Research On Frequency Specificity Of Mesial Temporal Lobe Epilepsy Based On FMRI

Medial temporal lobe epilepsy(mTLE)is one of the most common types of epilepsy syndromes.By the technique of blood oxygenation level dependent(BOLD)functional magnetic resonance imaging,it has been found that the aberrant neuronal synchronization and the extensive damage of functional networks caused by mTLE.However,the existing research works are limited to the amplitude analysis of BOLD signals.Few scholars were involved in the analysis of frequency specificity of BOLD signal.The analysis algorithm of frequency specificity of BOLD signal failed to well adapt to the characteristics of BOLD signal(non-linear and non-stationary),and the results of frequency specificity analysis failed to correspond to different brain functions and different physiological and pathological manifestations of mTLE.To address the problems mentioned above,we use the extreme-point symmetric mode decomposition(ESMD)method to decompose the BOLD signals into a series of intrinsic mode functions(IMFs)under different frequency bands.Then we use the direct interpolation strategy and the weighted sum method to obtain the modal frequency,the intrinsic frequency and the modal energy.In the course of the research,we use these parameters as indicators to evaluate the frequency specificity of subjects.We have the following specific findings and conclusions in the study:1)In the brain of normal controls,the neurons of the default mode network,the frontal lobe and bilateral calcarine gyrus are at a relatively low frequency to synchronize activities.Meanwhile,the brain neurons of these regions,such as temporal lobe,insular lobe and hippocampus are at a relatively high frequency.In the six functional networks,the value of intrinsic frequency is the highest in salience network,the value is the lowest in default mode network and visual network.This shows that the BOLD signals do exist different frequency specificity which corresponds to different functional networks and physiological functions.2)Compared with the normal controls,the frequency value of the brain areas which associated with the origin and spread of the epileptic activity is significantly higher in the mTLE patients.The average frequency in the six functional networks of the mTLE patients is generally higher than the normal control,which makes that the peak of frequency distribution curve of the mTLE patients moves towards the higher frequency than that of the healthy controls,the performance of the left mTLE patients is particularly prominent.3)There are differences in the frequency distribution curve of left and right hippocampus in normal controls.The distribution curve of the left hippocampus shows two peaks while that of the right hippocampus only has a single peak.At the same time,the mean frequency of the left hippocampus is higher than that of the right hippocampus.This phenomenon can be attributed to the asymmetry of the structure and function of the left and right hippocampus.4)The pathological characteristics of the two types of patients are different.The difference between the left mTLE patients and the normal controls is mainly in 0.044~0.06 Hz,while the difference between the right mTLE patients and the normal controls is mainly in 0.097~0.367 Hz.In comparison to the normal controls,the frequency distribution curve in the bilateral hippocampus of patients with left mTLE moves towards the higher frequency,while this phenomenon only appears in the right hippocampus of patients with right mTLE.This implies that there are different levels of cognitive impairment and different neurological pathology mechanism in the two types of patients.5)Compared with the normal controls,energy enhancements in these regions of the left mTLE patients,such as inferior temporal gyrus,part of the entorhinal cortex,bilateral insula,cingulate.Energy enhancements in these regions of the right mTLE patients,such as cerebellum,right insula,middle frontal gyrus.The increase of the energy in the part of the brain reflects the role of the brain in the generation and dissemination of epileptic activity.Because of the inhibitory receptors in the cerebellum,the increase of energy in the region may reflect its inhibitory effect on abnormal nerve discharges.In addition,the results of functional connectivity of instantaneous energy show that there are differences in the consistency of energy changes between the mTLE patients and the normal controls.This may be due to the interruption of the default mode network caused by epileptic seizure.In this paper,the frequency specificity of BOLD-fMRI signals is studied from the viewpoint of frequency and energy.Also,we compare the characteristics between the mTLE patients and the normal controls.The research work in frequency domain can be a supplement to the method of time domain analysis.It is helpful to reveal the neural mechanisms and pathological basis of cognitive impairment.It can provide more help for the diagnosis and treatment of mTLE.