Research into the use of handedness as an indirect measure
Research into the use of handedness as an indirect measure for speech laterality has formerly proved weak and inconclusive (Groen et al., 2013), predominately due to the variability of methodologies, and hand preference and skill definitions being highly dependent on the measurement and classification used (Groen et al., 2013). However, speech and motor control are said to share a common developmental trajectory (Iverson, 2010), sub served by overlapping neural pathways predominantly situated in the left hemisphere (see Binkofski and Buccino, 2004). Converging evidence underlines the relationship between language and motor function. For example, it has been shown that 4EGI-1 regions typically associated with movement (pre-motor cortex, supplementary motor area and cerebellum) are also activated by language tasks (e.g. Tremblay and Gracco, 2009; Petersen et al., 1989) and that classic speech production areas (i.e. Broca’s area/Brodmann areas 44 and 45) show increased activation during the execution of sequenced hand movements (Erhard et al., 1996). In addition, individuals with aphasia (Pedelty, 1987) and children with specific language impairments (Hill, 2001) frequently display co-occurring motor deficits.
Flowers and Hudson (2013) propose that motor and speech laterality are related where they involve a common feature of motor output, namely the co-ordination of sequences of movements or utterances to execute a plan or intention so as to achieve a goal, either limb movement or expression of an idea (Grimme et al., 2011). This rationale has demonstrated that measures of performance based hand skill are better at revealing the underlying commonalities between the two functions, and thus are more effective at informing on their neurological relationship (Flowers and Hudson, 2013; Groen et al., 2013). The present study investigated the speech and motor lateralisation profiles of children aged 3–10 years to determine whether the two functions develop in parallel and, specifically, whether younger children would show more variable laterality across these functions. It focussed on a direct measure of language lateralisation (fTCD) and a handedness task (electronic pegboard) which relies on the same concept of motor sequencing suggested to underlie speech and motor action. Specifically the research questions posed were as follows: 1. does age affect motor skill performance on the pegboard task? 2. Do speech lateralisation profiles vary with age? 3. Can skilled motor performance predict direction of hemispheric speech laterality?
Method and materials
Conclusions This study set out to answer three specific research questions regarding the effect of age on motor skill performance and speech lateralisation indices, as well as on the interaction between motor performance and speech lateralisation. The data showed that, 1) motor skill does vary with age, whereby younger children show significantly larger performance differences between their preferred and non-preferred hands; 2) Speech lateralisation indices do not vary with age in a sample of 3–10year old typically developing children; 3) greater performance differences between the hands significantly predicts atypical speech lateralisation, regardless of participant age or hand preference. In conclusion these data suggest that lateralisation of language and motor control is a process which begins very early in development, before the child is proficient at manual coordination or speech. Evidence from early lateralisation of auditory processing (Dehaene-Lambertz et al., 2002) may indicate the start of this hemispheric specialisation seen in later childhood; perhaps most critical is the period in which speech sound and motor output mappings are beginning to be formed and rehearsed. The specialisation of the left hemisphere for control of response sequences and timing integration also accounts for the patterns observed between speech laterality and motor performance (Serrien and Sovijarvi-Spape, 2015). Future work needs to focus on isolating the common components of the speech and motor tasks which may be driving this relationship and will also look at the performance of individuals with motor impairments.