BEHAVIORAL BIOLOGY, 13, 359-367 (1975), Abstract No. 4206
Gustatory Memory: Incubation and Interference 1
KENNETH F. GREEN and LINDA A. P A R K E R 2
California State University, Long Beach, California The present study explored the mechanism by which gustatory memory is encoded by employing a simple biological test. The test was based on the fact that rats presented with a novel taste demonstrate a marked neophobia for that taste, and, upon subsequent exposures, increase their consumption of that taste. The results indicated that flavor intake in a test increased as time since prior exposure increased up to 6 hr, with the greatest increase occurring within 90 min. Gustatory memory was also found to be susceptible to interference from another novel flavor introduced at a delay of 0 min or 45 rain, but not of 90 min. Some implications of the present data for conditioned taste aversions were discussed.
Conditioned taste aversions can be acquired when long intervals exist between ingestion of a flavored substance (CS) and the experience o f illness induced b y a toxic agent (US). In an early paper, Garcia, Ervin, and Koelling (1966) reported aversions to saccharin-flavored water with interstimulus intervals (ISis) occurring between the end o f ingestion and the injection of apomorphine o f up to 75 rain. Later workers obtained maximal ISis o f nearly 8 hr between sucrose and X-irradiation (Revusky, 1968), 1 2 h r between saccharin and X-irradiation (Smith and Roll, 1967), and 2 4 h r between saccharin and cyclophosphamide injection (Etscorn and Stephens, 1973). Hypotheses advanced to explain the phenomenon o f long-delay learning have dealt with temporal contiguity between aftertaste and illness, the lack of interference by other tastes in the ISI, the "belongingness" o f flavors and illness, and the organization o f the central nervous system (see reviews by Garcia, McGowan, and Green, 1972; Revusky, 1971; Revusky and Garcia, 1970; Rozin and Kalat, 1971; and Seligman and Hager, 1972). All the hypotheses depend to some extent upon the abilities o f animals to remember flavored substances long enough to associate them with illness, and it is to the 1Based on a Master's thesis by the second author under the direction of the first author. Committee members Ralph Hupka and Gilbert Padilla made helpful suggestions. 2Now at The Memorial University of Newfoundland, St. John's. 359 Copyright © 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
360
GREEN AND PARKER
nature of such a gustatory memory mechanism that the present work is directed. In particular, the time course over which gustatory memory becomes encoded is examined within the conceptual framework of consolidation theory. During the course of consolidation, memory traces are said to pass through a labile phase where they are easily disrupted by manipulations such as ECS (McGaugh and Dawson, 1971) and during which they may be easily associated with other cues (John, 1967). After passing through the labile phase, memory traces become fixated and are relatively hard to disrupt. Study of the time course of consolidation of gustatory memories might provide information on the time when the memory is in the labile phase-when it is presumably most associable-and thus might aid our understanding of the mechanism whereby taste aversions can be acquired with long ISis. No aversion test was used in the present study. Rather, the method chosen relied upon the rat's reluctance to ingest water flavored with novel tastes. Since this neophobic reaction occurs only to novel flavors and not to familiar flavors, a rat would be expected to ingest more of an initially novel substance as he became more familiar with it, and the decrement in neophobia could thus serve as an index of memory consolidation. The paradigm used in the present work permitted thirsty rats to drink a novel fluid for a brief period, then, using a two-bottle test to compensate for differential deprivation levels, measured the preferences for the novel fluid in separate groups of rats at varous times after the initial exposure. Increasing preference for the novel flavor, with increasing intervals between initial exposure and test exposure, was understood to reflect increasing familiarity with the flavor, which could be attributed to consolidation of the memory of that flavor. EXPERIMENT 1 Experiment 1 examined the span of the 1Sis providing maximum reduction in neophobia, as evidenced by increased intake of a novel solution relative to water in a two-bottle test. Eight intervals, selected on the basis of delay gradients reported in studies of taste aversion learning (e.g., Etscorn and Stephens, t973; Nachman, 1970; Revusky, 1968; Smith and Roll, 1967), were 0, 45, and 90 rain, 3, 6, 9, 12, and 24 hr. A ninth group, used as a control, received only a two-bottle test. Since preliminary work had indicated a possible linkage of intake to diurnal cycles, an attempt was made to compensate for this source of contamination by starting one S from every ISI condition every 2½ hr until all Ss had been run. The initial flavor exposure lasted for 10 min in all ISI groups, except the group given the 0-rain delay between initial exposure and two-bottle test: The 0-min group was given a 2½ min initial exposure to guard against satiation effects disrupting intake in the two-bottle test.
GUSTATORY MEMORY
361
Method
Ninety male Wistar rats (Hilltop; Chatsworth, CA) weighed 190-300 g (mean, 250 g) at the start of testing. Each rat was housed individually, with Purina Rat Chow available ad lib. The rats had previously been used in taste tests involving very dilute hydrochloric acid, and had never experienced administration of toxic agents. Before the experimental procedures began, the rats were placed on a water-deprivation schedule for 13 days: They were trained to ingest their entire daily ration of water in a single 1-hr session, except on the 13th day when they were allowed water for only 20 min. On the 14th day of deprivation the rats were well trained to drink immediately, and experimental procedures began. Every 2½ hr, eight rats were presented a saccharin solution (2 g/liter of tap water) as a novel flavor. Seven of the rats had saccharin available for 10 min and were then assigned to the 45- or 90-rain or 3-, 6-, 9-, 12-, or 24-hr group. One of the rats was given the 0.2% saccharin solution for 2½ min, and assigned to the 0-rain group. Posttests for all rats were two-bottle preference tests where 0.2% saccharin solution and plain tap water were presented for 20 min. For each rat the two solutions were presented one at a time, with the order counterbalanced across rats, to enable sampling o f each taste. After each solution had been sampled, both bottles were presented with the positioning on the cage counterbalanced across rats, and the rat's consumption of each was recorded to the nearest 0.1 g. A control group (n = 10) was used to check the assumption of neophobia for a novel solution. The rats were administered novel 0.2% saccharin solution and tap water simultaneously for 20 min in a two-bottle test, with no preexposure.
Results
The data were analyzed in terms of the percentage intake of saccharin relative to water in the two-bottle preference test (S/(S+W)= flavor score). The means and standard errors are shown in Fig. 1 as a function of ISI. The data indicate an increase, in intake with increasing ISis until reaching asymptote at 6 hr, after which no significant deviation occurred. A one-way analysis of variance indicated that the increase in flavor scores with increased ISis was significant (F(7,9) = 8.52; P < 0 . 0 0 5 ) . The increase was reflected in a significant linear trend (F(7,9) = 37.0; P < 0.001). A significant quadratic trend (F(7,9) = 11.0; P < 0.001) reflected the fact that flavor scores reached asymptote at 6 hr. Groups were compared using the Mann-Whitney U Test. A number of significant differences were found between groups, with the general indication
362
GREEN AND PARKER hO .9
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[~t x--xMoo~ T
Standard error
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i
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90rain.
3hr.
6 hr
9hK
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i
24hr
/¢/1__.~
Controls
Inter- stimulus intervals
Fig. 1. Mean and standard error of saccharin scores (see text) in tests conducted at the indicated postexposure intervals in Experiment 1.
being no difference among controls, 0-min, and 45-min groups, but with each of these groups differing from each of the other groups.
Discussion The data indicated a progressive increase in intake of a solution with increasing ISis until reaching asymptote at 6 hr. Such a result may indicate a consolidation process. That is, the gradual reduction in neophobia, evidenced by increased preference for saccharin over time, may reflect a process in which the memory for the taste is being consolidated into long-term storage. When the behavioral index reached a ceiling, consolidation was complete-the memory trace was fixated. Thus, the present paradigm seems to be a robust test for consolidation of memory traces of single stimuli, since it was sensitive to the reduction of neophobia that occurred over a long span of time and required only one trial.
EXPERIMENT 2 Consolidation models assume that a learning trial produces a state of gradual encoding that may be disrupted by an interfering event. The longer the process remains undisturbed, the more probably it will become fixated into long-term memory storage. Experiment 2 was conducted to determine whether such an interference mechanism operated upon consolidation of a gustatory memory trace. Six groups of rats drank novel milk and were tested
363
GUSTATORY MEMORY TABLE 1 The Presentation Schedule of the Solutions for Each of the Groups in Experiment 2 10-Min
Group Controls 0 rain 45 rain 90 rain 3 hr 6 hr
initial exposure Milk Milk Milk Milk Milk Milk
5-Min interfering exposure 0 min 45 min 90 rain 3 hr
6 hr
20-Min test 9 hr
Sego
Test Test Test Test Test Test
Sego Sego Sego Sego
for memory of it 9 hr later. Interference was a second novel taste given to five groups at different times after the initial drink o f milk; the sixth group was a no-interference control. It was expected that the less the time between initial exposure to milk and introduction of the second novel taste the greater would be the disruption of the memory o f milk, and the greater would be the neophobic response to milk in the 9-hr test.
Method The same Ss as in Experiment 1 were used. They were assigned to groups with care taken to control for previous experience by counterbalancing as much as possible. The rats weighed between 264 and 400 g (mean 340 g) at the start o f testing. Each group contained 15 rats maintained in individual cages without water b u t with Purina Rat Chow ad lib. Between experiments the rats had been maintained on a schedule o f 1 hr of water per day, except on the day preceding testing, when water was presented for 20 min. The experimental procedure employed six groups-five experimental and one control. All groups were given an initial 10-min exposure to novel Pet milk (mixed 1:1 with tap water) and were subsequently tested for memory of the milk 9 hr later, with milk and water presented in a two-bottle test as described in Experiment 1. The 9-hr interval was chosen because it appeared in Experiment 1 that memory had reached asymptote by that time. The groups varied with respect to the interval between exposure to the test solution and exposure to the interfering solution (Very Strawberry Sego, undiluted). Five 5-min interfering-solution intervals were used, with a separate group o f 15 rats assigned to each interval. The intervals were 0, 45, and 90 rain, and 3 and 6 h r . A control group (n = 15) was used for which no interfering solution was presented during the interval. The paradigm is depicted in Table 1.
364
GREEN AND PARKER 1.0
t _= .8
t
.7
o
.6
o
Means
x~ I
.5
Omln.
4 5 rain.
Standard error
90mln.
3hr.
6hr.
Controls
interference Introduction Times
Fig. 2. Mean and standard error in flavor scores in a 9-hr postexposure test for groups varying according to time of introduction of interference in Experiment 2.
Results
Flavor scores (M/(M+W) = flavor score) were analyzed as in Experiment 1. Means and standard errors are presented in Fig. 2, the ordinate o f which has been foreshortened to reflect the initial preferability of milk. Intake of milk was least when Sego was introduced 0 rain and 45 min after initial exposure to milk. At 90 min and thereafter, no significant effect of Sego exposure was demonstrated by reduced neophobia relative to controls. A one-way analysis of variance indicated that the interval between milk and Sego produced a significant effect on preference for milk (F(4,14) = 10.7; P < 0.001). The increase in flavor scores with increased ISis accounted for a significant linear trend (F(4,14) = 37.2; P < 0.001). The reduced interference after 90 rain contributed to a significant quadratic trend (F(4,14) = 7.4; P < 0.01). Analysis of comparisons between means by Mann-Whitney U Test showed that the 0-min and 45-min groups did not differ from each other, but did differ from all other groups. Discussion
The preferences for milk during the 9-hr memory test may indicate that consolidation of gustatory memory was interfered with by introduction of another novel solution during the interval between initial and subsequent exposures to the test solution. The earlier during the interval that the second
GUSTATORY MEMORY
365
solution was presented, the greater was the disruption, as shown by a greater neophobic response to the initial solution. The results are consistent with those of Experiment 1 in that the greatest disruption occurred when interference was introduced between 0 and 45 min after initial exposure to a novel solution. When the interfering solution was introduced with a delay of 90 rain or more, little decrement in consolidation occurred. Thus, gustatory memory seems most susceptible to interference, or most highly labile, during the first 90 min after initial exposure.
DISCUSSION The results suggest that gustatory memory may be encoded into long-term storage by a consolidation process, and that the encoding may be disrupted by the presentation of another novel taste soon after exposure to the test solution. In particular, consolidation of gustatory memory appears to be complete at the end of 6 hr, and the memory trace appears to be highly labile during the first 45 min. The time-dependent relationship in the results is consistent with studies which demonstrate with conventional learning paradigms that a process resembling consolidation encodes experiences into long-term storage (McGaugh and Herz, 1972). Direct comparison with the time course of consolidation of other memory traces is hampered by the fact that memory for an association rather than memory of a single stimulus (retentive rather than associative memory, to use Revusky's (1971) nomenclature) is usually examined. However, some comparison can be made with data from other taste experiments. For example, Holman (1973) reported that rats tended to alternate their choices between familiar isohedonic flavors-the tendency to alternate was strongest immediately after ingestion of one flavor, and waned steadily until an indifference point was reached at about 6 hr. Whether the 6-hr indifference point in Holman's work is comparable to the 6-hr consolidation time in the present work is not immediately clear, but one may assume that the operation of gustatory memory mechanisms is reflected in both sets of data. That gustatory memory traces are most highly labile before expiration of the first 90 postingestion rain is suggested by the present work in two ways. First, the rapid rise in intake in Experiment 1 between 45 and 90 rain indicates a rapid increase in the rats' recognition of flavors. Second, the disruption in the second experiment of consolidation through retroactive inhibition produced by a second flavor was maximal before 90 min had elapsed. The latter finding is consistent with the idea that a memory trace in the labile phase is not fixated, therefore it is more easily subject to disruption through retroactive inhibition than after it has been fixated (John, 1967; Ost, 1969; McGaugh and Herz, 1972).
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If a memory trace in the labile phase is also more associable than when it has been fixated, results o f the present work suggest that aversions may be conditioned most readily with ISis of 45 rain or less, and more readily with ISis under 6 hr than above 6 hr. In this regard it is of interest that Garcia, McGowan, and Green (1972) presented a scatter plot which showed no correlation at all between aversion strength and ISis up to 22 rain; it is also of interest that several investigators appear to find that the maximum 1SI that can be tolerated in a one-trial procedure is in the neighborhood of 6 hr (Kalat and Rozin, 1970; 1971; Nachman and Jones, 1974). Further data on this point are needed before conclusions can be drawn, however. It is not clear whether the decrements in neophobia observed in the present work with increasing exposure-test intervals were due to short-term, long-term, or both memory mechanisms. I f short-term mechanisms were operating, then more than 45 min were needed for taste information to become encoded sufficiently for the rats to utilize it. Such a suggestion is consistent with the idea that the gustatory system uses an unusually long time base, as exemplified by conditioned taste aversion learning, and as compared to telereceptor systems, where information in short-term memory is available for use shortly after events have been experienced. If short-term mechanisms were not operating, then one must attribute the decrements in neophobia primarily to long-term mechanisms. A choice between the mechanisms cannot be made at the present time. Kesner (1973) reviewed methods for separating short-term and long-term memory systems by means o f electrical stimulation of specific brain sites. Application of such methods in the present paradigm might help resolve the issue.
REFERENCES Etscorn, F., and Stephens, R. (1973). Establishment of conditioned taste aversions with a 24 hour CS-US interval. PhysioL Psychol. 1,251-253. Garcia, J., Ervin, F. R., and Koelling, R. A. (1966). Learning with prolonged delay of reinforcement. Psychon. Sci. 5, 121-122. Garcia, J., McGowan, B. K., and Green, K. F. (1972). Biological constraints on conditioning. In A. H. Black and W. Prokasy (Eds.), "Classical Conditioning II. Current Research and Theory," pp. 3-27. New York: Appleton-Century-Crofts. Holman, E. W. (1973). Temporal properties of gustatory spontaneous alternation in rats. J. Comp. Physiol. Psychol. 85,536-539. John, E. R. (1967). "Mechanisms of Memory." New York: Academic Press. Kalat, J. W., and Rozin, P. (1970). "Salience": A factor which can override temporal contiguity in taste-aversion learning. J. Comp. Physiol. Psychol. 71, 192-197. Kalat, J. W., and Rozin, P. (1971). Role of interference in taste aversion learning. J. Comp. Physiol. Psychol. 77, 53-58. Kesner, R. (1973). A neural system analysis of memory storage and retrieval. Psychol. Bull. 80, 177-203.
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McGaugh, J. L., and Dawson, R. G. (1971). Modification of memory storage processes. Behav. Sei. 16, 45-63. McGaugh, J. L., and Herz, M. J. (1972). "Memory Consolidation." San Francisco: Albion Publ. Co. Nachman, M. (1970). Learned taste and temperature aversions due to lithium chloride sickness after temporal delays. J. Comp. PhysioL Psyehol. 73, 22-30. Nachman, M., and Jones, D. R. (1974). Learned taste aversions over long delays in rats: The role of learned safety. J. Comp. Physiol. Psyehol. 86,949-956. Ost, J. W. (1969). Consolidation disruption and inhibition in classical conditioning. PsychoL Bull. 72, 379-383. Revusky, S. H. (1968). Aversion to sucrose produced by contingent X-irradiation. 3". Comp. Physiol. Psychol. 65, 17-22. Revusky, S. H. (1971). The role of interference in association over a delay. In W. K. Honig and H. James (Eds.), "Animal Memory," New York: Academic Press. Revusky, S. H., and Garcia, J. (1970). Learned associations over long delays. In G. H. Bower and J. T. Spence (Eds.), "The Psychology of Learning and Motivation: Advances in Research and Theory," Vol. 4, New York: Academic Press. Rozin, P., and Kalat, J. W. (1971). Specific hungers and poison avoidance as adaptive specializations of learning. P~yehol. Rev. 78,459-485. Seligman, M. E. P., and Hager, J. L. (Eds.) (1971). "Biological Boundaries of Learning," New York: Appleton-Century-Crofts. Smith, J. C., and Roll, D. L. (1967). Trace conditioning with X-rays as an aversive stimulus. Psyehon. SoL 9, 11-12.
Gustatory Memory: Incubation and Interference 1
KENNETH F. GREEN and LINDA A. P A R K E R 2
California State University, Long Beach, California The present study explored the mechanism by which gustatory memory is encoded by employing a simple biological test. The test was based on the fact that rats presented with a novel taste demonstrate a marked neophobia for that taste, and, upon subsequent exposures, increase their consumption of that taste. The results indicated that flavor intake in a test increased as time since prior exposure increased up to 6 hr, with the greatest increase occurring within 90 min. Gustatory memory was also found to be susceptible to interference from another novel flavor introduced at a delay of 0 min or 45 rain, but not of 90 min. Some implications of the present data for conditioned taste aversions were discussed.
Conditioned taste aversions can be acquired when long intervals exist between ingestion of a flavored substance (CS) and the experience o f illness induced b y a toxic agent (US). In an early paper, Garcia, Ervin, and Koelling (1966) reported aversions to saccharin-flavored water with interstimulus intervals (ISis) occurring between the end o f ingestion and the injection of apomorphine o f up to 75 rain. Later workers obtained maximal ISis o f nearly 8 hr between sucrose and X-irradiation (Revusky, 1968), 1 2 h r between saccharin and X-irradiation (Smith and Roll, 1967), and 2 4 h r between saccharin and cyclophosphamide injection (Etscorn and Stephens, 1973). Hypotheses advanced to explain the phenomenon o f long-delay learning have dealt with temporal contiguity between aftertaste and illness, the lack of interference by other tastes in the ISI, the "belongingness" o f flavors and illness, and the organization o f the central nervous system (see reviews by Garcia, McGowan, and Green, 1972; Revusky, 1971; Revusky and Garcia, 1970; Rozin and Kalat, 1971; and Seligman and Hager, 1972). All the hypotheses depend to some extent upon the abilities o f animals to remember flavored substances long enough to associate them with illness, and it is to the 1Based on a Master's thesis by the second author under the direction of the first author. Committee members Ralph Hupka and Gilbert Padilla made helpful suggestions. 2Now at The Memorial University of Newfoundland, St. John's. 359 Copyright © 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
360
GREEN AND PARKER
nature of such a gustatory memory mechanism that the present work is directed. In particular, the time course over which gustatory memory becomes encoded is examined within the conceptual framework of consolidation theory. During the course of consolidation, memory traces are said to pass through a labile phase where they are easily disrupted by manipulations such as ECS (McGaugh and Dawson, 1971) and during which they may be easily associated with other cues (John, 1967). After passing through the labile phase, memory traces become fixated and are relatively hard to disrupt. Study of the time course of consolidation of gustatory memories might provide information on the time when the memory is in the labile phase-when it is presumably most associable-and thus might aid our understanding of the mechanism whereby taste aversions can be acquired with long ISis. No aversion test was used in the present study. Rather, the method chosen relied upon the rat's reluctance to ingest water flavored with novel tastes. Since this neophobic reaction occurs only to novel flavors and not to familiar flavors, a rat would be expected to ingest more of an initially novel substance as he became more familiar with it, and the decrement in neophobia could thus serve as an index of memory consolidation. The paradigm used in the present work permitted thirsty rats to drink a novel fluid for a brief period, then, using a two-bottle test to compensate for differential deprivation levels, measured the preferences for the novel fluid in separate groups of rats at varous times after the initial exposure. Increasing preference for the novel flavor, with increasing intervals between initial exposure and test exposure, was understood to reflect increasing familiarity with the flavor, which could be attributed to consolidation of the memory of that flavor. EXPERIMENT 1 Experiment 1 examined the span of the 1Sis providing maximum reduction in neophobia, as evidenced by increased intake of a novel solution relative to water in a two-bottle test. Eight intervals, selected on the basis of delay gradients reported in studies of taste aversion learning (e.g., Etscorn and Stephens, t973; Nachman, 1970; Revusky, 1968; Smith and Roll, 1967), were 0, 45, and 90 rain, 3, 6, 9, 12, and 24 hr. A ninth group, used as a control, received only a two-bottle test. Since preliminary work had indicated a possible linkage of intake to diurnal cycles, an attempt was made to compensate for this source of contamination by starting one S from every ISI condition every 2½ hr until all Ss had been run. The initial flavor exposure lasted for 10 min in all ISI groups, except the group given the 0-rain delay between initial exposure and two-bottle test: The 0-min group was given a 2½ min initial exposure to guard against satiation effects disrupting intake in the two-bottle test.
GUSTATORY MEMORY
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Method
Ninety male Wistar rats (Hilltop; Chatsworth, CA) weighed 190-300 g (mean, 250 g) at the start of testing. Each rat was housed individually, with Purina Rat Chow available ad lib. The rats had previously been used in taste tests involving very dilute hydrochloric acid, and had never experienced administration of toxic agents. Before the experimental procedures began, the rats were placed on a water-deprivation schedule for 13 days: They were trained to ingest their entire daily ration of water in a single 1-hr session, except on the 13th day when they were allowed water for only 20 min. On the 14th day of deprivation the rats were well trained to drink immediately, and experimental procedures began. Every 2½ hr, eight rats were presented a saccharin solution (2 g/liter of tap water) as a novel flavor. Seven of the rats had saccharin available for 10 min and were then assigned to the 45- or 90-rain or 3-, 6-, 9-, 12-, or 24-hr group. One of the rats was given the 0.2% saccharin solution for 2½ min, and assigned to the 0-rain group. Posttests for all rats were two-bottle preference tests where 0.2% saccharin solution and plain tap water were presented for 20 min. For each rat the two solutions were presented one at a time, with the order counterbalanced across rats, to enable sampling o f each taste. After each solution had been sampled, both bottles were presented with the positioning on the cage counterbalanced across rats, and the rat's consumption of each was recorded to the nearest 0.1 g. A control group (n = 10) was used to check the assumption of neophobia for a novel solution. The rats were administered novel 0.2% saccharin solution and tap water simultaneously for 20 min in a two-bottle test, with no preexposure.
Results
The data were analyzed in terms of the percentage intake of saccharin relative to water in the two-bottle preference test (S/(S+W)= flavor score). The means and standard errors are shown in Fig. 1 as a function of ISI. The data indicate an increase, in intake with increasing ISis until reaching asymptote at 6 hr, after which no significant deviation occurred. A one-way analysis of variance indicated that the increase in flavor scores with increased ISis was significant (F(7,9) = 8.52; P < 0 . 0 0 5 ) . The increase was reflected in a significant linear trend (F(7,9) = 37.0; P < 0.001). A significant quadratic trend (F(7,9) = 11.0; P < 0.001) reflected the fact that flavor scores reached asymptote at 6 hr. Groups were compared using the Mann-Whitney U Test. A number of significant differences were found between groups, with the general indication
362
GREEN AND PARKER hO .9
.8 o
.7
o ,,~
.6
o .4
8 o
[~t x--xMoo~ T
Standard error
c
i
I
I
I
I
i
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Om|n.
45rain.
90rain.
3hr.
6 hr
9hK
12hr
i
24hr
/¢/1__.~
Controls
Inter- stimulus intervals
Fig. 1. Mean and standard error of saccharin scores (see text) in tests conducted at the indicated postexposure intervals in Experiment 1.
being no difference among controls, 0-min, and 45-min groups, but with each of these groups differing from each of the other groups.
Discussion The data indicated a progressive increase in intake of a solution with increasing ISis until reaching asymptote at 6 hr. Such a result may indicate a consolidation process. That is, the gradual reduction in neophobia, evidenced by increased preference for saccharin over time, may reflect a process in which the memory for the taste is being consolidated into long-term storage. When the behavioral index reached a ceiling, consolidation was complete-the memory trace was fixated. Thus, the present paradigm seems to be a robust test for consolidation of memory traces of single stimuli, since it was sensitive to the reduction of neophobia that occurred over a long span of time and required only one trial.
EXPERIMENT 2 Consolidation models assume that a learning trial produces a state of gradual encoding that may be disrupted by an interfering event. The longer the process remains undisturbed, the more probably it will become fixated into long-term memory storage. Experiment 2 was conducted to determine whether such an interference mechanism operated upon consolidation of a gustatory memory trace. Six groups of rats drank novel milk and were tested
363
GUSTATORY MEMORY TABLE 1 The Presentation Schedule of the Solutions for Each of the Groups in Experiment 2 10-Min
Group Controls 0 rain 45 rain 90 rain 3 hr 6 hr
initial exposure Milk Milk Milk Milk Milk Milk
5-Min interfering exposure 0 min 45 min 90 rain 3 hr
6 hr
20-Min test 9 hr
Sego
Test Test Test Test Test Test
Sego Sego Sego Sego
for memory of it 9 hr later. Interference was a second novel taste given to five groups at different times after the initial drink o f milk; the sixth group was a no-interference control. It was expected that the less the time between initial exposure to milk and introduction of the second novel taste the greater would be the disruption of the memory o f milk, and the greater would be the neophobic response to milk in the 9-hr test.
Method The same Ss as in Experiment 1 were used. They were assigned to groups with care taken to control for previous experience by counterbalancing as much as possible. The rats weighed between 264 and 400 g (mean 340 g) at the start o f testing. Each group contained 15 rats maintained in individual cages without water b u t with Purina Rat Chow ad lib. Between experiments the rats had been maintained on a schedule o f 1 hr of water per day, except on the day preceding testing, when water was presented for 20 min. The experimental procedure employed six groups-five experimental and one control. All groups were given an initial 10-min exposure to novel Pet milk (mixed 1:1 with tap water) and were subsequently tested for memory of the milk 9 hr later, with milk and water presented in a two-bottle test as described in Experiment 1. The 9-hr interval was chosen because it appeared in Experiment 1 that memory had reached asymptote by that time. The groups varied with respect to the interval between exposure to the test solution and exposure to the interfering solution (Very Strawberry Sego, undiluted). Five 5-min interfering-solution intervals were used, with a separate group o f 15 rats assigned to each interval. The intervals were 0, 45, and 90 rain, and 3 and 6 h r . A control group (n = 15) was used for which no interfering solution was presented during the interval. The paradigm is depicted in Table 1.
364
GREEN AND PARKER 1.0
t _= .8
t
.7
o
.6
o
Means
x~ I
.5
Omln.
4 5 rain.
Standard error
90mln.
3hr.
6hr.
Controls
interference Introduction Times
Fig. 2. Mean and standard error in flavor scores in a 9-hr postexposure test for groups varying according to time of introduction of interference in Experiment 2.
Results
Flavor scores (M/(M+W) = flavor score) were analyzed as in Experiment 1. Means and standard errors are presented in Fig. 2, the ordinate o f which has been foreshortened to reflect the initial preferability of milk. Intake of milk was least when Sego was introduced 0 rain and 45 min after initial exposure to milk. At 90 min and thereafter, no significant effect of Sego exposure was demonstrated by reduced neophobia relative to controls. A one-way analysis of variance indicated that the interval between milk and Sego produced a significant effect on preference for milk (F(4,14) = 10.7; P < 0.001). The increase in flavor scores with increased ISis accounted for a significant linear trend (F(4,14) = 37.2; P < 0.001). The reduced interference after 90 rain contributed to a significant quadratic trend (F(4,14) = 7.4; P < 0.01). Analysis of comparisons between means by Mann-Whitney U Test showed that the 0-min and 45-min groups did not differ from each other, but did differ from all other groups. Discussion
The preferences for milk during the 9-hr memory test may indicate that consolidation of gustatory memory was interfered with by introduction of another novel solution during the interval between initial and subsequent exposures to the test solution. The earlier during the interval that the second
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solution was presented, the greater was the disruption, as shown by a greater neophobic response to the initial solution. The results are consistent with those of Experiment 1 in that the greatest disruption occurred when interference was introduced between 0 and 45 min after initial exposure to a novel solution. When the interfering solution was introduced with a delay of 90 rain or more, little decrement in consolidation occurred. Thus, gustatory memory seems most susceptible to interference, or most highly labile, during the first 90 min after initial exposure.
DISCUSSION The results suggest that gustatory memory may be encoded into long-term storage by a consolidation process, and that the encoding may be disrupted by the presentation of another novel taste soon after exposure to the test solution. In particular, consolidation of gustatory memory appears to be complete at the end of 6 hr, and the memory trace appears to be highly labile during the first 45 min. The time-dependent relationship in the results is consistent with studies which demonstrate with conventional learning paradigms that a process resembling consolidation encodes experiences into long-term storage (McGaugh and Herz, 1972). Direct comparison with the time course of consolidation of other memory traces is hampered by the fact that memory for an association rather than memory of a single stimulus (retentive rather than associative memory, to use Revusky's (1971) nomenclature) is usually examined. However, some comparison can be made with data from other taste experiments. For example, Holman (1973) reported that rats tended to alternate their choices between familiar isohedonic flavors-the tendency to alternate was strongest immediately after ingestion of one flavor, and waned steadily until an indifference point was reached at about 6 hr. Whether the 6-hr indifference point in Holman's work is comparable to the 6-hr consolidation time in the present work is not immediately clear, but one may assume that the operation of gustatory memory mechanisms is reflected in both sets of data. That gustatory memory traces are most highly labile before expiration of the first 90 postingestion rain is suggested by the present work in two ways. First, the rapid rise in intake in Experiment 1 between 45 and 90 rain indicates a rapid increase in the rats' recognition of flavors. Second, the disruption in the second experiment of consolidation through retroactive inhibition produced by a second flavor was maximal before 90 min had elapsed. The latter finding is consistent with the idea that a memory trace in the labile phase is not fixated, therefore it is more easily subject to disruption through retroactive inhibition than after it has been fixated (John, 1967; Ost, 1969; McGaugh and Herz, 1972).
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If a memory trace in the labile phase is also more associable than when it has been fixated, results o f the present work suggest that aversions may be conditioned most readily with ISis of 45 rain or less, and more readily with ISis under 6 hr than above 6 hr. In this regard it is of interest that Garcia, McGowan, and Green (1972) presented a scatter plot which showed no correlation at all between aversion strength and ISis up to 22 rain; it is also of interest that several investigators appear to find that the maximum 1SI that can be tolerated in a one-trial procedure is in the neighborhood of 6 hr (Kalat and Rozin, 1970; 1971; Nachman and Jones, 1974). Further data on this point are needed before conclusions can be drawn, however. It is not clear whether the decrements in neophobia observed in the present work with increasing exposure-test intervals were due to short-term, long-term, or both memory mechanisms. I f short-term mechanisms were operating, then more than 45 min were needed for taste information to become encoded sufficiently for the rats to utilize it. Such a suggestion is consistent with the idea that the gustatory system uses an unusually long time base, as exemplified by conditioned taste aversion learning, and as compared to telereceptor systems, where information in short-term memory is available for use shortly after events have been experienced. If short-term mechanisms were not operating, then one must attribute the decrements in neophobia primarily to long-term mechanisms. A choice between the mechanisms cannot be made at the present time. Kesner (1973) reviewed methods for separating short-term and long-term memory systems by means o f electrical stimulation of specific brain sites. Application of such methods in the present paradigm might help resolve the issue.
REFERENCES Etscorn, F., and Stephens, R. (1973). Establishment of conditioned taste aversions with a 24 hour CS-US interval. PhysioL Psychol. 1,251-253. Garcia, J., Ervin, F. R., and Koelling, R. A. (1966). Learning with prolonged delay of reinforcement. Psychon. Sci. 5, 121-122. Garcia, J., McGowan, B. K., and Green, K. F. (1972). Biological constraints on conditioning. In A. H. Black and W. Prokasy (Eds.), "Classical Conditioning II. Current Research and Theory," pp. 3-27. New York: Appleton-Century-Crofts. Holman, E. W. (1973). Temporal properties of gustatory spontaneous alternation in rats. J. Comp. Physiol. Psychol. 85,536-539. John, E. R. (1967). "Mechanisms of Memory." New York: Academic Press. Kalat, J. W., and Rozin, P. (1970). "Salience": A factor which can override temporal contiguity in taste-aversion learning. J. Comp. Physiol. Psychol. 71, 192-197. Kalat, J. W., and Rozin, P. (1971). Role of interference in taste aversion learning. J. Comp. Physiol. Psychol. 77, 53-58. Kesner, R. (1973). A neural system analysis of memory storage and retrieval. Psychol. Bull. 80, 177-203.
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McGaugh, J. L., and Dawson, R. G. (1971). Modification of memory storage processes. Behav. Sei. 16, 45-63. McGaugh, J. L., and Herz, M. J. (1972). "Memory Consolidation." San Francisco: Albion Publ. Co. Nachman, M. (1970). Learned taste and temperature aversions due to lithium chloride sickness after temporal delays. J. Comp. PhysioL Psyehol. 73, 22-30. Nachman, M., and Jones, D. R. (1974). Learned taste aversions over long delays in rats: The role of learned safety. J. Comp. Physiol. Psyehol. 86,949-956. Ost, J. W. (1969). Consolidation disruption and inhibition in classical conditioning. PsychoL Bull. 72, 379-383. Revusky, S. H. (1968). Aversion to sucrose produced by contingent X-irradiation. 3". Comp. Physiol. Psychol. 65, 17-22. Revusky, S. H. (1971). The role of interference in association over a delay. In W. K. Honig and H. James (Eds.), "Animal Memory," New York: Academic Press. Revusky, S. H., and Garcia, J. (1970). Learned associations over long delays. In G. H. Bower and J. T. Spence (Eds.), "The Psychology of Learning and Motivation: Advances in Research and Theory," Vol. 4, New York: Academic Press. Rozin, P., and Kalat, J. W. (1971). Specific hungers and poison avoidance as adaptive specializations of learning. P~yehol. Rev. 78,459-485. Seligman, M. E. P., and Hager, J. L. (Eds.) (1971). "Biological Boundaries of Learning," New York: Appleton-Century-Crofts. Smith, J. C., and Roll, D. L. (1967). Trace conditioning with X-rays as an aversive stimulus. Psyehon. SoL 9, 11-12.
