Correlation Between Alpha Rhythm and Cognitive ... - IEEE Xplore

2010 3rd International Conference on Biomedical Engineering and Informatics (BMEI 2010)

Correlation Between Alpha Rhythm and Cognitive Processes Zhihua Chen

Safeng Zou

AI & Bioinformatics Technology Laboratory, Software Technology Institute of Dalian Jiaotong University Dalian 116052, China

Department of Neurology Xinhua Hospital Dalian University Dalian 116021, China

Li Zhao

Jie Hu Meichsner

AI & Bioinformatics Technology Laboratory, Software Technology Institute of Dalian Jiaotong University Dalian 116052 , China

Department of Computer Science, St Cloud State University, St Cloud MN 56301, USA Abstract—This study aims to confirm the relationship between the cognitive processes and frequency of alpha rhythm. Alpha frequencies were calculated based on two events: eyes close and eyes close calculating. These results were then analyzed to find their change. The frequency of alpha rhythm becomes high from eyes close only condition to eyes close calculating. The cognitive processes lead to the increase of the frequency of alpha rhythm. This result indicates that alpha frequency is an important parameter of cognitive performance. A higher alpha frequency is needed to complete a cognitive task Keywords- EEG; Alpha; Frequency; Cognitive;

I.

INTRODUCTION

The past research has shown that the frequency of alpha rhythm is related to subject’s age or health status. Alpha rhythm can be observed at approximately 6 Hz from newborn babies to six month infants. By the end of the first year, infants can have approximately 8 Hz of Alpha rhythm [1]. It is commonly known that alpha frequency increases with age from childhood to puberty [2]. Alpha frequency starts to decline after puberty. Although insignificant change can be found in adults from thirty to fifty, it is easy to confirm that alpha frequency declines in adults over sixty [3]. In other studies, the relationships between the period of EEG alpha rhythm and the reaction time, and between the period of EEG alpha rhythm and age are investigated. A study was performed on a group of 110 subjects aged four to seven years old and the result shows that the period of alpha rhythm decreased inversely with age. It also demonstrates that the major period (measured in seconds) varies as -0.253 Age + 181, while the intermediate period varies as -0.119 age + 127 [4] . In another study, the EEG data of 1416 healthy subjects aged six to thirty-nine years old was analyzed. The result indicates that the percentage time, amplitude and continuity of alpha rhythm in the three leads reached a stable level at 22-25 years of age [5]. Moreover, the study shows that the percentage time in the slower alpha band decreases slightly with age, while in the faster alpha band it increases with age. In many

978-1-4244-6498-2/10/$26.00 ©2010 IEEE

research reports it has been proved that alpha rhythm characteristics are related to subject’s age. The frequency of alpha rhythm is reported to be in relationship with subject’s brain size [6]. Why does alpha frequency change with age? What is the role of alpha frequency? Cognitive and memory performance is an important topic in EEG study. It was wildly discussed by a review of Wolfgang Klimesch that a good performance is related to alpha and theta rhythm [7]. Jokisch D and Jensen O believe that simultaneous alpha, beta and gamma frequency band oscillations are required for unified cognitive operations [8]. It has been found by Ciesielski et al that the alpha band synchronization has effect on cognitive processes. It decreases with high cognitive load and increases with low cognitive demand or with active inhibition of distracters [9]. It is suggested by Jokisch and Jensen that alpha rhythm increase is a result of functional inhibition on brain activity from working memory [10]. It is reported that alpha oscillatory is modulated by cognitive events. The relationship between memory performance and alpha rhythm has been widely studied for a long time. Maltseva and Masloboev demonstrate that shortterm memory span is associated with a ratio of the alpha rhythm frequency [11]. In some study of Williams et al, it was indicated that alpha-like EEG activity may subserve memory processes [12]. Several results have demonstrated that alpha frequency varies as a function of memory performance [13]. From the research on the relationship between cognitive performance and age, it shows that the reaction time of old subjects is slow [14]. Calculation is a complex mental act which requires the integration of multiple processes. It is relative to perception of number information, number operation, calculation and so on. The mental arithmetic, however, is a very important study to investigate the cognitive performance. In general, EEG is used to analyze the variables of different cognitive performance tasks because the characters of different tasks cause different EEG variations [15]. It is suggested that EEG peak alpha frequency neurofeedback can improve cognitive processing speed and executive function [16].

823

It's well known that the power spectral analysis and amplitude analysis are generally used to study EEG. A study has shown that power increases in the theta and lower-beta frequency bands, and an early power increase and subsequent decrease for the alpha band by comparing and analyzing the EEG responses to nouns with either visual semantic properties or with auditory semantic properties [17]. Using amplitude analysis, the gender differences in the functional lateralization are also found [18]. Using these techniques, the many important results have been obtained [19-22]. Similarly, the differences of left and right brain functions are also related to the oscillation amplitude of alpha rhythm. In EEG study, there are many studies based on alpha rhythm research using fixed frequency [23-27]. It has been demonstrated that the frequency of alpha rhythm is a variation depended on age [1, 4-5, 28-29]. The relationship between alpha frequency and age is that alpha frequency is higher in young adult and goes down in elderly. So it has been an interesting topic that why the frequency of alpha rhythm can be changed with age. Can it be changed in a short time range due to different environmental conditions? Is cognitive performance related to the frequency of alpha rhythm? In this study, the frequencies of alpha rhythms on two consecutive different events are analyzed in order to confirm the role of frequency of alpha rhythm in cognitive performance. The purpose is to investigate the relationship between cognitive processes and alpha frequency. II.

negative peak (Cn) is related to the half cycle of the period. The correlative value (C0) can be defined by

C 0 = Pp − Pn .

(1)

The maximum of correlation among the P3, P4, O1 and O2 is used to decide the initial mean frequency of alpha rhythm. The mean frequency (f0) of the alpha rhythm is estimated by the minimum cycle

f 0 = 1 / Cyclemin

(2)

The mean alpha frequency is calculated by applying the least square method to approximate to the power spectral density of filtered EEG data using the initial mean frequency (f0) of alpha rhythm Because some data sets may exhibit clear alpha rhythm, while other data sets may have no dominant alpha rhythm component, it require that absolute threshold of the correlative value be larger than 0.5 to make the initial value f0 of alpha rhythm be effective. Among the four f0 values calculated for the four different electrodes, one of them was selected when the value of f0 corresponding to C0 which is the largest. Another important requirement is that the value of the f0 has to fall in the traditional alpha frequency range.

7