Neuroscience Letters 402 (2006) 273–277 Using graph theoretical analysis of multi channel EEG to evaluate the neural efficiency hypothesis Sifis Micheloyannis a,d,∗ , Ellie Pachou a , Cornelis J. Stam b , Michael Vourkas c , Sophia Erimaki a , Vasso Tsirka a a University of Crete, Medical Division, 71409 Iraklion/Crete, Greece b Department of Clinical Neurophysiology, VU University Medical Center, P.O. Box 7057, 1007 MB Amsterdam, The Netherlands c Technical Education High School, Iraklion/Crete, Greece d Technical High School of Crete, Cnania/Crete, Greece Received 9 January 2006; received in revised form 9 April 2006; accepted 12 April 2006 Abstract Previous studies demonstrated that intelligence is significantly related to an impressive array of psychological, social, biological and genetic factors and that working memory (WM) can be considered as a general cognitive resource strongly related with a wide variety of higher order cognitive competencies and intelligence. Also, evaluating the WM of subjects might allow one to test the neural efficiency hypothesis (NEH). WM typically involves functional interactions between frontal and parietal cortices. We recorded EEG signals to study neuronal interactions during one WM test in individuals who had few years of formal education (LE) as compared to individuals with university degrees (UE). The two groups of individuals differed in the scores they obtained in psychological tests. To quantify the synchronization between EEG channels in several frequency bands, we evaluated the “synchronization likelihood” (SL), which takes into consideration nonlinear processes as well as linear ones. SL was then converted into graphs to estimate the distance from “small-world network” (SWN) organization, i.e., an optimally organized network that would give rise to the data. In comparison to LE subjects, those with university degrees exhibited less prominent SWN properties in most frequency bands during the WM task. This finding supports the NEH and suggests that the connections between brain areas of well-educated subjects engaged in WM tasks are not as well-organized in the sense of SWN. © 2006 Elsevier Ireland Ltd. All rights reserved. Keywords: Neural efficiency; Graph theory; Small-world network; Synchronization likelihood; Working memory Measurement of the brain’s glucose metabolism using positron emission tomography (PET) showed that cortical activation is more strongly focused and glucose consumption lower in more as compared to less intelligent individuals [9,10]. This observa- tion has been explained by the neural efficient hypothesis (NEH), which predicts that lower and more focused cortical activation reflects higher neural efficiency. Thus, more intelligent subjects are expected to require less brain activation to accomplish a task and easier tasks are expected to produce lower brain activation in relation to difficult tasks. Additional studies using fMRI and EEG to evaluate local brain activation during cognitive tasks also supported this hypothesis [5,7,14,17–20,32] (see however ref. [18] for a discussion of studies failing to support the NEH). ∗ Corresponding author. Tel.: +30 6932431138. E-mail address: michelosifis@yahoo.com (S. Micheloyannis). Working memory (WM) can be considered as one general cogni- tive resource strongly related with a wide variety of higher order cognitive competencies and intelligence [4,7]. If WM underlies the mental abilities of normal individuals its study could allow one to evaluate the NEH. Here, we used a WM test such as the 2Back to check if persons that differ in their mental abil- ities (as shown by psychometric testing) due to the education they received (lower or higher) also differ in terms of neural organization at the network level. On the basis of the NEH, we hypothesized that brain activation is less intense when more edu- cated individuals engage in cognitive tasks such as the 2Back WM test. This test is not particularly demanding so that less well-educated individuals do well albeit with longer reaction times and some failures. Our study is related to previous evalu- ations of the NEH in individuals with low or high intelligence. The latter term is used here in the sense of the empirical con- struct “g” (general intelligence) as discovered and described by 0304-3940/$ – see front matter © 2006 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2006.04.006