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Taste aversion learning is impaired by interpolated amygdaloid stimulation but not by posttraining amygdaloid stimulation
BEHAVIORAL BIOLOGY, 13,369-376 (1975), Abstract No. 4247
BRIEF REPORT Taste Aversion Learning Is Impaired by Interpolated Amygdaloid Stimulation but Not by Posttraining Amygdaloid Stimulation JAMES B. A R T H U R 1
Department of Psychology, Lakehead University, Thunder Bay P7B 5El, Ontario, Canada
Supraseizure threshold amygdaloid stimulation, when interpolated between the taste of saccharin and the administration of lithium chloride, disrupted the acquisition of a conditioned taste aversion. Amygdaloid stimulation administered 15 min after the injection of lithium chloride did n o t alter subsequent performance of the taste aversion. These f'mdings may indicate that the amygdala is involved in the mediation of learning over long delays.
Conditioned taste aversions are formed by pairing a distinctive taste (CS) with experimentally induced illness (US). One such pairing even with a CS-US interval several hours long can result in subsequent avoidance of that taste by rats (Garcia and Ervin, 1968). Numerous studies have shown that animals with amygdaloid lesions are impaired in the acquisition of a conditioned taste aversion (Arthur, 1974; Ashe and Nachman, 1974; Kemble and Nagel, 1973; McGowan, Hankins, and Garcia, 1972; Rolls and Rolls, 1973). Furthermore, when amygdaloid lesions are sustained after training, but before testing, the animals are impaired (Arthur, 1974; Ashe and Nachman, 1974), suggesting that the amygdala may be involved in the performance o f the task. The use of chronic amygdaloid disturbance, however, is insufficient to establish amygdaloid involvement in the learning of a taste aversion. To assess a possible role o f the amygdala in learning it is necessary to examine the effects o f a reversible treatment such as localized stimulation (Kesner and Wilburn, 1974). 1This research was partially supported by an operating research grant A8618 from the National Research Council of Canada and by a grant from the President's NRC Committee to Dr. J. L. Jamieson. The author expresses his thanks to Dr. J. L. Jamieson for his help in conducting the experiment. 369 Copyright (S) 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
370
JAMES B. ARTHUR
Amygdaloid stimulation administered after passive avoidance training has been shown to induce retrograde amnesia for the experience (Gold, Macri, and McGaugh, 1973; Kesner and Doty, 1968). Since passive avoidance and taste aversion learning both require response inhibition, similar brain mechanisms may be involved in the learning of these tasks. Thus, the amygdala may be implicated in the consolidation of taste aversion learning. Therefore, it may be worthwhile to investigate the effect of amygdaloid stimulation delivered after taste aversion training. Electroconvulsive shock (ECS) has been shown to disrupt taste aversion learning when administered within the CS-US interval (Kral, 1971). Since ECS may be assumed to disrupt amygdaloid functioning, electrical stimulation of the amygdala applied between the CS and US may be expected to interfere with taste aversion learning. The present study examined the effect on taste aversion learning of supraseizure threshold amygdaloid stimulation administered following training or within the CS-US interval. One hundred fourteen male hooded rats obtained from the Canadian Breeding Farm served as subjects. All animals had participated previously in an experiment investigating the effect of amygdaloid stimulation in a one-trial appetitive task for water (Jamieson and Arthur, in preparation). They were assigned to this experiment equating the effects of their previous experience across groups. Of the animals used in this experiment, 49 had been stimulated once previously. They weighed 220-300 g at the beginning of the experiment. All rats were implanted bilaterally with bipolar twisted electrodes (1-mm tip separation, 0.5 mm bared at the tips) aimed at the amygdalae (Pellegrino and Cushman, 1967, coordinates: 0.8 mm posterior to bregma, -+ 4.5 mm lateral to the midline, 7.8 mm ventral to the dura). For 85 of the rats Amphenol connectors were used, wire electrodes with pin connectors being used for the remainder. All electrodes were constructed so that the medial tips would be of like polarity during stimulation and, in the case of the electrodes with Amphenol connectors, so that electrical recordings could be taken from the amygdalae individually. Stereotaxic surgery was performed under sodium pentobarbital anesthesia (Diabutal, 60mg/kg) and was followed by a 20,000-IU intramuscular injection of penicillin G procaine (Strepenalean). All animals were housed in individual cages and had food (Purina Rat Chow) available ad lib. Taste aversion training occurred 5-7 days after appetitive testing so that prior to training all animals had had at least 11 days' experience with the restricted drinking schedule of 20 min daily access to water. The water bottles were weighed before and after the drinking sessions and the volume of fluid consumed was calculated to the nearest 0.5 ml. Training consisted of the presentation of sodium saccharin solution (0.1% w/w) for 20 minutes followed 30 min later by 10ml/kg intraperitoneal injection of 0.4M lithium chloride solution (LiC1). Electrical stimulation of
AMYGDALA STIMULATION AND TASTE AVERSION
371
the amygdalae (115/~A/electrode, 60-Hz sine waves, 10-sec duration) was administered either 15 rain after the end of the saccharin-drinking period (SW), i.e., within the CS-US interval or 45 rain after the end of the drinking period (SA), i.e.; 15 rain after the administration of lithium chloride. For the 2 days following training water was presented for 20 rain daily and on the third day 0.1% sodium saccharin solution was presented. The investigation of the effect of interpolated amygdaloid stimulation on taste aversion learning uses a 2 × 2 factorial design with two levels of stimulation (actual and sham) and two levels of training (actual and sham). Similarly, the investigation of the effect of posttraining amygdaloid stimulation on taste aversion learning uses a 2 × 2 factorial design with two levels of stimulation (actual and sham) and two levels of training (actual and sham). Sham stimulation consists of the connection of leads without passing the current and sham training omits the injection of lithium chloride. Following testing the animals were sacrificed with ether and their brains were removed and stored in 10% formalin. Sections at 40/~m were made with a freezing microtome and sections through the electrode tracks were mounted on glass slides. Electrode placements were determined by reference to the Pellegrino and Cushman (1967) rat brain atlas. In the course of the experiment the electrodes of seven rats came loose, so the data from these animals were discarded. Histological analysis revealed that in 66 of the remaining 107 rats both tips of both electrodes were located in the amygdaloid complex. The data from these 66 rats and from 15 rats receiving stimulation prior to injection of lithium chloride, but having electrodes not in the amygdala, are considered in the statistical analysis. The distribution of electrode placements is shown in Fig. 1. The data from the 26 rats in other experimental groups which had poor electrode placements were not analyzed. The daily fluid intake of the groups is presented in Fig. 2. Acquisition of a conditioned taste aversion is indicated by a reduction in fluid intake on the test day. Difference scores were calculated by subtracting the volume of saccharin solution ingested on the test day from the mean volume of water ingested on the 2 preceding days and these difference scores were compared statistically. The mean difference scores of the groups and the number of animals on which each mean difference score is based are presented in Table 1. An unweighted means analysis (Wirier, 1971, pp. 445-449) of the difference scores yielded a significant SW × LiC1 interaction (F(1,29) = 10.002, P < 0.005) indicating that amygdaloid stimulation within the CS-US interval tended to disrupt the acquisition of a lithium chloride-induced aversion to saccharin. The SA × LiC1 interaction was not statistically significant ( F < 1, unweighted means analysis). In the light of the strong effect of taste aversion training (F(1,29) = 62.021, P < 0 . 0 0 1 ) the above finding suggests that amygdaloid electrical stimulation administered 15 min
372
J A M E S B . ARTHUR
,;;' iI
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. r ' , e, ~
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Fig. 1. Distribution o f electrode placements drawn on section from Pellegrino and Cushman (1967) rat brain atlas. Electrode tips within the amygdaloid complex (66 rats) are found within the horizontally striped areas. The vertically striped areas represent the distribution o f electrode placements of those animals (N = 15) with electrode tips not in the amygdaloid complex but which received stimulation between the presentation of CS and US.
AMYGDALA STIMULATION AND TASTE AVERSION
373
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Fig. 2. (A) Mean daffy fluid intake of groups receiving amygdaloid stimulation or sham stimulation at the midpoint of a 30-rain CS-US interval. SW and SSW denote amygdaloid stimulation and sham amygdaloid stimulation, respectively, administered within the CS-US interval. The presence or absence of taste aversion training is denoted by LiCI and no LiC1, respectively. (B) Mean daily fluid intake of groups receiving amygdaloid stimulation or sham stimulation 15 min after taste aversion training. SA and SSA denote amygdaloid stimulation and sham amygdaloid stimulation, respectively. Taste aversion training and no training conditions are designated LiC1 and no LiCI, respectively.
374
JAMES B. ARTHUR TABLE 1 Mean Difference Score (ml) and Sample Size (N) of Each Group LiC1
No LiCI
SW
-1.32 (9)
-2.17 (6)
SSW
7.13 (9)
-0.79 (9)
SA SSA
7.62 (5) 5.83 (10)
-1.51 (7) -2.18 (11)
after the injection of lithium chloride did not disrupt the acquisition of the aversion. The mean difference scores of animals receiving stimulation prior to injection of lithium chloride, but having electrodes within the piriform cortex and corpus callosum, were calculated and compared with those of the SW-no LiC1, SSW-LiC1, and SSW-no LiC1 groups. These 15 animals with poorly placed electrodes tended to show good acquisition of the conditioned taste aversion (F(1,35) = 1.32, P > 0.25, unweighted means analysis). Subsequent analysis by the Newman-Keuls procedure indicated that the SA-LiC1 group and the trained, sham-stimulated groups showed better learning than did the other groups ( P < 0.01, cf. Table 1). It is particularly interesting to note that the performance of the SW-LiC1 group did not differ significantly from that of the untrained groups. The electrical activity of the amygdala was recorded during training just prior to and immediately following electrical stimulation in approximately half the animals. At the end of the experiment all animals were administered electrical stimulation identical to that used in the experiment and additional recordings were made. Of the 27 animals in stimulated groups (cf. Table 1), interference-free, interpretable recordings could not be obtained for six animals. Seizure activity was recorded in 18 animals and in three animals clear records showing no evidence of seizure activity were obtained. Examples of typical records are presented in Fig. 3. In most cases the behavioral manifestation of seizure activity included arrest reaction, gnawing movements, or orienting responses, but not clonic movements or running seizures. It is worth noting that in the case of stimulation administered within the CS-US interval amnesia did not seem to depend upon the presence of seizure afterdischarges. For example, one animal in the SW-LiC1 group for which no seizure was recorded showed the second greatest amnesia. In addition, seizure activity produced after taste aversion training did not necessarily result in amnesia for the learning. "Four animals in the SA-LiC1 group experienced high amplitude (500-1000 /RV) seizures lasting 11-30 sec and still showed good acquisition of the taste aversion. Performance of this
AMYGDALA STIMULATION AND TASTE AVERSION
~'~ll~I,~l~
~~.'-',''~'--'~'-'~ . . . .
~,...,
375
,
Fig. 3. Examples of afterdischaxges elicited in the amygdalae of two rats A and B, after the experiment. The upper trace of each pair was recorded from the right amygdala, the lower trace from the left amygdala. The traces show basal amygdaloid activity prior to stimulation, a blank period during amygdaloid stimulation when the polygraph leads were grounded, the seizure activity followed by a period of depression.
task did not seem to be affected by prior experience with amygdaloid stimulation. In every group the animals which had been stimulated prior to taste aversion training did not perform differently from those animals which had not been stimulated previously (all t's < 1). In general, the results indicate that amygdaloid stimulation administered at the midpoint of a 30-rain CS-US interval disrupts taste aversion learning while amygdaloid stimulation administered 15 min after taste aversion training has no disruptive effect. The latter finding is consistent with Kral and Beggerly's (1973) data indicating that ECS administered immediately after injection of lithium chloride (and, therefore, functionally within the CS-US interval) disrupted taste aversion learning while ECS administered 2.5-15 min after lithium chloride injection did not. Data showing that electrical disruption of neural activity up to 15 rain after training does not affect retention suggests that an association between taste and illness is consolidated quickly, and thus, is susceptible only briefly to disruption by electrical stimulation (cf., Mah and Albert, 1973). The effects of supraseizure threshold electrical stimulation are generally assumed to be either (a) localized, directly disrupting neural activity at the site of stimulation and indirectly alerting activity in connected systems, or (b) diffuse, spreading to many other neural systems via fiber tracts or volume conduction (Kesner and Wilburn, 1974). Since animals receiving electrical stimulation at sites other than the amygdala prior to the US tended not to be impaired in retention, it may be suggested that disruption of taste aversion learning by interpolated amygdaloid stimulation is a fairly localized effect. The disruption of learning by such stimulation is consistent with the view that amygdaloid disturbance results in a difficulty in establishing an appropriate
376
JAMES B. ARTHUR
context (Barrett, 1969) or in recognizing the significance of stimuli (Ashe and Nachman, 1974). Thus, the amygdala and, possibly, a related system may be implicated in the mediation o f long CS-US intervals. REFERENCES Arthur, J. B. (1974). An investigation of the role of the amygdala in taste aversion learning. Unpublished Master's thesis, Lakehead University, Thunder Bay, Ontario. Ashe, J., and Nachman, M. (1974). Effects of basolateral arnygdala lesions on neophobia learning taste aversions and sodium appetite in rats. Z Comp. Physiol. Psychol. 87, 622-643. Barrett, T. W. (1969). Studies of the function of the amygdaloid complex in macaca rnulatta. Neuropsychologia 7, 1-12. Garcia, J., and Ervin, F. R. (1968). Gustatory-visceral and telereceptor-cutaneous conditioning-Adaptation in internal and external milieus. Commun. Behav. Biol. Part A, 1,389-415. Gold, P. E., Macri, J., and McGaugh, J. L. (1973). Retrograde amnesia produced by subseizure amygdala stimulation. Behav. BioL 9, 671-680. Kemble, E. D., and Nagel, J. A. (1973). Failure to form a learned taste aversion in rats with amygdaloid lesions. Bull. Psychon. Soc. 2, 155-156. Kesner, R. P., and Doty, R. W. (1968). Amnesia produced in cats by local seizure activity initiated from the amygdala. Exp. NeuroL 21, 58-68. Kesner, R. P., and Wilburn, M. W. (1974). A review of electrical stimulation of the brain in context of learning and retention. Behav. Biol. 10, 259-293. Kral, P. A. (1971). Electroconvulsive shock during taste-illness interval: Evidence for induced disassociation. Physiol. Behav. 7,667-670. Kral, P. A., and Beggerly, H. D. (1973). Electroconvulsive shock impedes association formation: Conditioned taste aversion paradigm. Physiol. Behav. 10, 145-147. Mah, C. J., and Albert, D. J. (1973). Electroconvulsive shock-induced retrograde amnesia: An analysis of the variation in the length of the amnesia gradient. Behav. BioL 9, 517-540. McGowan, B. K., Hankins, W. G., and Garcia, J. Limbic lesions and control of internal and external environment. Behav. Biol. 7,841-852. Pellegrino, L. J., and Cushman, A. J. (1967). "A Stereotaxic Atlas of the Rat Brain," New York: Appleton-Century-Crofts. Rolls, E. T., and Rolls, B. J. (1973). Altered food preferences after lesions in the basolateral region of the amygdala in the rat. J. Cornp. Psychol. 83,248-259. Winer, B. J. (1971). "Statistical Principles in Experimental Design," New York: McGrawHill.
BRIEF REPORT Taste Aversion Learning Is Impaired by Interpolated Amygdaloid Stimulation but Not by Posttraining Amygdaloid Stimulation JAMES B. A R T H U R 1
Department of Psychology, Lakehead University, Thunder Bay P7B 5El, Ontario, Canada
Supraseizure threshold amygdaloid stimulation, when interpolated between the taste of saccharin and the administration of lithium chloride, disrupted the acquisition of a conditioned taste aversion. Amygdaloid stimulation administered 15 min after the injection of lithium chloride did n o t alter subsequent performance of the taste aversion. These f'mdings may indicate that the amygdala is involved in the mediation of learning over long delays.
Conditioned taste aversions are formed by pairing a distinctive taste (CS) with experimentally induced illness (US). One such pairing even with a CS-US interval several hours long can result in subsequent avoidance of that taste by rats (Garcia and Ervin, 1968). Numerous studies have shown that animals with amygdaloid lesions are impaired in the acquisition of a conditioned taste aversion (Arthur, 1974; Ashe and Nachman, 1974; Kemble and Nagel, 1973; McGowan, Hankins, and Garcia, 1972; Rolls and Rolls, 1973). Furthermore, when amygdaloid lesions are sustained after training, but before testing, the animals are impaired (Arthur, 1974; Ashe and Nachman, 1974), suggesting that the amygdala may be involved in the performance o f the task. The use of chronic amygdaloid disturbance, however, is insufficient to establish amygdaloid involvement in the learning of a taste aversion. To assess a possible role o f the amygdala in learning it is necessary to examine the effects o f a reversible treatment such as localized stimulation (Kesner and Wilburn, 1974). 1This research was partially supported by an operating research grant A8618 from the National Research Council of Canada and by a grant from the President's NRC Committee to Dr. J. L. Jamieson. The author expresses his thanks to Dr. J. L. Jamieson for his help in conducting the experiment. 369 Copyright (S) 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
370
JAMES B. ARTHUR
Amygdaloid stimulation administered after passive avoidance training has been shown to induce retrograde amnesia for the experience (Gold, Macri, and McGaugh, 1973; Kesner and Doty, 1968). Since passive avoidance and taste aversion learning both require response inhibition, similar brain mechanisms may be involved in the learning of these tasks. Thus, the amygdala may be implicated in the consolidation of taste aversion learning. Therefore, it may be worthwhile to investigate the effect of amygdaloid stimulation delivered after taste aversion training. Electroconvulsive shock (ECS) has been shown to disrupt taste aversion learning when administered within the CS-US interval (Kral, 1971). Since ECS may be assumed to disrupt amygdaloid functioning, electrical stimulation of the amygdala applied between the CS and US may be expected to interfere with taste aversion learning. The present study examined the effect on taste aversion learning of supraseizure threshold amygdaloid stimulation administered following training or within the CS-US interval. One hundred fourteen male hooded rats obtained from the Canadian Breeding Farm served as subjects. All animals had participated previously in an experiment investigating the effect of amygdaloid stimulation in a one-trial appetitive task for water (Jamieson and Arthur, in preparation). They were assigned to this experiment equating the effects of their previous experience across groups. Of the animals used in this experiment, 49 had been stimulated once previously. They weighed 220-300 g at the beginning of the experiment. All rats were implanted bilaterally with bipolar twisted electrodes (1-mm tip separation, 0.5 mm bared at the tips) aimed at the amygdalae (Pellegrino and Cushman, 1967, coordinates: 0.8 mm posterior to bregma, -+ 4.5 mm lateral to the midline, 7.8 mm ventral to the dura). For 85 of the rats Amphenol connectors were used, wire electrodes with pin connectors being used for the remainder. All electrodes were constructed so that the medial tips would be of like polarity during stimulation and, in the case of the electrodes with Amphenol connectors, so that electrical recordings could be taken from the amygdalae individually. Stereotaxic surgery was performed under sodium pentobarbital anesthesia (Diabutal, 60mg/kg) and was followed by a 20,000-IU intramuscular injection of penicillin G procaine (Strepenalean). All animals were housed in individual cages and had food (Purina Rat Chow) available ad lib. Taste aversion training occurred 5-7 days after appetitive testing so that prior to training all animals had had at least 11 days' experience with the restricted drinking schedule of 20 min daily access to water. The water bottles were weighed before and after the drinking sessions and the volume of fluid consumed was calculated to the nearest 0.5 ml. Training consisted of the presentation of sodium saccharin solution (0.1% w/w) for 20 minutes followed 30 min later by 10ml/kg intraperitoneal injection of 0.4M lithium chloride solution (LiC1). Electrical stimulation of
AMYGDALA STIMULATION AND TASTE AVERSION
371
the amygdalae (115/~A/electrode, 60-Hz sine waves, 10-sec duration) was administered either 15 rain after the end of the saccharin-drinking period (SW), i.e., within the CS-US interval or 45 rain after the end of the drinking period (SA), i.e.; 15 rain after the administration of lithium chloride. For the 2 days following training water was presented for 20 rain daily and on the third day 0.1% sodium saccharin solution was presented. The investigation of the effect of interpolated amygdaloid stimulation on taste aversion learning uses a 2 × 2 factorial design with two levels of stimulation (actual and sham) and two levels of training (actual and sham). Similarly, the investigation of the effect of posttraining amygdaloid stimulation on taste aversion learning uses a 2 × 2 factorial design with two levels of stimulation (actual and sham) and two levels of training (actual and sham). Sham stimulation consists of the connection of leads without passing the current and sham training omits the injection of lithium chloride. Following testing the animals were sacrificed with ether and their brains were removed and stored in 10% formalin. Sections at 40/~m were made with a freezing microtome and sections through the electrode tracks were mounted on glass slides. Electrode placements were determined by reference to the Pellegrino and Cushman (1967) rat brain atlas. In the course of the experiment the electrodes of seven rats came loose, so the data from these animals were discarded. Histological analysis revealed that in 66 of the remaining 107 rats both tips of both electrodes were located in the amygdaloid complex. The data from these 66 rats and from 15 rats receiving stimulation prior to injection of lithium chloride, but having electrodes not in the amygdala, are considered in the statistical analysis. The distribution of electrode placements is shown in Fig. 1. The data from the 26 rats in other experimental groups which had poor electrode placements were not analyzed. The daily fluid intake of the groups is presented in Fig. 2. Acquisition of a conditioned taste aversion is indicated by a reduction in fluid intake on the test day. Difference scores were calculated by subtracting the volume of saccharin solution ingested on the test day from the mean volume of water ingested on the 2 preceding days and these difference scores were compared statistically. The mean difference scores of the groups and the number of animals on which each mean difference score is based are presented in Table 1. An unweighted means analysis (Wirier, 1971, pp. 445-449) of the difference scores yielded a significant SW × LiC1 interaction (F(1,29) = 10.002, P < 0.005) indicating that amygdaloid stimulation within the CS-US interval tended to disrupt the acquisition of a lithium chloride-induced aversion to saccharin. The SA × LiC1 interaction was not statistically significant ( F < 1, unweighted means analysis). In the light of the strong effect of taste aversion training (F(1,29) = 62.021, P < 0 . 0 0 1 ) the above finding suggests that amygdaloid electrical stimulation administered 15 min
372
J A M E S B . ARTHUR
,;;' iI
iI
/ ~
i ~
~jj. > Ht,:--~
(~", ,n~
. r ' , e, ~
','
'-s-a
~ \
~ ~
__
,
,__ /
S
f !
,1//"
,
s/
Fig. 1. Distribution o f electrode placements drawn on section from Pellegrino and Cushman (1967) rat brain atlas. Electrode tips within the amygdaloid complex (66 rats) are found within the horizontally striped areas. The vertically striped areas represent the distribution o f electrode placements of those animals (N = 15) with electrode tips not in the amygdaloid complex but which received stimulation between the presentation of CS and US.
AMYGDALA STIMULATION AND TASTE AVERSION
373
A
/
D E
\ 10=
~ SW- LiCI i
--
~
S W - n o LiCI
',
S S W - LiCI S S W - no L i C I
'~
z_ ~ _ I ~ ......
D
5, h
TRAIN DAYS
18-
B
j_:~
~ . . ~
15-
E
" " ' " "~°2~-. . . . . . .
.'"
10-
hi V
:_-_ _o" s : o Lio,
_z _a
•
D .J h
TEST
OoL,o
,
5-
TRAIN DAYS
TEST
Fig. 2. (A) Mean daffy fluid intake of groups receiving amygdaloid stimulation or sham stimulation at the midpoint of a 30-rain CS-US interval. SW and SSW denote amygdaloid stimulation and sham amygdaloid stimulation, respectively, administered within the CS-US interval. The presence or absence of taste aversion training is denoted by LiCI and no LiC1, respectively. (B) Mean daily fluid intake of groups receiving amygdaloid stimulation or sham stimulation 15 min after taste aversion training. SA and SSA denote amygdaloid stimulation and sham amygdaloid stimulation, respectively. Taste aversion training and no training conditions are designated LiC1 and no LiCI, respectively.
374
JAMES B. ARTHUR TABLE 1 Mean Difference Score (ml) and Sample Size (N) of Each Group LiC1
No LiCI
SW
-1.32 (9)
-2.17 (6)
SSW
7.13 (9)
-0.79 (9)
SA SSA
7.62 (5) 5.83 (10)
-1.51 (7) -2.18 (11)
after the injection of lithium chloride did not disrupt the acquisition of the aversion. The mean difference scores of animals receiving stimulation prior to injection of lithium chloride, but having electrodes within the piriform cortex and corpus callosum, were calculated and compared with those of the SW-no LiC1, SSW-LiC1, and SSW-no LiC1 groups. These 15 animals with poorly placed electrodes tended to show good acquisition of the conditioned taste aversion (F(1,35) = 1.32, P > 0.25, unweighted means analysis). Subsequent analysis by the Newman-Keuls procedure indicated that the SA-LiC1 group and the trained, sham-stimulated groups showed better learning than did the other groups ( P < 0.01, cf. Table 1). It is particularly interesting to note that the performance of the SW-LiC1 group did not differ significantly from that of the untrained groups. The electrical activity of the amygdala was recorded during training just prior to and immediately following electrical stimulation in approximately half the animals. At the end of the experiment all animals were administered electrical stimulation identical to that used in the experiment and additional recordings were made. Of the 27 animals in stimulated groups (cf. Table 1), interference-free, interpretable recordings could not be obtained for six animals. Seizure activity was recorded in 18 animals and in three animals clear records showing no evidence of seizure activity were obtained. Examples of typical records are presented in Fig. 3. In most cases the behavioral manifestation of seizure activity included arrest reaction, gnawing movements, or orienting responses, but not clonic movements or running seizures. It is worth noting that in the case of stimulation administered within the CS-US interval amnesia did not seem to depend upon the presence of seizure afterdischarges. For example, one animal in the SW-LiC1 group for which no seizure was recorded showed the second greatest amnesia. In addition, seizure activity produced after taste aversion training did not necessarily result in amnesia for the learning. "Four animals in the SA-LiC1 group experienced high amplitude (500-1000 /RV) seizures lasting 11-30 sec and still showed good acquisition of the taste aversion. Performance of this
AMYGDALA STIMULATION AND TASTE AVERSION
~'~ll~I,~l~
~~.'-',''~'--'~'-'~ . . . .
~,...,
375
,
Fig. 3. Examples of afterdischaxges elicited in the amygdalae of two rats A and B, after the experiment. The upper trace of each pair was recorded from the right amygdala, the lower trace from the left amygdala. The traces show basal amygdaloid activity prior to stimulation, a blank period during amygdaloid stimulation when the polygraph leads were grounded, the seizure activity followed by a period of depression.
task did not seem to be affected by prior experience with amygdaloid stimulation. In every group the animals which had been stimulated prior to taste aversion training did not perform differently from those animals which had not been stimulated previously (all t's < 1). In general, the results indicate that amygdaloid stimulation administered at the midpoint of a 30-rain CS-US interval disrupts taste aversion learning while amygdaloid stimulation administered 15 min after taste aversion training has no disruptive effect. The latter finding is consistent with Kral and Beggerly's (1973) data indicating that ECS administered immediately after injection of lithium chloride (and, therefore, functionally within the CS-US interval) disrupted taste aversion learning while ECS administered 2.5-15 min after lithium chloride injection did not. Data showing that electrical disruption of neural activity up to 15 rain after training does not affect retention suggests that an association between taste and illness is consolidated quickly, and thus, is susceptible only briefly to disruption by electrical stimulation (cf., Mah and Albert, 1973). The effects of supraseizure threshold electrical stimulation are generally assumed to be either (a) localized, directly disrupting neural activity at the site of stimulation and indirectly alerting activity in connected systems, or (b) diffuse, spreading to many other neural systems via fiber tracts or volume conduction (Kesner and Wilburn, 1974). Since animals receiving electrical stimulation at sites other than the amygdala prior to the US tended not to be impaired in retention, it may be suggested that disruption of taste aversion learning by interpolated amygdaloid stimulation is a fairly localized effect. The disruption of learning by such stimulation is consistent with the view that amygdaloid disturbance results in a difficulty in establishing an appropriate
376
JAMES B. ARTHUR
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