Is there a continuum between speech and other oromotor tasks? evidence from motor speech disorders

Background Oral diadochokinetic (DDK) tasks are widely used in the assessment of patients with motor speech disorders (MSD). They require the repetition of homogeneous and of heterogeneous syllables at a maximum rate. The nature of DDK tasks is a matter of debate: although they are usually considered as "non-speech", given the involvement of real speech syllables, they have also been recognized to share more properties and requirements with speech as compared to other oromotor behaviours (i.e., non-speech gestures). Additionally, the production of heterogeneous syllables has been suggested to exhibit closer correspondence with speech given the production of a cluster of different syllables, as compared to the repetition of a monosyllable. Each DDK task being differently related to speech supports the assumption of a continuum between speech and other oromotor processes, in which tasks with increasingly speech-like properties may overlap to a larger extent with speech. Aim To test the potentially differential relationship between speech and DDK tasks in patients with MSD. Traditional views of MSD claim that speech and other oromotor behaviours are impaired in patients with dysarthria, while patients with AoS present with speech being exclusively compromised. This suggests that DDK tasks should be differently affected in these two MSD populations. Moreover, in the framework of a continuum, oromotor tasks sharing some characteristics with speech (i.e., heterogeneous DDK task) should also be impaired in patients with AoS. Method Syllabic rates of a sentence production (SP) task are contrasted with those of homogeneous and heterogeneous DDK tasks in three pathological populations: patients with dysarthria due to Parkinson's Disease (PD, n = 10) or due to amyotrophic lateral sclerosis (ALS, n = 10) and patients with apraxia of speech (AoS, n = 10); and 30 matched neurotypical controls. Results While patients with PD show similar rates across the SP and the DDK tasks relative to control speakers, patients with ALS display significantly slower rates across all tasks as compared to matched controls. Patients with AoS show reduced performances in the SP and in the heterogeneous DDK task, but not on the repetition of homogeneous syllables. Conclusions The findings of this study confirm that the repetition of heterogeneous syllables shares more principles and properties with speech than the homogeneous DDK task. This dissimilar relationship between speech and the two types of DDK tasks points to the existence of a continuum among oromotor tasks.