Animal Behaviour 78 (2009) e1–e2
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Do rats learn rules? Michael C. Corballis* Department of Psychology, University of Auckland
a r t i c l e i n f o Article history: Received 26 February 2009 Initial acceptance 21 April 2009 Final acceptance 4 May 2009 Published online 5 June 2009 MS. number: AF-09-00133 Keywords: language rat rule learning
In a recent paper, Murphy et al. (2008), as indicated in the title of their report, claim to have demonstrated ‘rule learning by rats’. They imply, moreover, that their results have implications for the evolution of language, generally regarded as a uniquely human accomplishment. Close inspection of their findings, though, suggest that their conclusions are overstated. The rats were exposed to three successive 10 s exposures to light, which could be either bright (A) or dim (B). The exposures were presented in six orders: ABA, BAB, AAB, BBA, ABB and BAA. The rats were reinforced with food according to three possible ‘rules’: An XYX rule in which only ABA and BAB were reinforced, an XXY rule in which only AAB and BBA were reinforced, and an XYY rule in which only ABB and BAA were reinforced. The authors examined the rats’ responses during the third exposure and found that the rats entered the food tray more often when the sequence followed the rule than when it did not. In a second experiment, pure tones of different frequency replaced the light stimuli, and rats reinforced for the XYX rule generalized to XYX sequences of different frequencies. The authors liken this kind of rule learning to the learning of such rules as subject–object–verb in sentences such as ‘The dog bit the woman’. Quite apart from the fact that grammar involves other, much more complex rules, the sequences used in this study differed in other significant ways.
* Correspondence: M. C. Corballis, Department of Psychology, University of Auckland, Private Bag 92019, Auckland 1042, New Zealand. E-mail address:
[email protected]
First, the elements differed along a single dimension (intensity in the first experiment, frequency in the second), rather than categorically, as in language. The generalization in the second experiment might then have been a matter of simple transposition (Hunter 1953), much as one might transpose a tune to a different key, rather than generalization of a rule. Second, each element lasted 10 s, so the sequences were much more prolonged than in language. This would militate against the learning of a true sequential rule, and one can scarcely imagine even human children learning language based on words 10 s long! Third, and most importantly, the sequences were composed of only two elements, not three. A closer analogue of ‘The dog bit the woman’ would have been sequences of the form XYZ, not XYX. The authors note that the discriminations could in principle have been based on unique pairs of stimuli. Thus the XYX sequences are distinguished by the fact that the first and last elements are the same, XXY by the fact that the first two are the same, and YXX by the fact that the last two are the same. In principle, then, the discriminations could have been based on same–different judgements or delayed matching to sample (e.g. Nakagawa 1993), along with learning to ignore one of the elements. Thus the XYX sequences, for example, might have been based on recognizing the first and last elements as the same, or on matching the last to the first. The authors argue against this on the grounds that the discrimination levels in the first experiment did not differ significantly between the three combinations; one might have expected YXX to be the easiest and the other two more difficult because of the delay in reinforcement. In fact, the level of discrimination was indeed higher in the YXX condition, although not significantly so.
0003-3472/$38.00 Ó 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2009.05.001
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M.C. Corballis / Animal Behaviour 78 (2009) e1–e2
Discrimination was measured in terms of the number of entries to the food tray, and the percentages of the total entries that were consistent with the rule were 51.78% for XYX, 51.58% for XXY and 52.46% for YXX. These percentages are barely above the chance level of 50% (although apparently reliably so), suggesting that differences may have been somewhat masked by a floor effect. I suggest, then, that it is more parsimonious to suppose that the rats weakly learned a strategy based on detection of identical pairs, rather than a rule involving combinations of three elements. But, in any case, there is little, if any, resemblance between what is learned here and what is involved in human language. Language requires assignment of symbols to different categories (noun, verb, article, etc.), and the application of rules that include the merging of elements in recursive fashion. And even if rats could learn a particular sequence of three elements, this does not imply an ability to form different meanings based on different combinations. This is not to say that nonhuman animals cannot learn rules. For example, Fountain & Benson (2006) showed that rats can chunk interleaved sequences into their two sequential streams, and thereby learn associations between elements that are not adjacent in the combined sequence. This demonstrates the use of simple rules to master complex patterns. Hauser et al. (2002) showed that cottontop tamarins, after habituation to two different patterns of consonant–vowel sequences, AAB (as in wi wi du) and ABB (as in le we we), later showed more dishabituation to patterns following different sequences than to those following the same sequences, indicating that they had learned sequential rules generalizing beyond the particular elements chosen. This is not to say, of course, that such examples have any bearing on the rules underlying language, and humans also learn rules unrelated to language. Hauser et al. (2002, page B15) concluded from their results that ‘the capacity to generalize rule-like patterns did not evolve specifically for language acquisition’. From Descartes (1964–1976/1985) to Chomsky (1975), there have long been claims that language is indeed uniquely human, and this of course challenges researchers to prove otherwise.
Arguments typically hinge on mastery of combinatorial rules, since there is little doubt that nonhuman species can respond to simple signals, or even learn associative combinations. The case of Clever Hans (Sebeok & Rosenthal 1981) should have taught us to be mindful of simpler explanations for what appear at first glance to be complex behaviours. Another case in point is the recent claim that starlings can parse recursive sequences involving centreembedding (Gentner et al. 2006), a claim that made international headlines, but which on closer inspection turns out to be explicable in terms of much simpler principles (Corballis 2007). The difference between what the rats learned in the study under investigation here and the rules underlying language requires an even larger leap. I thank Douglas Elliffe and an anonymous referee for helpful comments. References Chomsky, N. 1975. Reflections on Language. New York: Pantheon. Corballis, M. C. 2007. Recursion, language, and starlings. Cognitive Science, 31, 697–704. Descartes, R. 1964–1976/1985. Oeuvres de Descartes. Revised edn (Ed. by C. Adam & P. Tannery). Paris: Vrin/CNRS. Edited and Translated by J. Cottingham, R. Stoothoff & D. Murdock as The Philosophical Writings of Descartes. Cambridge: Cambridge University Press. Fountain, S. B. & Benson, D. M., Jr. 2006. Chunking, rule learning, and multiple item memory in rat interleaved serial pattern learning. Learning and Motivation, 37, 95–112. Gentner, T. Q., Fenn, K. M., Margoliash, D. & Nusbaum, H. C. 2006. Recursive syntactic pattern learning by songbirds. Nature, 440, 1204–1207. Hauser, M. D., Weiss, D. & Marcus, G. 2002. Rule learning by cotton-top tamarins. Cognition, 86, B15–B22. Hunter, I. M. L. 1953. The absolute and relative theories of transposition behaviour in rats. Journal of Comparative and Physiological Psychology, 45, 493–497. Murphy, R. A., Mondrago´n, E. & Murphy, V. A. 2008. Rule learning by rats. Science, 319, 1849–1851. Nakagawa, E. 1993. Matching and nonmatching concept-learning in rats. Psychobiology, 21, 142–150. Sebeok, T. A. & Rosenthal, R. (Eds) 1981. The Clever Hans phenomenon: communication with horses, whales, apes, and people. Annals of the New York Academy of Sciences, 364, 1–311.
