The Study Of Theta Oscillation Characteristic For Working Memory In Mild Cognitive Impairment

ObjectiveMild cognitive impairment(MCI)is an unstable clinnal status with cognitive impairment during normal brain aging and dementia.The ability of daily life is normal,but it has mild memory or cognitive impairment,and does not reach the degree of dementia.Patients with MCI often suffer from serious working memory(WM)dysfunction.Theta oscillation plays a key role in the WM tasks.MCI subjects were taken as the research subjects to study the changes of theta oscillations during WM in order to explore the neural mechanism of WM dysfunction in MCI.Methods1.Subjects and experiment: 28 MCI patients and 15 healthy subjects were divided into MCI group and control group.According to the Clinical Dementia Rating sum-of-box(CDR-SB)the MCI patients were divided into two groups named high CDR-SB and low CDR-SB MCI subgroups.Behavioral data and 34-channel electroencephalogram(EEG)were recorded while they performed a visual delayed matching-to-sample task.2.Analysis of behavioral data: Reaction time(RT)and accuracy of the 3 groups were calculated.3.Analysis of 34-channal EEG during WM:(1)Preprocess of EEGs: baseline drift,50 Hz noise,eyes and muscle electricity interference were removed from the original EEG.(2)The characteristic spectrum for WM: the percentage of power in every spectrum was calculated among the three groups to define the characteristic spectrum.(3)The characteristic channel and important brain region for WM: the spatial distribution of power in characteristic spectrum of EEG was calculated for WM among the three groups to define the characteristic channel and important brain region.(4)Calculated the power in characteristic channel and important brain region among the three groups.(5)Calculated time-frequency distributions of power in characteristic channel for WM amog the three groups.(6)Calculated spectral coherence value between characteristic channel and other channels in important region among the three groups.(7)Calculated phase synchronization value between characteristic channel and other channels in important region.(8)Compare the differences of power,time-frequency distributions of power,coherence value and phase synchronization value among the three groups.5.Statistical analysis: One-Way ANOVA was used to compare the difference among the three groups.P<0.05 was statistically significant.Results1.MCI group showed WM impairment: The accuracy of high CDR-SB MCI group,low CDR-SB MCI group and control group were 80.93±0.45%,86.14±0.64% and 90.06±0.70%.The accuracy of MCI group was lower than control group,and the accuracy of high CDR-SB MCI group was lower than low CDR-SB MCI group.The difference was statistically significant(P<0.01).The RT of high CDR-SB MCI group,low CDR-SB MCI group and control group were 1381.43±41.26ms、1218.57±50.94 ms and1131.67±36.81 ms.The RT of high CDR-SB MCI group was longer than control group and low CDR-SB MCI group,and the difference was statistically significant(P<0.05).The RT of low CDR-SB MCI group was longer than the control group,but the difference was not statistically significant.2.The percentage of power in theta spectrum was higher than the others among the 3 groups and the difference was statistically significant(P<0.01),indicateing that the characteristic spectrum for WM was theta spectrum.The three groups all showed obviously theta oscillatory activity in frontal region,with the power in Fz channel being the maximum,indicateing that Fz channel and frontal region were the characteristic channel and important region for WM.3.The power of MCI group in Fz channel and frontal region were lower than control group and the power of high CDR-SB MCI group was lower than that of low CDR-SB MCI group.The difference was statistically significant(P<0.01).4.The time-frequency distribution of power in Fz channel during WM for the three groups showed that power was focused in oscillatory theta activity.5.Theta spectral coherence values in frontal region for high CDR-SB MCI group,low CDR-SB MCI group and control group were 1.12±0.02、1.23±0.02 and 1.31±0.03.The value of MCI group was lower than control group and the value of the high CDR-SB MCI group was lower than low CDR-SB MCI group.The difference was statistically significant(P<0.01).6.Theta phase synchronization value in frontal region of high CDR-SB MCI group,low CDR-SB MCI group and control group was 0.92±0.06、1.13±0.05 and 1.26±0.03.The value of MCI group was lower than the control group,and the value of the high CDR-SB MCI group was lower than low CDR-SB MCI group.The difference was statistically significant(P<0.05).Conclusion1.MCI patients show WM dysfunction.2.The characteristic spectrum of WM is theta band.The characteristic channel is frontal midline Fz channel and the important region is frontal rgion for MCI patients.3.The absence of oscillatory theta activity in frontal region could be one of the important reasons of the WM dysfunction for MCI patients.