ReviewHuman gamma-band activity: A review on cognitive and behavioral correlates and network models
Introduction
Since the discovery of the electroencephalogram (EEG) by Berger (1929), oscillatory patterns can be observed in brain electrical activity. The most prominent oscillation in spontaneous EEG can be found in a frequency band of 8–12 Hz, which was considered by Berger as the basic rhythm and was named alpha-rhythm. Alpha oscillations appear with large amplitudes between 10 and 50 mV. The chronologically next identified frequency range between 12 and 30 Hz was named by Berger consequently with the Greek letter beta. Faster oscillations in the human EEG between 30 and 80 Hz could be identified only later, since the amplitudes of EEG oscillations decrease with increasing frequencies. The term gamma activity was coined for this higher frequency band. After it had been identified, the gamma frequency range was demonstrated to correlate with many cognitive functions which will be the topic of this review.
Section snippets
Cognitive functions of gamma-band activity
Before addressing the cognitive aspects of human EEG gamma oscillations, we need to briefly introduce an important property of these responses. In response to sensory stimulation, a gamma-band response (GBR) is typically evoked. This has been shown for auditory (Pantev, 1995), visual (Hoogenboom et al., 2006), and somatosensory (Chen and Herrmann, 2001) stimuli in humans and for olfactory stimuli in animals (Eeckman and Freeman, 1990). At an early latency, these stimuli evoke a GBR that can be
Behavioral and perceptual relevance of gamma oscillations
The section above clearly points out numerous task-dependent modulations of GBRs in different experimental conditions, tasks, and modalities. However, there is much less evidence for the actual behavioral and perceptual relevance of these oscillations. More precisely, as suggested by Sejnowski and Paulsen (2006), if oscillations are essential for and related to perception and behavioral responses, these processes should be impaired by disturbing the oscillations which should lead to behavioral
Neural mechanisms of gamma-band activity
In this section we will explore several models that have been proposed to account for the emergence of gamma-band activity.
In a series of studies on hippocampal brain slices, Traub et al. (1998) and Whittington et al. (2000) demonstrated that neural networks that include mutually interconnected inhibitory interneurons synchronize at high frequencies, predominantly in the gamma-band. They studied two extreme cases that are replicated in Fig. 4. Clearly, if a network does not incorporate any
Conclusions
In conclusion, we want to stress four aspects of human gamma-band activity. First of all, they reflect cognitive processes – among which memory-matches seem to be most prominent and probably fundamental for others, such as attention, binding, object representation, and language. Secondly, these cognitive modulations cannot only be found on the late induced GBR but already on the early evoked GBR which is generated in early sensory cortices. Thus, GBRs provide evidence that already very early
Acknowledgments
IF was supported by the BMBF Bernstein Group for Computational Neuroscience. CSH and DL were supported by the Deutsche Forschungsgemeinschaft (grants HE3353/2 and SFB-TRR/31-A09).
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