Journal of Comparative and Physiological Psychology June, 1975, Vol. 89, No. 4, 285-294 © 1975 by the American Psychological Association, Inc.
Olfactory Discrimination, Reversal Learning, and Stimulus Control in Rats Barry J. Nigrosh Columbia University
Burton M. Slotnick National Institute of Mental Health, Bethesda, Maryland John A. Nevin University of New Hampshire
Rats were trained to discriminate lights, tones, or odors and then given a series of discrimination reversals. Only rats trained with odors showed positive transfer on the first reversal and acquisition of a reversal set. Other experiments demonstrated that rats preferentially attend to odors when presented in compound with lights or tones; that odors exert more discriminative control than tones in tests using compound stimuli of competing sign; and that after pretraining on the positive stimulus, acquisition of an odor but not a light discrimination occurs with virtually no errors. These results demonstrate the importance of stimulus modality in the establishment of stimulus control and the need for more careful analysis of stimulus factors in cross-species comparisons of learning ability.
discrimination learning has not been examined systematically, studies have demonstrated not only that olfactory cues are used as discriminative stimuli when they are available but that they may be used in preference to visual cues (Thorne & O'Brien, 1971). In a recent experiment in our laboratory (Slotnick & Katz, 1974), learning-set formation of rats was examined using a series of 16 two-odor discrimination problems. Each of 12 rats tested showed a dramatic decrease in errors over the problem series, and asymptotic performance approached nearly errorless learning. Because the interproblem transfer shown in this study was far superior to that previously reported These experiments were conducted in the Lab- for rodents and carnivores tested on visual oratory of Brain Evolution and Behavior, Na- learning-set problems (Warren, 1973), it sugtional Institute of Mental Health, and were sup- gests that the degree of interproblem transfer ported in part by U.S. Public Health Service observed may, in part, be dependent upon Grant MH-18267 to B. M. Slotnick. Experiment 1 the type of cue available for discriminative is based in part on a thesis submitted by B. J. Nigrosh to the Department of Psychology, Columbia responding. The present experiment provides a more University, in partial fulfillment of the requirements for the PhD degree. We are pleased to direct test of this hypothesis by comparing acknowledge the technical assistance of William the ability of separate groups of rats to form Kump in the construction of the olfactometer and a successive discrimination reversal on the of Robert Gelhard in the gathering of data. B. J. Nigrosh is now with the Office of Staff Develop- basis of auditory, visual, or olfactory cues. ment, Northampton State Hospital, Northamp- The successive discrimination reversal task ton, Massachusetts. was chosen because it has been widely used Request for reprints should be sent to Burton to compare interproblem transfer ability M. Slotnick, who is now with the Department of Psychology, American University, Washington, among different species (Bitterman, 1965). However, these studies have usually used D.C. 20016. 285 The recent literature on biological constraints on learning describes many behavioral phenomena that depend on the particular stimuli, responses, and reinforcers employed (e.g., Seligman & Haber, 1972; Shettleworth, 1972). There is some reason to expect that macrosmatic animals such as rats might perform differently with olfactory cues than with the auditory and visual stimuli commonly used in the study of learning. For example, odors provide important information about the environment, and many aspects of intraspecific social behavior of rodents are governed by olfactory signals (Cheal & Sprott, 1971; Eisenberg & Kielman, 1972). Although the role of olfaction in
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B. J. NIGROSH, B. M. SLOTNICK, AND J. A. NEVIN
visual stimuli and have not examined the potential relationship between reversal learning and stimulus modality. A second and related aim of the present research was to evaluate the relative controlling power of olfactory and visual cues or olfactory and auditory cues when presented together during discrimination training. Finally, our data bear on some determinants of errorless learning after equivalent initial training with different modalities of stimulation. EXPERIMENT 1 Method Subjects Ten young adult male Wistar rats were housed individually and kept on a 23.75-hr water-deprivation schedule beginning 5 days after arrival in the laboratory.
Apparatus The chamber, diagrammed in Figure 1, consisted of a 10.2-cm-diam. glass funnel attached by a collar to a 13-cm-long Plexiglas tube. A perforated glass disk in the stem of the funnel served to diffuse the incoming air stream. A 2-mm-diam. tungsten wire extending 1.8 cm through the left wall of the chamber served as a response key. Contacts between a stainless steel floor plate and the response key were detected by a drinkometer circuit. A 10-mm-diam. glass cup connected to a capillary tube was sealed through the bottom of the funnel. Water reinforcement was delivered to the cup by a gravity-fed two-way solenoid valve located directly below the chamber. Air from the chamber was continuosuly exhausted to the building exhaust system by a 100-cfm fan connected to the chamber by a 13-cm-diam. flexible plastic hose. The orifice of the hose was fitted with a wire mesh screen and connected to the rear of the chamber
FIGURE 1. Schematic drawing showing a top view of the test chamber. (A, diffusing screen at air-inlet port; B, glass cup for water reinforcement; C, response key; D, collar connecting glass funnel to Plexiglas tube; E, stainless steel floor plate; F, exhaust hose screen; G, pressure-fit collar connecting exhaust hose to test chamber.)
by pressure fit. A photobeam located across the neck of the chamber was used to detect trialinitiating responses. The odorizing system was constructed of glass and Teflon and is diagrammed in Figure 2. Air from a pressure-regulated compressor was filtered through glass wool and a 60-jum pore fritted glass filter. The air stream was then dehydrated through silica gel and divided into a carrier and odorant stream. Both streams were deodorized through separate columns of activated coconut charcoal. Air in the carrier stream was partly rehydrated by being passed over distilled water and then led into the test chamber via a manifold at a rate of 4 1/min. The odor stream was divided into two channels, and odor stimuli were generated by bubbling air at 160 cc/min through 60 cc of liquid odorant contained in 250-ml gas-washing bottles. The odorized air was led to the common port of a three-way Teflon solenoid valve (Mace Corp.). The normally open port of each valve connected to an exhaust line, and the normally closed port to the manifold. As shown in Figure 2, air flow in the carrier stream and in each odorant stream was controlled by needle valves and monitored by flow meters. Fritted glass filters were placed in the line after the silica gel and activated charcoal columns. The inputs to the gas-washing bottles were fitted with glass stopcocks to isolate them when the system was not in service. All glass-to-glass connections were made of precision-ground glass joints; glass-to-Teflon connections were made by pressure fit.
Odor Stimuli Odorants used were isoamyl acetate diluted 1:1,000 with distilled water and ethyl acetate diluted 1:100 with distilled water. Pilot studies indicated that these concentrations are several orders of magnitude above threshold for the rat. The two stimuli were sampled at the entrance of the test chamber by three human observers and judged to be pleasant, moderately easy to discriminate, and approximately equal in intensity. The chemicals are both of the ethereal class (Harper, Smith, & Land, 1968). The odorants were replaced with fresh solutions after each rat had been tested. Gas chromatographic measures of odor intensity performed subsequent to this experiment showed that the strength of the stimuli did not diminish significantly during the 30-45 min required for a training session. Additional details concerning odor control and the odorizing system are given by Slotnick and Nigrosh (1974), Control by odors rather than extraneous cues was verified by surgical procedures. After unilateral bulbectomy, rats showed little or no deficits in acquisition or retention of an olfactory discrimination. After bilateral bulbectomy, rats had no retention of a previously acquired odor discrimination and no ability to reacquire the discrimination even with prolonged training (Slotnick, 1972; Slotnick & Nigrosh, 1974). Bilateral bulbectom-
OLFACTORY DISCRIMINATION LEARNING
287
o Fritted Filter * Needle Valve r '••••' -i Flow Meter * Stopcock •—> To Exhaust . @ Solenoid Valve
Manifold Gas Wash Bottle
FIGURK 2. Schematic representation of odorizer. (Odor stimuli are generated in gas-washing bottles A and B. The gas-washing bottle in the carrier stream contained water for rehydrating the air.) ized rats were, however, able to acquire a visual discrimination in the same apparatus as well as normal animals.
Tone and Light Stimuli Tone stimuli of 1 kHz and 2.5 kHz were produced by a BRS-Foringer tone generator through a 7.5-cm.-diam. loudspeaker suspended 7.5 cm. above the test chamber. A 7-W, 115-V frosted white light in series with a 2,000-fl resistor was located just above the front of the chamber for presentation of visual stimuli. The light could be flashed at 5/sec (equal on/off duty time) or illuminated steadily during a trial. Procedure Experimental design. Eight subjects were initially trained on a light-discrimination task and on the basis of their performance divided into two equal-sized matched groups and given further training on tone or odor stimuli. After achieving criterion performance the animals were given a series of discrimination reversals. Two additional animals were first trained on an olfactory discrimination task and then tested for reversal learning with light stimuli (Group L). Training procedures for tone and odor groups. In the first session, standard operant procedures were used to shape a paw or mouth contact with the response key for a .04-cc water reward. In the next session, responses were reinforced only when made in the presence of the positive visual stimulus (flashing light). The S+ was presented for a maximum of 10 sec at variable intervals averaging 20 sec, and a response during the trial terminated the stimulus simultaneous with the delivery of
reinforcement. When responses occurred reliably soon after the onset of the stimulus, further trials were made contingent upon the animal's breaking the photobeam at the neck of the chamber (trialinitiating response). Training was continued until the rat made 30 beam-break key responses. In the third session, 200 S+ trials were allowed during which the trial duration was progressively decreased to 4 sec, an .8-sec fixed-interval requirement was introduced at the beginning of the trial, and intertrial-interval responses were punished by a 2-sec delay before the next trial could be initiated. Responses during the fixed-interval period were ignored. The minimum intertrial interval was 4 sec. In the next session, the negative (S—) stimulus (steady light) was introduced after the animal had completed 20 additional S+only trials. Errors (responding in the presence of S— or not responding in the presence of S+) were followed by a fixed-ratio-(FR) 6 requirement for the trial initiating response for the trial following the error. The S+ and S— trials were presented in a random order with the restriction that no more than three of each type of trial would occur consecutively and that there were an equal number of pairs and triplets of each type. Sessions were run for 200 trials per day until a criterion of 85% correct responses occurred in 50 consecutive trials. One week after the achievement of criterion by the last animal, all rats were given 100 additional light-discrimination trials and, on the basis of error scores, were divided into two equal-sized tone and odor groups. Beginning on the next day, the two groups were run on alternate days such that each group was run every other day. Fifteen minutes of ad-lib water was given in the home cage to a subject that had finished a session and
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B. J. NIGROSH, B. M. SLOTNICK, AND J. A. NEVIN other, ethyl acetate and steady light were positive. Several aspects of the training procedures were altered for this group as pilot studies indicated that rats frequently failed or showed extremely slow acquisition of the first reversal with visual stimuli. These changes consisted of eliminating the FR-6 requirement after an error and increasing the fixed-interval requirement at the beginning of a trial from .8 to 2 sec and the trial duration from 4 to 5 sec. The FR-6 requirement appeared to have no beneficial effect on performance and for some animals resulted in disruption of trial-initiating behavior. The longer fixed-interval requirement was used to ensure further exposure to the stimulus before responses were effective in terminating the trial.
500n
450-
400-
350-
300-
250-
as 200 H 150-
100-
50-
3
4 5 6 7 REVERSALS
8
9
10
FIGURE 3. Median number of errors on original learning and successive discrimination reversals for groups trained with tones, lights, or odors. to its assigned partner from the group not run that day. For the tone group, original acquisition was carried out with the 1-kHz tone as S+ and the 2.5-kHz tone as S—. For the odor group, ethyl acetate served as S+ and isoamyl acetate as S—. Training was carried out for 200 trials per session until a criterion of 9 out of 10 correct S+ concurrent with 9 out of 10 correct S— trials was achieved. A session was terminated immediately after the last criterion trial. In the following session, the reinforcement values of the S+ and S— stimuli were reversed, and training was continued until criterion was reached. This procedure was repeated until the odor group had completed 10 reversals. Testing of the tone group, which showed much slower learning, was continued until three rats had completed five reversals and one had completed three reversals. Training procedures for the light group. The two rats in this group were initially trained to discriminate the amyl acetate and ethyl acetate odors and then given 10 serial reversals after acquisition of the light discrimination. For one rat, the amyl acetate odor and flashing light were positive on original acquisition, and for the
Results Serial Reversal Discrimination Median error scores for serial reversal training for each group are illustrated in Figure 3. The three groups did not differ in the number of errors made in original acquisition. Each rat in the tone and light groups showed negative transfer on the first reversal and continued to make more errors on the first five reversals than in initial acquisition. The light group showed improvement in the last five reversals with asymptotic performance at approximately the level of initial learning. In contrast to the performance of the tone and light groups, all subjects in the odor group showed positive transfer on the first reversal and asymptotic performance of only a few errors by the fifth reversal. One animal in this group made only two errors in each of the last three reversals. Analysis of individual acquisition functions revealed that all subjects in the odor group responded more often to the S — odor than to the S+ odor in the first odor-training session. This apparent preference for the S — odor was reversed in 20-40 trials and was followed by a decrease in S — responding. On the first reversal, two animals in the odor group perseverated on the response habit of original acquisition for 30 and 60 trials. The remaining two animals in this group initially responded to both S+ and S— on the first reversal and then showed rapid acquisition. On subsequent reversals, all subjects in the odor group showed few or no perseverative errors. For each rat in the tone and light groups, behavior on original acquisition was charac-
OLFACTORY DISCRIMINATION LEARNING
terized by a period of responding to both S + and S— followed by a gradual decrease in S— responding. This pattern was observed on each of the reversals and there was little or no indication of persevcrative responding on the previous habit. Initial Acquisition of the Light and Odor Discriminations It is instructive to compare the acquisition of the eight rats originally trained on the light discrimination with that of the two rats first trained on odors. It will be recalled that discrimination training in these cases was preceded by 200 trials with S+ before S — was introduced. Animals trained on light stimuli required 4-11 sessions (median errors = 335) before reaching a 90 % discrimination criterion. The two rats initially trained on odors made only one and three errors, respectively, in reaching criterion performance after introduction of S — . The performance of these two subjects demonstrates virtually errorless learning of the odor discrimination and stands in marked contrast to that of animals initially trained with light cues and that of rats in the odor group in which odor discrimination training was not preceded by a series of S+-only odor trials. Errorless acquisition of olfactory discrimination is examined further in Experiment 3. EXPERIMENT 2 This experiment compares the degree to which tone and odor stimuli gain control over discrimination performance during compound odor-tone stimulus training. The tests of cue reduction and cue competition used are modified from those described by Reynolds (1961) and Diamond (1967) and provide an evaluation of selective attention. Method Subjects Three young adult Wistar rats were maintained on a 23.75-hr water-deprivation schedule. Apparatus and Procedure The odorizing system and wind tunnel described in Experiment 1 were used. The animals were first trained on a light discrimination and then trained using a compound of tone and odor stimuli. Ethyl acetate odor presented simul-
289
taneously with a 1-kHz tone served as S+. The S— was a compound of amyl acetate odor presented simultaneously with a 2.5-kHz tone. The training procedures described in Experiment 1 for the odor and tone groups were followed except that the FR-error requirement was eliminated and that for compound-stimulus training, the reinforcement probability was reduced to 70%. This partial-reinforcement schedule was used in an effort to slow discrimination learning so that the course of behavioral control by each element of the stimulus compound could be more easily determined during acquisition by interpolated cue-reduction tests and also to reduce the likelihood that the rats would learn to discriminate test trials from training trials. A cue-reduction trial consisted of presenting one of the tones or odors alone. The four element types were presented in random order and in random positions throughout compound-stimulus training with the restriction that only one cue-reduction trial was given in each block of nine compound trials. Responses during cue-reduction trials were not reinforced. Following criterion performance of 86% correct responding in 50 compound trials, an additional 350 compound trials with interpolated cue-reduction tests were given in a single session. Tone training, odor training, and competition tests. On the day after completion of the above procedures, alternating blocks of 40 trials on the tone stimuli alone and on the odor stimuli alone were given until each rat achieved a criterion of 9 out of 10 correct concurrent S+ and S— trials for each modality. During this training, all correct responses were reinforced. In the next session, 350 trials of compound stimulus training were given during which reinforcement probability for correct responding was 70% and nonreinforced competition test trials were inserted. On these test trials, the S+ odor and the S— tone or the S— odor and the S+ tone were presented together in random order so that one competition test was given in each block of nine compound-training trials.
Results
Compound-Stimulus Training and Cue Reduction In spite of the partial-reinforcement schedule, all three rats reached criterion so rapidly (400-450 trials) that there were too few interpolated cue-reduction trials to allow analysis of the order in which the separate elements gained stimulus control. As illustrated in the left-hand side of Figure 4, each rat responded to 95 %-100 % of the S+ compound trials and to the S+ odor element of the compound during the postacquisition test session. Two of the rats showed little or no responding to the tone clement of the S+
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B. J. NIGROSH, B. M. SLOTNICK, AND J. A. NEVIN COMPETITION
CUE REDUCTION
as-o
I
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1 .
n
+0 +T
-0 -T
L 1 II +0 +T -0
-T
+0 +T
—0 -T
+0 —0 -T +T
STIMULUS COMPONENTS
FIGURE 4. Percent correct responses for three rats to compound stimuli (open bars) and test stimulus components (solid bars) for the cue-reduction and cue-competition phases of Experiment 2. (Abbreviations: O = odor; T = tone.)
compound, but one responded on 70% of these cue-reduction trials. As expected, few responses were made on the S— compound trials or S— cue-reduction test trials. Tone Training, Odor Training, and Competition Tests On training with tones alone and odors alone, each rat reached criterion during the third block of trials of each modality (medians: tone, 118 trials; odors, 106 trials). The right-hand portion of Figure 4 shows the percent responses during the 350 compound training trials of this session and for the competition trials. On the competition trials consisting of the S+ odor and S— tone elements, each rat responded as frequently as it did on the S+ compound trials. However, considerable inhibition of responding was observed when the S—- odor was paired with the S+ tone. EXPERIMENT 3 This experiment tests the degree to which olfactory cues are learned when they are su-
perimposed upon an established visual discrimination. In addition, a comparison between animals initially trained on olfactory or visual stimuli after S+-only training examines further the "errorless" learning of an olfactory discrimination described in Experiment 1. Method Four young adult Wistar rats (the light-odor group) were initially trained on a visual discrimination. The procedures described in Experiment 1 for the light group were followed: After 220-250 S+-only trials, S— was introduced, and training was continued until stable performance at 90% accuracy had been achieved. In the next session, after 40 visual discrimination trials, odors were added as discriminative stimuli. After 160 trials of the compound stimulus, the lights were turned off, and odors only were available as discriminative cues. Training was continued for 60 trials, and if necessary, additional sessions were given until criterion performance of 9 out of 10 correct S+ concurrent with 9 out of 10 correct S— trials was achieved. Four other animals (the odor-light group) were given identical training except that they were first trained to discriminate odors, given compound odor-light trials, and tested on the visual stimuli alone. The positive stimuli used for each animal in the two groups are given in the right-hand column of Table 1. Results
Initial Acquisition Table 1 lists the number of errors to criterion on the original problem for each subj ect in the light-odor and odor-light groups. Following the introduction of the S — stimuTABLE 1 ERRORS TO CRITERION AND POSITIVE STIMULI USED FOR RATS IN EXPERIMENT 3 Subject
AcquisiInitial tion after acquisi- compound tion training
Group L-O 123 125 126 131
119 171 95 130
lldn
124.5
Group O-L 127 128 129 130 Mdn
0 2 2 1 1.5
1 0 28 14 7.5 135 40 103 127 115
Positive stimuli
Flashing light; amyl acetate Flashing light; ethyl acetate Flashing light; ethyl acetate Steady light; ethyl acetate Ethyl acetate; steady light Amyl acetate; flashing light Amyl acetate; steady light Ethyl acetate; flashing light
OLFACTORY DISCRIMINATION LEARNING
lus, animals trained on lights (the light-odor group) initially responded on almost all trials and only gradually began to show response inhibition on S— trials. In contrast, animals trained on odors made only zero to two errors in reaching criterion after the S— stimulus was introduced. Compound Training In the light-odor group, two rats made only zero and one errors, respectively, in reaching discrimination criterion for odors after the 160 compound-stimulus trials. These rats had clearly learned the significance of the odor cues during compoundstimulus training. The other two animals in this group made more errors (see Table 1), but still showed rapid acquisition of the odor discrimination after visual stimuli were removed. In contrast, the four rats in the odor-light group made 40-135 errors before reaching criterion on visual stimuli after compound-stimulus training. As the median number of errors to reach criterion for light stimuli for this group \\ as virtually identical to that made by the light-odor group, it is clear that rats in odor-light group showed no acquisition of the light discrimination during their compound-stimulus training. Observation of the animals during the introduction of discriminative stimuli in each stage of this study also revealed striking differences in the behavior of the two groups. During S+-only training, animals given visual stimuli typically made key responses immediately after initiating a trial. These animals either made multiple responses or maintained contact with the key through the 2-scc fixed-interval period until reinforcement was delivered. For the first two or three presentations of the S— stimulus, each animal showed a startlelike response and either did not make a key contact (three rats) or responded only after a delay of several seconds. Thereafter, the animals responded quickly to both S+ and S— stimuli for 100 or more trials until discriminative behavior began to emerge. When odor stimuli were first introduced, each animal in this group made no key responses for 3-10 trials, but instead, spent the entire trial period sniffing at the air-inlet port. Performance accuracy dropped to chance or near chance levels in
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the first block of 20 compound-stimulus trials for each rat in the light-odor group. Animals in the odor-light group typically sniffed at the air-inlet port for 1-2 sec on each of the S+-only trials before responding. When the S — odor was first introduced, each animal spent the entire trial period sniffing at the air-inlet port. During continued odordiscrimination training, these animals showed stimulus-sampling behavior on every trial. On S— trials, they either maintained sampling behavior through the trial period or, after a 1-2 sec sample, backed away from air-inlet port and licked at the water cup, groomed, or turned around to face the back of the chamber. In the first 100 odor-discrimination trials, these rats made only three to six S— errors. During compound-stimulus training, each animal showed a startlelike reaction to the visual stimuli for the first two or three trials. One animal (Rat 127) dropped to chance performance in the first block of 20 compound trials. The remaining three animals continued to perform at or near criterion level throughout the compound-stimulus training. The results of this experiment arc best summarized in Figure 5 which shows the complete performance record for one rat from each group. Note that Rat 0-L 129 (Figure 5, top) shows no decrement in performance when the S — odor is introduced or during compound odor-light training. However, performance drops to chance levels when only light stimuli are available. In contrast, the performance of Rat L-0 123 drops to chance levels when the S— light is introduced during initial light training, and performance is again disturbed when odors are first presented during the light-odor compound trials. However, after the light stimuli are removed, the animal continues to perform at or near criterion level. To determine if errorless acquisition of the light discrimination would occur after extensive experience with S +, we allowed three additional rats 1,500 S+ light trials before introducing S — . These three rats made 24, 27, and 114 errors, respectively, in reaching criterion. Although they acquired the discrimination faster (p < .06, one-tailed Utest) than those allowed only 200 S+ light trials (light-odor group), their performance
B. J. NIGROSH, B. M. SLOTNICK, AND J. A. NEVIN
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00
Olfactory Discrimination, Reversal Learning, and Stimulus Control in Rats Barry J. Nigrosh Columbia University
Burton M. Slotnick National Institute of Mental Health, Bethesda, Maryland John A. Nevin University of New Hampshire
Rats were trained to discriminate lights, tones, or odors and then given a series of discrimination reversals. Only rats trained with odors showed positive transfer on the first reversal and acquisition of a reversal set. Other experiments demonstrated that rats preferentially attend to odors when presented in compound with lights or tones; that odors exert more discriminative control than tones in tests using compound stimuli of competing sign; and that after pretraining on the positive stimulus, acquisition of an odor but not a light discrimination occurs with virtually no errors. These results demonstrate the importance of stimulus modality in the establishment of stimulus control and the need for more careful analysis of stimulus factors in cross-species comparisons of learning ability.
discrimination learning has not been examined systematically, studies have demonstrated not only that olfactory cues are used as discriminative stimuli when they are available but that they may be used in preference to visual cues (Thorne & O'Brien, 1971). In a recent experiment in our laboratory (Slotnick & Katz, 1974), learning-set formation of rats was examined using a series of 16 two-odor discrimination problems. Each of 12 rats tested showed a dramatic decrease in errors over the problem series, and asymptotic performance approached nearly errorless learning. Because the interproblem transfer shown in this study was far superior to that previously reported These experiments were conducted in the Lab- for rodents and carnivores tested on visual oratory of Brain Evolution and Behavior, Na- learning-set problems (Warren, 1973), it sugtional Institute of Mental Health, and were sup- gests that the degree of interproblem transfer ported in part by U.S. Public Health Service observed may, in part, be dependent upon Grant MH-18267 to B. M. Slotnick. Experiment 1 the type of cue available for discriminative is based in part on a thesis submitted by B. J. Nigrosh to the Department of Psychology, Columbia responding. The present experiment provides a more University, in partial fulfillment of the requirements for the PhD degree. We are pleased to direct test of this hypothesis by comparing acknowledge the technical assistance of William the ability of separate groups of rats to form Kump in the construction of the olfactometer and a successive discrimination reversal on the of Robert Gelhard in the gathering of data. B. J. Nigrosh is now with the Office of Staff Develop- basis of auditory, visual, or olfactory cues. ment, Northampton State Hospital, Northamp- The successive discrimination reversal task ton, Massachusetts. was chosen because it has been widely used Request for reprints should be sent to Burton to compare interproblem transfer ability M. Slotnick, who is now with the Department of Psychology, American University, Washington, among different species (Bitterman, 1965). However, these studies have usually used D.C. 20016. 285 The recent literature on biological constraints on learning describes many behavioral phenomena that depend on the particular stimuli, responses, and reinforcers employed (e.g., Seligman & Haber, 1972; Shettleworth, 1972). There is some reason to expect that macrosmatic animals such as rats might perform differently with olfactory cues than with the auditory and visual stimuli commonly used in the study of learning. For example, odors provide important information about the environment, and many aspects of intraspecific social behavior of rodents are governed by olfactory signals (Cheal & Sprott, 1971; Eisenberg & Kielman, 1972). Although the role of olfaction in
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B. J. NIGROSH, B. M. SLOTNICK, AND J. A. NEVIN
visual stimuli and have not examined the potential relationship between reversal learning and stimulus modality. A second and related aim of the present research was to evaluate the relative controlling power of olfactory and visual cues or olfactory and auditory cues when presented together during discrimination training. Finally, our data bear on some determinants of errorless learning after equivalent initial training with different modalities of stimulation. EXPERIMENT 1 Method Subjects Ten young adult male Wistar rats were housed individually and kept on a 23.75-hr water-deprivation schedule beginning 5 days after arrival in the laboratory.
Apparatus The chamber, diagrammed in Figure 1, consisted of a 10.2-cm-diam. glass funnel attached by a collar to a 13-cm-long Plexiglas tube. A perforated glass disk in the stem of the funnel served to diffuse the incoming air stream. A 2-mm-diam. tungsten wire extending 1.8 cm through the left wall of the chamber served as a response key. Contacts between a stainless steel floor plate and the response key were detected by a drinkometer circuit. A 10-mm-diam. glass cup connected to a capillary tube was sealed through the bottom of the funnel. Water reinforcement was delivered to the cup by a gravity-fed two-way solenoid valve located directly below the chamber. Air from the chamber was continuosuly exhausted to the building exhaust system by a 100-cfm fan connected to the chamber by a 13-cm-diam. flexible plastic hose. The orifice of the hose was fitted with a wire mesh screen and connected to the rear of the chamber
FIGURE 1. Schematic drawing showing a top view of the test chamber. (A, diffusing screen at air-inlet port; B, glass cup for water reinforcement; C, response key; D, collar connecting glass funnel to Plexiglas tube; E, stainless steel floor plate; F, exhaust hose screen; G, pressure-fit collar connecting exhaust hose to test chamber.)
by pressure fit. A photobeam located across the neck of the chamber was used to detect trialinitiating responses. The odorizing system was constructed of glass and Teflon and is diagrammed in Figure 2. Air from a pressure-regulated compressor was filtered through glass wool and a 60-jum pore fritted glass filter. The air stream was then dehydrated through silica gel and divided into a carrier and odorant stream. Both streams were deodorized through separate columns of activated coconut charcoal. Air in the carrier stream was partly rehydrated by being passed over distilled water and then led into the test chamber via a manifold at a rate of 4 1/min. The odor stream was divided into two channels, and odor stimuli were generated by bubbling air at 160 cc/min through 60 cc of liquid odorant contained in 250-ml gas-washing bottles. The odorized air was led to the common port of a three-way Teflon solenoid valve (Mace Corp.). The normally open port of each valve connected to an exhaust line, and the normally closed port to the manifold. As shown in Figure 2, air flow in the carrier stream and in each odorant stream was controlled by needle valves and monitored by flow meters. Fritted glass filters were placed in the line after the silica gel and activated charcoal columns. The inputs to the gas-washing bottles were fitted with glass stopcocks to isolate them when the system was not in service. All glass-to-glass connections were made of precision-ground glass joints; glass-to-Teflon connections were made by pressure fit.
Odor Stimuli Odorants used were isoamyl acetate diluted 1:1,000 with distilled water and ethyl acetate diluted 1:100 with distilled water. Pilot studies indicated that these concentrations are several orders of magnitude above threshold for the rat. The two stimuli were sampled at the entrance of the test chamber by three human observers and judged to be pleasant, moderately easy to discriminate, and approximately equal in intensity. The chemicals are both of the ethereal class (Harper, Smith, & Land, 1968). The odorants were replaced with fresh solutions after each rat had been tested. Gas chromatographic measures of odor intensity performed subsequent to this experiment showed that the strength of the stimuli did not diminish significantly during the 30-45 min required for a training session. Additional details concerning odor control and the odorizing system are given by Slotnick and Nigrosh (1974), Control by odors rather than extraneous cues was verified by surgical procedures. After unilateral bulbectomy, rats showed little or no deficits in acquisition or retention of an olfactory discrimination. After bilateral bulbectomy, rats had no retention of a previously acquired odor discrimination and no ability to reacquire the discrimination even with prolonged training (Slotnick, 1972; Slotnick & Nigrosh, 1974). Bilateral bulbectom-
OLFACTORY DISCRIMINATION LEARNING
287
o Fritted Filter * Needle Valve r '••••' -i Flow Meter * Stopcock •—> To Exhaust . @ Solenoid Valve
Manifold Gas Wash Bottle
FIGURK 2. Schematic representation of odorizer. (Odor stimuli are generated in gas-washing bottles A and B. The gas-washing bottle in the carrier stream contained water for rehydrating the air.) ized rats were, however, able to acquire a visual discrimination in the same apparatus as well as normal animals.
Tone and Light Stimuli Tone stimuli of 1 kHz and 2.5 kHz were produced by a BRS-Foringer tone generator through a 7.5-cm.-diam. loudspeaker suspended 7.5 cm. above the test chamber. A 7-W, 115-V frosted white light in series with a 2,000-fl resistor was located just above the front of the chamber for presentation of visual stimuli. The light could be flashed at 5/sec (equal on/off duty time) or illuminated steadily during a trial. Procedure Experimental design. Eight subjects were initially trained on a light-discrimination task and on the basis of their performance divided into two equal-sized matched groups and given further training on tone or odor stimuli. After achieving criterion performance the animals were given a series of discrimination reversals. Two additional animals were first trained on an olfactory discrimination task and then tested for reversal learning with light stimuli (Group L). Training procedures for tone and odor groups. In the first session, standard operant procedures were used to shape a paw or mouth contact with the response key for a .04-cc water reward. In the next session, responses were reinforced only when made in the presence of the positive visual stimulus (flashing light). The S+ was presented for a maximum of 10 sec at variable intervals averaging 20 sec, and a response during the trial terminated the stimulus simultaneous with the delivery of
reinforcement. When responses occurred reliably soon after the onset of the stimulus, further trials were made contingent upon the animal's breaking the photobeam at the neck of the chamber (trialinitiating response). Training was continued until the rat made 30 beam-break key responses. In the third session, 200 S+ trials were allowed during which the trial duration was progressively decreased to 4 sec, an .8-sec fixed-interval requirement was introduced at the beginning of the trial, and intertrial-interval responses were punished by a 2-sec delay before the next trial could be initiated. Responses during the fixed-interval period were ignored. The minimum intertrial interval was 4 sec. In the next session, the negative (S—) stimulus (steady light) was introduced after the animal had completed 20 additional S+only trials. Errors (responding in the presence of S— or not responding in the presence of S+) were followed by a fixed-ratio-(FR) 6 requirement for the trial initiating response for the trial following the error. The S+ and S— trials were presented in a random order with the restriction that no more than three of each type of trial would occur consecutively and that there were an equal number of pairs and triplets of each type. Sessions were run for 200 trials per day until a criterion of 85% correct responses occurred in 50 consecutive trials. One week after the achievement of criterion by the last animal, all rats were given 100 additional light-discrimination trials and, on the basis of error scores, were divided into two equal-sized tone and odor groups. Beginning on the next day, the two groups were run on alternate days such that each group was run every other day. Fifteen minutes of ad-lib water was given in the home cage to a subject that had finished a session and
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B. J. NIGROSH, B. M. SLOTNICK, AND J. A. NEVIN other, ethyl acetate and steady light were positive. Several aspects of the training procedures were altered for this group as pilot studies indicated that rats frequently failed or showed extremely slow acquisition of the first reversal with visual stimuli. These changes consisted of eliminating the FR-6 requirement after an error and increasing the fixed-interval requirement at the beginning of a trial from .8 to 2 sec and the trial duration from 4 to 5 sec. The FR-6 requirement appeared to have no beneficial effect on performance and for some animals resulted in disruption of trial-initiating behavior. The longer fixed-interval requirement was used to ensure further exposure to the stimulus before responses were effective in terminating the trial.
500n
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as 200 H 150-
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FIGURE 3. Median number of errors on original learning and successive discrimination reversals for groups trained with tones, lights, or odors. to its assigned partner from the group not run that day. For the tone group, original acquisition was carried out with the 1-kHz tone as S+ and the 2.5-kHz tone as S—. For the odor group, ethyl acetate served as S+ and isoamyl acetate as S—. Training was carried out for 200 trials per session until a criterion of 9 out of 10 correct S+ concurrent with 9 out of 10 correct S— trials was achieved. A session was terminated immediately after the last criterion trial. In the following session, the reinforcement values of the S+ and S— stimuli were reversed, and training was continued until criterion was reached. This procedure was repeated until the odor group had completed 10 reversals. Testing of the tone group, which showed much slower learning, was continued until three rats had completed five reversals and one had completed three reversals. Training procedures for the light group. The two rats in this group were initially trained to discriminate the amyl acetate and ethyl acetate odors and then given 10 serial reversals after acquisition of the light discrimination. For one rat, the amyl acetate odor and flashing light were positive on original acquisition, and for the
Results Serial Reversal Discrimination Median error scores for serial reversal training for each group are illustrated in Figure 3. The three groups did not differ in the number of errors made in original acquisition. Each rat in the tone and light groups showed negative transfer on the first reversal and continued to make more errors on the first five reversals than in initial acquisition. The light group showed improvement in the last five reversals with asymptotic performance at approximately the level of initial learning. In contrast to the performance of the tone and light groups, all subjects in the odor group showed positive transfer on the first reversal and asymptotic performance of only a few errors by the fifth reversal. One animal in this group made only two errors in each of the last three reversals. Analysis of individual acquisition functions revealed that all subjects in the odor group responded more often to the S — odor than to the S+ odor in the first odor-training session. This apparent preference for the S — odor was reversed in 20-40 trials and was followed by a decrease in S — responding. On the first reversal, two animals in the odor group perseverated on the response habit of original acquisition for 30 and 60 trials. The remaining two animals in this group initially responded to both S+ and S— on the first reversal and then showed rapid acquisition. On subsequent reversals, all subjects in the odor group showed few or no perseverative errors. For each rat in the tone and light groups, behavior on original acquisition was charac-
OLFACTORY DISCRIMINATION LEARNING
terized by a period of responding to both S + and S— followed by a gradual decrease in S— responding. This pattern was observed on each of the reversals and there was little or no indication of persevcrative responding on the previous habit. Initial Acquisition of the Light and Odor Discriminations It is instructive to compare the acquisition of the eight rats originally trained on the light discrimination with that of the two rats first trained on odors. It will be recalled that discrimination training in these cases was preceded by 200 trials with S+ before S — was introduced. Animals trained on light stimuli required 4-11 sessions (median errors = 335) before reaching a 90 % discrimination criterion. The two rats initially trained on odors made only one and three errors, respectively, in reaching criterion performance after introduction of S — . The performance of these two subjects demonstrates virtually errorless learning of the odor discrimination and stands in marked contrast to that of animals initially trained with light cues and that of rats in the odor group in which odor discrimination training was not preceded by a series of S+-only odor trials. Errorless acquisition of olfactory discrimination is examined further in Experiment 3. EXPERIMENT 2 This experiment compares the degree to which tone and odor stimuli gain control over discrimination performance during compound odor-tone stimulus training. The tests of cue reduction and cue competition used are modified from those described by Reynolds (1961) and Diamond (1967) and provide an evaluation of selective attention. Method Subjects Three young adult Wistar rats were maintained on a 23.75-hr water-deprivation schedule. Apparatus and Procedure The odorizing system and wind tunnel described in Experiment 1 were used. The animals were first trained on a light discrimination and then trained using a compound of tone and odor stimuli. Ethyl acetate odor presented simul-
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taneously with a 1-kHz tone served as S+. The S— was a compound of amyl acetate odor presented simultaneously with a 2.5-kHz tone. The training procedures described in Experiment 1 for the odor and tone groups were followed except that the FR-error requirement was eliminated and that for compound-stimulus training, the reinforcement probability was reduced to 70%. This partial-reinforcement schedule was used in an effort to slow discrimination learning so that the course of behavioral control by each element of the stimulus compound could be more easily determined during acquisition by interpolated cue-reduction tests and also to reduce the likelihood that the rats would learn to discriminate test trials from training trials. A cue-reduction trial consisted of presenting one of the tones or odors alone. The four element types were presented in random order and in random positions throughout compound-stimulus training with the restriction that only one cue-reduction trial was given in each block of nine compound trials. Responses during cue-reduction trials were not reinforced. Following criterion performance of 86% correct responding in 50 compound trials, an additional 350 compound trials with interpolated cue-reduction tests were given in a single session. Tone training, odor training, and competition tests. On the day after completion of the above procedures, alternating blocks of 40 trials on the tone stimuli alone and on the odor stimuli alone were given until each rat achieved a criterion of 9 out of 10 correct concurrent S+ and S— trials for each modality. During this training, all correct responses were reinforced. In the next session, 350 trials of compound stimulus training were given during which reinforcement probability for correct responding was 70% and nonreinforced competition test trials were inserted. On these test trials, the S+ odor and the S— tone or the S— odor and the S+ tone were presented together in random order so that one competition test was given in each block of nine compound-training trials.
Results
Compound-Stimulus Training and Cue Reduction In spite of the partial-reinforcement schedule, all three rats reached criterion so rapidly (400-450 trials) that there were too few interpolated cue-reduction trials to allow analysis of the order in which the separate elements gained stimulus control. As illustrated in the left-hand side of Figure 4, each rat responded to 95 %-100 % of the S+ compound trials and to the S+ odor element of the compound during the postacquisition test session. Two of the rats showed little or no responding to the tone clement of the S+
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B. J. NIGROSH, B. M. SLOTNICK, AND J. A. NEVIN COMPETITION
CUE REDUCTION
as-o
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L 1 II +0 +T -0
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STIMULUS COMPONENTS
FIGURE 4. Percent correct responses for three rats to compound stimuli (open bars) and test stimulus components (solid bars) for the cue-reduction and cue-competition phases of Experiment 2. (Abbreviations: O = odor; T = tone.)
compound, but one responded on 70% of these cue-reduction trials. As expected, few responses were made on the S— compound trials or S— cue-reduction test trials. Tone Training, Odor Training, and Competition Tests On training with tones alone and odors alone, each rat reached criterion during the third block of trials of each modality (medians: tone, 118 trials; odors, 106 trials). The right-hand portion of Figure 4 shows the percent responses during the 350 compound training trials of this session and for the competition trials. On the competition trials consisting of the S+ odor and S— tone elements, each rat responded as frequently as it did on the S+ compound trials. However, considerable inhibition of responding was observed when the S—- odor was paired with the S+ tone. EXPERIMENT 3 This experiment tests the degree to which olfactory cues are learned when they are su-
perimposed upon an established visual discrimination. In addition, a comparison between animals initially trained on olfactory or visual stimuli after S+-only training examines further the "errorless" learning of an olfactory discrimination described in Experiment 1. Method Four young adult Wistar rats (the light-odor group) were initially trained on a visual discrimination. The procedures described in Experiment 1 for the light group were followed: After 220-250 S+-only trials, S— was introduced, and training was continued until stable performance at 90% accuracy had been achieved. In the next session, after 40 visual discrimination trials, odors were added as discriminative stimuli. After 160 trials of the compound stimulus, the lights were turned off, and odors only were available as discriminative cues. Training was continued for 60 trials, and if necessary, additional sessions were given until criterion performance of 9 out of 10 correct S+ concurrent with 9 out of 10 correct S— trials was achieved. Four other animals (the odor-light group) were given identical training except that they were first trained to discriminate odors, given compound odor-light trials, and tested on the visual stimuli alone. The positive stimuli used for each animal in the two groups are given in the right-hand column of Table 1. Results
Initial Acquisition Table 1 lists the number of errors to criterion on the original problem for each subj ect in the light-odor and odor-light groups. Following the introduction of the S — stimuTABLE 1 ERRORS TO CRITERION AND POSITIVE STIMULI USED FOR RATS IN EXPERIMENT 3 Subject
AcquisiInitial tion after acquisi- compound tion training
Group L-O 123 125 126 131
119 171 95 130
lldn
124.5
Group O-L 127 128 129 130 Mdn
0 2 2 1 1.5
1 0 28 14 7.5 135 40 103 127 115
Positive stimuli
Flashing light; amyl acetate Flashing light; ethyl acetate Flashing light; ethyl acetate Steady light; ethyl acetate Ethyl acetate; steady light Amyl acetate; flashing light Amyl acetate; steady light Ethyl acetate; flashing light
OLFACTORY DISCRIMINATION LEARNING
lus, animals trained on lights (the light-odor group) initially responded on almost all trials and only gradually began to show response inhibition on S— trials. In contrast, animals trained on odors made only zero to two errors in reaching criterion after the S— stimulus was introduced. Compound Training In the light-odor group, two rats made only zero and one errors, respectively, in reaching discrimination criterion for odors after the 160 compound-stimulus trials. These rats had clearly learned the significance of the odor cues during compoundstimulus training. The other two animals in this group made more errors (see Table 1), but still showed rapid acquisition of the odor discrimination after visual stimuli were removed. In contrast, the four rats in the odor-light group made 40-135 errors before reaching criterion on visual stimuli after compound-stimulus training. As the median number of errors to reach criterion for light stimuli for this group \\ as virtually identical to that made by the light-odor group, it is clear that rats in odor-light group showed no acquisition of the light discrimination during their compound-stimulus training. Observation of the animals during the introduction of discriminative stimuli in each stage of this study also revealed striking differences in the behavior of the two groups. During S+-only training, animals given visual stimuli typically made key responses immediately after initiating a trial. These animals either made multiple responses or maintained contact with the key through the 2-scc fixed-interval period until reinforcement was delivered. For the first two or three presentations of the S— stimulus, each animal showed a startlelike response and either did not make a key contact (three rats) or responded only after a delay of several seconds. Thereafter, the animals responded quickly to both S+ and S— stimuli for 100 or more trials until discriminative behavior began to emerge. When odor stimuli were first introduced, each animal in this group made no key responses for 3-10 trials, but instead, spent the entire trial period sniffing at the air-inlet port. Performance accuracy dropped to chance or near chance levels in
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the first block of 20 compound-stimulus trials for each rat in the light-odor group. Animals in the odor-light group typically sniffed at the air-inlet port for 1-2 sec on each of the S+-only trials before responding. When the S — odor was first introduced, each animal spent the entire trial period sniffing at the air-inlet port. During continued odordiscrimination training, these animals showed stimulus-sampling behavior on every trial. On S— trials, they either maintained sampling behavior through the trial period or, after a 1-2 sec sample, backed away from air-inlet port and licked at the water cup, groomed, or turned around to face the back of the chamber. In the first 100 odor-discrimination trials, these rats made only three to six S— errors. During compound-stimulus training, each animal showed a startlelike reaction to the visual stimuli for the first two or three trials. One animal (Rat 127) dropped to chance performance in the first block of 20 compound trials. The remaining three animals continued to perform at or near criterion level throughout the compound-stimulus training. The results of this experiment arc best summarized in Figure 5 which shows the complete performance record for one rat from each group. Note that Rat 0-L 129 (Figure 5, top) shows no decrement in performance when the S — odor is introduced or during compound odor-light training. However, performance drops to chance levels when only light stimuli are available. In contrast, the performance of Rat L-0 123 drops to chance levels when the S— light is introduced during initial light training, and performance is again disturbed when odors are first presented during the light-odor compound trials. However, after the light stimuli are removed, the animal continues to perform at or near criterion level. To determine if errorless acquisition of the light discrimination would occur after extensive experience with S +, we allowed three additional rats 1,500 S+ light trials before introducing S — . These three rats made 24, 27, and 114 errors, respectively, in reaching criterion. Although they acquired the discrimination faster (p < .06, one-tailed Utest) than those allowed only 200 S+ light trials (light-odor group), their performance
B. J. NIGROSH, B. M. SLOTNICK, AND J. A. NEVIN
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