New Science Behind Speech Impairments in Parkinson's Disease

The study covered in this summary was published in medRxiv.org as a preprint and has not yet been peer reviewed.

This study suggests and integrates new methods of spectral brain mapping and speech impairment quantification to characterize the functional neural pathology that plays a role in speech impairment in Parkinson’s disease (PD).
In a large group of patients with PD, a pathological relationship between articulation impairments and spectral deviations in the left inferior frontal cortex (LIFC) was identified, especially in the alpha and delta bands.
In healthy adults, the LIFC is a focal point that features multifrequency interactions with a number of language network regions.
The more significant the spectral deviations in the LIFC, the more evident the articulation deficits in patients with PD.
The data also revealed that neurophysiological connectivity between LIFC and a network of somatomotor cortices in the beta band independently predicted articulation impairments and fully mediated the effect of cognitive abilities on these impairments.
As a group, these results supply a spatially- and spectrally-resolved cortical network underlying articulatory impairments in PD.
These findings may be valuable in future biomarker research and therapeutic targeting in PD.
The researchers also expect that the new, individualized modeling approach of spectral brain pathology for each patient may translate and be worthwhile for other clinical populations.

On a global scale, PD is the second-most common neurodegenerative disorder. It is marked by progressive declines in motor function and cognition.
A frequent and debilitating symptom of PD is difficulty uttering intelligible speech. It is one of the earliest impairments of PD.
Speech production is innately complex, resulting in multidimensional symptoms in patients with PD. Common symptoms include hoarse voice, imprecise articulation, and monotonous prosody.
However, it is challenging to robustly quantify pathological changes in speech impairments as well as to identify the affected brain systems.
Researchers believe their data further the knowledge of fundamental mechanisms involved in speech production in health and disease.
They also call attention to two new dissociable neurophysiological markers of symptom-specific clinical decline in PD, which improve the targeting of non-invasive neuromodulatory therapies.

Researchers explored the spectral and spatial definitions of the functional neuropathology underlying decreased speech quality in 59 patients with PD.
With this goal in mind, they initially quantified patient speech impairments with a novel interactive tool designed for nonspecialists, showing that this approach surpasses measures based on the automatic extraction of acoustical features.
The researchers then introduced a new brain mapping technique of spectral neurophysiological deviations (Spectral Deviation Index; SDI) between each patient and a group of 65 demographically-matched healthy controls.
They speculated that deviant neurophysiological manifestations would map to brain regions involved in speech production and scale with the severity of speech impairments in patients with PD.
Researchers also expected that neurophysiological connectivity across the brain circuit for speech production would also be altered in those patients with greater speech difficulties.

The investigators found that the interactive scoring of speech impairments in PD is reliable across non-expert raters and better related to the hallmark motor and cognitive impairments of PD than automatically-extracted acoustical features.
By relating these speech impairment ratings to neurophysiological deviations from healthy adults, they demonstrated that it is possible to robustly predict articulation impairments in patients with PD based on aberrant activity in the LIFC. Functional connectivity of this region with somatomotor cortices mediates the influence of cognitive decline on speech deficits.