Infant directed speech expands F1/F2 space for/i a u: but what happens to central vowels /ɜ ʊ/ in Australian English?

Relative to their adult-directed speech (ADS), mothers expand the triangle circumscribed by the reference vowels (/i a u/) in F1/F2 space when interacting with their infants. This vowel space expansion in infant-directed speech (IDS) is thought to support word learning and phonological development. But what about non-reference (i.e., ‘central’) vowels such as /ɜ ʊ/? Are they also located differently in F1/F2 space in IDS and ADS? We compared /ɜ ʊ/ in Australian English, in which they fall posterior to the vowel triangle, produced by 15 mothers addressing their 4-month-old versus an adult. We extracted F1 and F2 values and calculated the Euclidean distance between the vowels for IDS and ADS. The distance was significantly shorter in IDS than ADS, rather than longer as with the /i a u/ triangle. F2 (but not F1) was also lower in IDS than ADS, suggesting more posterior tongue placement for /ɜ ʊ/ in IDS.