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Parkinson’s disease and impairment in Theory of Mind:


Ameya Naik


Parkinson’s disease (PD) is characterised as a progressive neurodegenerative disease that leads to tremors, stiffness, and in later stages difficulties in balancing, coordination, and walking. Apart from motor symptoms, PD also shows the manifestation of non-motor symptoms like cognitive impairment (APA Dictionary of Psychology, n.d.). One of these cognitive impairments of PD is postulated to be the impairment in Theory of Mind (ToM). ToM is defined as one’s ability to ascribe mental states such as beliefs, intentions, desires, and knowledge to others, even when those mental states are different from their own (APA Dictionary of Psychology, n.d.). It plays an essential role in predicting others' behaviour and is comprised of two components— the affective component of ToM and the cognitive component of ToM. Hence, this blog post aims to analyse the impairment of the cognitive and affective components of ToM in patients with PD and the implications and future directions of ToM deficits in PD. 

 

The cognitive component of ToM is found to be impaired in PD patients. Cognitive ToM refers to an individual’s ability to deduce the beliefs and intentions of others (Sebastian et al., 2011). Impairment in cognitive ToM would suggest that the individual is unable to rationally differentiate between their own mental states and the mental states of others, as the individual would be more likely to impose their thoughts and beliefs onto others (Shamay-Tsoory & Aharon-Peretz, 2007). A study by Bodden et al. (2010) demonstrated how non-demented PD patients scored lower on cognitive ToM tasks as compared to the healthy control subjects, thereby suggesting that there was an impairment in the cognitive ToM in PD patients without dementia. Other studies have also reported that non-demented PD patients performed poorly on cognitive ToM tasks since their cognitive component of ToM was impaired (Bora et al., 2015; Roca et al., 2010; Santangelo et al., 2011). It has also been found that difficulties involved with cognitive ToM usually develop in the early stages of PD (Poletti et al., 2011). However, other research has also suggested that only more advanced stages of non-demented PD show impairments in cognitive ToM (Péron et al., 2009). Hence, while research agrees that non-demented PD patients have an impairment in the cognitive component of ToM, the onset of the impairment is still not agreed upon. 

 

The affective component of ToM is also observed to be impaired due to PD. Affective ToM demonstrates the ability of an individual to ascertain another individuals' feelings and emotions (Sebastian et al., 2011). If an individual’s affective ToM is impaired, then it would be difficult for that individual to differentiate their emotions from those of others. When non-demented PD patients were compared to the healthy control group on affective ToM based tasks, results showed how PD patients had more difficulty than healthy individuals in accurately describing and understanding the mental states of others through their facial expressions (Poletti et al., 2013; Bodden et al., 2010). Additionally, affective ToM was associated with the health-related quality of life of PD patients (Bodden et al., 2010). Another study by Romosan et al. (2019) delineated how the relationship between affective ToM and PD is mediated by other variables such as visuospatial functioning, attention, and executive functions. They also found that impairment in affective ToM could be predicted by deficits in the cognitive functioning of a patient. Moreover, abilities of affective ToM were preserved in patients with early stages of PD whereas those abilities progressively declined as the severity of the disease increased. Poletti et al. (2011) also indicated that deficiencies in affective ToM were seen only in advanced stages of PD. Therefore, PD does affect the abilities associated with the affective component of ToM in non-demented patients, however, this impairment manifests as the disease progresses. 

 

There are major implications associated with the impairment in ToM, specifically because it plays a crucial role in developing effective human interactions. Given that ToM helps individuals understand and predict other people’s cognitions and emotions, research suggests that deficits in any component of ToM can lead to social seclusion and can negatively impact a patient’s quality of life (Saltzman et al., 2000). A patient's quality of life may be impacted because patients with PD may not just experience difficulty due to physical symptoms, but may also face struggles while interacting with others and maintaining existing relationships. This is because the impairment in affective and cognitive components of ToM would prevent patients from accurately differentiating their intentions, beliefs, and emotions from others. Also, research regarding PD and ToM impairment is very relevant because ToM impairments in PD patients are seen to precede the development of dementia in the patients (Freedman & Stuss, 2011). However, it is important to note that the onset of deficiencies of either affective or cognitive ToM are not found to be the same. While there is a lot of debate around the manifestation of impairment of cognitive ToM, several studies have noted that impairment of affective ToM manifests in advanced stages of PD. Hence, future studies should pay specific emphasis on the developmental markers of impairments in both the components of ToM. Moreover, most studies testing the relationship between ToM and PD have had a relatively small sample size (< 100), which could affect the generalisability of the findings. To make the results more externally valid, future research could test ToM impairment in more non-demented patients with PD. Nonetheless, current research suggests how Parkinson’s disorder does impair a patient's theory of mind, specifically their affective and cognitive components of ToM. 


References:

APA Dictionary of Psychology. (n.d.). Parkinson’s Disease. Retrieved April 16, 2022, from 

https://dictionary.apa.org/parkinsons-disease

APA Dictionary of Psychology. (n.d.). Theory of Mind. Retrieved April 16, 2022, from 

https://dictionary.apa.org/theory-of-mind

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Oertel, W. H., Kessler, J., Dodel, R., & Kalbe, E. (2010). Affective and cognitive theory of mind in patients with parkinson’s disease. Parkinsonism & Related Disorders, 16(7), 466–470. https://doi.org/10.1016/j.parkreldis.2010.04.014

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Jeune, F., Sauleau, P., & Vérin, M. (2009). Are dopaminergic pathways involved in theory of mind? A study in Parkinson’s disease. Neuropsychologia, 47(2), 406–414. https://doi.org/10.1016/j.neuropsychologia.2008.09.008

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    cognitive and affective theory of mind: A lesion study. Neuropsychologia, 45(13), 3054–3067.                 

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