Parkinson’s disease is a progressive, neurodegenerative disease that results from nigrostriatal dopamine deficiency (Lewis et al. 1998). The disease causes damage to nerve cells within the brain, resulting in a drop of dopamine levels leading to the deterioration of the individual’s motor skills. Parkinson’s was once viewed as a disease that solely affected the motoric system of the body, but recent studies have revealed the complexity of the disease. Studies over the past few decades have begun to examine the brain’s correlation with Parkinson’s, and have found evidence supporting impairments to the individual’s executive cognitive functioning. These findings have led to the revelation of a direct relationship between motoric and cognitive deficits with speech impairments presented by these individuals.
Lewis et al. (1998), examined both cognitive and linguistic impairments in Parkinson’s disease, a condition characterized in the past primarily by impairment of the motoric aspects. This specific study begins to describe the subtle language impairments of an individual with Parkinson’s disease (PD). Lewis and colleagues, hypothesized that cognitive and linguistic impairments in PD are a result of impaired motor aspects present within the frontal lobe of language function (Lewis et al. 1998). This study examined 20 Parkinson’s subjects and 20 matched-controls (9 males, 11 females) ranging from 49-82 years in age. Subjects with PD were divided into two groups depending on their cognitive statuses: normal and below normal cognitive status (Lewis et al. 1998). Cognitive function was measured using the Mattis Dementia Rating Scale (MDRS). The MDRS assesses cognitive performance by measuring abilities of 5 parameters: attention, initiation/perseveration, construction, conceptualization/abstraction, and short-term memory (Lewis et al. 1998).
An array of language tests were used to assess the subtle language impairments associated with PD. The Boston-Naming Test, Word Test, Test of Language Competence-Expanded edition, and phonemic verbal fluency tasks were all performed on both PD subjects and their matched-controls (Lewis et al. 1998). The Boston-Naming Test was selected because of confrontation naming impairments. The Word Test was selected to test the ability of one to recognize and verbally express vocabulary through tasks of: categorization, definition, verbal reasoning, and selection of appropriate words. The Test of Competence-Expanded edition was used to assess divergent language production, cognitive linguistic flexibility, and planning (e.g. figurative language, sentence recreation, ambiguous sentences) (Lewis et al. 1998). These tests were performed to reveal evidence on the specific language impairments associated with executive cognitive processing.
As a whole, both groups showed impairments in naming and definitional abilities; along with difficulties in interpreting ambiguity and figurative language (Lewis et al. 1998). When PD subjects were divided into normal and below normal cognitive status, the below normal cognitive subjects performed significantly worse on language tasks that had a direct correlation with executive cognitive processing. They showed deficits in interpreting/providing novel explanations for metaphors, processing ambiguity, defining words, constructing sentences, and naming. Normal cognitive subjects only showed impairments in providing definitions and in sentence structure (Lewis et al. 1998). Verbal fluency went unimpaired along with selection of semantic association, synonyms/antonyms, and multiple definitions for both groups of subjects with PD. The cognitive deficits presented within this study are directly influenced by impairments to individual’s frontal lobes.
Vanhoutte et al. (2012), examined the quantitative analysis of language production in Parkinson’s disease. This specific study examined the language production skills of PD patients using a unique cued sentence generation task. This task was used to assess the differences in sentence production abilities according to disease severity and cognitive impairments (Vanhoutte et al. 2012). Language samples were taken from 20 right-handed PD patients in which consisted of two subgroups: early stage of PD and advanced stage of PD, along with 20 matched-controls. All individuals within the study were considered “cognitively normal”.
The cued sentence generation task was the first task administered to the participants. All individuals then underwent three cognitive tests, which were randomized across patients. These tests aimed to assess language production on a broad sentential level, with a restricted influence of cognition (Vanhoutte et al. 2012). Restricted influence of cognition just means researchers didn’t want to mentally fatigue subjects so certain adaptations were made to ensure this. Researchers then asked participants to “retell” the text Meneer Meijer from Logotherapia, a Dutch therapeutic program. The text was shortened from 20 to 11 sentences and participants were allowed to read the text twice. These were the adaptations made to reduce memory and attention demands of individuals within the study (Vanhoutte et al. 2012). Cues were provided in the sentence generation task to facilitate reproduction. Participants were asked to recall 10 lexical words from the text which were visually presented in chronological order. Lexical words are words that covey information within speech (Harley, 2010). They were also asked to give one sentence summarizing the general idea of the text (Vanhoutte et al. 2012). The Asta, an analysis of spontaneous language in aphasia, was used to measure the length of utterances, proportion of verbs, type/token ratio of notional verbs, nouns, and proportion of grammatical correct sentences (Vanhoutte et al. 2012). Results revealed that only advanced PD patients differed significantly from matched-controls in areas of: less diverse notional verbs, proportion of complex sentences, and proportion of grammatical correct sentences. According to research, moderately affected PD patients tended to produce more verbs than mildly affected PD patients. Overall, as Parkinson’s advances so do speech impairments.
Cardona et al. (2013) focused on the potential role of subcortical areas in language processing and in the semantic integration of actions. This specific study examined the effects on action-verb processing during early stages of Parkinson’s disease. This disorder-related language impairment frequently goes unrecognized and untreated by clinicians (Cardona et al. 2013). Cardona and colleagues put an emphasis on how early detection of this specific impairment is crucial for diagnosing PD and potential development of treatment strategies.
According to research, individuals with this impairment show difficulties in action-word naming, action-verb production, action-verb identification, and contextual interaction between action-verb comprehension and motor response (Cardona et al. 2013). These findings led to the support of ‘motor-language interaction theories’ and the role of the basal ganglia (BG). Cardona and colleagues state the evidence found remains relevant because “it makes it possible to connect clinical aspects of PD with neurocognitive models of motor-language coupling (Cardona et al. 2013). Understanding the role of the brain in a disorder like Parkinson’s is essential to not only understanding the impairments developed by these individuals but also in helping develop intervention strategies that may be effective in delaying cognitive decline.
Evidence also shows that the BG and fronto-striatal are involved in language processing. Studies of the fronto-striatal networks have emphasized the role of BG in semantic processing as well (Cardona et al. 2013). Semantic processing is our ability to encode the meaning of words and phrases we hear (Harley, 2010). Cardona hypothesized that the BG-thalamocortical circuitry may regulate the motor-language integration observed in action-verb processing. In other words, both our language and motor systems must work together in order for us to process certain forms of speech (e.g. actions verbs). This is done through loops of interactions that project from the cortical areas to BG, to the thalamus, and back to the cortex (Cardona et al. 2013). BG impairments and preserved fronto-striatal circuitry could be responsible for the action-language impairments in early PD. These findings open up a new path for research. The organization of motor-language networks will lead to important insights regarding the role of BG in “motor grounding” of language (Cardona et al. 2013). Motor grounding is referring to the connection present between basal ganglia and language. This complex pathway leaves work to be done by researchers – brain correlates that underlie the role of BG must be further examined in order to develop more effective prevention strategies for PD.
To further explain the role of basal ganglia in language, Macoir et al. (2013) conducted a study to prove whether or not it plays a role in both language processing and verb production. Macoir and colleagues examined 15 French participants with PD and 15 matched-controls. All individuals were assessed using the Unified Parkinson’s disease rating scale part III. Following the initial assessment of the disease, the Neuropsychological assessment battery was administered to PD participants during two different testing sessions – separated by a gap of 1 week. Cognitive global functioning of participants was then assessed using the Montreal Cognitive Assessment, to identify mild cognitive impairments in the elderly (Macoir et al. 2013). Construction abilities were assessed using the Rey-Osterrieth complex figure test and were presented as impaired. Impaired construction means individuals struggled to form language (e.g. meaningful sentences). Episodic memory was assessed using the French adaptation of the Grober and Buschke paradigm and also presented as impaired for PD participants (Macoir et al. 2013). Your episodic memory has to do with the memory of autobiographical events (e.g. times, places) (Harley, 2010). Battery tests were performed to measure participants working memory and executive cognitive functioning. Executive cognitive functioning is associated with skills of mental control and self-regulation (Harley, 2010). PD revealed no deficits in verbal short-term memory, but deficits were present in working memory and executive functioning (Macoir et al. 2013).
Lastly, participants were asked to conjugate subsets of verbs and non-verbs. This was done to minimize the possibility of lexical information and more directly assess the application of conjugation rules. In other words, this task was not supposed to be meaningful in any way for subjects, it only wanted to assess whether or not individuals could conjugate verbs. Participants were presented 72 stimuli (48 regular verbs, 24 irregular verbs) (Macoir et al. 2013). They were asked to conjugate each stimuli in either the 3rd person present, or the 3rd person future tense. Results revealed that all participants struggled to conjugate irregular verbs, but all performed better when conjugating regular verbs. Overall, PD participants made more errors for both subsets of verbs (Macoir et al. 2013). Findings within this study suggest BG are involved in the application of rules required for conjugation of verbs and language processing, but has no specific role in verb production.
As you can see, Parkinson’s is a far more complex disease than research has portrayed it to be in the past. Recent studies have begun to shed a lot of light on the brain’s correlation with Parkinson’s, and has found a significant amount of supporting evidence regarding the impairments individuals develop to their executive cognitive function. Findings associated with cognitive function have led to the revelation of a direct relationship between cognitive and motoric deficits present in individuals with PD. Most importantly, this new found relationship has begun to explain the complexity surrounding the speech impairments presented by these individuals.

References

Cardona, J. F., Gershanik, O., Gelormini-Lezama, C., Houck, A. L., Cardona, S., Kargieman, L., Trujillo, N.,
Arevalo, A., Amoruso, L., Manes, F., & Ibanez, A. (2013). Action-verb processing in Parkinson’s disease:
new pathways for motor-language coupling. Brain Struct Funct, 218, 1355-1373. doi:10.1007/s00429-013-0510-1

Harley, T. A. (2010). Talking the Talk: Language, Psychology, and Science. New York, NY: Psychology Press.

Lewis, F. M., Lapointe, L. L., Murdoch, B. E., & Chenery, H. J. (1998). Language impairment in Parkinson’s
disease, Aphasiology, 12(3), 193-206. doi:10.1080/02687039808249446

Macoir, J., Fossard, M., Merette, C., Langlois, M., Chantal, S., & Auclair-Ouellet, N. (2013). The Role of
Basal Ganglia in Language Production: Evidence from Parkinson’s disease. Journal of Parkinson’s Disease,
393-397. doi:10.3233/JPD-130182

Vanhoutte, S., Letter, M. D., Corthals, P., Borsel, J. V., & Santens, P. (2012). Quantitative analysis of
language production in Parkinson’s disease using a cued sentence generation task. Clinical Linguistics &
Phonetics,26(10), 863-881. doi:10.3109/02699206.2012.711420