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Animal Behaviour 78 (2009) e1–e2

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Animal Behaviour

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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 2009Initial acceptance 21 April 2009Final acceptance 4 May 2009Published online 5 June 2009MS. number: AF-09-00133

Keywords:languageratrule learning

* Correspondence: M. C. Corballis, Department oAuckland, Private Bag 92019, Auckland 1042, New Ze

E-mail address: m.corballis@auckland.ac.nz

0003-3472/$38.00 � 2009 The Association for the Studoi:10.1016/j.anbehav.2009.05.001

In a recent paper, Murphy et al. (2008), as indicated in the title oftheir report, claim to have demonstrated ‘rule learning by rats’.They imply, moreover, that their results have implications for theevolution of language, generally regarded as a uniquely humanaccomplishment. Close inspection of their findings, though, suggestthat their conclusions are overstated.

The rats were exposed to three successive 10 s exposures tolight, which could be either bright (A) or dim (B). The exposureswere 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, anXXY rule in which only AAB and BBA were reinforced, and an XYYrule in which only ABB and BAA were reinforced. The authorsexamined the rats’ responses during the third exposure and foundthat the rats entered the food tray more often when the sequencefollowed the rule than when it did not.

In a second experiment, pure tones of different frequencyreplaced the light stimuli, and rats reinforced for the XYX rulegeneralized to XYX sequences of different frequencies.

The authors liken this kind of rule learning to the learning ofsuch rules as subject–object–verb in sentences such as ‘The dog bitthe woman’. Quite apart from the fact that grammar involves other,much more complex rules, the sequences used in this studydiffered in other significant ways.

f Psychology, University ofaland.

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First, the elements differed along a single dimension (intensityin the first experiment, frequency in the second), rather thancategorically, as in language. The generalization in the secondexperiment might then have been a matter of simple transposition(Hunter 1953), much as one might transpose a tune to a differentkey, rather than generalization of a rule.

Second, each element lasted 10 s, so the sequences were muchmore prolonged than in language. This would militate against thelearning of a true sequential rule, and one can scarcely imagineeven human children learning language based on words 10 s long!

Third, and most importantly, the sequences were composed ofonly two elements, not three. A closer analogue of ‘The dog bit thewoman’ would have been sequences of the form XYZ, not XYX. Theauthors note that the discriminations could in principle have beenbased on unique pairs of stimuli. Thus the XYX sequences aredistinguished by the fact that the first and last elements are thesame, XXY by the fact that the first two are the same, and YXX bythe fact that the last two are the same. In principle, then, thediscriminations could have been based on same–different judge-ments or delayed matching to sample (e.g. Nakagawa 1993), alongwith learning to ignore one of the elements. Thus the XYXsequences, for example, might have been based on recognizing thefirst and last elements as the same, or on matching the last to thefirst.

The authors argue against this on the grounds that thediscrimination levels in the first experiment did not differ signifi-cantly between the three combinations; one might have expectedYXX to be the easiest and the other two more difficult because ofthe delay in reinforcement. In fact, the level of discrimination wasindeed higher in the YXX condition, although not significantly so.

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M.C. Corballis / Animal Behaviour 78 (2009) e1–e2e2

Discrimination was measured in terms of the number of entries tothe food tray, and the percentages of the total entries that wereconsistent with the rule were 51.78% for XYX, 51.58% for XXY and52.46% for YXX. These percentages are barely above the chancelevel of 50% (although apparently reliably so), suggesting thatdifferences may have been somewhat masked by a floor effect.

I suggest, then, that it is more parsimonious to suppose thatthe rats weakly learned a strategy based on detection of identicalpairs, rather than a rule involving combinations of threeelements. But, in any case, there is little, if any, resemblancebetween what is learned here and what is involved in humanlanguage. Language requires assignment of symbols to differentcategories (noun, verb, article, etc.), and the application of rulesthat include the merging of elements in recursive fashion. Andeven if rats could learn a particular sequence of three elements,this does not imply an ability to form different meanings based ondifferent combinations.

This is not to say that nonhuman animals cannot learn rules. Forexample, Fountain & Benson (2006) showed that rats can chunkinterleaved sequences into their two sequential streams, andthereby learn associations between elements that are not adjacentin the combined sequence. This demonstrates the use of simplerules to master complex patterns. Hauser et al. (2002) showed thatcottontop tamarins, after habituation to two different patterns ofconsonant–vowel sequences, AAB (as in wi wi du) and ABB (as in lewe we), later showed more dishabituation to patterns followingdifferent sequences than to those following the same sequences,indicating that they had learned sequential rules generalizingbeyond the particular elements chosen. This is not to say, of course,that such examples have any bearing on the rules underlyinglanguage, and humans also learn rules unrelated to language.Hauser et al. (2002, page B15) concluded from their results that ‘thecapacity to generalize rule-like patterns did not evolve specificallyfor language acquisition’.

From Descartes (1964–1976/1985) to Chomsky (1975), therehave long been claims that language is indeed uniquely human, andthis of course challenges researchers to prove otherwise.

Arguments typically hinge on mastery of combinatorial rules, sincethere is little doubt that nonhuman species can respond to simplesignals, or even learn associative combinations. The case of CleverHans (Sebeok & Rosenthal 1981) should have taught us to bemindful of simpler explanations for what appear at first glance tobe complex behaviours. Another case in point is the recent claimthat starlings can parse recursive sequences involving centre-embedding (Gentner et al. 2006), a claim that made internationalheadlines, but which on closer inspection turns out to be explicablein terms of much simpler principles (Corballis 2007). The differencebetween what the rats learned in the study under investigationhere and the rules underlying language requires an even largerleap.

I thank Douglas Elliffe and an anonymous referee for helpfulcomments.

References

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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 multipleitem memory in rat interleaved serial pattern learning. Learning and Motivation,37, 95–112.

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