Laterality and Babble: Does asymmetry in lip opening during babble indicate increasing left hemisphere dominance as babies gain articulatory experience?

Speech and language are supported by task-dependent neural networks that are predominantly lateralised to the left hemisphere of the brain, whilst emotion is supported by predominantly right hemispheric networks. This is reflected in the asymmetry of lip openings during speech and facial expressions in adults. One cross-sectional orofacial asymmetry study found an analogous distinction between 5-12-month-old babies’ lip openings during reduplicated babble and during positively valenced emotional facial expressions and this has been interpreted as evidence to support the hypothesis that babble is fundamentally linguistic in nature (Holowka & Petitto, 2002). However, a similar distinction is also observed in orofacial behaviours in some non-human primates. Differential hemispheric specialisation for emotional and vocal communicative functions may then be an ancient trait, long predating human language. Additionally, characterising babble as babies’ immature attempts to do language marginalises the critical role of endogenously motivated vocal exploration and may assume a degree of goal-directedness in infant behaviour around the time of babble emergence for which we have little other supporting evidence. This thesis explores laterality in eight 5-12-month-old’s babble, positive facial expressions, and other vocalisations longitudinally. Singleton and variegated babble are captured as well as reduplicated babble, and an alternative method for analysing orofacial asymmetry – hemimouth measurement – is used. Overall, Holowka and Petitto’s between-category distinction was replicated. However, babble was found to show right laterality at emergence and become left lateralised gradually over developmental time. Some interactional effect of utterance complexity was also observed. Bisyllabic babbles showed significant leftward shift over developmental time, whilst monosyllabic and polysyllabic babbles did not. Furthermore, hemimouth measurement revealed a degree of real-time variability in the laterality of babble not previously observed. An alternative theory of the underlying nature of babble – the Old Parts, New Machine hypothesis – is proposed.