BOLD Response and EEG Gamma Oscillations.

The rhythmic activities in the resting or ˵spontaneous″ EEG are usually divided into several frequency bands (delta: <4 Hz; theta: 4–8 Hz; alpha: 8–12 Hz; beta: 12–30 Hz; and gamma: 30–70 Hz or higher, centred at 40 Hz), which are associated with different behavioural states, ranging from sleep to relaxation, heightened alertness and mental concentration (Lindsley 1952; Niedermeyer and Lopes Da Silva 2004; Nunez 1995). High-frequency EEG oscillations such as gamma oscillations with relatively small amplitudes can be measured on the scalp due to the fact that scalp EEG recording sensors are physically separated from intracranial activities by the resistive skull tissue, which acts as a low-pass filter. Since the amplitudes of the EEG oscillations decrease with increasing frequency, the importance of high-frequency EEG oscillations like gamma oscillations with respect to cognitive functions and disorders is often underestimated compared to slower oscillations. However, in recent years, a special interest in oscillations in the gamma frequency range has emerged in neuroscience, because there is a lot of evidence for a close correlation between gamma activity and cognitive functions (Engel and Singer 2001). Evidence from neuropsychological and physiological studies suggests that consciousness and its different aspects, like sensory awareness for example, can be understood as a cooperating system involving several different brain regions, such as structures responsible for sensory perception, memory functions, executive control, or manipulation of emotion and motivation (Delacour 1997; Young and Pigott 1999). Theories about the neural correlates of consciousness must explain how multiple component processes can be integrated and which mechanisms underlie the dynamic selection of specific components of neuronal responses that gain access to consciousness from all available information. For both aspects, so called ˵neuronal binding″ seems to play an important role (Crick and Koch 1990; Engel and Singer 2001). The concept of dynamic binding by synchronising neuronal discharges has developed mainly in the context of perceptual processing and was first introduced in the context of feature integration (Gray et al. 1989; Treisman 1996) and perceptual segmentation (von der Malsburg 1994). The synchronisation of activity in neuronal assemblies appears to support specific processes during neural communication, whereas the behavioural specificity of synchronisation phenomena suggests a functional role of synchronised activity in neural information processing (Fries 2005). Meanwhile, the concept of binding has been applied to many different domains and is now employed in theories on object recognition (Hummel and Biederman 1992), arousal (Struber et al. 2000), attention (Fell et al. 2003; Niebur 1993; Pantev et al. 1991; Tiitinen et al. 1993), memory formation and recall (Damasio 1990; Herrmann et al. 2004), motor control (Murthy and Fetz 1992), sensorimotor integration (Roelfsema et al. 1997) and language processing (Eulitz et al. 1996; Pulvermuller 1999; Pulvermuller et al. 1995). [ABSTRACT FROM AUTHOR]