Last week, I talked about Part 1 of this study, which compared the initial, babbling stage of infant language development for infants with hearing loss and normally-hearing infants. This week, I want to talk about Part 2 of the study, which looked at how babies, as they got older, transitioned from babbling to producing words. Here’s a link to a full PDF of the study.
Part 1 of this study found that infants with hearing loss (HL) generally are delayed relative to normally hearing (NH) infants in the babbling stage of language development. HL infants took longer to begin babbling, and, once they began babbling, were slower to acquire particular types of consonants, such as fricatives (“sss,” “shhh,” “f,” etc.). The researchers wanted to then look at older babies to see whether HL infants were also delayed in transitioning from babbling to producing words relative to NH infants.
The infants included in this study were the same as those in Part 1 – to recap, there were 21 NH infants and 12 HL infants. The HL infants varied a lot in degree of hearing loss, and three received cochlear implants (CIs) during the course of the study. For all infants, language productions were monitored during play sessions with a caregiver (typically the infant’s mother), and these sessions were generally conducted every 6 weeks. In Part 2 of the study, data from sessions when the infants were between 10 and 36 months old were used.
Let’s get to the results!
The researchers analyzed the infants’ language productions during the sessions in 2 broad categories: the proportion of different utterance types at different ages and the structural characteristics of words produced at 24 months.
To look at the proportion of different utterance types at different ages, the researchers coded each utterance produced by an infant during a session as belonging to one of 3 utterance types:
- Non-communicative – these were speechlike sounds but were more vocal play than attempts to communicate. Examples include babbling that wasn’t directed to an adult.
- Unintelligible communicative attempts – these were vocalizations that were a) directed to an adult and b) served a communicative purpose, such as getting the adult to do something, seeking attention, etc. Some of these might have been attempts by the infant to say a particular word, but weren’t recognized by the caregiver or the researchers as a word.
- Words – the researchers pointed out that it’s tricky to decide what constitutes a word. For this study, utterances were classified as words if: 1) at least one vowel and consonant in the word attempted by the infant matched the “real” word (e.g., “baba” for “bottle”), 2) the utterance was a communicative attempt (see #2 above), and 3) it was clear that the child was attempting to say a word, for example, that the infant was imitating the parent or that the parent recognized the word and repeated it.
FIG. 1 of Moeller, et al. (reproduced below) shows the results of the analysis of utterance type for NH and HL infants at 16 months old and 24 months old.
As you can see in FIG. 1, at a given age, the pattern of the proportion of different response types was different for NH infants compared to HL infants. For example, at 16 months, the NH infants were producing more unintelligible communicative attempts as well as more words compared to the HL infants. As another example, at 24 months, a greater fraction of utterances for the NH infants were words compared to the HL infants. Additionally, while both the NH infants and the HL infants produced more words at 24 months compared to 16 months, the researchers found that the magnitude of the increase was larger for NH infants. Interestingly, the researchers found that the pattern of utterance types for the HL infants at 24 months was similar to that of the NH infants at 16 months (I highlighted these in the red boxes in FIG. 1 above), indicating that the HL infants might have a similar pattern of improvement over time, but delayed.
To look at the structure of word attempts by the infants at 24 months, the researchers randomly selected 25 words from each child’s transcripts during the experimental session and compared the word attempt with the actual, target word to assess both the complexity of the word attempt and how accurate the attempt was. They computed 7 different metrics:
- Mean syllable structure level (MSSL) – this metric was used in Part 1, as well, and I described this in more detail here. As a quick recap, words with only vowels were scored with 1 point, words with a single consonant type were scored with 2 points (e.g., “ba” or “baba”) and words with two or more consonant types were scored with 3 points (e.g., “bada” or “dago”).
- Percentage of vowels correct – this indicates the percentage of the time that the infant’s vowel productions in their word productions matched the “correct” vowels in the corresponding word. For example, if the target word was “mama,” the child would get 100% for saying “gaga” or “baba” but 0% for saying “momo.”
- Percentage of consonants correct (PCC) – this is similar to as above, but with consonants. As an example, if the target word was “shoe,” the child would get 100% for “shoo,” “shee,” “shaw,” etc., but 0% for “too.”
- Phonological mean length of utterance (PMLU) – This measure is intended to identify children who attempt longer, more complicated words but produce them less accurately as compared to children who attempt shorter, simpler words but produce them more accurately. To calculate this, the child received 1 point for each vowel and consonant produced, and an additional point for each correctly produced consonant. For example, if the target word was “cat,” at the child produced “cat,” they would receive 3 points for producing “c,” “a,” and “t,” and an additional 2 points for correctly producing the “c” and “t,” for a total of 5 points. However, if the child had instead produced “da” for “cat,” they’d receive only 2 points – one each for the “d” and “a,” but no points for accuracy. In this way, the PMLU reflects both accuracy of the word production as well as the length of the word.
- Proportion of whole word proximity (PWWP) – This measure is intended to give an overall reflection of how accurately the child produced a particular word. It is calculated by dividing the PMLU of the word attempt into the PMLU of the target word. As descried above, “cat” produced correctly would have a PMLU of 5, and “cat” produced as “da” would receive a PMLU of 2. Therefore, if a child produced “da” for “cat,” the corresponding PWWP would be 2/5, or 0.4.
- Word shape match – This measure indicates how accurate a child’s production of a word was in turns of shape/number of syllables. For example, if the target word was “cookie,” the target shape would be “consonant-vowel-consonant-vowel.” (CVCV). If, instead of producing a word that had a CVCV shape, the child produced one with just a CV shape (e.g., “di,” “koo,” “da,” etc.), this would not be a match.
- Words with final consonants – Word productions were given points for this metric if a target word ended with a consonant, and the child’s production of the word also ended with a consonant, even if the consonant wasn’t totally accurate. So, for example, if the target word was “goat,” the child would get points for producing “goat,” “got,” “god,” “goad,” etc.
The results of the structural analysis of the children’s word productions are shown in Table 1 of Moeller, et al. (reproduced below).
This table shows that, for every measure of word structure (the rows in the table), the NH hearing children performed better than the HL children. The difference between the NH and HL children was statistically significant (this is indicated by the crosses in next to the score for the NH children in each row).
One of the things that I really like about this table is that they indicate the Effect Size for each metric of word structure (this is indicated in the right-most column of the table). The effect size tells you the strength of of the finding. For the measure of effect size used in this paper (called “Cohen’s d”), an effect size of around 0.2-0.3 is considered a small effect, an effect size of around 0.5 is considered a medium effect, and an effect size of more than 0.8 is considered a large effect. So, as you can see from this table, for every metric of word structure, the researchers found that not only was there a statistically significant difference in performance between the NH children and the HL children, but that the size of this difference was large.
So, overall, the data in Table 1 indicates that compared to age-matched NH children, HL children were producing words that were less complex (contained fewer different types of consonants, were less likely to end in a consonant, and were shorter) and that tended to be less accurate representations of the target word (an incorrect number of syllables or producing an incorrect vowel or consonant).
The researchers also looked at the number of words each child could produce as a function of age. FIG. 4 of Moeller, et al. (reproduced below) shows this data (the top two panels of FIG. 4 show data from this study; the bottom two panels show data from two other studies for comparison). The number of words was determined by asking the child’s caregiver to fill out an evaluation at home at each time point.
In FIG. 4, you can see that the curves for the NH children (top right panel) are both steeper and shifted to the left compared to the curves for the HL children (top left panel). This indicates that the NH children began producing words at a younger age relative to the HL children, and that, once they began producing words, their vocabularies expanded at a faster rate. The researchers noted that there was considerable variability in the data (for example, you can see that some of the NH children had much shallower curves than others, indicating that they were acquiring words more slowly than their peers), but that the individual data collected in this study “suggest a much slower rate of early vocabulary development compared with NH children.” (Moeller, et al. p. 636).
One cool thing – in the panel for the HL children (upper left), the curves with unfilled symbols indicate children with CIs – one of the best performing children in this group had a CI! I thought this was pretty remarkable!
Since there was so much variability within the group of HL children regarding degree of hearing loss, the researchers weren’t really able to say much about how degree of hearing loss affected language production in this study.
T has been babbling up a storm for a few months now, but this paper made me think about the different contexts of his babbling (e.g., non-communicative, unintelligible communicative, and words/word attempts). Of course, at this age, T’s babbling is essentially entirely non-communicative or unintelligible communicative (and no words/word attempts). Reflecting on these distinctions, I think that T tends to babble in the non-communicative category primarily when he’s relaxing – like riding in the stroller or in his crib at night (or the wee hours of the morning) – at these times, he’ll go on a long, uninterrupted soliloquy, complete with big variations in vocal inflection. T’s babbles that fall in the unintelligible communicative category seem to happen when we’re playing interactively with him (to tell us to do something again), when he wants something (usually food), or when he’s excited about something (he’ll often shout “DAY-DA!” while looking at us when he’s excited – usually when we open the refrigerator door). I think the distinction in types of communication based on activity/mood makes sense – if non-communicative babbling is a form of vocal play, (that is, allowing T to play with making different sounds), it makes sense that this would come most naturally to him when he’s just chilling.
At 10 months, T is on the young age compared to the ages of the children studied here. However, I think he’s allllllmost on the cusp of his first word. At least a couple times, it seems like he was fairly consistently saying “a-ga” for “again” (to ask us to do something again) and saying “bah-bol” for “bubble” (to ask us to blow more bubbles). I’m not sure these are consistent enough to count as his first word (for example, he’ll say “a-ga” at other times too), but it seems like he might be close. We try to really reinforce when we think he’s saying something that might have meaning – for example, if he says “dada” and it seems plausible that he’s saying something to or about his dad, we’ll make a big production of saying the word “dad.” We do the same thing for “again” and “bubble,” and I think this repetition is helping him connect the sound of the word to the concept/object.
One thing this study made me excited about – I didn’t realize how rapidly vocabulary grows once children start talking! I get the feeling that T is thinking some pretty fun thoughts (like when he starts grinning when he sees the trash can and races over to look inside), and I can’t wait to hear what he’s thinking once he starts talking.