Marcus built on some earlier studies of his (e.g., pdf here) in which he familiarized seven-month infants with a list of nonsense "words" like latala or gofifi. Importantly, all of the words heard by any one infant had the same structure, such as A-B-B ("gofifi") or A-B-A ("latala"). The infants heard two minutes of these type of words, and then were presented with a new set of words using different syllables, half of which followed the same pattern as before, half of which followed a new pattern. Marcus found that infants listened longer and paid more attention to the words with the unfamiliar structure, which they could have done only if they successful abstracted that structure (not just the statistical relationships between particular syllables). Thus, for instance, an infant who heard many examples of words like "gofifi" and "bupapa" would be more surprised to hear "wofewo" than "wofefe"; they have abstracted the underlying rule. (The question of how and to what extent they abstracted the rule is rather debated, and I'm not going to address it here).

The BUCLD talk focused instead on another question: did it matter at all that the stimuli they heard were linguistic rather than, say, tone sequences? To answer this question, Marcus did the same experiment with sequences of various kinds of different tones and tambors in the place of syllables (e.g. "blatt blatt honk" instead of "gofifi"). His finding? Infants did not respond differently in testing to the structures they had heard - that is, they didn't seem to be abstracting the underlying rule this time.

There is an unfortunately large confounding factor, however: infants have a great deal more practice and exposure to language than they do to random tones. Perhaps the failure was rather one of discrimination: they didn't actually perceive different tones to be that different, and therefore of course could not abstract the rule. To test this, Marcus trained infants on syllables but tested them on tones. His reasoning was that if it was a complete failure of discrimination, they shouldn't be able to perceive the pattern in tones when presented in testing any more than they could when presented in training. To his surprise, they did respond differently to the tones in testing, as long as they were trained on syllables. His conclusion? Not only can infants do cross-modal rule transfer, but they can only learn rules when they are presented linguistically, though they can then apply them to other domains. Marcus argued that this was probably due to an innate tendency in language, not a learnt effect.

It's fascinating work, though rather counterintuitive. And, quite honestly, I remain unconvinced (at least about the innate tendency part). Research on analogical mapping has shown that people who have a hard time perceiving underlying structure in one domain can nevertheless succeed in perceiving it if they learn about the same structure in another and map it over by analogy. (This is not news to good teachers!) It's entirely possible - and indeed a much simpler hypothesis - that babies trained on tones lack the experience they have with language and hence find it more difficult to pick up on the differences between the tones and therefore the structural rule they embody. But when first trained on language - which they do have plenty of practice hearing - they can learn the structure more easily; and then when hearing the tones, they know "what to listen for" and can thus pick out the structure there, too. It's still rule learning, and even still biased to be easier for linguistically presented things; but that bias is due to practice rather than some innate tendency.