Another study found that seniors who had the worst test scores on a smell test were 2.2 times more likely to show signs of mild cognitive decline. Olfactory cells in the nose that are responsible for a sense of smell require a constant supply of new stem cells from the olfactory bulb, a region in the forebrain. NOTE: Many other factors can lead to a poor sense of smell so other biomarkers need to be looked at in addition to sense of smell.