

REFERENCES

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5. Landfield, P. W., J. L. McGaugh and R. J. Tusa. Theta rhythm: A temporal correlate of memory storage processes in the rat. Science 175: 87-89, 1972.

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6. Lee-Teng, E. and S. Giaquinto. Electrocorticograms following threshold transcranial electroshock for retro-grade amnesia in chicks. Expl Neurol. 23:485 -490, 1969.

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