There have been several clinical studies focused on participants and their olfactory function over a prolonged period of time.
The medical term for decreased sense of smell is “anosmia”. The term “aMCI” is defined as amnestic mild cognitive impairment and indicates memory problems more severe than normal for one’s age but not serious enough to affect daily life. This term is distinguishable from “naMCI” which is nonamnestic mild cognitive impairment and affects thinking skills other than memory. These skills include executive functioning, planning and organizing and judgment. My understanding is that these naMCI skills are located primarily in the prefrontal regions of the frontal lobe of the brain. This is distinct from the area of the brain responsible for the processing of memory. There are many different types of memories and many are reliant upon the medial temporal lobe or the hippocampus and entorhinal cortex regions. Other types of memories are reliant upon other areas of the brain including the neocortex, amygdala, basal ganglia, cerebellum and prefrontal cortex. It appears as if different areas of the brain decline at different rates.
Most of the clinical studies investigated “odor investigation” and relied upon the UPSIT (University of Pennsylvanis smell identification test). The studies looked at cognitively normal individual and tracked the association between declining olfaction and declining cognitive performance. A number of the studies suggested a strong association of impaired olfaction with progression from aMCI to AD dementia.