Psychopharmacologia (Berl.) 43, 175-179 (1975) - 9 by Springer-Verlag 1975
Colour Preferences in the Pigeon" A Behavioural and Psychopharmacological Study A. SAHGAL and S. D. IVERSEN University of Cambridge Received April 4, 1975 Abstract. Ten London pigeons were tested in an operant apparatus, and found to have marked colour preferences, blue being preferred over green, followed by yellow and red. The effects of 2 tranquilizers, diazepam and chlorpromazine, were studied and it was found that diazepam released responding, and chlorpromazine suppressed it. Diazepam also tended to
abolish colourpreference, whilst chlorpromazinewas effective only at the highest dose used (5.00 mg/kg). The implications of these findings, particularly in relation to the design of experiments and to the aversiveness of certain stimuli, are discussed.
Key words." Colour Preferences - Counterbalanced Design - Diazepam - Chlorpromazine - Aversive Stimuli - Pigeons.
Research into the various aspects of visually controlled behaviour, such as studies on the reinforcing and aversive properties of conditioned stimuli, discrimination learning, generalisation studies, matching to sample and psychophysical threshold determination, almost always necessitates the use of more than one training stimulus. An initial decision as to which particular values of stimuli are to be selected for the purposes of an experiment, must therefore be taken. These choices tend to be from within the same stimulus dimension, such as angular orientation, colour and curvature, for example. It has been shown that such intra-dimensional choices result in excellent stimulus control over behaviour (see Thomas, 1969), and furthermore, allow the experimenter to define his stimuli in an objective manner. Nonetheless, no guidelines exist to enable the selection of the appropriate stimuli from within such a dimension. For example, within the colour dimension, red and green may be selected as stimuli for a generalisation study, it being assumed that both these stimuli would be equally capable of either eliciting or suppressing responding, depending on the training paradigms. This assumption is of central importance in a strictly empiricist psychology, where any observed differences in the rates of responding to two or more stimuli would have to be ascribed to the animal's past experience. We have accumulated some evidence in our laboratory, indicating that such an assumption may not always be correct (Sahgal et al., in preparation). Briefly, the colour preferences of four naive rhesus
monkeys were measured, it being found that they preferred blue to red. Most importantly, these preferences appeared to be quite stable, and resisted attempts to abolish them. One group of animals were trained to press a red panel for food, whilst the other pressed a blue panel; subsequently, both groups were tested for responses to red and blue, presented sequentially. Whereas the "red" trained animals responded equally to both colours, the "blue" trained animals responded solely to blue. Thus the outcome of a discrimination experiment, where red was the reinforced stimulus, would have been to conclude that the animals had failed to attend to colour cues. Had blue been the reinforced stimulus, however, the conclusion would have been the exact opposite. Obviously, therefore, the possibility of such preferences must be taken into account in the design of experiments. The work reported here was undertaken to study the existence of any such colour preferences in the pigeon. It was also decided, should such preferences exist, to study the underlying mechanism of the preferences using the 2 tranquilizers, diazepam and chlorpromazine. Experiment 1 Methods Ten adult London pigeons were maintained at approximately 80 ~o of their free feeding weights. The birds had had prior autoshaping experience in an undergraduate practical class; no bird, however, had served in an experiment where colour was a discriminative stimulus.
176
Psychopharmacologia (Berl.), Vol. 43, Fasc. 2 (1975)
A Grason Stadler two key pigeon station (EI184JA-I), with the left side key blanked out with tape, was used as the testing apparatus. The stimuli were back projected, using a Kodak Carousel S-AV projector, and consisted of neutral density or colour filters as appropriate. The colours used were blue, green, yellow and red, each of which was equated for brightness at 216 cd/m 2. A house light provided background illumination at all times except during the inter-trial interval. Reinforcement consisted of a 4 sec access to grain, and the magazine was illuminated during this period. This chamber had been rewired in order to allow it to be controlled by an on-line computer program, ONLI, which was resident in a Modular one computer (Computer Technology Ltd.). Thus, the birds' key pecks were monitored by the computer, which apart from controlling the experiment, also recorded the birds' data. The birds were first autoshaped to peck the response key (Brown and Jenkins, 1968). This involved a simple program which operated the feeder mechanism whilst simultaneously, the key illumination was turned on and off at 10 sec cycles. The birds were then transferred to the next stage, which consisted of presenting them with the two neutral density stimuli of brightness 272 and 3426 cd/m z, successively, and in a random order. Each bird received ten 2 min trials daily, each trial being separated by a 4 sec inter-trial interval. The reinforcement was scheduled on a variable interval 1 min (VI 1'). Such training was given for 3 reasons: (a) it renders brightness cues irrelevant, since the two training stimuli were equally reinforced, (b) it raises the birds' typically low spontaneous response rate, without any specific discrimination being learned and (c) makes responding very resistant to extinction (see Ferster and Skinner, 1957, pp. 3 2 6 - 390). This training continued until all the birds had reached stable rates of responding, i.e., the overall number of responses to the two training stimuli were within a 10~ margin over 4 consecutive days. Responses to the colour stimuli were measured as follows : the birds were given two 1 min warm up trials, during which the training stimuli appeared successively, and responses were reinforced as before. Immediately after the second trial, the test stimuli were presented, sequentially, for a further eight 1 min trials, during which responses were recorded, but never reinforced. The following day was devoted to normal training, followed in turn by a further test session. This continued until each bird had completed 4 test days. The colour stimuli were presented in such a way that for each session over the 4 days of testing, each subject produced two 4• 4 Latin squares, the first being derived from test trials 1 - 4 and the second from trials 5 - 8 . On each day, the order of stimuli for trials 5 - 8 was the reverse of that for trials 1 - 4. Such a design was necessary in order to allow for any extinction effects, different Latin squares being used for different birds. The statistical tests employed in the analysis of the results were the Friedman two-way analysis of variance (see Siegel, 1956), Kendall coefficient of concordance, W, (see Friedman, 1940) and the Jonckheere trend statistic (Jonckheere, 1954). Two tailed significance levels were used with the trend statistic, as any monotonic trend would have been of interest.
Results The cumulative scores for each bird on each o f the 4 colours are shown in Table 1. All the birds tested responded m o r e to blue than to green, yellow and red, This distribution is significantly different f r o m chance
Table I Bird
1 2 3 4 5 6 7 8 9 10
Total responses per colour blue
green
yellow
red
347 234 362 453 218 380 199 479 97 245
83 140 109 247 108 46 97 392 83 109
37 16 30 138 29 11 62 257 46 76
131 12 56 175 9 22 9 185 39 214
Individual scores for each of the 10 birds, cumulated over sessions and days. Thus, each value represents the total number of responses in 8 min presentation (2 min per day for 4 test days).
( F r i e d m a n ) ~ = 23.880, d r = 3, P < 0.001, W = 0.742), indicating that the pigeons have very m a r k e d colour preferences. F u r t h e r m o r e , these preferences follow a m o n o t o n i c trend with wavelength, with blue being preferred over green, followed by yellow and red. This trend is also highly significant (Jonckheere tau = 0.77, z = 4.83, P < 0.0001).
Discussion It could be argued that the highly systematic preferences observed in these pigeons simply reflects some accidental conditioning. Evidence does n o t support this view. T r a c y (1970) raised ducklings in two conditions f r o m birth; one g r o u p was raised in light o f wavelength 589 nm, whilst the other was raised in white light. B o t h g r o u p s showed a preference for green light, despite the obvious lack o f any discriminative training. M o n k e y s f r o m different laboratories, and therefore with different histories, also s h o w identical preferences for blue light ( H u m p h r e y , 1971 ; Sahgal et al., in preparation). The safest course is to accept the existence o f such preferences, and to use suitable c o u n t e r b a l a n c e d designs. Such a precaution has too frequently been ignored, resulting in a possible misinterpretation o f data. A recent example illustrates this point. In their experiment, M u l v a n e y et al, (1974) wished to determine the effect o f a negative discriminative stimulus on the response p r o d u c i n g it. Pecks on an observing key p r o d u c e d either a red or green stimulus, where green alone was reinforced (positive discriminative stimulus). In subsequent sessions, it was f o u n d that if red (the negative discriminative stimulus) was withheld as one o f the consequences o f pecking the observing key, the rate o f responding on that key
A. Sahgal and S. D. Iversen: Colour Preferences in the Pigeon increased. They therefore argued that a negative discriminative stimulus has a suppressive effect on the response that precedes it, i.e., acts as a form of punisher. Their result can, of course be predicted on the basis of colour preference alone, in the absence of any discriminative training. Before arriving at their conclusions, Mulvaney et al. ought to have employed a counterbalanced design, where some of their birds received green as the negative discriminative stimulus.
177 Table 2 Drug
A stimulus can, by its presentation, suppress responding without being aversive. F o r example, Terrace (1965) trained 2 groups of pigeons in a horizontal/vertical line discrimination; one group (error) learned the discrimination with errors, i.e., made several responses to the negative stimulus ( S - ) during acquisition, whereas the second group learned the discrimination without errors (errorless), this being achieved by gradual introduction of the S - . When, in subsequent tests, the S - appeared, Terrace found that the error group showed typical emotional responses, such as wing flapping; the errorless group showed no such emotional behaviour. Terrace argued that in the error group, the S - was regarded as an aversive stimulus, whereas in the errorless group, it simply suppressed responding without being aversive. A question therefore arose concerning colour preferences: was the low response rate to the non-preferred colour due to some aversive property? It was decided to explore this problem with the aid of 2 drugs, diazepam (valium) and chlorpromazine (largactil, CPZ). Both these drugs are known to affect responding (see Margules and Stein, 1967) and are regarded as minor and major tranquilizers respectively. Benzodiazepines such as diazepam are known to disinhibit responding in a wide variety of situations, such as responding suppressed by electric shock, responding for food adulterated by the addition of quinine sulphate (Margules and Stein, 1967), and responding in a go/ no-go type of discrimination (Hasegawa et al., 1973). We might therefore expect diazepam to disirihibit responding to the non-preferred stimuli, i.e., to abolish colour preferences. Terrace (1963a) has shown that CPZ will also disinhibit responding provided the relevant stimuli are aversive in some way. Thus, he found that CPZ had no effect on the responding to the S - if the discrimination was learned without errors, but released responding to the S - when the discrimination had been learned with errors (the S now being regarded as an aversive stimulus). Administration of CPZ might therefore help to further elucidate the status of the non-preferred colours.
Tau
z
P
0.00 0.62 1.25 2.50 5.00
0.42 0.38 0.27 0.07 0.27
2.57 2.36 1.60 0.32 1.60
0.0102 0.0182
0.1096
0.00 0.62 1.25 2.50 5.00
0.52 0.65 0.60 0.62 0.28
3.22 4.08 3.76 3.86 1.72
0.0014 0.0001 0.0001 0.0001 0.0854
(mg/kg, i.m.) Diazepam
Chlorpromazine
Experiment 2
Dose
0.7490 0.1096
Jonckheere trend statistic values (2 tailed) for each dose of the 2 tranquilizers, diazepam and chlorpromazine. A high value of P (> 0.05) indicates that the colour preference has been abolished. Thus, medium and high doses of diazepam abolished colour preferences, whereas chlorpromazine had little effect. Methods The same subjects and apparatus were used. At the conclusion of experiment 1, the birds were retrained on the two neutral density stimuli for a further period, whilst receiving 1 ml/kg of saline injections intramuscularly, 1 hr prior to testing. Once the birds were accustomed to being injected, they were switched to propylene glycol based injection solution (Crankshaw and Raper, 1971), and trained as before. They were then divided into 2 groups, one to receive CPZ (dissolved in saline), and the other to receive diazepam (dissolved in the glycol vehicle). Five doses of each drug (including placebo) were administered to each bird at the following doses: 0, 0.62, 1.25, 2.50 and 5.00 mg/kg. These were arranged in a 5 x 5 Latin square design. Each bird received 1 dose of the drug per test day; each test day was followed by a normal training day, on which the bird received a saline or vehicle injection. The test procedure was the same as that followed in experiment 1, except that the colours were presented in a random manner. There were 5 test days per drug, and at the conclusion, the 2 groups were inter-changed. This change over was separated by 14 days of training on the neutral density stimuli, during which the birds received the appropriate injection of placebo. The results were analysed using the same tests as employed for experiment 1. Results Statistical analysis of the data showed that C P Z had an overall drug-dose effect (Friedman)~2 = 27.18, d f = 16, P < 0.05), whereas diazepam did not (Z2 --- 11.22, d f = 16, P < 0.80). This indicates that CPZ has a sharp dose response curve, whilst diazepam has a broad one. It was also noted that there was a large variability amongst the different birds, some of them having reached very low rates of responding, due to gradual extinction. Table 2 lists the values of tau, z and the associated significance levels on the Jonckheere trend statistic for the two drugs. It can
178
Psychopharmacologia (Berl.), Vol. 43, Fasc. 2 (1975)
DOSE mg/kg
+--e
PLACEBO
~-..-~
2 . 50
c,- ,.o
0.62
~ . ~
5 9 00
c:.-.a
1 +25
~'"~... +.+ 9 "+. "-++ 9 9 -.. %% "-+,. %9 +"-.. % % q 9 "..,. \ 9 ",.. \ % -.. . 9 ".,. \ \
5.
\ +"-..-.+
\~'\ ~n
+
4 ",.
Z O m ua
,,...+ . ~ + ' + ~ - , o
-~-~+
i, I
9 -++.
9
-+a
A
3
o 2.
9
-\ -. \\ -, "..\ ". \~ ~
" ' - " ...................... l+
BLUE
.
GREEN
t
L
YELLOW
RED
DIAZEPAM
BLUE
GREEN
2.-Z~:y~-:.-;-~ YELLOW
RED
CHLORPROMAZINE
Fig. 1. Combined scores for the 10 birds are plotted at each dose of the 2 drugs, diazepam and chlorpromazine. This illustrates the nonspecific release of responding obtained with diazepam, compared to the systematic reduction in the rate of responding observed with chlorpromazine
be seen that diazepam tended to abolish colour preferences, whilst chlorpromazine did not, except at the highest dose. Fig. 1 illustrates the finding that diazepam tended to abolish preferences by causing an increase in the overall rates of responding, whereas CPZ suppressed responding. The preferences, however remained intact at all but the highest dose level, i.e., CPZ did not disinhibit responding to the non-preferred colours. Discussion The results obtained with diazepam are in agreement with the findings of Margules and Stein (1967) and Hasegawa et al. (1973), i.e., that benzodiazepines such as diazepam cause a release of suppressed behaviour in a wide variety of situations. Our results, however, failed to pinpoint an especially effective dose level for the drug. Even relatively high doses of CPZ did not abolish colour preferences. Margules and Stein (1967) have found that a dose as low as I mg/kg was effective in suppressing most behaviour; our results show that CPZ in low doses neither abolished colour preferences nor suppressed responding although at higher doses (5 mg/kg) a general behavioural depression was observed.
A conclusion in terms of the aversiveness of the non-preferred colours is difficult. Terrace (1963b) argued that previously neutral stimuli may become aversive if the animal had made several non-reinforced responses to them. He also showed (1963a) that CPZ caused a release of responding to such aversive stimuli, whilst having no effect on non-aversive negative stimuli. If we accept Terrace's (1963b) hypothesis, then it may be concluded that the non-preferred colours are not aversive, in any simple sense, since CPZ did not disinhibit responding. Care must, however, be exercised when using such terms as "aversive stimuli". Several workers, such as Morse (1964), and Margules and Stein (1967) have found that CPZ does not disinhibit behaviour suppressed by such presumably aversive stimuli as electric shock and quinine sulphate, nor did CPZ administration result in an increase of responding in extinction. On the basis of such findings, the possibility remains that the nonpreferred colours are mildly aversive.
Acknowledgements. It is a pleasure to thank Dr. S. E. G. Lea for his invaluable advice on statistical analysis, and for his detailed comments on an earlier draft of this paper. Dr. M. J. Morgan and Mr. N. C. Tye made many suggestions, for which we are very grateful. We are also indebted to Mr. S. T. Mason, who wrote the computer program. On-line control of experi-
A. Sahgal and S. D. Iversen: Colour Preferences in the Pigeon ments used the ONLI system developed by C. Crook and S. E. G. Lea with the help of grant MRC G970/297/B to A. J. Watson. We also wish to thank Professor O. L. Zangwill for extending to us the facilities of the Psychological Laboratory. References
Brown, P., Jenkins, J. : Autoshaping of the pigeons key peck. J. exp. Anal. Behav. I1, 1 - 8 (1968) Crankshaw, D. P., Raper, C. : The effect of solvents on the potency of chlordiazepoxide, diazepam, medazepam and nitrazepam. J. Pharmac. Pharmacol. 23, 313-321 (1971) Ferster, C. B., Skinner, B. F. : Schedules of reinforcement. New York:Appleton-Century-Crofts 1957 Friedman, M. : A comparison of alternative tests of significance for the problems of rn rankings. Ann. Math. Statist. 11, 8 6 - 9 2 (1940) Hasegawa, Y., Ibuka, N., Iwahara, S. : Effects of chlordiazepoxide upon successive red-green discrimination responses in Japanese monkeys, macaca fuscata. Psychopharmacologia (Berl.) 30, 8 9 - 9 4 (1973) Humphrey, N. K.: Colour and brightness preferences in monkeys. Nature (Lond.) 229, 615-617 (1971) Jonckheere, A. R. : A test of significance for the relation between m rankings and k ranked categories. Brit. J. Statist. Psychol. 7, 93-100 (1954)
179 Margules, D. L., Stein, L.: Neuroleptics vs. tranquilizers: evidence from animal studies of mode and site of action. In: Neuro-Psychopharmacology, H. Brill, J. O. Cole, P. Deniker, H. Hippius, and P. B. Bradley, eds., pp. 108- 120. Amsterdam: Excerpta Medica Foundation 1967 Morse, W. H. : Effect of amobarbital and chlorpromazine on punished behaviour in the pigeon. Psychopharmacologia (Berl.) 6, 286-294 (1964) Mulvaney, D. E., Dinsmoor, J. A., Jwaideh, A. R., Hughes, L. H. : Punishment of observing by the negative discriminative stimulus. J. exp. Anal. Behav. 21, 3 7 - 4 4 (1974) Siegel, S.: Nonparametric statistics. New York: McGrawHill 1956 Terrace, H. S. : Errorless discrimination learning in the pigeon: effects of chlorpromazine and imipramine. Science 140, 318-319 (1963a) Terrace, H. S.: Discrimination learning with and without "errors". J. exp. Anal. Behav. 6, 1 - 2 7 (1963b) Terrace, H. S.: Stimulus control. In: Operant behaviour: Areas of research and application, W. K. Honig, ed., pp. 271-344. New York: Appleton-Century-Crofts 1965 Thomas, D. R. : The use of operant conditioning techniques to investigate perceptual processes in animals. In: Animal discrimination learning, R. M. Gilbert and N. S. Sutherland, eds., pp. 1-33. New York: Academic Press 1969 Tracy, W. K. : Wavelength generalisation and preference in monotonically reared ducklings. J. exp. Anal. Behav. 13, 163-178 (1970)
A. Sahgal, Department of Experimental Psychology, University of Cambridge Cambridge, England, CB2 3EB
Colour Preferences in the Pigeon" A Behavioural and Psychopharmacological Study A. SAHGAL and S. D. IVERSEN University of Cambridge Received April 4, 1975 Abstract. Ten London pigeons were tested in an operant apparatus, and found to have marked colour preferences, blue being preferred over green, followed by yellow and red. The effects of 2 tranquilizers, diazepam and chlorpromazine, were studied and it was found that diazepam released responding, and chlorpromazine suppressed it. Diazepam also tended to
abolish colourpreference, whilst chlorpromazinewas effective only at the highest dose used (5.00 mg/kg). The implications of these findings, particularly in relation to the design of experiments and to the aversiveness of certain stimuli, are discussed.
Key words." Colour Preferences - Counterbalanced Design - Diazepam - Chlorpromazine - Aversive Stimuli - Pigeons.
Research into the various aspects of visually controlled behaviour, such as studies on the reinforcing and aversive properties of conditioned stimuli, discrimination learning, generalisation studies, matching to sample and psychophysical threshold determination, almost always necessitates the use of more than one training stimulus. An initial decision as to which particular values of stimuli are to be selected for the purposes of an experiment, must therefore be taken. These choices tend to be from within the same stimulus dimension, such as angular orientation, colour and curvature, for example. It has been shown that such intra-dimensional choices result in excellent stimulus control over behaviour (see Thomas, 1969), and furthermore, allow the experimenter to define his stimuli in an objective manner. Nonetheless, no guidelines exist to enable the selection of the appropriate stimuli from within such a dimension. For example, within the colour dimension, red and green may be selected as stimuli for a generalisation study, it being assumed that both these stimuli would be equally capable of either eliciting or suppressing responding, depending on the training paradigms. This assumption is of central importance in a strictly empiricist psychology, where any observed differences in the rates of responding to two or more stimuli would have to be ascribed to the animal's past experience. We have accumulated some evidence in our laboratory, indicating that such an assumption may not always be correct (Sahgal et al., in preparation). Briefly, the colour preferences of four naive rhesus
monkeys were measured, it being found that they preferred blue to red. Most importantly, these preferences appeared to be quite stable, and resisted attempts to abolish them. One group of animals were trained to press a red panel for food, whilst the other pressed a blue panel; subsequently, both groups were tested for responses to red and blue, presented sequentially. Whereas the "red" trained animals responded equally to both colours, the "blue" trained animals responded solely to blue. Thus the outcome of a discrimination experiment, where red was the reinforced stimulus, would have been to conclude that the animals had failed to attend to colour cues. Had blue been the reinforced stimulus, however, the conclusion would have been the exact opposite. Obviously, therefore, the possibility of such preferences must be taken into account in the design of experiments. The work reported here was undertaken to study the existence of any such colour preferences in the pigeon. It was also decided, should such preferences exist, to study the underlying mechanism of the preferences using the 2 tranquilizers, diazepam and chlorpromazine. Experiment 1 Methods Ten adult London pigeons were maintained at approximately 80 ~o of their free feeding weights. The birds had had prior autoshaping experience in an undergraduate practical class; no bird, however, had served in an experiment where colour was a discriminative stimulus.
176
Psychopharmacologia (Berl.), Vol. 43, Fasc. 2 (1975)
A Grason Stadler two key pigeon station (EI184JA-I), with the left side key blanked out with tape, was used as the testing apparatus. The stimuli were back projected, using a Kodak Carousel S-AV projector, and consisted of neutral density or colour filters as appropriate. The colours used were blue, green, yellow and red, each of which was equated for brightness at 216 cd/m 2. A house light provided background illumination at all times except during the inter-trial interval. Reinforcement consisted of a 4 sec access to grain, and the magazine was illuminated during this period. This chamber had been rewired in order to allow it to be controlled by an on-line computer program, ONLI, which was resident in a Modular one computer (Computer Technology Ltd.). Thus, the birds' key pecks were monitored by the computer, which apart from controlling the experiment, also recorded the birds' data. The birds were first autoshaped to peck the response key (Brown and Jenkins, 1968). This involved a simple program which operated the feeder mechanism whilst simultaneously, the key illumination was turned on and off at 10 sec cycles. The birds were then transferred to the next stage, which consisted of presenting them with the two neutral density stimuli of brightness 272 and 3426 cd/m z, successively, and in a random order. Each bird received ten 2 min trials daily, each trial being separated by a 4 sec inter-trial interval. The reinforcement was scheduled on a variable interval 1 min (VI 1'). Such training was given for 3 reasons: (a) it renders brightness cues irrelevant, since the two training stimuli were equally reinforced, (b) it raises the birds' typically low spontaneous response rate, without any specific discrimination being learned and (c) makes responding very resistant to extinction (see Ferster and Skinner, 1957, pp. 3 2 6 - 390). This training continued until all the birds had reached stable rates of responding, i.e., the overall number of responses to the two training stimuli were within a 10~ margin over 4 consecutive days. Responses to the colour stimuli were measured as follows : the birds were given two 1 min warm up trials, during which the training stimuli appeared successively, and responses were reinforced as before. Immediately after the second trial, the test stimuli were presented, sequentially, for a further eight 1 min trials, during which responses were recorded, but never reinforced. The following day was devoted to normal training, followed in turn by a further test session. This continued until each bird had completed 4 test days. The colour stimuli were presented in such a way that for each session over the 4 days of testing, each subject produced two 4• 4 Latin squares, the first being derived from test trials 1 - 4 and the second from trials 5 - 8 . On each day, the order of stimuli for trials 5 - 8 was the reverse of that for trials 1 - 4. Such a design was necessary in order to allow for any extinction effects, different Latin squares being used for different birds. The statistical tests employed in the analysis of the results were the Friedman two-way analysis of variance (see Siegel, 1956), Kendall coefficient of concordance, W, (see Friedman, 1940) and the Jonckheere trend statistic (Jonckheere, 1954). Two tailed significance levels were used with the trend statistic, as any monotonic trend would have been of interest.
Results The cumulative scores for each bird on each o f the 4 colours are shown in Table 1. All the birds tested responded m o r e to blue than to green, yellow and red, This distribution is significantly different f r o m chance
Table I Bird
1 2 3 4 5 6 7 8 9 10
Total responses per colour blue
green
yellow
red
347 234 362 453 218 380 199 479 97 245
83 140 109 247 108 46 97 392 83 109
37 16 30 138 29 11 62 257 46 76
131 12 56 175 9 22 9 185 39 214
Individual scores for each of the 10 birds, cumulated over sessions and days. Thus, each value represents the total number of responses in 8 min presentation (2 min per day for 4 test days).
( F r i e d m a n ) ~ = 23.880, d r = 3, P < 0.001, W = 0.742), indicating that the pigeons have very m a r k e d colour preferences. F u r t h e r m o r e , these preferences follow a m o n o t o n i c trend with wavelength, with blue being preferred over green, followed by yellow and red. This trend is also highly significant (Jonckheere tau = 0.77, z = 4.83, P < 0.0001).
Discussion It could be argued that the highly systematic preferences observed in these pigeons simply reflects some accidental conditioning. Evidence does n o t support this view. T r a c y (1970) raised ducklings in two conditions f r o m birth; one g r o u p was raised in light o f wavelength 589 nm, whilst the other was raised in white light. B o t h g r o u p s showed a preference for green light, despite the obvious lack o f any discriminative training. M o n k e y s f r o m different laboratories, and therefore with different histories, also s h o w identical preferences for blue light ( H u m p h r e y , 1971 ; Sahgal et al., in preparation). The safest course is to accept the existence o f such preferences, and to use suitable c o u n t e r b a l a n c e d designs. Such a precaution has too frequently been ignored, resulting in a possible misinterpretation o f data. A recent example illustrates this point. In their experiment, M u l v a n e y et al, (1974) wished to determine the effect o f a negative discriminative stimulus on the response p r o d u c i n g it. Pecks on an observing key p r o d u c e d either a red or green stimulus, where green alone was reinforced (positive discriminative stimulus). In subsequent sessions, it was f o u n d that if red (the negative discriminative stimulus) was withheld as one o f the consequences o f pecking the observing key, the rate o f responding on that key
A. Sahgal and S. D. Iversen: Colour Preferences in the Pigeon increased. They therefore argued that a negative discriminative stimulus has a suppressive effect on the response that precedes it, i.e., acts as a form of punisher. Their result can, of course be predicted on the basis of colour preference alone, in the absence of any discriminative training. Before arriving at their conclusions, Mulvaney et al. ought to have employed a counterbalanced design, where some of their birds received green as the negative discriminative stimulus.
177 Table 2 Drug
A stimulus can, by its presentation, suppress responding without being aversive. F o r example, Terrace (1965) trained 2 groups of pigeons in a horizontal/vertical line discrimination; one group (error) learned the discrimination with errors, i.e., made several responses to the negative stimulus ( S - ) during acquisition, whereas the second group learned the discrimination without errors (errorless), this being achieved by gradual introduction of the S - . When, in subsequent tests, the S - appeared, Terrace found that the error group showed typical emotional responses, such as wing flapping; the errorless group showed no such emotional behaviour. Terrace argued that in the error group, the S - was regarded as an aversive stimulus, whereas in the errorless group, it simply suppressed responding without being aversive. A question therefore arose concerning colour preferences: was the low response rate to the non-preferred colour due to some aversive property? It was decided to explore this problem with the aid of 2 drugs, diazepam (valium) and chlorpromazine (largactil, CPZ). Both these drugs are known to affect responding (see Margules and Stein, 1967) and are regarded as minor and major tranquilizers respectively. Benzodiazepines such as diazepam are known to disinhibit responding in a wide variety of situations, such as responding suppressed by electric shock, responding for food adulterated by the addition of quinine sulphate (Margules and Stein, 1967), and responding in a go/ no-go type of discrimination (Hasegawa et al., 1973). We might therefore expect diazepam to disirihibit responding to the non-preferred stimuli, i.e., to abolish colour preferences. Terrace (1963a) has shown that CPZ will also disinhibit responding provided the relevant stimuli are aversive in some way. Thus, he found that CPZ had no effect on the responding to the S - if the discrimination was learned without errors, but released responding to the S - when the discrimination had been learned with errors (the S now being regarded as an aversive stimulus). Administration of CPZ might therefore help to further elucidate the status of the non-preferred colours.
Tau
z
P
0.00 0.62 1.25 2.50 5.00
0.42 0.38 0.27 0.07 0.27
2.57 2.36 1.60 0.32 1.60
0.0102 0.0182
0.1096
0.00 0.62 1.25 2.50 5.00
0.52 0.65 0.60 0.62 0.28
3.22 4.08 3.76 3.86 1.72
0.0014 0.0001 0.0001 0.0001 0.0854
(mg/kg, i.m.) Diazepam
Chlorpromazine
Experiment 2
Dose
0.7490 0.1096
Jonckheere trend statistic values (2 tailed) for each dose of the 2 tranquilizers, diazepam and chlorpromazine. A high value of P (> 0.05) indicates that the colour preference has been abolished. Thus, medium and high doses of diazepam abolished colour preferences, whereas chlorpromazine had little effect. Methods The same subjects and apparatus were used. At the conclusion of experiment 1, the birds were retrained on the two neutral density stimuli for a further period, whilst receiving 1 ml/kg of saline injections intramuscularly, 1 hr prior to testing. Once the birds were accustomed to being injected, they were switched to propylene glycol based injection solution (Crankshaw and Raper, 1971), and trained as before. They were then divided into 2 groups, one to receive CPZ (dissolved in saline), and the other to receive diazepam (dissolved in the glycol vehicle). Five doses of each drug (including placebo) were administered to each bird at the following doses: 0, 0.62, 1.25, 2.50 and 5.00 mg/kg. These were arranged in a 5 x 5 Latin square design. Each bird received 1 dose of the drug per test day; each test day was followed by a normal training day, on which the bird received a saline or vehicle injection. The test procedure was the same as that followed in experiment 1, except that the colours were presented in a random manner. There were 5 test days per drug, and at the conclusion, the 2 groups were inter-changed. This change over was separated by 14 days of training on the neutral density stimuli, during which the birds received the appropriate injection of placebo. The results were analysed using the same tests as employed for experiment 1. Results Statistical analysis of the data showed that C P Z had an overall drug-dose effect (Friedman)~2 = 27.18, d f = 16, P < 0.05), whereas diazepam did not (Z2 --- 11.22, d f = 16, P < 0.80). This indicates that CPZ has a sharp dose response curve, whilst diazepam has a broad one. It was also noted that there was a large variability amongst the different birds, some of them having reached very low rates of responding, due to gradual extinction. Table 2 lists the values of tau, z and the associated significance levels on the Jonckheere trend statistic for the two drugs. It can
178
Psychopharmacologia (Berl.), Vol. 43, Fasc. 2 (1975)
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Fig. 1. Combined scores for the 10 birds are plotted at each dose of the 2 drugs, diazepam and chlorpromazine. This illustrates the nonspecific release of responding obtained with diazepam, compared to the systematic reduction in the rate of responding observed with chlorpromazine
be seen that diazepam tended to abolish colour preferences, whilst chlorpromazine did not, except at the highest dose. Fig. 1 illustrates the finding that diazepam tended to abolish preferences by causing an increase in the overall rates of responding, whereas CPZ suppressed responding. The preferences, however remained intact at all but the highest dose level, i.e., CPZ did not disinhibit responding to the non-preferred colours. Discussion The results obtained with diazepam are in agreement with the findings of Margules and Stein (1967) and Hasegawa et al. (1973), i.e., that benzodiazepines such as diazepam cause a release of suppressed behaviour in a wide variety of situations. Our results, however, failed to pinpoint an especially effective dose level for the drug. Even relatively high doses of CPZ did not abolish colour preferences. Margules and Stein (1967) have found that a dose as low as I mg/kg was effective in suppressing most behaviour; our results show that CPZ in low doses neither abolished colour preferences nor suppressed responding although at higher doses (5 mg/kg) a general behavioural depression was observed.
A conclusion in terms of the aversiveness of the non-preferred colours is difficult. Terrace (1963b) argued that previously neutral stimuli may become aversive if the animal had made several non-reinforced responses to them. He also showed (1963a) that CPZ caused a release of responding to such aversive stimuli, whilst having no effect on non-aversive negative stimuli. If we accept Terrace's (1963b) hypothesis, then it may be concluded that the non-preferred colours are not aversive, in any simple sense, since CPZ did not disinhibit responding. Care must, however, be exercised when using such terms as "aversive stimuli". Several workers, such as Morse (1964), and Margules and Stein (1967) have found that CPZ does not disinhibit behaviour suppressed by such presumably aversive stimuli as electric shock and quinine sulphate, nor did CPZ administration result in an increase of responding in extinction. On the basis of such findings, the possibility remains that the nonpreferred colours are mildly aversive.
Acknowledgements. It is a pleasure to thank Dr. S. E. G. Lea for his invaluable advice on statistical analysis, and for his detailed comments on an earlier draft of this paper. Dr. M. J. Morgan and Mr. N. C. Tye made many suggestions, for which we are very grateful. We are also indebted to Mr. S. T. Mason, who wrote the computer program. On-line control of experi-
A. Sahgal and S. D. Iversen: Colour Preferences in the Pigeon ments used the ONLI system developed by C. Crook and S. E. G. Lea with the help of grant MRC G970/297/B to A. J. Watson. We also wish to thank Professor O. L. Zangwill for extending to us the facilities of the Psychological Laboratory. References
Brown, P., Jenkins, J. : Autoshaping of the pigeons key peck. J. exp. Anal. Behav. I1, 1 - 8 (1968) Crankshaw, D. P., Raper, C. : The effect of solvents on the potency of chlordiazepoxide, diazepam, medazepam and nitrazepam. J. Pharmac. Pharmacol. 23, 313-321 (1971) Ferster, C. B., Skinner, B. F. : Schedules of reinforcement. New York:Appleton-Century-Crofts 1957 Friedman, M. : A comparison of alternative tests of significance for the problems of rn rankings. Ann. Math. Statist. 11, 8 6 - 9 2 (1940) Hasegawa, Y., Ibuka, N., Iwahara, S. : Effects of chlordiazepoxide upon successive red-green discrimination responses in Japanese monkeys, macaca fuscata. Psychopharmacologia (Berl.) 30, 8 9 - 9 4 (1973) Humphrey, N. K.: Colour and brightness preferences in monkeys. Nature (Lond.) 229, 615-617 (1971) Jonckheere, A. R. : A test of significance for the relation between m rankings and k ranked categories. Brit. J. Statist. Psychol. 7, 93-100 (1954)
179 Margules, D. L., Stein, L.: Neuroleptics vs. tranquilizers: evidence from animal studies of mode and site of action. In: Neuro-Psychopharmacology, H. Brill, J. O. Cole, P. Deniker, H. Hippius, and P. B. Bradley, eds., pp. 108- 120. Amsterdam: Excerpta Medica Foundation 1967 Morse, W. H. : Effect of amobarbital and chlorpromazine on punished behaviour in the pigeon. Psychopharmacologia (Berl.) 6, 286-294 (1964) Mulvaney, D. E., Dinsmoor, J. A., Jwaideh, A. R., Hughes, L. H. : Punishment of observing by the negative discriminative stimulus. J. exp. Anal. Behav. 21, 3 7 - 4 4 (1974) Siegel, S.: Nonparametric statistics. New York: McGrawHill 1956 Terrace, H. S. : Errorless discrimination learning in the pigeon: effects of chlorpromazine and imipramine. Science 140, 318-319 (1963a) Terrace, H. S.: Discrimination learning with and without "errors". J. exp. Anal. Behav. 6, 1 - 2 7 (1963b) Terrace, H. S.: Stimulus control. In: Operant behaviour: Areas of research and application, W. K. Honig, ed., pp. 271-344. New York: Appleton-Century-Crofts 1965 Thomas, D. R. : The use of operant conditioning techniques to investigate perceptual processes in animals. In: Animal discrimination learning, R. M. Gilbert and N. S. Sutherland, eds., pp. 1-33. New York: Academic Press 1969 Tracy, W. K. : Wavelength generalisation and preference in monotonically reared ducklings. J. exp. Anal. Behav. 13, 163-178 (1970)
A. Sahgal, Department of Experimental Psychology, University of Cambridge Cambridge, England, CB2 3EB
