Is language acquisition different depending on how many people speak a child? We used three methods to answer this question. First, we ran three laboratory experiments with infants, and a fourth experiment is ongoing. Overall, we have found relatively little evidence that the number of speakers babies are routinely exposed to impacts their language development. This might entail that infants have ways of dealing with talker variability beyond what we had given our computer models, either as part of their innate language skills or because something in their experience allows them to compensate for that variation.
Our second method is called a “meta-analysis”, and it consists of a detailed, quantitative review of all previous experiments on one topic, which allows us to combine all evidence into one metric. We carried out three of them, on slightly different questions, which helped us identify good methods to use in our experimental work. In the process, we also made the same data available to the broader community (see
http://metalab.stanford.edu(s’ouvre dans une nouvelle fenêtre)). These meta-analyses helped design more informative infant experiments.
Finally, we used computational models to check how speech processing was affected “in theory” when there were multiple talkers. Using a computational model, rather than human participants, is very informative: Since we design the model ourselves, we know exactly how it perceives sounds, and we can thus check what an unbiased ear may do with a single-talker or multi-talker experience. Another advantage is that we can do many more tests than would be feasible with actual human listeners. In a nutshell, the computational models showed us that at the level of the human inner ear, we should find it a bit harder to distinguish sounds when they are spoken by different people than when they are spoken by a single person. Another, rather incidental, result was that it mattered a lot which sound pair is compared, and this is important because studies with adults and infants often cannot compare more than a handful of sound pairs. The results from such studies might not reflect reality across the board, because we find that some sound pairs are always easy to distinguish while others are always hard, both extremes show little influence of the differences between talkers. Sound pairs that are a bit difficult become harder to distinguish when multiple voices are involved. A second modeling study was conducted on data from meta-analyses to find out whether and how knowing words and knowing sounds interacts over the first years of life.
The results have been disseminated widely: two papers that have been accepted for publication in peer-reviewed journals and are available as preprints, three papers are already published in peer-reviewed proceedings; further manuscripts are in preparation. The results have also been disseminated in five invited lectures and at ten international conferences.
