I wanted to talk about a really cool article that I first read in grad school about infant language development. Although this article was published in 1984, it’s from a classic series of experiments, and is very relevant to T at his current age (8.5 months). Note that the infants studied in the article all had normal hearing, and I’m not sure how the results would change for infants with hearing loss!
The article is “Cross-Language Speech Perception: Evidence for Perceptual Reorganization During the First Year of Life” and is available for free as a PDF through the link. (Werker and Tees. “Cross-Language Speech Perception: Evidence for Perceptual Reorganization During the First Year of Life.” Infant Behavior and Development, Vol. 7, Pages 49-63, 1984).
All languages have consonants – consonants are speech sounds that are articulated with a partial or full closure of the vocal tract. One defining feature of a consonant is its place of articulation, or where in the vocal tract the obstruction occurs. For example, the consonant sounds “p” and “b” are called “bilabial” because both lips close to form the obstruction. Another example is “alveolar” consonants, where the tongue presses against the gum ridge just behind the upper teeth – examples of alveolar consonants are “d” and “t”.
Place of articulation can be a defining feature for distinguishing two consonants – for example, a key difference between “ba” and “da” is that “ba” is made with the lips pressed together and “da” is made with the tongue pressed up against the alveolar ridge (just behind the upper teeth). “Ba” and “da” are also considered “contrastive” in English – this means that if you substitute one for the other in a word, the meaning changes (for example, “bang” and “dang”). So, in English, a bilabial articulation (“ba”) is contrastive with an alveolar articulation (“da”), but there are other pairs of places of articulation that are not contrastive in English.
For example, Hindi has a “retroflex” place of articulation – this position is created by curling the tongue backward toward the hard palate, and is made in conjunction with sounds that are similar to the English consonants of “t” and “d.” In Hindi, the retroflex articulation is contrastive with a “dental” articulation, where the tongue is pressed just behind the upper teeth. So, in Hindi, a “t” sound can be made with the tongue just behind the upper teeth (“dental”), or with the tongue curled way back (“retroflex”), and these two types of “ts” are different letters, and one substituted for the other in a word creates a different word.
English doesn’t have a retroflex consonant (only 11% of languages have retroflex consonants!) – in fact, native adult English speakers can’t really even hear the difference between a retroflex “t” and a dental “t” (which are different sounds and different letters in Hindi) – they tend to label both the retroflex “t” and the dental “t” as an alveolar articulation, which corresponds to the “t” sound in English.
What’s really interesting is that newborn babies (<6-8 months old) in native English-speaking families, can hear the difference between a retroflex “t” and a dental “t” – and sometime between early infancy and adulthood, they lose this ability. The general idea is that all babies, regardless of language spoken at home, are born with the ability to hear the difference between all these different consonant contrasts, and based on the language they hear around them, the brain “prunes” out the ability to hear the contrasts that aren’t needed to learn their native language. Werker and Tees studied when infants lose this ability.
In this post, I’ll focus on Experiment 2 described in the article. The authors had previously found that 6-8 month old infants could discriminate (or, hear the difference) between the Hindi retroflex “t” (which I’ll lablel here as tr) from the Hindi dental “t” (which I’ll label here as (td). In a pilot study, they found that English-speaking 4-year old children performed similarly to English-speaking adults – that is, they couldn’t hear the difference. So, in Experiment 2, the authors looked at whether 8-10 month old infants being raised in English-speaking homes could hear the difference between tr and td, and whether 10-12 month old infants raised in English-speaking homes could hear the difference between tr and td. They also compared the results to those of babies being raised in Hindi-speaking homes.
To test whether the babies could hear the difference between tr and td, they used a conditioned head-turn procedure. A string of one of the consonants was played in a loop, and then suddenly changed to be the other consonant (for example, “tr tr tr tr td“). The babies were conditioned to turn their head to look at a toy animal when they detected a change in the consonant being played. (This is actually kind of similar to the procedure for Visually Reinforced Audiometry used to measure audiograms in babies!)
The authors found that, of the babies being raised in English-speaking homes, most of the 6-8 month old infants could discriminate tr and td, some of the 8-10 month old infants could discriminate tr and td, and only a few of the 10-12 month old infants could discriminate tr and td. The 10-12 month old infants were significantly worse at discriminating the two consonants than either the 6-8 month old infants and the 8-10 month old infants. Additionally, the authors found that all of the 10-12 month old babies being raised in Hindi-speaking homes could discriminate tr and td. The figure below shows the proportions of infants in the different age groups that could discriminate these two consonants (it’s FIG. 4 from the article).
(Note that, in the above figure from the article, the graphs show both results using the Hindi consonants, as well as two consonants from a different language called Salish. Additionally, the top row of graphs (labeled “cross-sectional data”) shows results from different babies, and the bottom row of graphs (labeled “longitudinal data”) shows results from the same group of babies followed over time from 6-8 months through 10-12 months).
The results of this study show that infants up until 6-8 months of age can hear the difference between consonants that aren’t contrastive in their native language, but that they lose this ability somewhere between 8-12 months of age. A lot of important language development happens in the first year of life!
Testing Myself, My Husband, and My Baby
I had originally read this article because I was interested in T’s ability to discriminate retroflex and dental consonant since he’s in the interesting 8-10 month old age range where he might or might not be able to hear the difference.
The Adults – Me and My Husband
Before testing T, I decided to see whether my husband and I could hear the difference. I found synthesized audio files of retroflex and dental consonants here (although the synthesized consonants were the retroflex and dental “d” consonants rather than “t” consonants as used in the article; the retroflex/dental “d” consonants are also present in Hindi). If you click on that link, you can hear the consonants that I used (they’re labeled CV1 and CV7) – can you hear the difference?
To test myself and my husband, I randomly played either the same consonant (both retroflex or both dental) or different consonants and asked whether they were the same or different. If we were just guessing randomly, we’d get about 50% correct.
Let’s start with me – after testing myself on two sets of 20 comparisons, I got 47% correct – just as if I’d guessed randomly – it turns out, I can’t hear the difference between retroflex and dental consonants at all! And now for my husband – over two sets of 20 comparisons, he got 90% correct! He said that the difference was in the beginning part of the consonant – they sounded like they had different “attacks,” and that this difference was very clear to him.
It kind of irritated me that my husband was so much better than me at hearing the difference! So, I did a little digging – this  study by Pruitt et al. found that adult native Japanese speakers are better than adult native English speakers at hearing the difference between Hindi retroflex and dental consonants. They hypothesized that this is because Japanese contains a consonant contrast that is similar to the retroflex/dental contrast in Hindi (in Japanese, it’s the /d/ vs. flapped /r/, which is sometimes produced as a retroflex consonat). My husband went to a Japanese immersion school for several years as a child, so my current hypothesis is is that the reason he could discriminate the Hindi consonants so much better than me is his early exposure to Japanese!
To test T, I first tried to see if he could tell the difference between the English “ba” and “da.” These two consonants also have a difference in place of articulation, but since T hears this every day, he should be able to tell the difference, regardless of age. I produced a stream of “ba”s and then switched to “da” (or vice versa), and he quickly looked up, indicating that he’d noticed the difference. (This is different than the way the babies were tested in the article, since I don’t have the same equipment that they had!).
I then repeated this with the Hindi retroflex consonants. A few times, it seemed like his attention shifted coincident with the change in the consonant, indicating that he might have heard the difference. However, there were also several instances where his attention didn’t shift at all. It’s hard to say whether this is because he didn’t hear the difference, or because his attention was drawn elsewhere (for example, to the laptop producing the sounds!).
Overall, I can’t say whether or not T can discriminate between Hindi retroflex and dental consonants – it might be that he can, but that the way I tested him wasn’t thorough enough to detect his ability to discriminate the two consonants. Alternatively, he might not be able to discriminate the two consonants, but we don’t know whether that’s because he previously had this ability and has since lost it with age (as in the case of the babies studied in the article), or whether he was never able to discriminate the two consonants (since I never tested him on this when he was younger). I wish I had done this earlier, when he was 6 months old, to see how he’d reacted then!
 Pruitt J.S., Jenkins J.J., and Strange W. “Training the perception of Hindi dental and retroflex stops by native speakers of American English and Japanese.” J. Acoust. Soc. Am., 119, 1684, 2006.