Final answer:
Decreased axonal white matter is most notably a characteristic finding in individuals with Alzheimer's disease due to the neurodegeneration of nerve cells. This reduction in white matter correlates with cognitive and memory deficits observed in patients.
Step-by-step explanation:
Decreased Axonal White Matter in Brain Disorders
Neuroscientists have found decreased axonal white matter in the brains of people with various mental and neurological disorders. In the context of neurodegenerative diseases, such as Alzheimer's disease, neuronal death occurs, which leads to a decline in the brain's white matter. Similarly, autoimmune diseases like multiple sclerosis (MS) affect the myelin sheath, a component of white matter. In the case of mental illnesses such as schizophrenia, anatomical abnormalities in both white and gray matter can be observed. While the question mentions decreased white matter in relation to depression, migraines, Alzheimer's, and schizophrenia, it is in the context of Alzheimer's disease that we typically see a pronounced neurodegeneration, particularly within the white matter regions of the brain, which underlies cognitive and memory deficits.
Given the specific conditions mentioned, the correct answer is c) Alzheimer's disease, where reduction in white matter is a characteristic finding due to the degeneration of nerve cells.