Table 2. An overview of studies analysing the early vocal behaviours of infants later diagnosed with Rett Syndrome.

Authors	Participants (M/F)	Control group (M/F)	Age range (in months)	Behaviours analysed	Results
Marschik et al. (2009)	PSV-RTT: 1 (0/1)	–	0-24	Vocalisations Vocalisation atypicalities Babbling Word production.	At 6 months old, repetitive un-modulated vocalisations were present. Inspiratory vocalisations were observed. First words occurred around the first birthday; word combinations were not observed before regression Instances of normal babbling were observed interspersed with atypical babbling. All vocalisations were double coded, k=0.91.
Marschik et al. (2012a)	PSV-RTT: 6 (0/6)	–	T1: 7-8 T2: 9-10 T3: 11-12 T4: 13-24	Canonical babbling Volubility Vocalisation atypicalities Proto-word combinations Lexical diversity.	Presence of atypical vocalisations in all participants from 13-24 months old 4/6 females demonstrated canonical syllables and produced proto-words, one participant developed proto-word combinations, complex consonant-vowel clusters were rarely seen Volubility (total) increased from 7-12 months, then decreased from 13-24 months. Vocalisations scored by three observers, Fleiss’ coefficient= 0.92.
Marschik et al. (2012b)	PSV-RTT: 5 (0/5)	–	12-24	Non-linguistic and (pre-)linguistic vocalisations used for communicative purposes*.	Non-verbal behaviours dominated over verbal behaviours for most communicative purposes. All vocalisations double coded, k=0.86.
Marschik et al. (2013a)	Typical RTT: 10 (0/10) PSV-RTT: 5 (0/5)	–	0-24	Cooing Babbling (Proto-)words Word combinations Vocalisation atypicalities.	12/15 participants demonstrated cooing, 9 participants demonstrated canonical babbling, Proto-words were observed in 3 of the typical RTT group and 3 of the PSV group, 1 PSV-RTT participant demonstrated word combinations. The more complex the vocalisation, the fewer participants presented it. Cooing and babbling was frequently observed to be atypical. Vocalisations scored by four observers, agreement reported as 0.97.
Marschik et al. (2013b)	Typical RTT: 1 (0/1) PSV-RTT: 1 (0/1)	1 (0/1)	T1: 9-12 T2: 13-18 T3: 19-24	Non-linguistic and (pre-)linguistic vocalisations used for communicative purposes*.	Repertoire of pre-linguistic vocalisations greater in TD child> female with PSV> female with typical RTT (where no pre-linguistic vocalisations were observed). All vocalisations double coded, disagreements (7%) discussed.
Bartl-Pokorny et al. (2013)	Typical RTT: 6 (0/6)	–	9-12	Non-linguistic and (pre-)linguistic vocalisations used for communicative purposes*.	Non-linguistic vocalisations were used to gain attention and answer, also to request an object, reject, and to respond in the social convention; pre-linguistic vocalisations for communicative purposes were not observed. All vocalisations double coded, disagreements (13%) discussed.
Marschik et al. (2014a)	PSV-RTT: 1 (0/1)	–	0-24	Babbling Volubility Vocalisation atypicalities (Proto-)words Word combinations.	Typical babbling (appeared at 7 months) was interspersed with atypical vocalisations First words produced around 12 months; mental lexicon of 12 proto-words; word combinations produced around 21 months. Echolalia was also observed Quantitative and qualitative analysis demonstrated reduced volubility and complexity of vocalisations. All vocalisations double coded, disagreements discussed.
Pokorny et al. (2016)	4 (0/4)	4 (0/4)	6-12	Acoustic (signal level) characteristics of pre-linguistic vocalisations.	An unweighted average recall of 76.5% was achieved for automatically identifying the RTT group vs. the TD group. Disagreements discussed.

^*i.e. unspecified vocalisations, pleasure vocalisations, laughing, crying, fussing, canonical and variegated babbling, onomatopoetics, (proto-)words, word combinations. ICC: Intra-class coefficients, k: Kappa coefficient.