Abstract

Introduction: Although the relationship between language lateralization and handedness has long been a tenet of neuropsychological investigations, exactly how the two behaviors are related is seldom examined. It is widely reported that for the vast majority of right-handed individuals the left hemisphere is said to be dominant for speech production (Wada & Rasmussen, 1960). Although the left hemisphere is dominant for the majority of left-handers also (Annett, 1975) there is a higher incidence of right hemisphere dominance and bilateral speech representation than that found in right-handers (Rasmussen & Milner, 1975), but it is unclear what determines the variation between individuals. Fundamental to drawing conclusions about the relationship between speech lateralization and handedness is how the latter is defined and measured. Previous research in the area has varied between simple definitions consisting of preferred writing hand, to reliance on self report handedness questionnaires to determine a handedness classification for participants. These approaches have been criticized over issues of reliability, simplicity and validity. A suggestion by Flowers and Hudson (2013) is 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. It was argued that there must be some central control function in one hemisphere that organizes the co-ordination and timing of both limbs, and that it would then be natural for this mechanism to control speech output too. If so, a motor task that reliably indicated the lateralization of the former would also indicate the latter. From previous work (Annett, 1970; Annett, Annett, Hudson, & Turner, 1979) it was suggested that pegboard scores provide such a measure of motor laterality, independent of the effects of practice. This was examined in a recent study of epilepsy patients who underwent Wada testing. It was reported that those with clear speech laterality could be discriminated from those with anomalous speech representation by their performance on a pegboard test (Flowers & Hudson, 2013). This study was designed to replicate these findings in a group of normal participants, using functional Transcranial Doppler ultrasonography (fTCD) to determine speech laterality, correlating this with hand preference and pegboard measures of motor laterality as in the previous study. Functional TCD uses ultrasonography to measure through a cranial ‘window’ blood flow velocities in the cerebral arteries. The technique has high temporal resolution, enabling the detection of rapid changes in blood flow that result from functional changes in brain activity under defined conditions of stimulation

Method: Forty-one adults participated in the study. Motor laterality was measured by a handedness questionnaire to indicate manual preference and an electronically timed pegboard task to assess performance. Speech laterality was determined using a word generation paradigm with (fTCD). Word generation has been validated in numerous neuroimaging studies as an effective paradigm to elicit speech lateralization (Benson et al., 1999). Within fTCD it has been used extensively by Knecht and colleagues (Knecht et al., 1998; Knecht et al., 1996) and is widely considered to be a reliable paradigm for determining language dominance in this technique. The paradigm consist of a letter presented on a computer screen for a 15 second period during which the participants had to silently think of as many words beginning with that letter as possible. This was followed by a 5 second verbal recall period, which was then followed by a rest period.

Results: Functional TCD categorized 23 cases as clearly lateralized and 15 as having anomalous speech representation (3 data sets were unusable). For the whole sample, regardless of handedness, peg moving times were faster with the preferred than the non-preferred hand. The mean between-hand difference on the pegboard task for the lateralized group was markedly larger than the anomalous group [F (1,36) = 5.338, p < 0.027]. Indeed, the between-hand difference for the group with anomalous speech representation did not differ as a function of hand skill. Categorizing individuals as either left or right handed, or indeed ambilateral, on the basis of self-report questionnaire responses failed to effectively discriminate between the lateralized and anomalous groups.

Conclusion: Our findings suggest that if one aims to derive conclusions regarding speech representation on the basis of motor laterality, it is not hand preference per se that is pivotal but the between-hands difference in sequencing skill. It is suggested that motor and speech laterality involve a common aspect of motor sequencing and a measure of the former will indicate the representation of the latter. The same pattern of results has now been observed with neuropsychological and healthy participants using methods of inactivation and activation. These findings are clinically relevant and speak to debates on evolutionary theories of handedness and language development.