The subjects repeatedly sniffed pleasant scents of rose, lemon, clove, eucalyptus, coffee and cinnamon twice a day, 6-min per time, with focus, until the completion of the study. Meanwhile, all patients will also continue on their existing dose and regimen of glucose- lowering schemes throughout the study. Twice weekly phone calls will be performed to evaluate the blood glucose situation, assess the safety of intervention and confirm the compliance of participants.

The subjects will maintain the original diet and lifestyle, and also continue on their existing dose and regimen of glucose- lowering schemes throughout the study. Phone calls will be made to evaluate the blood glucose situation.

The subjects repeatedly sniffed pleasant scents of rose, lemon, clove, eucalyptus, coffee and cinnamon twice a day, 6-min per time, with focus, until the completion of the study.

Whether MoCA scores in the two groups of Type 2 diabetes mellitus patients with MCI after intervention were different from those before treatment and the difference of changes between the two groups. The MoCA scale evaluates overall cognitive function, with a total score of 30. Generally, subjects scoring ≥26 points are considered as normal cognition, while between 19-25 points as mild cognitive impairment. An extra point is added If the subject has less than 12 years of education.

Whether the activation degree of olfactory task fMRI brain area in the two groups after intervention was different from that before treatment and the difference of changes between the two groups. All patients underwent odor-induced task fMRI on a 3.0T MR scanner with 222 volumes for task fMRI and 230 volumes for resting-state fMRI. The odor-induced task consisted of "fresh air" "rest" and "scent". Odor-induced brain activation was assessed by a general linear model using Statistical Parametric Mapping 12 (SPM12) software. Following extraction of the three separate conditions "fresh air," "scent," and "rest" from the whole sequence, contrasts were made for each participant between "fresh air > rest" and "scent > rest." Odor-induced fMRI data were analyzed in the mask of the olfactory network, including the regions of bilateral parahippocampus, amygdala, piriform cortex, insula, orbitofrontal cortex, hippocampus, and entorhinal cortices.