Monday, July 29, 2013

Bergelson & Swingley (2013)

A recent paper by Elika Bergelson and Dan Swingley at UPenn reports that 10 - 13 month old infants show some evidence of understanding common abstract terms like "uh oh," "bye bye" and "all gone." This finding is big news because previous research hadn't found evidence for these terms until substantially later. Although parents report children's knowledge of these words very early on, parent report measures may be unreliable (an idea ratified by the current paper).

Bergelson & Swingley tested children from 6 - 16 months using a standard eye-tracking paradigm that measures whether infants look more at a movie when they hear a word that matches it. This work builds on a previous paper from the same authors that used the same method to show that 6 - 9 month olds showed some evidence of knowing simple common nouns. In the current study, the 6 - 9 month group showed no evidence of knowledge, but the 10 - 13 mos were above chance and the 14 - 16 mos were quite robust.

As someone who is fascinated by the emergence of early language, I like this paper and its predecessor quite a bit. It takes a common method for measuring infant language comprehension and alters in a few ways that may make it sensitive enough to measure comprehension in very young babies. The researchers use a corneal reflection eye-tracker to get accurate moment-to-moment data on babies' gaze. In addition, rather than using standard audio recordings of words, parents themselves produce the words after hearing a cue on headphones. And finally, stimuli like "all gone" (peering into an empty bowl) and "hi" (waving to the camera) are paired throughout the experiment, and what the authors measure is not whether the infants look more than chance (50%) at the "all gone" movie when they hear the matching words. Instead they ask whether, whatever the infants' bias is initially, this bias is changed by hearing the target item.

One thing that I don't love about this paired-item design, however, is that it doesn't let us make inferences about individual words. I'd be very interested to see whether children understand "all gone" in particular. This common term is one of the first that has negative content ("all gone" = "not there"), and it's an interesting and open question whether very young children understand negation. Some new research from my lab has tried to look into this question using a related method with older children and found significant challenges in children's comprehension of negative sentences. So it would be nice to compare our results to the Bergelson and Swingley findings. But because in their study "all gone" was paired with "hi," we can't really know which one the children understood (and it seems likely that it might have been "hi" in this case).

The framing of the Bergelson & Swingley paper rejects a traditional sounds -> forms -> meanings approach to acquisition, where young infants are assumed to know nothing about what words mean when they are mastering basic skills of speech perception. I strongly agree with this theoretical shift and think it dovetails nicely with two points I've been trying to make in my work.

First, natural language has a frequency structure that stands alongside its hierarchical organization into phonemes, morphemes, and words, such that some sounds, morphemes, and words are much more frequent than the vast majority of others. The Zipfian distribution of natural language means that some units at each level should be much easier to learn than others. So it stands to reason that some word meanings might be learned early, because they are just so frequent (and meaningful in the infant's life) even if the majority are mastered quite a bit later.

Second, my collaborator Mark Johnson has been writing about what he calls "synergies" in language acquisition - cases where it's better to try and solve two problems at once rather than one at a time. For example, in one piece of work we did together, we showed that models that tried to segment word forms from continuous speech performed better if they tried to link those word forms to word meanings at the same time. The word meanings acted as anchors that helped in the segmentation process. What this means is that if infants are trying to learn word meanings, learn sounds, and segment word forms all at the same time, they may see more success than if they follow a kind of "staged" strategy (as was previously assumed).

Two other minor points:

1. Since the sample size was very asymmetric across age groups (34, 46, and 18 babies in the three age groups respectively), I wondered what the recruitment strategy was and how the authors decided to terminate data collection. Given this asymmetry, I also wondered whether it would have been useful to use a more sophisticated analysis (e.g. some kind of non-parametric curve fitting) to estimate the shape of the developmental curve and when it differed from chance.

2. The authors included a nice corpus analysis of what sorts of interactions were happening when the children heard particular abstract words, as well as the frequencies of the words themselves. (I was also a bit surprised here that the authors didn't cite our work on interaction feature coding). Given that they now have comprehension data for both concrete and abstract words, it seems like they should be able to enter a number of different social and frequency predictors into a regression together to see which ones best predict the age of word acquisition. Perhaps Elika Bergelson has done this in her thesis already.

Overall, the two papers by Bergelson and Swingley make a strong addition to the literature on early word learning. I'm very curious to see what kinds of uses their method can be put to in future. One thing I have been considering, for example, is trying to do this kind of method longitudinally (say once a month) with M, as soon as she gets old enough. I'd love to see the emergence of knowledge of word pairs over time - though of course we'd have to think about whether we were testing her knowledge or adding to it!

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