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Olfactory cues in visual discrimination problems
Physiology and Behavior Vol 3, pp. 683-685 Pergamon Press, 1968. P r m t e d m Great B n t a m
Olfactory Cues in Visual Discrimination Problems D A V I D S. P H I L L I P S
Department of Medical Psychology, University of Oregon Medical School, Portland, Oregon (Received 10 April 1968) D. S. Olfactory cues in visualdiscriminationproblems. PHYSIOL.BEHAV.3 (5) 683--685, 1968.--The purpose of this study was to assess the possible role of olfactory cues in a situation designed to test visual behavior. Two groups of rats were tested on visual discrimination problems where the relevance of olfactory cues was varied. Animals tested under conditions where both visual and olfactory cues led to the same response displayed learning set formation similar to that reported by other investigators. Animals tested under conditions where the visual and olfactory cues led to conflicting responses also demonstrated learning set formation; however, their performance was consistently at a lower level than that of the group where both cues led to the same response. PHILLIPS,
Olfactory cues
Visual discrimination
IT HAS BEEN KNOWN FOR more than ten years that rats and other non-primates are capable of forming learning sets [1, 2, 4]. A favorite method of demonstrating this learning in rats has been to require the subject to nose open the hinged door which carries an odd stimulus pattem for a food reward [2, 5]. In a previous study involving rats, the author noticed that once the learning set pattern of behavior was established, the visual stimuh could be removed after a few trials and the animal would continue to respond to the correct door at levels above chance. It was hypothesized that the animal left his "scent" on the correct door and then proceeded to respond mainly on the basis of olfactory rather than visual cues. Since this type of test situation has been used in many studies where the investigator presumed only visual cues to be present, a further investigation of the above hypothesis was deemed necessary. The study reported here is an attempt to ascertain the relevance of visual and olfactory cues in this type of test situation.
doors were mounted inside the box. The first of these was opaque and was mounted immediately in front of the stimulus doors; the second, made of clear plexiglass, was mounted four inches from the stimulus doors. A 15 W fluorescent bulb was mounted six inches behind the stimulus doors to provide illumination. The visual stimuli were ten of the patterns used by Weaver and Michels [5]. The patterns were cut from black construction paper and mounted on white paper. Each stimulus card was sealed in plastic and the stimuli within a given set were equated for area to eliminate brightness cues. Three of the stimuli within a set were identical; the fourth, the correct stimulus, was different.
Subjects The subjects were 18 male, hooded rats. The animals were 90 days of age when first introduced to the discrimination apparatus.
METHODS
Procedure
Apparatus
At 70 days of age the animals were randomly divided into two groups and housed in individual cages. On the 88th day of age all animals were placed on a restricted diet and on the 90th day they were introduced to the test apparatus. Each animal received ten days of pre-training in the test apparatus so that on the 100th day of age each animal was required to make 25 responses; a response being to open one of the stimulus doors and retrieve a 45 mg Noyes pellet. During the pre-training, blank pieces of white cardboard were attached to the stimulus doors. One set of stimuli was randomly selected from the ten sets to be used as the training problem and this set served as the training problem for all animals. The animals were started on the training problem on the 101st day of age. The testing procedure consisted of placing the animal in the apparatus with both the guillotine doors down, all the food wells baited,
The apparatus consisted of a four-choice visual discrimination box modelled after the one used by Weaver and Michels [5] and ten sets of visual stimuli. The test apparatus was a wooden box 12 in. high by 12 in. wide by 16 in. long, painted a flat black. Four 2 in. square holes were cut in one end and clear plexiglass stimulus doors were mounted on the outside of the box so as to cover these openings. The doors were hinged from above so that the animal could nose them open to gain access to the food well placed immediately behind each door. The doors were constructed to allow the stimulus patterns to be clipped behind them so that after a given trial the stimulus pattern alone or the door with the stimulus pattern attached could be moved to a new position. A locking device was installed over each door so that on a given trial only the correct door could be opened. Two guillotine 683
684
PHILLIPS
the stimulus doors attached and the incorrect doors locked. The opaque door was raised to allow the animal to observe the stimuli and ten seconds later the transparent door was raised. If the subject made a correct response, both guillotine doors were lowered as soon as he had retrieved his pellet; if an incorrect response was made, both doors were lowered immediately before another response could be attempted. After both guillotine doors had been lowered, the stimuli were positioned for the next trial and the sequence repeated. The position of the correct stimulus was determined by a random sequence with the restriction that the correct stimulus appear at each door an equal number of times. Each animal was given 25 trials a day on the training problem until he made 13 or more correct responses on two out of three days. After each animal had been tested, the stimulus doors were cleaned with alcohol and checked for scratches. All testing was done in a darkened room. The remaining nine sets of stimuli were randomly arranged in a sequence and as an animal reached criterion on the training problem he was randomly assigned a test problem on which to begin the sequence. This randomization was restricted so that within each group one subject was tested on each sequence in the Latin Square. Each animal was given 25 trials a day for four days (100 trials) on a test problem and then started on the next problem in the sequence. This was continued until he had been tested on all nine problems. For animals in Group I, the stimulus doors always remained in the same position but the stimulus patterns were moved behind them after each trial. In this case both olfactory and visual cues were available to the animal but would lead to conflicting responses. Animals in Group II were trained and tested under conditions identical to those used by Weaver and Michels [5], that is, the stimuli were attached to the stimulus doors and the whole configuration of stimulus and door was moved after each trial. Thus, the possibility existed for these animals to use both olfactory and visual cues to solve the problems.
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FIG. 1. Mean per cent correct responses for two experimental groups on nine test problems.
RESULTS
The mean per cent correct responses on each problem (block of 100 trials) for each group is shown in Fig. 1. The Group II animals, the group in which both olfactory and visual cues led to the same response, consistently performed at a higher level than Group I animals. The corresponding data from the Weaver and Michel's study is also shown for comparison. The curves for all these groups are typical of learning set data in that they show a rising trend and then reach a plateau. The data were submitted to an analysis of variance and differences significant at the 0.01 level were found between Groups (F = 16.96, dr= 1,6) and between Problems (F = 31.66, df= 8,128). The Groups-by-Problems interaction was not significant (F -- 0.56, dr= 8,128). It may be seen in Fig. 1 that even on the first test problem the two groups show a marked difference in performance. Since all the animals were obtained from the same source and randomly assigned to the two groups, it must be assumed that this initial difference is generated as a function of the two procedures during the training problem. The two groups did not differ significantly on the mean number of trials required to reach criterion on the training problem.
DISCUSSION
The lack of significant Groups × Problems interaction indicates that the curves for the two groups have essentially the same shape. The significant Problems effect is typical of learning set data and tests were not deemed necessary to determine which individual difference were significant since the main interest of this study was the Group effect. This Group effect was significant with Group II consistently surpassing Group I. This difference is evident with the first test problem and continues throughout testing. Comparison of these two groups indicates that rats do form learning sets when limited solely to visual cues (Group I) but that the addition of a relevant olfactory cue can help performance significantly (Group II; Weaver and Michels) [5]. Since the two groups do not differ on the mean number of trials required to reach criterion on the training problem, it appears that the utilization of the olfactory cue does not occur immediately but develops over time. The exact nature of the olfactory cues involved in this study have not as yet been elucidated. It is possible that the rat is simply leaving his "scent" on the correct door and following that door, in which case the cue could be considered either positive or neutral. On the other hand, if the subject is responding to "scent" left on doors at which he was not rewarded, the cue could be considered to be a negative one and he is making an avoidance response. Ludvigson and Sytsma [3] have shown that rats can use the negative type of cue in maze learning. However, the data here do not allow for this distinction. Regardless of whether the cues involved are considered positive or negative, this study and the one by Ludvigson and Sytsma clearly indicate that animal subjects will pick the cues that allow them to solve the problem posed by the experimenter in the most efficient manner and that these cues may not be the same ones that the experimenter has selected as most relevant.
OLFACTORY CUES IN VISUAL D I S C R I M I N A T I O N PROBLEMS
685
REFERENCES 1. Johnson, J. I. Jr. and K. M. Michels. Learning sets and size effects in visual discrimination learning by racoons. J. comp. physioL Psychol. 51 : 376--379, 1958. 2. Koronakos, C. and W. J. Arnold. The formation of learning sets in rats. J. comp. physiol. Psychol. 50:11-14, 1957. 3. Ludvigson, H. W. and D. Sytsma. The sweet smell of success:
Apparent double alternation in the rat. Psychonom. Sci. 9: 283-284, 1967. 4. Warren, J. M. and A. Baron. The formation of learning sets by cats. J. comp. physiol. Psychol. 49: 227-231, 1956. 5. Weaver, L. A. and K. M. Michels. Methodological factors effecting the formation of learning sets by rats. Anim. Behao. 9: 4-7, 1961.
Olfactory Cues in Visual Discrimination Problems D A V I D S. P H I L L I P S
Department of Medical Psychology, University of Oregon Medical School, Portland, Oregon (Received 10 April 1968) D. S. Olfactory cues in visualdiscriminationproblems. PHYSIOL.BEHAV.3 (5) 683--685, 1968.--The purpose of this study was to assess the possible role of olfactory cues in a situation designed to test visual behavior. Two groups of rats were tested on visual discrimination problems where the relevance of olfactory cues was varied. Animals tested under conditions where both visual and olfactory cues led to the same response displayed learning set formation similar to that reported by other investigators. Animals tested under conditions where the visual and olfactory cues led to conflicting responses also demonstrated learning set formation; however, their performance was consistently at a lower level than that of the group where both cues led to the same response. PHILLIPS,
Olfactory cues
Visual discrimination
IT HAS BEEN KNOWN FOR more than ten years that rats and other non-primates are capable of forming learning sets [1, 2, 4]. A favorite method of demonstrating this learning in rats has been to require the subject to nose open the hinged door which carries an odd stimulus pattem for a food reward [2, 5]. In a previous study involving rats, the author noticed that once the learning set pattern of behavior was established, the visual stimuh could be removed after a few trials and the animal would continue to respond to the correct door at levels above chance. It was hypothesized that the animal left his "scent" on the correct door and then proceeded to respond mainly on the basis of olfactory rather than visual cues. Since this type of test situation has been used in many studies where the investigator presumed only visual cues to be present, a further investigation of the above hypothesis was deemed necessary. The study reported here is an attempt to ascertain the relevance of visual and olfactory cues in this type of test situation.
doors were mounted inside the box. The first of these was opaque and was mounted immediately in front of the stimulus doors; the second, made of clear plexiglass, was mounted four inches from the stimulus doors. A 15 W fluorescent bulb was mounted six inches behind the stimulus doors to provide illumination. The visual stimuli were ten of the patterns used by Weaver and Michels [5]. The patterns were cut from black construction paper and mounted on white paper. Each stimulus card was sealed in plastic and the stimuli within a given set were equated for area to eliminate brightness cues. Three of the stimuli within a set were identical; the fourth, the correct stimulus, was different.
Subjects The subjects were 18 male, hooded rats. The animals were 90 days of age when first introduced to the discrimination apparatus.
METHODS
Procedure
Apparatus
At 70 days of age the animals were randomly divided into two groups and housed in individual cages. On the 88th day of age all animals were placed on a restricted diet and on the 90th day they were introduced to the test apparatus. Each animal received ten days of pre-training in the test apparatus so that on the 100th day of age each animal was required to make 25 responses; a response being to open one of the stimulus doors and retrieve a 45 mg Noyes pellet. During the pre-training, blank pieces of white cardboard were attached to the stimulus doors. One set of stimuli was randomly selected from the ten sets to be used as the training problem and this set served as the training problem for all animals. The animals were started on the training problem on the 101st day of age. The testing procedure consisted of placing the animal in the apparatus with both the guillotine doors down, all the food wells baited,
The apparatus consisted of a four-choice visual discrimination box modelled after the one used by Weaver and Michels [5] and ten sets of visual stimuli. The test apparatus was a wooden box 12 in. high by 12 in. wide by 16 in. long, painted a flat black. Four 2 in. square holes were cut in one end and clear plexiglass stimulus doors were mounted on the outside of the box so as to cover these openings. The doors were hinged from above so that the animal could nose them open to gain access to the food well placed immediately behind each door. The doors were constructed to allow the stimulus patterns to be clipped behind them so that after a given trial the stimulus pattern alone or the door with the stimulus pattern attached could be moved to a new position. A locking device was installed over each door so that on a given trial only the correct door could be opened. Two guillotine 683
684
PHILLIPS
the stimulus doors attached and the incorrect doors locked. The opaque door was raised to allow the animal to observe the stimuli and ten seconds later the transparent door was raised. If the subject made a correct response, both guillotine doors were lowered as soon as he had retrieved his pellet; if an incorrect response was made, both doors were lowered immediately before another response could be attempted. After both guillotine doors had been lowered, the stimuli were positioned for the next trial and the sequence repeated. The position of the correct stimulus was determined by a random sequence with the restriction that the correct stimulus appear at each door an equal number of times. Each animal was given 25 trials a day on the training problem until he made 13 or more correct responses on two out of three days. After each animal had been tested, the stimulus doors were cleaned with alcohol and checked for scratches. All testing was done in a darkened room. The remaining nine sets of stimuli were randomly arranged in a sequence and as an animal reached criterion on the training problem he was randomly assigned a test problem on which to begin the sequence. This randomization was restricted so that within each group one subject was tested on each sequence in the Latin Square. Each animal was given 25 trials a day for four days (100 trials) on a test problem and then started on the next problem in the sequence. This was continued until he had been tested on all nine problems. For animals in Group I, the stimulus doors always remained in the same position but the stimulus patterns were moved behind them after each trial. In this case both olfactory and visual cues were available to the animal but would lead to conflicting responses. Animals in Group II were trained and tested under conditions identical to those used by Weaver and Michels [5], that is, the stimuli were attached to the stimulus doors and the whole configuration of stimulus and door was moved after each trial. Thus, the possibility existed for these animals to use both olfactory and visual cues to solve the problems.
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FIG. 1. Mean per cent correct responses for two experimental groups on nine test problems.
RESULTS
The mean per cent correct responses on each problem (block of 100 trials) for each group is shown in Fig. 1. The Group II animals, the group in which both olfactory and visual cues led to the same response, consistently performed at a higher level than Group I animals. The corresponding data from the Weaver and Michel's study is also shown for comparison. The curves for all these groups are typical of learning set data in that they show a rising trend and then reach a plateau. The data were submitted to an analysis of variance and differences significant at the 0.01 level were found between Groups (F = 16.96, dr= 1,6) and between Problems (F = 31.66, df= 8,128). The Groups-by-Problems interaction was not significant (F -- 0.56, dr= 8,128). It may be seen in Fig. 1 that even on the first test problem the two groups show a marked difference in performance. Since all the animals were obtained from the same source and randomly assigned to the two groups, it must be assumed that this initial difference is generated as a function of the two procedures during the training problem. The two groups did not differ significantly on the mean number of trials required to reach criterion on the training problem.
DISCUSSION
The lack of significant Groups × Problems interaction indicates that the curves for the two groups have essentially the same shape. The significant Problems effect is typical of learning set data and tests were not deemed necessary to determine which individual difference were significant since the main interest of this study was the Group effect. This Group effect was significant with Group II consistently surpassing Group I. This difference is evident with the first test problem and continues throughout testing. Comparison of these two groups indicates that rats do form learning sets when limited solely to visual cues (Group I) but that the addition of a relevant olfactory cue can help performance significantly (Group II; Weaver and Michels) [5]. Since the two groups do not differ on the mean number of trials required to reach criterion on the training problem, it appears that the utilization of the olfactory cue does not occur immediately but develops over time. The exact nature of the olfactory cues involved in this study have not as yet been elucidated. It is possible that the rat is simply leaving his "scent" on the correct door and following that door, in which case the cue could be considered either positive or neutral. On the other hand, if the subject is responding to "scent" left on doors at which he was not rewarded, the cue could be considered to be a negative one and he is making an avoidance response. Ludvigson and Sytsma [3] have shown that rats can use the negative type of cue in maze learning. However, the data here do not allow for this distinction. Regardless of whether the cues involved are considered positive or negative, this study and the one by Ludvigson and Sytsma clearly indicate that animal subjects will pick the cues that allow them to solve the problem posed by the experimenter in the most efficient manner and that these cues may not be the same ones that the experimenter has selected as most relevant.
OLFACTORY CUES IN VISUAL D I S C R I M I N A T I O N PROBLEMS
685
REFERENCES 1. Johnson, J. I. Jr. and K. M. Michels. Learning sets and size effects in visual discrimination learning by racoons. J. comp. physioL Psychol. 51 : 376--379, 1958. 2. Koronakos, C. and W. J. Arnold. The formation of learning sets in rats. J. comp. physiol. Psychol. 50:11-14, 1957. 3. Ludvigson, H. W. and D. Sytsma. The sweet smell of success:
Apparent double alternation in the rat. Psychonom. Sci. 9: 283-284, 1967. 4. Warren, J. M. and A. Baron. The formation of learning sets by cats. J. comp. physiol. Psychol. 49: 227-231, 1956. 5. Weaver, L. A. and K. M. Michels. Methodological factors effecting the formation of learning sets by rats. Anim. Behao. 9: 4-7, 1961.