[The following appeared as a Letter in Science, 284, April 16, 1999)]

Do Infants Learn Grammar with Algebra or Statistics?

The report "Rule learning by seven-month-old infants" by G. F. Marcus et al. (1 Jan., p. 77) adds to a growing body of evidence concerning the remarkable learning abilities of infants. This evidence indicates that children acquire much more knowledge of language from experience than one might assume (1). However, the conclusion by Marcus et al. that the infants had learned rules rather than merely statistical regularities is unwarranted.

In the experiments in the report by Marcus et al., infants were familiarized with sequences of syllables that conformed to patterns such as ABB or AAB (for example, "wo fe fe" versus "wo wo fe"). They were then tested on sequences containing different syllables that either matched these patterns or not. Infants preferred (2) novel sequences that violated the pattern to which they had been pre-exposed, and so were said to have learned the rule governing the sequences' "grammar." This conclusion rests on the fact that the test sequences contained novel syllables; thus, the infants could not have learned anything about their statistical properties. However, these "grammatical rules" created other statistical regularities. AAB, for example, indicated that a syllable would be followed by another instance of the same syllable and then a different syllable. Thus, in the pretraining phase, the infant was exposed to a statistical regularity governing sequences of perceptually similar and different events. The report's discussion focused on what the infants could learn about the particular syllables used in training, but there is no reason to deny these infants the capacity to learn these same-different contingencies.

There is also no reason to deny connectionist neural network models for this capacity. In our view, the goal of modeling is to understand children's behavior by endowing networks with the same capacities and experiences as children. The networks that Marcus et al. studied were not provided with either, so it is not unexpected that they behaved differently. A 7-month-old child has already developed a rich representation of the structure of acoustic and speech events on the basis of several thousand hours of exposure to examples, including the "novel" test syllables. In the model used by Marcus et al., in contrast, there was no knowledge of the structure of utterances, no exposure to these syllables, and no way to represent phonological similarity.

A model with the same kinds of capacities and experiences as infants will perform in a similar manner. To demonstrate this, we implemented a simple model (3), which is not a general account of all aspects of the phenomena, but serves to illustrate that the limitations that Marcus et al. described are not intrinsic to all connectionist models.

Rather than showing that rule learning is "there from the start" (4), the findings in Marcus et al.'s report indicate that infants are able to encode multiple types of statistical regularities. This feat places them squarely on the path toward acquiring a central aspect of the adult's linguistic competence (5).

Mark S. Seidenberg
Neuroscience Program,
University of Southern California,
Los Angeles, CA 90089-2520, USA.
E-mail: marks@gizmo.usc.edu

Jeff L. Elman
Department of Cognitive Science,
University of California, San Diego,
La Jolla, CA 92093-0515, USA
E-mail: elman@crl.ucsd.edu

References and Notes

  1. N. Chomsky, Knowledge of Language (Praeger, New York, 1986).
  2. As described on page 78 of the report, preference was indicated by an
     infant "looking longer at the flashing side light during presentations
     of [novel] sentences."
  3. Discussion and model are at crl.ucsd.edu/~elman/Papers/MVRVsim.html .
  4. As stated in the Perspective "Out of the minds of babes" (S. Pinker, p.
     40) that accompanied the report.
  5. M. S. Seidenberg, Science 275, 1599 (1997); J. L. Elman et al.,
     Rethinking Innateness (MIT Press, Cambridge, MA, 1996).