The influence of ageing on complex brain networks: a graph theoretical analysis

Abstract

Objective: To determine the functional connectivity of different EEG bands at the "baseline" situation (rest) and during mathematical thinking in children and young adults to study the maturation effect on brain networks at rest and during a cognitive task.

Methods: Twenty children (8-12 years) and twenty students (21-26 years) were studied. The synchronization likelihood was used to evaluate the interregional synchronization of different EEG frequency bands in children and adults, at rest and during math. Then, graphs were constructed and characterized in terms of local structure (clustering coefficient) and overall integration (path length) and the "optimal" organization of the connectivity i.e., the small world network (SWN).

Results: The main findings were: (i) Enhanced synchronization for theta band during math more prominent in adults. (ii) Decrease of the optimal SWN organization of the alpha2 band during math. (iii) The beta and especially gamma bands showed lower synchronization and signs of lower SWN organization in both situations in adults.

Conclusion: There are interesting findings related to the two age groups and the two situations. The theta band showed higher synchronization during math in adults as a result of higher capacity of the working memory in this age group. The alpha2 band showed some SWN disorganization during math, a process analog to the known desynchronization. In adults, a dramatic reduction of the connections in gray matter occurs. Although this maturation process is probably related to higher efficiency, reduced connectivity is expressed by lower synchronization and lower mean values of the graph parameters in adults.

Figure 1

Presented are the mean SL values in children and adults of the different frequency bands during rest and the cognitive task (math); error bars = 95% of the CI. Significant differences are: (1) Increase of theta band during math in students [Rest values 0.0373 (SD = 0.0072); Math values 0.0407 (SD = 0.0117) i.e., 8.35% higher, P = 0.029]. (2) Increase of gamma band during math in comparison with rest in children [Rest values 0.0289 (SD = 0.0068); Math values 0.0378 (SD = 0.0035) i.e., 23.54% higher, P = 0.010]. (3) At rest the values of beta band were higher in children [Children 0.0379 (SD = 0.0089); Students 0.0300 (SD = 0.0052) i.e., 20.84% higher, P = 0.002]. (4) At rest the values of gamma band were higher in children [Children 0.0289 (SD = 0.0068); Students 0.0237 (SD = 0.0035) i.e., 17.99% higher in children, P = 0.010]. (5) During math, the values of beta band were higher in children [Children 0.0379 (SD = 0.0112); Students 0.0307 (SD = 0.0072) i.e., 18.99% higher in children, P = 0.025]. (6) During math, the values of gamma band were higher in children [Children 0.0378 (SD = 0.0184); Students 0.0262 (SD = 0.0117) i.e., 30.68% higher in children, P < 0.001].

Figure 2

The bars present the mean values of S (the small world index) during the rest situation and the cognitive task (math) in children and adults for all the frequency bands studied (error bars = 95% of the CI). Significant higher are the values of S at rest for the beta band in children in relation to the students [9.37% higher, P = 0.025].