Operant conditioning studies of taste discrimination in the pigeon (Columba livia)

Physiology & Behavior, Vol. 24, pp. 163--168. Pergamon Press and Brain Research Publ., 1980. Printed in the U.S.A.

Operant Conditioning Studies of Taste Discrimination in the Pigeon (Columba livia) G. M A R I O T T I A N D L . F I O R E Istituto di Biologia Generale dell' Universitd di Pisa 1 56100 Pisa, Italia R e c e i v e d 30 D e c e m b e r 1978 MARIOTTI, G. AND L. FIORE. Operant conditioning studies of taste discrimination in the pigeon (Columba livia).

PHYSIOL. BEHAV. 24(1) 163-168, 1980.--In the conditioning procedure employed in this study, pigeons were trained to dip their bills into a container and then to peck either at a left or at a right key, depending on whether the container was full of distilled water or of a solution of the substance being tested. The discrimination thresholds found (from one bird each) were 2 grl (0.034 M) for NaCI, 2 g/I (0.024 M) for NaHCO3, 1 g/1 (0.013 M) for KC1, 0.5 g/l (0.005 M) for KHCO3. Following the same procedure, two birds were then faced with the choice between the solution of the known substance and of a novel one. A higher degree of discrimination was found between NaHCOa and KHCOa than between NaHCO3 and NaCI; in the latter case, choices appeared to be mostly based on the concentration of the two substances. Discrimination learning Pigeon

Taste

Gustatory acuity

OUR knowledge of taste in birds is at present incomplete with respect both to gustatory acuity and to discrimination between stimuli. This fact appears to be largely due to difficulties of experimentation in this field. The most commonly used behavioral method [5, 6, 8] consists in facing the birds, simultaneously or successively, with a solution of the substance being tested and with alternative fluids, mostly tap water or distilled water; if there is a difference between the consumption of the various types of fluid, that proves the ability of the birds to perceive them differently. Preference tests, in fact, determine whether a dissolved substance exercises an attractive or aversive effect over the birds, and assess the concentration at which it is active in this sense. They are not adequate, however, if the presence of a substance is perceived by the bird, but does not induce attraction or aversion, and consequently does not provoke significant differences in the amount of fluids assumed. Moreover, any effects which are observed may conceivably be influenced by the dietary or metabolic conditions of the birds tested. Methods based on illness, artificially induced after ingestion of food or water added by the chemical being tested, and consequent aversion for this particular substance (conditioned taste aversion), could overcome these difficulties, but they present other disadvantages which are linked to the general effects of a negative reinforcement. In the present study our aim has been to test the gustatory capabilities of pigeons by means of a method of operant conditioning, which was based on positive reinforcement and implied a clear and unequivocal response by the birds (left-right choice). The choice was either between distilled water and water with a substance added, or between two different substances dissolved in water.

Successive discrimination

Gustatory discrimination

A short report on this study has already been published [3]. METHOD

Animals Six homing pigeons, obtained from the stock of our Institute, were used in the experiments. They varied in age and sex. The birds were reared together in an aviary, but were fed singly in order to control the amount of food consumed (mixed grain). Their weights were maintained at 80% of their free-feeding weights.

Apparatus A test chamber was used, in which the front panel contained a white houselight at the top, two symmetrically placed, translucent keys, a left-hand green one and a right-hand yellow one, at the level of the pigeon's head, and a central food hopper between the keys. A hole was placed under the food hopper through which a small container (30 ml) could be introduced from the outside. The keys were illuminated from behind. They were lit manually, and operated only when lit; a peck at either of them turned off both, and could activate the hopper at the same time. When the keys were lit, the houselight was switched off, automatically, and the vice versa.

Procedure Pretraining. Standard shaping procedures were used to produce stable, food-reinforced key pecking. In some cases, it was necessary to disconnect either one or the other key

C o p y r i g h t © 1980 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/80/010163-06502.00/0

164 from food as a correction procedure, to prevent perseveration of responding to only one key. In the next stage, the container was introduced into the test chamber, full of distilled water; the keys were lit when the bird came near it, and later when it dipped its bill into it. This made it possible for the bird to peck at one key and get food. Finally, the pigeon was trained to dip its bill into the container three times in rapid succession before the keys were lit. Pretraining sessions were repeated daily, and lasted about 30 minutes. Preliminary training took 15-20 days altogether. Experimental sessions. A successive discrimination procedure was used. In the first series of experiments, in successive trials the container was introduced full of either distilled water or of a dissolved chemical, according to quasirandom sequences (sequences which presented the same choice five or more consecutive times were discarded); in all, a session comprised 15 presentations of pure water and 15 presentations of the dissolved chemical. After the container had been introduced and the pigeon had dipped its bill into it, the two keys were lit; responses on the left or right key were correct in the presence of pure water or of the dissolved chemical, respectively. After the bird had made its choice, and obtained food for correct choices, a correction procedure was employed; the container remained in the chamber, and the pigeon had the opportunity of dipping its bill again and of making a second choice. The second choices were not recorded; they were correct in most cases. Sessions were held daily. In the second series of experiments, the container was alternatively filled with solutions of two chemicals, and the procedure followed was the same as in the first series. Two pigeons were used in these experiments, chosen from among those used in the first series; they were faced with a choice between the chemical they had been acquainted with before and a novel chemical, which differed from the other substance only with regard to its cationic or its anionic part. The correct response to the known chemical continued to be the right-hand key, as in the first series, whereas the correct response to the novel chemical was the left-hand key. The novel chemical was initially presented at a low concentration, but its concentration was then progressively raised; after this, the concentration of the known chemical could be lowered. Some blank sessions were held among the experimental sessions; in these, distilled water was given in all trials, or, in one case, the test substance was presented on every trial. In these cases the performance fell to about the 50% level. These controls were not made in discrimination tests between two substances. As a further control some sessions were run by two experimenters, with the one lighting the keys unaware of the contents of the containers. The substances tested were NaCl, NaHCO3, KCI, KHCO3 in the first series of experiments, and NaHCO3 (novel substance) vs NaCl (known substance), NaC1 (n.s.) vs NaHCO3 (k.s.), KHCO.~ (n.s.) vs NaHCO3 (k.s.) in the second series. For statistical evaluation, the z parameter, which applies to individual sessions, was not adopted, because of the large fluctuations in performance from session to session and of the infrequent occurrence of very high levels of performance. For every group of successive sessions carded out in the same conditions, the mean, standard error, and confidence interval of a correct choices were computed; if the random level (50%) lay below the confidence interval, it was concluded that a discrimination had taken place.

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GUSTATORY CONDITIONING IN PIGEONS

165 TABLE 1

RESULTS OF THE EXPERIMENTS WITH ONE SUBSTANCE. THE SUBSTANCES EMPLOYED AND THE LETTERS USED TO NAME THE PIGEONS TESTED ARE LISTED AT TOP OF TABLE*

KHCO3

KCi

NaHCO3

NaHCO3

NaC1

NaCI

M

N

G

C

V

B

100 mM (10 g/l) 22.70 ± 0.60 (36)

134 mM (10 g/l) 23.80 ± 0.83 (49)

238 mM (20 g/i) 22.20 +- 0.65 (32)

238 mM (20 g/l) 25.10 - 0.55 (62)

342 mM (20 g/l) 24.80 _ 0.88 (22)

342 mM (20 g/I) 27.30 - 0.68 (30)

40mM (4 g / l ) 25.84 - 0.58 (13)

54mM (4g/l) 25.69 ± 0.57 (13)

ll9mM(10g/l) 22.12 ± 0.77 (8)

ll9mM(10g/i) 25.69 ± 0.80 (13)

171 mM(10g/1) 24.30 ± 0.68 (10)

171 mM(10g/I) 23.57 ± 0.66 (14)

20 mM (2 g/l) 25.15 ± 0.41 (13)

27 mM (2 g/l) 22.45 ± 0.64 (11)

60 mM (5 g/l) 20.50 ± 0.76 (12)

60 mM (5 g/l) 24.50 ± 0.76 (8)

85 mM (5 g/l) 20.04 ± 0.70 (23)

85 mM (5 g/I) 20.35 ± 0.76 (17)

10 mM (1 g/l) 22.38 ± 0.83 (13)

13 mM (1 g/l) 18.53 ± 0.87 (15)

24 mM (2 g/l) 22.00 ± 0.82 (7)

5 mM (0.5 g/l) 18.26 ± 0.86 (15)

7 mM (0.5 g/1):~ 15.25 ± 0.49 (12)

12 mM (1 g/l)~: 15.50 - 0.87 (8)

34 mM (2 g/l) 18.62 ± 0.94 (8) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

34 mM (2 g/l) 20.69 ± 0.64 (13)

2 mM (0.25 g/1)'~ 16.50 ± 0.54 (10) *Sessions carried out with the same concentration are grouped together; for each group of sessions, the concentrationof the chemical, the mean ± standard error of correct choices, and the number of sessions (in parentheses) are listed. At the highest concentration tested with each bird only the results of the 10 last sessions were computed. The dashed line for pigeon B corresponds to some sessions carded out at 85 mM (5 g/I) NaC1 concentration; the relative results are not given. The random level [15] lies below the 99% confidence interval, except for t, where it lies below the 95% confidence interval, and for :~, where the difference from the random level is not significant.

RESULTS

Discrimination Between a Dissolved Chemical and Pure Water Two birds (pigeons B and V) were tested with NaCI, two (C and G) with NaHCO3, one (N) with KCI, and one (M) with KHCO3. The sessions with pigeons V (NaCI) and G (NaHCO3) were not pursued until meeting threshold concentrations; threshold values, therefore, were obtained from one bird for each substance. The results are summarized in Table 1. Discrimination Between Two Dissolved Chemicals Two birds, pigeons C and B, were tested, Pigeon C was initially tested with the pair NaCI (novel chemical)NaHCO3 (known chemical). Figure 1 shows the results, plotted together in part A, plotted separately for the NaC1 and NaHCO3 presentations in part B. The same pigeon was then tested with the pair KHCO3 (novel chemical)NaHCO3 (known chemical) (Fig. 2A, B). Pigeon B was tested with the pair NaHCO3 (novel chemical)--NaCl (known chemical). This pair, in reverse relationship, had been tested with pigeon C, too. Figure 3A, B summarizes the results. Note that the level of correct choices fell below 66.7% in the first series of presentations at 119:171 mM; the 48:171 mM situation was therefore resumed, until the 66.7% level was consistently surpassed again. DISCUSSION

The sensitivity thresholds obtained from the experiments with one substance can be compared with values derived from preference tests. As regards NaHCO3 and NaC1, pre-

vious research carried out with chickens [4] had given an identical threshold for NaHCOz (24 mM, 2 g/l), a higher one for NaC1 (not less than 171 mM, 10 g/l, compared to 34 mM, 2 g/l, in our experiments). A threshold or 171 mM NaCI had also been found in the great tit (Parus major) [7]. A study carried out with feral pigeons [1,2] had shown aversion to NaCI beginning at a similar concentration (140 mM); however, it had revealed that this substance was also perceived at lower concentrations, but in this case, at 43 mM concentration, it gave rise to a slight preference. Concerning KCI, an analogous, still less clear-cut alternation between preference and aversion was observed [1,2]; aversion occurred at about 50 mM, was followed by preference at 200 mM, and, finally, by rejection at higher concentrations. The possibility of the occurrence of such alternations in preference and aversion according to concentration represents a further limit to the efficacy of the preference tests, particularly when the attractive and aversive effects are weak; on the other hand, they are not expected to interfere with the conditioning method used here. In discrimination tests between two substances, results from the experiments with the NaHCOr-KHCO3 pair, in which the cationic part was different, were dissimilar from those obtained with the NaHCO~-NaCI pair, in which the anionic part was different. A clear discrimination was obtained with the KHCO3NaHCOa pair, which was tested with pigeon C alone. The possibility that the choices were chiefly based on differences in pH or concentration between the two solutions was ruled out, since discrimination did not deteriorate as the KHCO3 concentration approached that of NaHCOa, so that those differences were progressively reduced; moreover, discrimination remained in the 100:48 mM situation, when the pH and concentration differences were inverted. The main cue

166

MARIOTTI A N D F I O R E

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GUSTATORY CONDITIONING IN PIGEONS

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168

MARIOTTI A N D F I O R E

used by the bird therefore appeared to be the presence of either K + or Na + ions. The deterioration of performance in response to NaHCO3 presentations which occurred in the 100:24 mM situation should, however, probably be ascribed to the marked decrease in the concentration of this chemical with respect to the value the bird had become accustomed to in a long series of sessions; the same reason could also explain the loss of discrimination in the successive 100:12 mM case. This indicates that variations in concentration had some influence, too. Concerning the NaCI-NaHCO3 pair, pigeon C gave a good performance in the 17:119 mM situation, but, as the NaCI concentration was raised to 34 and 68 mM, it gradually shifted its choices to presentations of this chemical, turning from the left-hand to the right-hand key. In these cases, it tended to choose in response to NaCI presentations the same key which was correctly chosen when the NaHCO3 solution was introduced, and therefore appeared to be no longer able to discriminate. The immediate reversibility of this effect, ascertained several times by alternating 34 and 68 mM NaCI concentrations, ruled out the possibility that perseveration with one key had taken place, due to the difficulty of the task. It can be deduced that the cues upon which the pigeon based its choices did not involve the presence of either CI- or HCO3- ions, but features which became less and less dissimilar as the difference in concentration between the two solutions was reduced. Most probably, these cues involved either the amount of Na ÷ ions present, or the total ionic concentration. Results from pigeon B, tested with this same pair of chemicals in reverse relationship (NaHCO3-NaCI), were less clear-cut. The bird showed good performance up to the 119:171 mM situation; in these sessions, however, discrimination could be due to differences between the types of ions

involved or to other cues, such as concentration differences. In the 119:68 mM situation, in which for the first time the value of NaCI concentration was varied and fell below that of the NaHCO3 solution, a kind of behavior was observed which was similar to that of pigeon C tested in the same situation. Pigeon B tended to give wrong responses to NaCI presentations, choosing the same key that was correctly pecked when NaHCO3 was presented (left-hand key). However, in a few sessions interspersed among the others the choices in response to NaC1 were mostly correct, indicating that, in these cases, the bird based its behavior on specific stimuli. This provides evidence in favor of the ability of the bird to perceive differences in ionic composition between the two solutions, even if it tended to rely on other features. The possibility that other, non-gustatory cues (olfactory, trigeminal) were involved in discrimination seems unlikely, since the substances used were non-volatile, non-irritant, in several cases were used in very low concentration, and the two concentrations were often similar in the experiments with two substances. The conditioning method used did not seem to facilitate discrimination performance. This is indicated by the fact that very high levels of performance were only rarely attained, and that remarkable oscillations from session to session often took place, even when learning had already occurred. Nonetheless, these difficulties may be overcome by pursuing training for a sufficiently long time, and seem to be counterbalanced by the opportunity the method offers of a detailed analysis of the gustatory function in birds. ACKNOWLEDGEMENTS We wish to express our gratitude to Dr. B. M. Wenzel for her very helpful advice and criticism.

REFERENCES 1. Duncan, C. J. Preference tests and the sense of taste in the feral pigeon (Columba livia Var Gmelin). Anim. Behav. 8: 54-60, 1960. 2. Duncan, C. J. Salt preferences of birds and mammals. Physiol. Zool. 35: 120--132, 1962. 3. Fiore, L. and G. Mariotti. Taste discrimination in pigeons studied by operant conditioning. Monitore zool. ital. (N.S.) 12: 65-66, 1978. 4. Kare, M. R. The special senses. In: Avian Physiology, edited by P. D. Sturkie, second edition. Ithaca, N. Y.: Cornell University Press, 1965, pp. 406--446.

5. Kare, M. R. Comparative study of taste. In: Handbook of SensoJy Physiology, Vol. IV, Part 2, edited by H. Autrum et al. Berlin: Springer Verlag, 1971, pp. 278-292. 6. Kare, M. R. and M. S. Ficken. Comparative studies on the sense of taste. In: OIfaction and Taste 1, edited by Y. Zotterman. New York: MacMillan, 1963, pp. 285--297. 7. Warren, R. P. and M. A. Vince. Taste discrimination in the Great Tit (Parus major). J. comp. physiol. Psychol. 56: 910--913, 1963. 8. Wenzel, B. M. Chemoreception. In: Avian Biology, Vol. III, edited by D. S. Farner et al. New York/London: Academic Press, 1973, pp. 389-415.