Evaluation of olfactory activation patterns in anosmic patients with peripheral and central injuries
Mohsen Kohanpour, Seyed Amir Hossein Batouli, Mohammad Ali Oghabian Introduction:
Functional study of brain, using fMRI as a method for evaluating important regions of brain and determining the exact locations of activities, is not only helpful in medical diagnosis of neurological diseases, but is also substantial in studying the functions of different brain areas.
Olfaction is a crucial sense in humans, and fMRI has opened a new horizon in studying brain regions which play a role in this sense.
The effect of head trauma as well as peripheral damage on the olfactory network and sense of smell is not well understood yet. The aim of this study is twofold: determining brain areas responsible for the olfactory sense, as well as examining the effect of trauma on disruption of this function.
This is a clinical study on unaffected volunteers as well as on patients who have a damaged olfaction after a head trauma. First, we performed fMRI on 20 healthy volunteers after olfactory stimulus using an Olfactometer device. At this stage we determined the active olfactory areas of normal brain. Then we performed fMRI on 60 head trauma patients. Of these, 28 had confirmed anosmia using standard diagnosis tests. In pre-analysis step, noises and artifacts were removed and then signal to noise ratio was increased. Raw images were analyzed with GLM Analysis in FSL Software and functionality of each voxel was determined. Finally in individual-level analysis, we did statistical hypothesis testing and multiple comparisons error correction to identify active brain regions during olfaction. In group-level analysis, the mean activity of voxels was calculated using linear regression analysis and linear mixed effects model to find core olfactory regions of the brain. In the last stage, brain activity patterns in normal subjects was compared to patients, to recognize possible brain activity differences.
Comparing brain activity in the two groups revealed meaningful differences. Figure 1 shows the brain regions controlling olfactory cognition in normal subjects which include piriform cortex, enthorhinal cortex and amygdalae. There were also several secondary regions that olfactory projections seem to enter them over time. These secondary regions include orbitofrontal cortex, hypothalamus, thalamus, hippocampus and insular cortex. In patient’s group, no activity was observed in the above mentioned areas, suggesting a significant disruption of this network in their brain.
The results of this study revealed brain regions involved in the perception of smells in our brain. In addition, we observed that the anosmic patients had a significantly different brain activation pattern for sensing the smells, in comparison to the healthy group. Based on these results, fMRI could be regarded as a reliable tool for identifying neural problems relating to sensing smells.
Comparing brain activations relevant to olfactory stimulus between healthy and anosmic groups.