General introduction

Abstract

Brain oscillations and cognitive behavior Complex behavior and cognition in humans and animals require timed activity of networks of neurons in different brain areas. Groups of neurons that are involved in a task, fire synchronously and repeatedly. Depending on the behavior, synchronization of activity of these neurons occurs at particular rhythms. These rhythms are reflected in brain oscillations in the electroencephalogram (EEG). How these rhythms are generated and how rhythmic activity is involved in complex behavior are central questions in neuroscience research, which are still largely unanswered. The research in this thesis is aimed at understanding how synchronized and rhythmic activity is generated in neuronal networks. Neuronal oscillations have been observed over a wide range of frequencies from 0.05 to 500 Hz (Buzsaki and Draguhn, 2004). They are categorized in frequency bands that are present in the EEG depending on the behavioral state (Figure 1.1). Alpha waves (8-13 Hz) occur more prominently during relaxed wake states, while theta (4-8 Hz) and delta (0.5-4 Hz) band activity occurs especially during different states of sleep (Kandel et al., 2000). Faster oscillations in the beta (13-30) and gamma (30-80 Hz) range are more prominent during active behavior. Rhythms such as alpha, theta and gamma often occur simultaneously and interact with each other (Bragin et al., 1995, Csicsvari et al., 2003, Jensen and Colgin, 2007). Gamma oscillations are observed during active states and are thought to be involved in short-term memory and attention. Gamma oscillations are for example present in humans during working memory tasks (Tallon-Baudry et al., 1998). In several brain disorders there are abnormalities in gamma activity, such as ADHD, schizophrenia, autism, Alzheimer's disease and epilepsy (Herrmann and Demiralp, 2005). Furthermore between healthy individuals there are large variations in oscillations properties as well as in cognitive performance (Vogel, 1981). Thus, it is becoming clear that there are correlations between the occurrence of neuronal network oscillations and cognitive behavior, however to understand their exact relation, one needs to consider three (non-exclusive) aspects of brain oscillations. Functions of neuronal oscillations First, brain oscillations in the gamma range may play a role in the temporal binding of information (Gray et al., 1989, Engel et al., 1999, Engel et al., 2001). This theory predicts that neurons that respond to the same sensory object might fire in temporal synchrony with a precision in the millisecond range. In this way information can be stored, processed and