As T’s (9 months) babbling has taken off, I’ve started to become interested in the order in which infants tend to acquire different speech sounds as well as how this might differ for infants with hearing loss vs. normally-hearing infants. I started doing a little Googling, and found this study (link to Abstract only) that compares vocalizations of infants with hearing loss to infants with normal hearing. (Moeller, M.P., Hoover, B., Putman, C., Arbataitis, K., Bohnenkam, G., Peterson, B., Wood, S., Lewis, D., Pittman, A., and Stelmachowicz, P. “Vocalizations of Infants with Hearing Loss Compared with Infants with Normal Hearing: Part 1- Phonetic Development.” Ear & Hearing, Vol. 28 No. 5, 605-627. 2007).
This study actually has two parts, the first looking at babbling with younger infants (up to 2 years old), and the second looking at older children and how they transition from babbling to acquiring words. This week, I’ll talk about part 1, and will hopefully write about part 2 next week.
It’s well-known that infants with hearing loss develop spoken vocabulary later than normally-hearing children. However, there’s a lot of language development happens before children start speaking words. For example, infants typically start off making vowel sounds, and then progress to babbling (like “bababa,” “dadada,” etc.). Less is known about how hearing loss affects this earlier stage of language development.
The researchers enrolled a group of normally-hearing (NH) infants and a group of infants identified as having hearing loss (HL). This was a longitudinal study, so each infant was followed over time – their spoken language was measured in an experimental session conducted every 1.5-2 months, from when the study began (generally when infants were 4 months old) until they were 36 months old. There were 21 NH infants, and 12 HL infants. All of the infants with HL had assistive technology, typically hearing aids, although 3 received cochlear implants (CIs) during the course of the study. The degree of hearing loss varied a lot for the HL group; on average, across the group of HL infants, they had a 67 dB HL Better Ear Pure Tone Average (BEPTA – meaning that the average audiogram for the infant’s better ear measured at 500, 1000, and 2000 Hz was 67 dB HL). All of the HL infants were involved in some form of early intervention.
To collect the data, at each session, each infant played with a caregiver while their interaction was taped and then transcribed. The infants would play with a parent or guardian, and the researchers transcribed each vocalization by the infant – for example, identifying a particular vowel or consonant, whether a sound was a grunt, cry, or squeal, etc.
There were 3 main categories of metrics the researchers looked, which were:
- Volubility – this indicates how much the infants vocalized over a session – were they pretty chatty during the session, or fairly quiet?
- Age at which the infant began babbling
- Speech complexity – here, the researchers looked at what types of consonants the infants were producing at a particular age, as well as whether they were able to string different types of sounds together to make more complex sounds.
Let’s get to the results!
To measure volubility, for each experimental session, the researchers calculated the infant’s vocalizations per minute. Vocalizations could be any sounds other than stuff like grunts, screams, cries, etc. So, an infant with a higher volubility score would have vocalized more during the session compared to an infant with a lower volubility score.
FIG. 1 of the article (shown below) shows the volubility results for both NH infants (left) and HL infants (right). In the figure, volubility scores are shown for infants at 3 different ages – 8.5 months, 10 months, and 12 months. As you can see in FIG. 1, the volubility scores for HL infants was really similar to that of NH infants, and the researchers found no significant difference between the two groups. I thought it was pretty interesting that, at each age, the HL infants seemed to be vocalizing as much as the NH infants!
FIG. 1 of Moeller, et al. – Volubility of NH and HL infants as a function of age
Age of Babbling Onset
The researchers then quantified the age at which the infants began babbling. Although we (or at least, I!) tend to think of babbling as any infant pre-word “talking,” babbling technically requires a consonant-vowel (CV) pairing – examples include “ba,” “da,” “ga,” etc. CV pairs can also be chained together, either the same consonant and vowel (“baba”) or different consonants and/or vowels (“babo,” “bada,” etc.)
In order to set a criteria to define the age of babbling onset, the researchers identified the age at which the proportion of babbles out of the total vocal utterances exceeded 0.2 – so this was the age at which, during an experimental session, more than 20% of the infant’s vocalizations were consonant-vowel pairings.
FIG. 2 of the article (shown below) shows, at each age, the proportion of infants in the NH group (black bars) and HL group (white bars) who had started babbling (defined as having more than 20% of their vocalizations during the session include a CV-pairing). As you can see, NH infants tended to begin babbling much earlier than HL infants – it took roughly 6ish additional months for the HL group to reach the milestone of having 50% of the infants in the group babbling compared to the NH group. The researchers also stated that, for the HL group, there was a correlation between the age at which the infants first received hearing aids and the age at which they began babbling, although this wasn’t statistically significant (possibly because there were only 12 infants in the group, and they varied a lot in degree of hearing loss).
FIG. 2 of Moeller et al. – Proportion of infants who had began babbling by age
The researchers quantified the complexity of the sounds the infants were producing by scoring each utterance as follows:
- 1 point for utterances that were vowels or primarily vowels – (like “ahhh,” “eeee,” “waaa,” etc.) – this was labeled SSSL1
- 2 points for utterances that had 1 type of consonant – (like “ba,” “da,” “baba,” etc.) – this was labeled SSSL2
- 3 points for utterances that had 2 or more types of consonants – (like “bada,” “gaba,” gabo,” etc.) – this was labeled SSSL3
- 4 points for utterances with consonant blends, like “spun.” – this was labeled SSSl4
FIG. 4 of Moeller et al. shows the proportion of utterances that belonged to each point category for both NH infants (top) and HL infants (bottom).
Adapted from FIG. 4 of Moeller et al. – proportion of utterances in each babble complexity category as a function of age
As you can see, for both NH infants and HL infants, vocalizations by the youngest babies (10-12 months) were dominated by the simplest type of vocalization – primarily vowels. Both groups tended to increase the proportion of more complex vocalizations – those containing consonants and multiple types of consonants – with age. One really interesting thing you can see in the above figure is that HL infants at 18-20 months had a babble complexity pattern that was similar to the NH infants at 10-12 months (I highlighted these in the red boxes above) – this is a pretty substantial delay. However, by the time the HL infants were 22-24 months old, the pattern resembles that of the NH infants at 18-20 months (highlighted in the green boxes above), indicating that the HL infants were closing the gap! This could be the result of amplification for the HL infants, early intervention services, as well as the fact that three of the HL infants received cochlear implants during this time period.
The researchers then looked at whether NH infants and HL infants differed in the rates at which they started saying vowels and different types of consonants. FIG. 5 of Moeller et al. (reproduced below) shows the infants’ progression in acquiring both vowels and consonants broken into different classes based on place of articulation. A consonant’s place of articulation indicates what part of the mouth is involved in obstructing the vocal tract – I wrote more about it here. Here’s a quick overview of the different classes of consonants shown in FIG. 5 below:
- bilabials – these are consonants produced with the lips pressed together (e.g., p, b, m, and w).
- labiodentals & interdetals – labiodentals are produced with the lower lip between the teeth (e.g., f and v). interdentals are produced with the tongue between the teeth (e.g., th).
- alveolars – these are produced with the tip of the tongue behind the top teeth (e.g., d and t).
- palatals – these are produced with the body of the tongue raised against the hard palate (e.g., j).
- velars – these are produced with the back part of the tongue against the soft palate (e.g., k and g).
Each panel in FIG. 5 shows the percent of sounds within a given category that the infants produced at a particular age. So, for example, there are 4 bilabial consonants (p, b, m, and w), and infants who could produce 2 out of the 4 at a particular age would receive a score of 50% for that age.
Adapted from FIG. 5 of Moeller, et al. – % of sounds produced in different phonetic categories as a function of age.
One thing that was interesting to me is that bilabial consonants seemed to be one of the “easier” sounds to produce in general (look at the top row, middle panel) – for both NH and HL infants, scores were fairly high at every age range, and the gap between NH and HL infants was fairly small as well. The researchers said that this might be because bilabial consonants tend to be very visually salient compared to other places of articulation – it’s pretty easy to see lips pressed together compared to where your tongue is inside your mouth! This might make it easier for infants to acquire bilabial consonants, since they can more easily see how they are formed.
Another interesting thing about Fig. 5 – the researchers found that acquisition of these different classes of sounds generally fell into 3 different categories, which I’ve highlighted by color in the above figure. For vowels and alveolar consonants, the HL infants were generally delayed relative to the NH infants, but their rate of acquisition was parallel (this is highlighted in blue above). For bilabial consonants and velar consonants, the HL infants seemed to be closing an initial gap relative to the NH infants – that is, their acquisition of these classes of consonants was converging with the NH infants (this is highlighted in green above). Conversely, for palatal consonants and labiodentals/interdetals, the HL infants seemed to be acquiring consonants in these classes at a slower rate than the NH infants – that is, over time, the gap between the HL infants and the NH infants widened. One thing to note is that, for both NH and HL infants, palatal and labiodental/interdental consonants (highlighted in red above) occurred less often in general compared to other consonants – regardless of hearing, children tend to take longer to produce these types of sounds, perhaps because they tend to be less common in English.
The researchers then broke the consonants up in a different way – into fricatives and non-fricatives. Fricatives are consonants that are produced by forming a small opening with the mouth and forcing air through – they include sounds like “ssss,” “shhhh,” “f,” and “zzz” – fricatives are the ones that sound kind of “hissy”! This hissyness also makes fricatives generally hard for people with hearing loss to hear – fricatives tend to have a lot of high frequency components and are often low in intensity. FIG. 6 of Moeller, et al. (reproduced below) shows the rate of acquisition of non-fricatives (left) and fricatives (right) for both NH and HL infants.
FIG. 6 of Moeller, et al. – Acquisition of non-fricative and fricative consonants.
As you can see, acquisition of the non-fricative consonants was parallel for both the HL and NH infants – both groups had a steady increase in production of non-fricative sounds. However, for fricatives, while the NH infants steadily increased their production of these sounds, the HL infants didn’t – they seemed sort of stuck from 10 months to 24 months and, in general, didn’t really add many consonants from this group into their repertoire. As I mentioned above, this might be because fricatives tend to be really hard to hear for people with hearing loss, so the HL infants might have not had enough exposure to these types of sounds to begin producing them.
I was particularly interested to read this study since T’s consonant inventory seems to have grown a lot just in the past 2 weeks. Although he’s been saying “da” for awhile (EVERYTHING is “dada”!), he’s started more consistently saying “ba” and “ma” (both are bilabial) and, just in the past few days, has started saying “la” (I think this is alveolar). From the data presented in this study, it seems like bilabials tend to be one of the “easiest” categories of consonants – babies tend to produce the highest proportion of consonants in this class at earlier ages relative to other categories, and this might be because of how easy it is to see the lips pressed together when producing bilabial consonants. Although T’s preferred consonants (the ones we hear more often) are “da” (alveolar) and “ga” (velar), I think we’ve heard him produce most of the bilabial consonants at least a few times now. And, lately, if we really emphasize the position of our lips while saying “pa,” “ba,” or “mmm,” he’ll try to imitate us!
One of the things I think I gained from reading this study was an appreciation for the activities we do at speech therapy and a deeper understanding of how those activities will help T acquire different speech sounds. One thing we really focus on is drawing T’s attention to different sounds by pairing the sound with something interesting and visually salient – this gets him to really listen to the sound rather than just have it be background noise that he might not pay attention to. We’ll do this in different ways, for example, pointing at our mouths, waving toys or ribbons around as we make the sound, etc. I think that, especially for children with hearing loss, merely passively hearing different sounds isn’t quite enough, and having their attention drawn to the sound and the way your mouth looks when you make the sound can help tie everything together.
Once again, this study highlighted the importance of T wearing his hearing aids! I think it’s really important for him to get as much good, high-quality exposure to all these different speech sounds so that he can start to produce them, and this is especially important for fricatives (like, “sss,” “shhh,” “f,” etc.). The “s” sound in particular is really important for English grammar – it’s what turns a singular noun into a plural – and the study that I wrote about here found that children with hearing loss tend to have more trouble with this grammar rule than normally-hearing children.
Finally, on a happy (for me) note – there are a few bad words I’ve been known to accidentally say in front of T that start with fricatives (I’ll let you figure out what they are) – I’ve been thinking I need to clean up my language, since I’m been worried that once T really starts talking, he’ll out me by repeating something he’s heard me say totally out of the blue. But, from the results of this study, it looks like children, whether normally-hearing or with hearing loss, don’t tend to really start producing fricatives until they are quite a bit older than T is now – so it looks like I have a little while before I have to be worried about T surprising me by dropping a fricative-bomb!