Depression is a mental health disorder characterized by persistent feelings of sadness, hopelessness, and a lack of interest or pleasure in activities once enjoyed. It can significantly impact cognitive functions such as memory, attention, and decision-making, which are crucial for daily functioning. In the context of neurodegenerative diseases, depression often emerges as a common comorbidity, complicating the clinical picture and affecting the overall quality of life for individuals with conditions like Alzheimer's disease or Parkinson's disease.