Internally-generated responses and the role of apathy and depression in Motor Neuron Disease

Abstract

Motor Neuron Disease (MND) is a rare, progressive neurodegenerative disorder characterised by the degeneration of motor neurons in the brain or spinal cord, leading to muscle wasting. In addition to the physical changes, multiple studies have found that approximately 50% of MND patients experience mild changes in cognition. These include reductions in memory, language, attention, and executive functioning (Phukan, Pender, & Hardiman, 2007). The most commonly documented change noted is a reduction in word fluency; however, it remains unclear as to why this reduction exists (Abdulla et al., 2014; Lepow et al., 2010; Machts et al., 2014). Apathy and depression have also been investigated and found to be significantly higher in patients with MND than in healthy controls (Atassi et al., 2011; Caga et al., 2016; Lillo, Mioshi, Zoing, Kiernan, & Hodges, 2011). The current study aimed to investigate verbal and non-verbal fluency in MND, and broader cognition, and whether differences in cognitive functions exist between types of MND. The study also aimed to understand whether the presence of apathy/ depression influences response generation. Sixty four MND patients and 28 healthy controls completed an extensive neuropsychological test battery, including apathy and depression questionnaires. A novel non-verbal fluency task was used to assess non-verbal response generation, and overcome limitations of a design fluency task. The results showed no significant differences between MND patients and controls on fluency tasks when controlling for speech/ motor rate and premorbid IQ. MND patients were comparable to healthy controls in their level of apathy; however, they reported significantly higher levels of depression. Significant relationships were found between behavioural apathy and the novel non-verbal fluency task. Depression was also found to be significantly associated with semantic word fluency. No differences were found in cognition, apathy, or depression between types of MND. The results indicated that verbal and non-verbal fluency are not influenced by executive processes, but rather premorbid IQ, apathy, and depression.