Cognitive tasks augment gamma EEG power S.P. Fitzgibbon a,b , K.J. Pope c , L. Mackenzie a , C.R. Clark b , J.O. Willoughby a, * a Centre for Neuroscience and Department of Medicine (Neurology), Flinders University, P.O. Box 2100 Adelaide, SA, Australia b Cognitive Neuroscience Laboratory, School of Psychology, Flinders University, P.O. Box 2100 Adelaide, SA, Australia c School of Informatics & Engineering, Flinders University, P.O. Box 2100 Adelaide, SA, Australia Accepted 3 March 2004 Abstract Objective: Gamma EEG oscillations are low amplitude rhythms in the 30 – 100 Hz range that correlate with cognitive task execution. They are usually reported using time-locked averaging of EEG during repetitive tasks. We tested the hypothesis that continuous gamma EEG would be measurable during mental tasks. Methods: We investigated sustained human gamma EEG oscillations induced by 8 cognitive tasks (Visual Checkerboard, Expectancy, Reading, Subtraction, Music, Expectancy, Word learning, Word recall, and a Video Segment) in 20 subjects using standard digital EEG recording and power spectral analysis. Results: All of the cognitive tasks augmented gamma power relative to a control condition (eyes open watching a blank computer screen). This enhancement was statistically significant at more than one scalp site for all tasks except checkerboard. The Expectancy, Learning, Reading and Subtraction tasks expressed the most impressive gamma response, up to 5 fold above the control condition and there was some task-related specificity of the distribution of increased gamma power, especially in posterior cortex with visual tasks. Conclusions: Widespread gamma activation of cortical EEG can easily be demonstrated during mental activity. Significance: These results establish the feasibility of measuring high frequency EEG rhythms with trans-cranial recordings, demonstrate that sustained gamma EEG activity correlates with mentation, and provides evidence consistent with the temporal binding model. q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Spectral analysis; Theta; Alpha; Beta 1. Introduction Gamma EEG oscillations (low amplitude rhythms in the 30–100 Hz range) became a topic of intense interest in humans (Aoki et al., 1999; Joliot et al., 1994; Keil et al., 1999; Revonsuo et al., 1997; Sauve et al., 1998; Tallon- Baudry et al., 1998) after it was established in animal models that synchronous recurrent discharge bursts within the gamma range are involved in perception and cognition and are correlated with cognitive task execution (Engel and Singer, 2001). In several animal studies, gamma EEG derived from cortex using small electrodes has been closely correlated with both local multi-unit and single-unit discharges. Synchronous gamma discharges have been identified in the visual, auditory, somatosensory, olfactory, motor and memory modalities in a wide range of animal species (Engel and Singer, 2001). Within these areas, there were synchro- nous bursts within groups of neurons in different cortical columns (Gray et al., 1990), spatially distributed within the same hemisphere (Engel et al., 1991a; Frien et al., 1994), between inter-hemispheric sites (Engel et al., 1991b) and in sub-cortical structures (Alonso et al., 1996). Other animal experiments have demonstrated the functional significance of synchronous gamma oscillations by showing that synchroneity of gamma discharges correlates with cognitive function (Fries et al., 1997; Murthy and Fetz, 1996; Roelfsema et al., 1997) and that disrupted synchroneity of gamma discharges correlates with diminished cognitive function (Roelfsema et al., 1994; Stopfer et al., 1997). In humans, trans-cranial EEG integrates measures summed electrical fields which are volume conducted from a large population of neurons within an extensive region around a recording site. Such measurement is unlikely to detect EEG rhythms derived from small cell Clinical Neurophysiology 115 (2004) 1802–1809 www.elsevier.com/locate/clinph 1388-2457/$30.00 q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2004.03.009 * Corresponding author. E-mail address: john.willoughby@flinders.edu.au (J.O. Willoughby).